SPATIAL AND TEMPORAL DYNAMICS OF BENTHIC CHLOROPHYLL FORMATION
ON THE NORTHWEST FLORIDA CONTINENTAL SHELF
Ebenezer S. Nyadjro
A Thesis submitted to the
University of North Carolina Wilmington in Partial Fulfillment
of the Requirements for the Degree of
Master of Science
Center for Marine Science
University of North Carolina Wilmington
2009
Approved by
Advisory Committee
Dr. Frederick M. Bingham
Dr. Daniel Kamykowski
Dr. John M. Morrison
Chair
Accepted
by
DN: cn=Robert D. Roer, o=UNCW, ou=Dean of the
Graduate School & Research, email=roer@uncw.edu,
c=US
Date: 2009.12.16 09:50:02 -05'00'
Dean, Graduate School
JOURNAL PAGE
This thesis is written following the style guide of UNCW Thesis Format Manual and the
referencing style of the journal Limnology and Oceanography.
ii
TABLE OF CONTENTS
ABSTRACT ................................................................................................................................... iv
ACKNOWLEDGMENTS ............................................................................................................. vi
DEDICATION ............................................................................................................................. vii
LIST OF TABLES ....................................................................................................................... viii
LIST OF FIGURES ....................................................................................................................... ix
INTRODUCTION ...........................................................................................................................1
Overview ..............................................................................................................................1
Objectives of Study ..............................................................................................................3
Background ..........................................................................................................................4
METHODS AND ANALYSES .......................................................................................................9
Site Description ....................................................................................................................9
Field Sampling ...................................................................................................................10
Data Analysis ....................................................................................................................15
RESULTS .....................................................................................................................................15
DISCUSSION ................................................................................................................................24
CONCLUSION ..............................................................................................................................29
REFERENCES ..............................................................................................................................32
iii
ABSTRACT
Two cruises, BenDim I (May 2008) and BenDim II (October 2008), were carried out to
study the effects of physical water properties and motion on the spatial and temporal dynamics of
near-bottom chlorophyll on the northwest Florida continental shelf. Data were collected with a
Sea Sciences’ AcrobatTM towed undulating profiling system. Wind data from NOAA buoys and
QuikSCAT ocean surface wind fields obtained from NASA were also analyzed. May results
showed Chl a concentrations were generally slightly higher than October values and exhibited a
clear horizontal distribution as compared to a vertically stratified distribution in the later. In the
October study, two patterns that were separated by the passage of an atmospheric front were
observed; pre-frontal distributions that looked more like the May ones (only with weaker
stratification) and post-frontal distributions that were more vertically stratified. In most cases,
surface Chl a concentrations ranged between ~0.2 and 0.6 µg l-1 while near-bottom
concentrations were between ~0.5 and 1.6 µg l-1. During BenDim II, near-bottom Chl a max
were observed between 25 m and 30 m, from 30.10° N to 30.15° N. Patterns in the distributions
were mainly due to the effects of water stratification and the impacts that wind speed and
direction had on it. The results also suggest a seasonal evolution of Chl a maxima on the shelf. It
begins to form in the spring within the main thermocline as a result of primary production in the
surface layer when the water column is replete with nutrients. As the spring and summer
progresses, the maxima sink further in the water column below the thermocline. By the time of
the fall cruise, phytoplankton find another source of nutrients, the bottom benthic layer, where
Chl a maximum formed in very low light conditions. I speculate that during this period
phytoplankton acquire nutrients within the bottom layer, migrate up into the water column during
the day to get light for photosynthesis. These conditions remain until the passage of storms
iv
during the late fall which bring nutrients back up from the bottom into the water column setting
the stage for the spring bloom.
v
ACKNOWLEDGMENTS
I am most grateful to the Lord Almighty for the strength and endurance to go through this
academic exercise. A dozen thanks to my advisor Dr. John Morrison for his patience,
supervision, constructive criticism, and diverse contributions that made this work see its last ink.
I would also like to thank my committee members, Dr. Frederick Bingham and Dr. Daniel
Kamykowski, for their patience, support, guidance and immense help throughout this work. My
sincere gratitude to Dr. Dylan McNamara for introducing me to Matlab and for helping with this
work. I am grateful to Dr. Wendy Woods for data collection with the AcrobatTM and for Matlab
codes and to my lab mate Michael Taylor for his support, notes and Matlab codes. My
appreciation also goes to Dr. Joan Willey, CMS Associate Director of Education for her
numerous help throughout my stay here at UNCW. Many thanks to Irene Acheampong for the
encouragement that helped me stay focused to persevere and finish my MS. To Philip Baiden
and Linda Denkyi, your support is very much appreciated. Last but not the least, I am grateful to
the National Science Foundation (NSF) for their funding support for this study and the crew of
RV Pelican for their assistance in data collection.
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DEDICATION
I would like to dedicate this thesis to my parents, Mr. John Nyadjro and Mrs. Matilda
Nyadjro and my entire family for their unflinching support and interest in what I do.
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LIST OF TABLES
Table
Page
1. Summary of measurements made. .............................................................................................14
viii
LIST OF FIGURES
Figure
Page
1. A depiction of MPB, nutrient and light fields (blue) on the continental shelf...........................36
2. Schematic of a stratified shelf sea showing the profile of temperature, nutrient, PAR and the
trapping of chlorophyll. The spiral sizes depict the varying turbulent intensity within the water
column............................................................................................................................................37
3. A map showing the Gulf of Mexico with schematic cartoons showing the Loop Current, and
eddy that has been shed from the Loop current and the Gulf Stream. (source:
http://ocean.tamu.edu/Quarterdeck/QD6.1Graphics/eddy-map.gif) ..............................................38
4. Figure showing the northeastern Gulf of Mexico region and AcrobatTM tracks for the two
cruises, BenDim I (red lines) and BenDim II (yellow lines). (source: Map modified after
http://pubs.usgs.gov/of/2000/of00-019/htmldocs/images/browse/bathyc.gif)...............................39
5. Plots of tow M1 variables measured with AcrobatTM at time indicated. Arrow shows the
direction of tow relative to shore ...................................................................................................40
6. Plots of tow M2 variables measured with AcrobatTM at time indicated. Arrow shows the
direction of tow relative to shore ...................................................................................................41
7. Plots of tow M3 variables measured with AcrobatTM at time indicated. Arrow shows the
direction of tow relative to shore. There were no valid data on sections indicated as tow was done
in the night .....................................................................................................................................42
8. Wind field in Pensacola between 5/12/2008-5/25/2008 ............................................................43
9. 05/15/2008 QuikScat wind vector plot for NW Florida ............................................................44
10. 05/16/2008 QuikScat wind vector plot for NW Florida ..........................................................45
11. 05/18/2008 QuikScat wind vector plot for NW Florida ..........................................................46
12. 05/19/2008 QuikScat wind vector plot for NW Florida ..........................................................47
13. 05/22/2008 QuikScat wind vector plot for NW Florida ..........................................................48
14. 05/15/2008-05/22/2008 averaged QuikScat wind vector plot for Gulf of Mexico..................49
15. Plots of tow O1 variables measured with AcrobatTM at time indicated. Arrow shows the
direction of tow relative to shore ...................................................................................................50
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16. Plots of tow O2 variables measured with AcrobatTM at time indicated. Arrow shows the
direction of tow relative to shore. There were no valid data on sections indicated as tow was done
in the night .....................................................................................................................................51
17. Plots of tow O3 variables measured with AcrobatTM at time indicated. Arrow shows the
direction of tow relative to shore ...................................................................................................52
18. Plots of tow O4 variables measured with AcrobatTM at time indicated. Arrow shows the
direction of tow relative to shore ...................................................................................................53
19. Plots of tow O5 variables measured with AcrobatTM at time indicated. Arrow shows the
direction of tow relative to shore ...................................................................................................54
20. Plots showing the extinction of corrected PAR (CPAR) at depth for (a) 10/17/2008 and
(b)10/20/2008 .................................................................................................................................55
21. Wind field in Panama City between 10/12/2008-10/25/2008..................................................56
22. Wind field in Pensacola between 10/12/2008-10/25/2008 ......................................................57
23. 10/15/2008 QuikScat wind vector plot for NW Florida ..........................................................58
24. 10/16/2008 QuikScat wind vector plot for NW Florida ..........................................................59
25. 10/17/2008 QuikScat wind vector plot for NW Florida ..........................................................60
26. 10/19/2008 QuikScat wind vector plot for NW Florida ..........................................................61
27. 10/20/2008 QuikScat wind vector plot for NW Florida ..........................................................62
28. 10/21/2008 QuikScat wind vector plot for NW Florida ..........................................................63
29. 10/15/2008-10/22/2008 averaged QuikScat wind vector plot for Gulf of Mexico ..................64
30. WinRiver II screen plots of ADCP 1 data showing (a) current magnitude (m s-1),
(b) current direction (°) and (c) 125° earth projected current velocity (m s-1). Plots are oriented
with reference to the direction of the survey; offshore (deeper water) is on the west and inshore
(shallower water) is on the east of the plots ...................................................................................65
31. WinRiver II screen plots of ADCP 2 data showing (a) current magnitude (m s-1),
(b) current direction (°) and (c) 125° earth projected current velocity (m s-1)...............................66
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INTRODUCTION
Overview
Chlorophyll has been used in the inference and study of phytoplankton. This is usually
based on the fact that all phytoplankton contain chlorophyll and also on chlorophyll’s absorption
and fluorescence (re-emission of absorbed light energy) properties that are quite easy to measure
and quantify (Tester et al. 2008). They can thus be used for the inference of spatial and temporal
dynamics of phytoplankton biomass. Chlorophyll acts as an proxy for photosynthesis and
therefore in places and conditions where phytoplankton primary productivity occurs, chlorophyll
is most certain to be present. Its action allows primary producers to obtain light energy and it is
as a result most likely to be found in the photic zones of the aquatic ecosystem. It has however
been noted that benthic microalgae are sometimes major primary producers on continental
shelves. This occurs in areas where there is sufficient light flux to the benthic zone to support
photosynthesis. Enhanced chlorophyll concentrations in near-bottom waters can therefore be
observed on some continental shelves (Cahoon et al. 1990; Nelson et al. 1999). The contributors
to near-bottom chlorophyll are the microphytobenthos (MPB) and phytoplankton in the water
column. When present, the MPB lie in the sediments in the euphotic zone, above the nutrient
sources (Figure 1) and are able to contribute to near-bottom chlorophyll via suspension. The
contributions from water column phytoplankton becomes even more enhanced in the presence of
clear water where light is able to reach deeper depths. This study sought to investigate the water
column characteristics and flow dynamics that favor the occurrence of near-bottom chlorophyll
on the continental shelf off Panama City, Florida.
The formation of near-bottom chlorophyll maxima is dependent on the sources of the
chlorophyll and the water dynamics that dictate its distribution in time and space. Chlorophyll in
near-bottom waters thus come from several sources including resuspension from sediments,
settling phytoplankton from overlying waters, suspension of microphytobenthos, and in situ
growth in the lower water column (Cahoon 1990; Verity et al. 1998; Yamaguchi et al. 2007).
The mechanisms by which chlorophyll from the above sources reach the near-bottom are an
interplay between varied processes such as estuarine and river run-off, surface gravity waves,
bottom shear stress and tidal turbulence. These processes could inhibit or promote the
occurrence of near-bottom chlorophyll. For example they could limit the sinking of
phytoplankton or change the quality of water and hence the penetration of photosynthetically
active radiation (PAR). Some relations between chlorophyll and these processes include
concentrations being inversely correlated to wave actions, water currents and storm activities
(Pamatmat 1968; Colijn and Dijkema 1981) and a positive correlation between winds and
chlorophyll (Verity et al. 1998). Yamaguchi et al. (2007) found that spatial and temporal
variations in benthic chlorophyll concentrations were negatively correlated to surface values and
that they could be two to four times less than surface values. In the development and persistence
of thermoclines and nutriclines, however, benthic values could rise substantially (Darrow et al.
2003). During these times, the depth at which chlorophyll maxima occur coincides with the
location at which the concentration of limiting nutrient element begins to increase with depth.
Darrow et al. (2003) added that chlorophyll values within the upper 1 cm of sediment in
about 30 m depth on the West Florida Shelf (WFS) could be two to four times higher than the
overlying water column. This was as a result of about 10% of surface PAR reaching that depth.
Thus notwithstanding the contributions of primary production from settling phytoplankton,
which is further enhanced by sufficient PAR in shallow waters, there is enough cause to believe
that microphytobenthos are a significant source of benthic chlorophyll. In summary, the
2
formation of chlorophyll maxima at the benthic zone could be attributed to photoadaptation that
causes cell chlorophyll content to increase, the presence of a nutricline that causes increased
phytoplankton growth rates and physical buildup of phytoplankton cells at water interfaces such
as thermoclines and fronts (Marcias et al. 2008).
The study area is a broad and gently sloping continental shelf whose physical water
characteristics are under the influence of local wind-driven upwelling, local outflow of turbid,
low-salinity river and estuarine plumes. The semi-enclosed nature of the west Florida basin
affects circulation features that in turn makes the effects on the advection of biological materials
quite complex to decipher (He and Weisberg 2003). Occasionally, the Loop current also
influences processes on the WFS. The interactions between the Loop current with the topography
of the shelf lead to the upwelling of nutrients and the dispersion of chlorophyll into the interior
of the shelf (Biggs and Muller-Karger 1994). The hydrographic conditions driving chlorophyll
distribution in time and space in the eastern Gulf of Mexico (GoM) shelf are somewhat different
than those in the western GoM (Elliot 1982; Vidal et al. 1994).
This study sought to investigate the distribution of benthic chlorophyll in relation to the
hydrographic dynamics on the continental shelf off Panama City, Florida.
Objectives Of Study
This work was one component of a larger-scale multidisciplinary and multi-institutional
project Benthic Dinoflagellate Migration (BenDiM): Occurrence and Processes and it involved a
team made up of investigators from University of North Carolina Wilmington (UNCW) and
North Carolina State University (NCSU). The team from UNCW focused on collection,
processing and interpretation of the physical regime as observed on the continental shelf and the
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use of this data to describe the water mass distribution and flow characteristics defining the
spatio-temporal dynamics of benthic chlorophyll on the northwest Florida shelf (NWFS). My
thesis involved the primary responsibility for processing, display and analysis/interpretation of
the unique set of data that was collected by the AcrobatTM Towed-Undulation Profiler.
My specific objectives were to:
1. Examine the spatial and temporal distribution of near-bottom chlorophyll that developed
during the study period.
2. Use hydrographic and current measurements to characterize water masses and shelf dynamics
to help explain the spatial and temporal distribution of near-bottom chlorophyll on the NWFS.
Background
Near-bottom chlorophyll can occur on continental shelves under the influence of varied
oceanographic processes (Nelson et al. 1999; Yamaguchi et al. 2007), including tides, turbulent
mixing, wind stress, offshore Ekman transport, and upwelling and downwelling of nutrients.
These are usually seasonally driven and their effects are reflected in the spatial and temporal
variability of benthic chlorophyll dynamics. The presence of nutrients in the euphotic zone and
water column plays an active role in the occurrence of chlorophyll in the water column.
Consequently, any process affecting the availability of nutrients could affect the dynamics of
chlorophyll too. Figure 2 for example, depicts a profile for nutrients, temperature, PAR and some
physical processes affecting their distribution. This snapshot is for a water-stratified season (e. g
summer time in temperate regions) during which chlorophyll is entrained in the thermocline and
the advection of nutrients from the bottom layer is also restrained. In such times, the wind mixed
surface layer is separated from the tidally mixed bottom layer. Nutrients can however be
4
sequestered from the sediments and be transported into the chlorophyll-entrained thermocline by
tidal turbulence. This tends to promote primary productivity in the thermocline. In the GoM, the
aforementioned scenario will occur between May and early October when the water column is
stratified as a result of high solar radiation (Müller-Karger et al. 2000). Beginning early October
when winds become strong, the thermocline collapses leading to mixing of nutrients and
plankton cells in the water column. Chlorophyll can therefore not be expected to concentrate in a
particular section of the water column.
The strength of current velocities affects the distribution of chlorophyll in the benthic
region and its overlying water column. Uneven distribution of benthic chlorophyll has been
reported in low-water flow rates (Snow et al. 2000). This usually occurs when the boundary layer
between the water and sediment bed below the euphotic zone is well developed and structured.
However, as the water flow rate increases, the thickness of the boundary layer is reduced
enabling the transport of chlorophyll from the sediment bed into the overlying water column.
The strength of the current velocities affects the depth in the water column at which nutrients
from the sediments are acted upon and also determine the distribution of chlorophyll. If they are
strong enough they push the nutrients below the bottom of the euphotic zone. If they are only
able to reach below the euphotic zone, the nutrients from the sediments become readily available
to pelagic producers in the water column.
The GoM is generally said to be oligotrophic (Steidinger 1973; Lohrenz et al. 1999;
Darrow et al. 2003). The West Florida continental shelf is however quite productive at some
times in the year as a result of land-based nutrients and a shallow, broad shelf that allows the
effective penetration of PAR (Gilbes et al. 1996; Darrow et al. 2003). In many such oligotrophic
waters, vertical patterns in the distribution of chlorophyll, and especially in the near-bottom, are
5
not linked to just the productivity of photic region but also to the hydrographic conditions at
varied spatio-temporal scales (Estrada et al. 1999; Marcias et al. 2008). Steidinger (1973)
estimated that chlorophyll a concentrations in the nearshore waters of the WFS averaged about
1–2 μg l-1. This concentration reduces seaward and rarely exceeds 0.3 μg l-1 beyond the 40-m
isobath. Darrow et al. (2003) reported that surface chlorophyll concentrations of up 6 μg l-1 had
been recorded on the WFS. High concentrations, between 2 to 30 μg l-1, have also been observed
during a bloom of the toxic Karenia brevis. When upwelling had resulted in favorable
conditions, the intrusions of slope water have produced up to 10 μg l-1of chlorophyll in nearbottom waters on the WFS (Walsh et al. 2003).
The interactions among convective mixing, river runoff and fronts and benthic
chlorophyll in the eastern GoM can be quite diverse and significantly define the thermohaline
structure in the NWFS (He and Weisberg 2002b; Walsh et al. 2003). These processes are
essential in productivity and other biological activities in the benthic communities along the
upper slope and outer continental shelf regions of the northeastern GoM (Tester and Steidinger
1997) by virtue of the importance of the thermohaline structure in water motion and entrainment,
mixing and the distribution of pigments (e.g. chlorophyll), and particles and organisms. The
structure of the thermocline controls the advection of materials. In spring when the water column
starts to be stabilized, chlorophyll begins to accumulate in lower euphotic layers where sufficient
light and nutrients enable the sustained growth of phytoplankton. In winter when stratification is
eliminated, vertical circulation intensifies and causes the distribution of chlorophyll in the water
column. The distribution of benthic chlorophyll is greatly enhanced by the dynamics of intrusion
(e.g. the loop current, salt tongues) and the onset of stratification. In stratified conditions, bottom
Ekman layers have been noted to aid in the across-shelf transport of biologically important
6
materials such as chlorophyll (He and Weisberg 2003). Near-bottom chlorophyll concentration
has also been known to peak during intense stratification (Verity et al. 1998). This has partly
been attributed to the increase in current velocity away from the boundary layer. This usually
occurs along an axis that is perpendicular to the substrate and the extent of which is influenced
by the ambient current velocity and the angle at which it is directed at the substrate (Stevenson
1983).
Chlorophyll dynamics in the coastal environment involves an interplay between benthic
ecology and nearshore oceanography. For example, the broad WFS is affected more on the inner
shelf by local coastal wind forcings and more on the middle and outer shelf by the interactions
with other oceanographic processes that affect the eastern GoM (Weisberg et al. 2000). Storm
events supply upwelled nutrient-rich water to the WFS that affect the productivity of
phytoplankton (Tester and Steidinger 1997). Nutrients and the light availability at shallower
water depths also contribute to high chlorophyll in the upwelling areas on the shelf break of the
NWFS. He and Weisberg (2003) observed this to extend inshore along the bottom.
Chlorophyll distribution dynamics over continental shelves are varied as a result of
influence of surface tongues of low-salinity and chlorophyll-rich coastal waters, intense mixing,
coastal upwelling and across shelf transport. Coastal waters are usually more productive than
deeper waters and therefore can contain more chlorophyll. The interactions between these two
water types result in the redistribution of biological materials. In the GoM, it has been said that
the interaction of eddies with the slope and the convergence of along-shelf currents are
responsible for the cross-shelf transport of chlorophyll (Müller-Karger et al. 1991; Biggs and
Müller-Karger 1994). While eddies usually transport waters whose chlorophyll contents are
determined by shelf circulation, along-shelf currents transport chlorophyll when coastal wind
7
stress components collide with the concave shaped eastern GoM (Martínez-López and ZavalaHidalgo 2009). Because eddies are not usually present in the northwestern Florida shelf, the later
scenario is more likely to occur there. The intrusion of the Loop current into the coastal waters of
the NWFS appears to be rare (Weisberg and He 2003). However when it does occur, it tends to
be obstructed by interactions with the topography of the coast influencing the transport of
materials in the northeastern GoM. This influence causes a stronger on-shore transport of
chlorophyll within the bottom Ekman layer by intensifying the mid-shelf currents. This also
brings deep nutrient rich water to the inner shelf that enhances biomass production (He and
Weisberg 2003).
Fronts have been recognized as contributing to the distribution of chlorophyll in water
columns. They mark the transition between water masses and cause an increase in chlorophyll
concentrations and pelagic primary production especially if they are well positioned (Janowitz
and Kamykowski 2006). A favorable position is usually determined by the balance between the
currents and the scale of fresh water inflow (Josefson and Conley 1997). With currents in the
coastal waters of the NWFS being strongly wind driven and with the large influence of
freshwater, the occurrence of fronts, subsequent enhancement of chlorophyll and its transport to
the benthic zone cannot be ruled out. Stegmann and Ulmann (2004) for example, observed
chlorophyll fronts in the inner shelf waters south of New England to be more persistent in spring
and summer than in fall and in winter and attributed this to the stability of the water column.
Josefson and Conley (1997) also showed a time persistent pattern of increased chlorophyll in
benthic waters near a pelagic front and in areas with a mixed water column. Chlorophyll
concentrations were increased in the immediate stratified side of the front and the pattern of this
and the entire pelagic front show that the orientation of high values of chlorophyll in the benthic
8
waters was about the same as the average orientation of the pelagic front. Janowitz and
Kamykowski (2006) also showed that cells tend to accumulate near the nearshore of a front in
western Florida and this could contribute to enhanced chlorophyll, especially if adequate
nutrients and light are present. Summarizing, the occurrence of a front could enhance the
pelagic-benthic coupling of chlorophyll when productivity is strong at the front, when mixing
contributes cold, nutrient rich deep water and when there is a good balance between currents and
fresh water flux (Josefson and Conley 1997; Janowitz and Kamykowski 2006).
METHODS AND ANALYSES
Site Description
The study was carried out in the waters of the continental shelf off the coast of Panama
City, Western Florida, U.S.A (Figure 3). The isobaths on the WFS run parallel to the coastline
and the spacing between them decreases from southwest to northwest reaching their minimum at
the DeSoto Canyon in the outer continental shelf off Pensacola (Weisberg and He 2003). The
tides in the GoM are dominated by diurnal (K1, O1) constituents in clear contrast to what occurs
on the east coast of the U.S.A. Nevertheless, semidiurnal constituents (M2, S2) are noticeable in
the eastern GoM but they are more evident in southwest Florida than northwest Florida (He and
Weisberg 2002a). The shelf also has well-defined winter upwelling and summer downwelling
seasons (Yang and Weisberg 1999; Weisberg and He 2003).
The large-scale circulation in the eastern Gulf of Mexico is dominated by the Loop
boundary Current (Behringer et al. 1977; Schmitz and Richardson 1991; Meyers et al. 2001).
Locally, the circulation on the WFS is relatively weak and driven by tidal currents, winds and
buoyancy fluxes (He and Weisberg 2003). Its oceanography is occasionally influenced by the
9
Loop current that periodically (between 3 to 17 months) sheds westward traveling anticyclonic
warm core rings of about 100 to 200 km in diameter (Weisberg and He 2003; Oey at al. 2005;
Lipphardt Jr. et al. 2008). The Loop current together with its frontal eddies, is the largest
circulation feature (Ohlmann and Niiler 2005) and the main source of energy for mesoscale
variability of the Gulf of Mexico.
Field Sampling
Two 7-day cruises (BenDim I and BenDim II) on board R/V Pelican took place in May
2008 and October 2008, between 20 m and 60 m contours within a 64 km by 16 km sampling
area on the continental shelf of Panama City (Figure 4). This timing falls between the upwelling
and downwelling seasons of Weisberg and He (2003) which aided the study of the effects of the
thermocline and nutricline on near-bottom chlorophyll dynamics by examining the presence and
extent of the thermohaline structure. The May transects were west of the October transects.
The primary instrument used in this study was UNCW’s Sea Sciences’ Acrobat
towed
undulating profiling system Malolo. The AcrobatTM was designed to be towed at up to 12 knots,
profiling to 100 m depths. The AcrobatTM provides 5 (five) standard underwater profiles:
constant depth, constant altitude above the bottom, constant undulation between two depths,
adapting undulation between a set depth and varying bottom, and maximum number of
undulations between two survey positions. UNCW’s AcrobatTM is currently instrumented with:
CTD – SeaBird CTD – Conductivity (Salinity), Temperature and Pressure
Dissolved Oxygen – SeaBird O2,
PAR - Biospherical PAR (photosynthetically active radiation; 400-700nm) surface &
underwater
Chl-a - SeaPoint chlorophyll fluorometer
10
CDOM - SeaPoint CDOM fluorometer
Turbidity - SeaPoint turbidity
Plankton Distribution - Brooke-Oceans Laser Optical Plankton Counter (LOPC); size
detection range of 100 μm - 35 mm, shape profiles for plankton from 1.5 - 35 mm in size,
coincidence limit of 106 counts m-3
Flow Volume - General Oceanics flow meter (to record the volume of flow through the
LOPC)
Navigation - integrated GPS and echo sounding location system
Data Display - real-time graphical display.
SeaPoint CDOM fluorometer measures in-situ chromophoric dissolved organic matter
(CDOM). It uses modulated ultraviolet LED lamps and excitation filter to excite CDOM present
in water. The fluorescent light emitted by the CDOM then passes through a blue emission filter
and is detected by a silicon photodiode. The low level signal is then processed using synchronous
demodulation circuitry which generates an output voltage proportional to CDOM concentration
in units of QSDE (CDOM Fluorescence in ppb). The CDOM in-situ data are calibrated against a
quinine sulfate dehydrate equivalent standard hence the units QSDE. The fluorometer has a
minimum detectable level of 0.1 μg l-1 of quinine sulfate.
SeaPoint chlorophyll fluorometer has a minimum detectable level of 0.02 μg l-1. It uses
modulated blue LED lamps and a blue excitation filter to excite chlorophyll a. The fluorescent
light emitted by the chlorophyll passes through a red emission filter and is detected by a silicon
photodiode. The signal is then processed using synchronous demodulation circuitry which
generates an output voltage proportional to chlorophyll a concentration (Chl a).
11
Sea-Bird Electronics PAR sensor measures photosynthetic active radiations between 400
and 700 nm in units of μmol of photons per m2 per second. SEASOFT software then computes
PAR as:
PAR = [multiplier x (109 x 10(V-B) / M) / calibration constant] + offset
where V is the light signal voltage, and M and B are calibration constants.
As per the sensor’s manual, the following were the settings made:
M = 1.0; B = 0.0; calibration constant = 105 / Cw ; multiplier = 1.0; offset = - (104 x Cw x 10V)
When corrected PAR (CPAR) was needed, the following equation was used:
CPAR= 100 x ratio multiplier x underwater light / surface light.
For the purposes of BenDim cruise, the ratio multiplier was set to 1.
The AcrobatTM was towed using different underwater modes: constant depth, constant
altitude above the bottom and maximum number of undulations between two survey positions.
Eight AcrobatTM tows are presented in this work; tows M1, M2 and M3 for the May cruise and
tows O1, O2, O3, O4 and O5 for the October cruise.
Objective 1: Examine the spatial and temporal distribution of near-bottom chlorophyll that
developed during the study period.
The ship’s Multiple Instrument Data Acquisition System (MIDAS) underway system was
used to collect data at 1-2 m depth continuously along the ship’s track. MIDAS recorded winds,
barometric pressure, ambient temperature, relative humidity, in addition to water temperature,
salinity, chlorophyll a fluorescence, transmissometry data and photosynthetically active radiation
(PAR).
12
During AcrobatTM sampling days, a preliminary survey was done once during the day and
once at night along the transects/stations (20 m to 60 m contours) in Figure 4 to find locations
with high benthic chlorophyll and to characterize the water properties. The AcrobatTM and the
ship’s echo sounders were used to guide the run between these contours. Plots from AcrobatTM
data were displayed in real time. Once a “hot spot” of chlorophyll fluorescence was found along
the track, the vessel was stopped for other data collection to be done at the site of the “hot spot”.
This included using a CTD rosette water sampler to collect water samples at 2 m intervals from
the bottom to ~10 m above the bottom, at mid-depth of the remaining water column and then at
the surface.
During the October cruise (BenDim II), two along track surveys (ADCP 1 and ADCP 2)
between the sampling contours were done with a Teledyne RD Instruments vessel-mounted 600
kHz Acoustic Doppler Current Profiler (ADCP) in bottom tracking mode. On this cruise, current
data was collected in blocks of 1 ping per ensemble with each ensemble being 2 seconds. Every
2 seconds, a new profile is recorded using a single ping. A 600 kHz ADCP typically has a depth
range of ~ 50 m with a velocity single-ping standard deviation of ~ 6 cm s-1 when used with
depth cell size of 1 m.
ACDP 1 was done in the afternoon of 17 October 2008 (between AcrobatTM tows O1 and
O2) and lasted for about 4 hours. ADCP 2 was done on the night of 20 October 2008 (after
AcrobatTM tow O5) and lasted for about 2.5 hours.
13
Table 1. Summary of measurements made.
Measurement
Instrumentation/Data
Chlorophyll a
Acrobat
Currents
600 kHz ADCP
Hydrography
AcrobatTM, CTD
In-water light field, PAR
Sea-Bird Electronics PAR sensor
M
Objective 2: Use hydrographic and current measurements to characterize water masses and shelf
dynamics to help explain the spatial and temporal distribution of near-bottom chlorophyll on the
NWFS.
Hydrographic and current data were collected in order to characterize the water’s
physical parameters and short-term variability within which the near-bottom chlorophyll maxima
developed. The AcrobatTM and CTD profilers were used to perform continuous alongtrack and
water column surveys. The information was used to ascertain temporal and spatial changes in
benthic chlorophyll and water properties by examining concentration changes in water column,
near-bottom peaks and nutricline intercept with the sediment. This helped determine where
benthic chlorophyll was concentrated within the water column along the survey tracks.
Marine Meteological Data: Winds
Wind data from the buoy stations closest to the survey area were acquired from the National
Data Buoy Data Center (http://www.ndbc.noaa.gov/):
a. Station PACF1 – 8729108-- is located on the nearshore of Panama City, FL at 30° 9'7" N
85° 40'1" W. The station was not operational during May 2008.
b. Station 42039 is located at 115 nm east-southeast of Pensacola, FL, 28° 47'28" N
14
86° 0'29" W.
Also daily and weekly QuikSCAT ocean surface wind fields collected by the SeaWinds
scatterometer at a horizontal resolution of 12.5 km were obtained from NASA Jet Propulsion
Laboratory (http://poet.jpl.nasa.gov/) for the study periods.
Data Analysis
Data analyses focused on interpretation of the physical regime as observed on the
continental shelf and a description of the water mass distribution and flow characteristics
defining and affecting enhanced near-bottom chlorophyll. Data are displayed as latitudinal-depth
sections and cross section profiles using MATLAB software. ADCP data was processed with
Teledyne RD Instruments WinRiver II software.
The light field in the water was determined from the diffuse attenuation coefficient
(DAC), PAR, surface and profile chlorophyll a estimates, and backscatter coefficients from the
data collected by the CTD and AcrobatTM. Beer’s Law Iz = Ioe-kz, (where Io is surface PAR, k is
the DAC, z is water depth, and Iz is PAR at z) was used to calculate the DAC within the water
column from daytime PAR data. This information determined the depth of the euphotic zone, the
bottom of which receives just about 1% of PAR calculated as Z1% = 1/Kd.ln(1/0.01) (Kirk 1994).
RESULTS
May values were generally slightly higher than October values and showed a clear
horizontal stratification as compared to a more vertical stratification in the later period.
15
May Cruise (BenDim I):
Three AcrobatTM tows from the May cruise (Figure 4) are presented:
M1 (Figure 5) -16 May 2008, 12:18 GMT -16 May 2008,15:44 GMT
M2 (Figure 6) -19 May 2008, 6:48 GMT - 19 May 2008, 10:40 GMT
M3 (Figure 7) -19 May 2008, 20:48 GMT - 19 May 2008, 23:05 GMT
Vector stick plots of winds (Figure 8) from buoy station 42039 show that wind speeds for
12 to 25 May 2008 ranged from ~0.2 m s-1 to ~9 m s-1. Four different wind periods are
discernable in this plot: northerly and northwesterly fields observed between 12 May and midday
of 13 May with speed at ~ 3 m s-1, a predominately southeasterly field between midday of 13
May and midday of 16 May at ~ 5 m s-1, a mixture of northerly, northwesterly and southwesterly
fields between 16 May and 20 May with speeds averaging 3.5 m s-1 and a period of mostly
southwesterly and southeasterly winds between 20 May and 25 May with speed of about 4 m s-1.
QuikSCAT wind vector plots (Figure 9 - Figure 14) with daily and a weekly morning
passes for the region show mostly southerly winds at Panama City on 15 May and 16 May with
speeds of about 5-7 m s-1. The direction then changed to northeasterly between 18 May and 20
May with speed remaining about the same. On the whole (full week averaged plot; Figure 14),
winds were stronger (6-14 m s-1) in the western Gulf of Mexico than the eastern end of it where
speeds were between 2-12 m s-1 with an average of about 6 m s-1. The direction was mostly
easterly and southeasterly in the western GoM as compared to a more southerly in the eastern
GoM.
Chl a concentration in the surface and bottom layers ranged between ~0.75 and 2.0 µg l-1,
with the highest values recorded closer to shore at about 20 m depth. Surface Chl a values were
lower than bottom ones with maximum Chl a values found within the colder bottom water.
16
During tow M1, May 16 12:18 GMT – 15:44 GMT (Figure 5), two clear horizontally delineated
layers of Chl a were observed; a ~0-20 m layer of Chl a with concentration between ~0-0.5 µg l-1
and a ~20-55 m layer of Chl a with concentration between ~0.5-1.2 µg l-1. On the next tow (M2),
19 May 6:48 GMT to 10:40 GMT (Figure 6), three layers were observed with slightly higher
concentrations than in the previous tow. There was a surface layer (0-20 m) and a bottom layer
(~30-45 m) with Chl a concentrations between ~0-0.75 µg l-1. The third layer occurred between
these two and had a higher concentration in the range of ~0.8-1.5 µg l-1. In tow M3, 19 May
20:48 to 23:05 GMT (Figure 7), there were two horizontally delineated layers. Chl a
concentration of the surface layer was ~0-0.6 µg l-1 and the lower layer ~0.6-2.0 µg l-1. The
largest Chl a max (~2.0 µg l-1) was observed in this tow between 15-20 m and at 30.23o N, at
about 10 m from the bottom and right below the thermocline (Figure 7). This tow also had the
widest salinity (32.5-36) and potential density anomaly (24-25.8 kg m-3) range.
During tow M1 (Figure 5), temperature and salinity showed similar distribution with
values ranging between 21-23 C and 34.4-36.4, respectively. The depth distribution of Chl a
concentration followed that of the temperature and salinity distributions with the highest Chl a
concentration occurring at the point of highest salinity and lowest temperature, i.e. 30.05 N and
40-55 m depth. Temperature and salinity plots reveal a well defined thermocline at about 10 m
depth and at 30.25° N and extending to about 30 m depth at 29.95° N. The same stratification
persisted during tow M2 (19 May 6:48 to 10:40 GMT; Figure 6) but was less pronounced. The
warmest temperatures, 22.5-23° C, were observed from surface down to about 20 m depth with a
smaller patch of cold water occurring between 20-45 m depth at 30.10 to 30.15° N. By the end of
the tow M3 (Figure 7), the surface warm water layer had thinned and the cold water had shifted
17
closer to shore with highest values observed between 30.18° N and 30.23° N at 10-25 m depth. It
was also at this point that the highest Chl a concentrations were observed.
Oxygen saturation, potential density anomaly, CDOM, and dissolved oxygen (DO)
followed similar spatial and temporal patterns as that of temperature and salinity. Plots of
potential density anomaly, σθ, show a stable water column throughout the entire study period.
Values ranged between 24-25.5 kg m-3 during the first two tows and became more pronounced in
the last tow with an almost uniform value of 24.5 kg m-3. CDOM values for the entire cruise
ranged between 0.1-0.55 QSDE. In tow M1 (Figure 5), higher values were closer to the shore
and near-bottom. This trend changed a little in tow M2 where the highest values were recorded
near shore and surface waters. In tow 3 the highest values recorded, 0.45-0.60 QSDE, occurred
in the surface layer at about 0-8 m depth. Turbidity was mostly between ~0-1.4 FTU (EPA
Formazin Turbidity Units) in tows M1 and M2 and ~0.5-2.0 FTU in tow M3.
In tow M1, irradiance fraction, computed as a ratio between underwater and surface PAR,
was mostly in the ~0-0.35 range throughout the water column except for a thin surface (0-5 m
depth) layer where values were about 0.5-0.9. Underwater PAR was very low (2-3 µmol m-2 s-1)
compared to values obtained in the earlier tow.
Dissolved Oxygen (DO) was observed to be higher in surface waters than bottom waters
with values in the entire water column ranging between 6 and 8 mg l-1. Differences between
surface and bottom values in tow three were however not as sharp as in the first two tows.
Oxygen saturation was very high especially in surface waters. It ranged between 94% and 104%
with lower values recorded in zones with higher Chl a concentrations.
18
October Cruise (BenDim II):
Five tows from the BenDim II cruise are presented:
O1 (Figure 15)- 16 October 2008, 5:02 GMT -16 October 2008, 10:20 GMT
O2 (Figure 16)-18 October 2008, 2:11 GMT -18 October 2008, 5:41 GMT
O3 (Figure 17)- 18 October 2008, 5:45 GMT -18 October 2008, 9:59 GMT
O4 (Figure 18)- 20 October 2008, 5:42 GMT -20 October 2008, 10:10 GMT
O5 (Figure 19)- 20 October 2008, 10:30 GMT -20 October 2008, 14:48 GMT
Two patterns were observed in the plots of the October cruise; transects (tows O1-O3)
that looked more like the May ones with somewhat weaker horizontal stratification and transects
(tow 4-5) that were vertical. The two were separated following a frontal passage that was
observed on 19 October 2008.
Vector stick plot of winds (Figure 21) from buoy station PACF1 show wind speed ranged
between 0.05 and 7.2 m s-1 for the period 12 to 25 October 2008. During this time five different
periods of wind conditions are noticeable. From 12 October to about midday of 15 October,
winds were predominately northeasterly (of southwest direction) and with speeds from 1.3 m s-1
to 5 m s-1. The second period occurred between midday of 15 October and 18 October where
they were predominantly southwesterly with occasional northerly periods on the 16 October and
part of 17 October. Wind speed during this period ranged between 0.05 m s-1 and 5.7 m s-1 with
quite significant rapid changes. For example it dropped from 5.7 m s-1 at 16 GMT on the 15
October to about 0.5 m s-1 in just about 12 hours. The third period is seen from 18 to 22 October
where the winds were northerly. Speed reduced from 4.6 m s-1 at the start of the period to about 1
m s-1 at the end of it. The wind field then changed briefly (22 to 23 October) to southerly origin
before finally returning to northeasterly with an average speed of about 4 m s-1.
19
Plots from the buoy station 42039 (Figure 22) which is about 155 km northwest of
Panama City, show three different periods of wind conditions with speeds between ~0.5 m s-1 to
~6.5 m s-1. Between 12-17 October, winds were mostly northeasterly with occasional
southeasterly gusts. Wind speeds were rising and falling quite significantly between 0.5 m s-1 and
4 m s-1. Between 17-22 October, wind direction was mostly northerly to northeasterly with
speeds between 0.5 m s-1 and 4.5 m s-1. The third period, 22-25 October, shows winds that were
mostly southeasterly and southwesterly with speeds from 1 m s-1 to 6.5 m s-1.
In order to get a wider view of winds in the Gulf of Mexico, QuikSCAT wind vector
plots with daily and a weekly morning passes are shown (Figures 23-29). On the whole, for the
one week period studied, winds in the eastern Gulf of Mexico were stronger (~7 m s-1 to ~ 13 m
s-1) than those of the western end of it (~1 m s-1 to ~ 10 m s-1). The direction was also mostly
northeasterly in the former and a mixture of northerly and northeasterly in the later. Plots show a
reduction (10 m s-1 to 3.5 m s-1) in the speed of wind to Panama City from 15 October (Figure
23) to 17 October (Figure 25). On the average these winds were mostly northeasterly. Between
19 and 20 October, winds were more northeasterly with speeds gusting to between 12 m s-1 and
14 m s-1 and then reduced to about 6 m s-1 with a mixture of northeasterly to southeasterly
directions on 21 October.
Plots from ADCP 1 (Figure 30) show four identifiable sections of different current
magnitude. One stratum is observed at the eastern end of the plot (inshore) between ensemble
~5150-7000 and 4-20 m depth. Currents here were mostly northward and flowed at about 0.050.15 m s-1. Two strata were observed at the western end (offshore) of the plot; a 4-~15 m depth
of northwestward flow at 0.2-0.35 m s-1 and a 15-40 m depth of northward flow at 0.1-0.15 m s-1.
The other stratum occurred in the central portion of the transect and was made up of mostly
20
currents flowing towards the southwest at 0.01-0.05 m s-1. A 125° earth projected velocity plot
(Figure 30c) show current velocity vectors rotated into along-shelf coordinates to match the
bottom bathymetry of the continental shelf. Onshore currents were mostly flowing towards the
southeast at ~0-0.02 m s-1. The western end of the plot show mostly northwestward currents
flowing at ~0.05-0.22 m s-1. Between these two were mostly southeastward currents that flowed
at ~0.02-0.12 m s-1.
Plots from ADCP 2 (Figure 31) show four regions with distinctively different currents.
The eastern end of the plot (offshore) show two strata; a surface layer (4-15 m depth) with
currents that were mostly southeastward and flowed at 0.15-0.20 m s-1 and a bottom layer (15-40
m depth) with currents that were mostly southwestward with velocity of ~0.10 m s-1. The rest of
the plot is made up of a thin surface strata (4-6 m depth) of northward currents flowing at ~0.00.05 m s-1, a mid-column strata of 6-15 m depth at the western end and 6-25 m depth in the
central portion with eastward currents flowing at 0.05-0.10 m s-1 and a bottom strata (20-25 m
depth) on the eastern end made up of mostly westward currents flowing at 0.05 m s-1. The 125°
earth projected velocity (rotated to along- and cross-shelf coordinates) plot (Figure 31c) show a
small patch of eastward currents of 0.25 m s-1 magnitude at 4-12 m depth on the eastern end of
the transect. A thin surface layer (4-5 m depth) of northwestward currents of speed ~0.05 m s-1 is
observed throughout the rest of the transect. The rest of the water column is made up of
southeastward currents of magnitude between ~0.0-0.15 m s-1.
Chl a distribution in the water column was more homogenous in October than May as the
vertical stratification observed in October was weaker. Higher concentrations were observed
closer to shore than offshore where waters were colder and lower salinity. Surface Chl a values
were usually not significantly different than bottom values in most of the water column except
21
where a near-bottom Chl a max was observed. Concentrations ranged between 0 and 0.8 µg l-1
over most of the water column, except in near-bottom maxima where concentrations were 1.01.6 µg l-1. Near-bottom Chl a max were observed in AcrobatTM tow O1 and tow O2 between 25
m and 30 m, from 30.10° N to 30.15° N latitude (Figure 15 and Figure 16, respectively).
Between 05:45 and 9:59 GMT on 18 October 2008 (tow O3) and 5:42 and 14:48 GMT on 20
October 2008 (tows O4 and O5), plots show quite distinct partitions of Chl a in time and space.
During tow O3 (Figure 17) for example two partitions are noticed; a lower Chl a concentration
(0-0.5 µg l-1) at 30.22° N to 30.24° N, 30° N and 30.11° N and about 20.97° N and 30° N with a
thin layer of mixed concentration in-between this latitude and the former. The other partition, a
mixed Chl a water column, is observed between 30.12° N and 30.21° N. Chl a concentration in
this partition was between 0.6 and 1.4 µg l-1. During tow O4 and tow O5, Figures 18 and 19,
respectively, a Chl a mixed water layer with concentrations between 0.6 to 1.6 µg l-1 is observed
closer to shore (30.25° N to 30.05° N) while a lower concentration (0-0.4 µg l-1) is observed more
offshore (30.05° N to 29.85° N).
Temperature values for the entire cruise in October were higher than those of the May
cruise (as expected) and ranged between 24.5° C and 26.6° C. There was no clearly defined
thermocline as temperature and density increased offshore and seemed to offset each other hence
an almost homogenous potential density anomaly distribution with little to no vertical
stratification. While there was little vertical stratification, the water column was highly
horizontally stratified during this cruise. On each of the tows during the October 2008 cruise,
about five different waters are recognizable based on the temperature and salinity profiles, as
well as potential density. During tow O2 (Figure 16) for example, these occurred between north
latitudes 30.20° and 30.25°, 30.15° and 30.20°, 30.05° and 30.15° and 29.99° and 30.05°. Within
22
the last latitude range, specifically between north latitudes 29.97° and 30° at about 30-42 m
depth, the occurrence of the highest temperature and salinity values for this transect were
observed. A front is recognizable in the plots of tow O1 (Figure 15) between 30° N and 30.15°
N with the Chl a max occurring to the west of it. This Chl a max persisted in the next tow (O2)
until it mixed with the water column in subsequent tows.
Potential density anomaly for the entire cruise ranged between 22-24 kg m-3 with trends
in plots resembling those of temperature, salinity and Chl a. CDOM fluorescence concentrations
showed some variations among the tows. Generally values ranged between 0.15-0.45 QSDE with
the widest range being recorded in the 18 October, 5:45-9:59 GMT tow with values between
0.15-0.6 QSDE. On the average turbidity in the water column was between ~0-2.5 FTU and was
relatively higher than in the May cruise.
For the entire cruise, underwater PAR was low (~0-8 µmol m-2 s-1) except for tow O5
which recorded values between ~0-3000 µmol m-2 s-1. Irradiance fraction values generally
increased offshore and varied from moderate to extreme indicating that the water becomes
clearer the further offshore the section went. During tows O1 and O2, irradiance fraction was
between 0.7- ~1.0. The range was larger during tows O4 and O5 with values between ~0 and ~1.
Plots of corrected PAR (CPAR) versus depth (Figure 20) show that a higher percentage of light
reached deeper depth on 20 October than 17 October. For example whereas on 17 October 10%
of surface PAR reached ~2 m on the shelf, this extended to ~7 m on 20 October. 1% CPAR
reached ~1 m and ~2 m on 17 October and 20 October, respectively.
23
DISCUSSION
This study shows that physical and chemical water properties affect the distribution of
Chl a in both time and space on the continental shelf off Panama City, Florida. Plots of variables
measured by the AcrobatTM in May show the distribution to be quite different than those
observed during the October cruise. Among the most significant difference is a strong
thermocline that separated the surface and bottom layers, with the Chl a profiles coupled to this
thermal structure. The distinct gradient between surface and bottom values occurred at about
18 m in tow M1 and about 15 m in tows M2 and M3. By inference, the water column was more
vertically stable in May than in October.
During the May cruise, the existence of temperature and salinity stratification enabled the
delineation of Chl a by restraining the advection of bottom water to the surface. This prevented
mixing and accounted for the observance of higher Chl a values in the near-bottom water layer
than the surface (Figures 5-7). In the October cruise however, the absence of clear stratification
and the occurrence of wind mixing, especially after the October 19 storm, accounted for the
vertically homogenous Chl a in the water column. This is a seasonal cycle that is observed in the
GoM. The northeastern GoM shelf is known to be highly vertically stratified in the late spring to
early fall when solar radiation is really high (Muller-Karger et al. 2000). As shown in Figure 2,
the presence of a thermocline separates the water column into a light-rich euphotic region and a
nutrient-rich benthic region (Sharples et al. 2001; Ross and Sharples 2007). During this season,
nutrients are generated from the sediment via tidal currents. These become available to
near-bottom producers and if sufficient light is available, lead to the observance of Chl a max
during this season. Additionally, the tidal currents also erode entrained chlorophyll from the
thermocline into the near-bottom. As the stratification starts to disappear with the onset of strong
24
winds in early October (Muller-Karger et al. 2000), the near-bottom Chl a max that was
established in the late spring to early fall begins to be mixed into the water column as seen in
Figure 16C. This mixing will be expected to intensify during the winter months when solar
radiation is minimal, the surface waters are colder and the water column completely destabilized.
Chl a max will therefore be absent. During this season, benthic biota are also suspended and this
leads to increase productivity and Chl a concentration in the water column (Sharples et al. 2001).
The highest mid-water column Chl a max (~1.6-2.0 µg l-1) in BenDim I was observed in
tow M3 (Figure 7). This occurred closer to shore at about 20 m depth and also right under the
thermocline. In a similar observation of Chl a max, Sharples et al. (2001) attributed this to a
spring-neap cycle that modulated stratification and turbulent fluxes. In such situations,
near-bottom nutrients can be transported to the Chl a max but at the same time also cause the
reduction of phytoplankton concentration in the thermocline by mixing it into the near-bottom. It
is also at the thermocline depth that limiting nutrient begins to increase with depth. It can also be
added that in such times when stratification restrict the effective supply of nutrients from below,
pelagic phytoplankton can follow nutrients sources down through the water column and
photosynthesize if optimum light is available. This is further enhanced by the clarity of water
during stratification which will enable deeper penetration of light for photosynthesis. Turbidity
ranged between ~0-1.4 FTU in May when stratification was stronger and ~0-2 FTU in October
when stratification was weaker. If the phytoplankton that get mixed into the near-bottom or those
that migrate there by themselves die, they will decay and add to the nutrients there. If they
survive, they may contribute to Chl a max when the onset of mixing breaks the stratification.
The reduction in frequency of observed near-bottom Chl a max and subsequent
homogenous distribution in BenDim II suggest the contribution of near-bottom biota. This is
25
supported by the immediate response of benthic phytoplankton to the wind events. Lawrence et
al. (2004) suggests that when benthic phytoplankton accumulate during stratified conditions,
they significantly contribute to early mixing periods but are observed less as mixing progresses.
Although the range in salinity and for that matter potential density anomaly for tow M3
was wider (~32.5-36 and 24-25.8 kg m-3, respectively; Figure 7 B, E) than in tow M1 (35.4-36.4
and 24.4-24.6 kg m-3, respectively; Figure 5 B, E) and tow M2 (35-36.5 and 24-25.5 kg m-3,
respectively; Figure 6 B, E), nevertheless the thermocline was the weakest as generally the
change in salinity was gradual. Signoret et al. (2006) attributed such an occurrence to occasional
advection of low salinity waters from the coastal region which tended to modify the density field.
They concluded that the depth of the thermocline and thickness of the euphotic layer were the
main abiotic variables that influenced Chl a distribution and maximal formation of this type.
The circulation on the WFS is relatively weak and driven by winds, tidal currents and
buoyancy fluxes (He and Weisberg 2003). Variability in wind speed and direction therefore
contributes significantly to the spatial and temporal dynamics of Chl a on the Panama City
continental shelf. QuikSCAT averaged weekly plots (Figure 14 and Figure 29) show that the
winds around Panama City were slower (2-4 m s-1) and more northeasterly in May and reversed
to a more southwesterly direction at a faster speed (5-7 m s-1) in October. In shallow waters such
as continental shelf regions, changes in coastal wind direction, stress and curl are essential
processes in the variability and distribution of nutrients and chlorophyll via advection and
mixing. This observation is in agreement with Weisberg et al. (2000) conclusion that the western
Florida shelf is affected more on the inner shelf by local coastal wind forcings and more on the
middle and outer shelf by the interactions with other oceanographic processes. This is further
supported by Verity et al. (1998) conclusions that the distribution of Chl a is enhanced when
26
nearshore circulation is aided by a strong northerly wind to cause surface and mid-surface
currents to move offshore and the bottom currents to move onshore. These are upwelling
favorable conditions and could influence the advection of chlorophyll from benthic waters to the
surface and across shelf. The effects of the winds could also be seen in the nearly homogenous
Chl a distribution that was observed in post-front tows. Winds averaged 2-5 m s-1 before and
during the passage of the atmospheric front but rose to about 10 m s-1 after the front passage.
The occurrence of a westward tilting thermohaline front in tow O1 can be attributed to
the meeting of what appears to be cooler and more saline onshore water with a warmer and less
saline patch of offshore water. A Chl a max formed west of it and was persistent in tow O2
before being mixed into the water column. Such coastal fronts have been observed to accumulate
plankton cells over the WFS (Janowitz and Kamykowski 2006). With adequate light and
nutrients, Chl a concentration can be enhanced in such regions. Coastal fronts are created and
sustained by buoyancy input from freshwater discharge (Blanton et al. 1994). When present they
restrain the exchange of biological materials with the offshore water (Verity et al. 1998; Gibbs
2000). The breakdown of the front in tow O1 is attributed to the strength of the prevailing wind
conditions which led to vertical mixing (Chao 1987). On 16 October the northerly winds of
~2.05 m s-1 had more than doubled to ~5.14 m s-1 on 18 October. The effects show in tow O2
profiles appearing more vertically mixed than in tow O1.With the break of the fronts, the amount
of Chl a was observed to increase and to spread seaward in tow O2. Also by early BenDim II,
the seasonal vertical stratification had weakened considerably and the Chl a max (observed in
BenDim I) while still present was considerably weaker. Later in BenDim II (tow O2, Figure 16),
and during the period of the strong winds mentioned earlier, the vertical stratification was totally
destroyed, the shelf had become horizontally stratified in all variables except Chl a (Figure 16)
27
where Chl a max was still present at ~ 20-25 m depth of the section. From the ADCP I results, it
appears that this was locally generated in a region of weak flow, between stronger along shelf
flows to the N and S.
The Chl a maximum observed in tows O1 and O2 (Figure 15 and Figure 16) occurred
under similar conditions. They both occurred between 20 m and 30 m, 26-26.2° C and 35-35.2
salinity. The main difference was that the max in tow O2 was more horizontally extensive. It was
also observed that the westward tilting front seen during tow O1 was not present in tow O2. The
occurrence of Chl a max and its spread (especially in the horizontal direction) in tow O2 (Figure
16C) was observed to have occurred when alongshore current speed increased from ~0.02 m s-1
to ~0.1 m s-1 and were mostly eastward and southeastward (Figure 30C). The small patch of Chl
a amount in the eastern end of that tow also seems to have coincided with the high alongshore
current speed (0.15-0.25 m s-1) that was observed offshore in ADCP 1 survey (Figure 30C).
Comparatively also, Chl a was higher in swifter currents. For example Chl a max in tow O2 was
~1.4 µg l-1 when current was ~0.025 m s-1 while Chl a was ~2 µg l-1 in a current of ~0.15 m s-1.
Increase in near-bottom current velocities enhances the flux of nutrients and the consequent
primary productivity of benthic plankton. The observed lateral distribution of near-bottom Chl a
max can also be explained by the vertical-horizontal coupling of the water properties. Wilson
and Okubo (1980) reported an increase in variance of the horizontal distribution of Chl a in
central Long Island sound. This was correlated with a significant increase in salinity stratification
which was believed to have been caused by horizontal advection.
The accumulation of Chl a in the water column could be as a result of the resuspension of
near-bottom phytoplankton. This could also be the aftermath of turbulent periods caused by
strong winds or strong currents (Abbott et al. 1984; Janowitz et al. 2008). A storm was observed
28
on 19 October 2008 and the effects are seen in the extensive distribution of Chl a in the water
column on 20 October as compared to the 18 October plots. The Chl a max is also linked to
increased sinking of surface phytoplankton biomass especially in times of stratification when the
water column is stable (Stegmann and Ulmann 2004). When this occurs, the near-bottom
phytoplankton with favorable light conditions could take advantage of the shallowness of the
inner shelf environment to enhance Chl a. If light availability becomes reduced in deeper areas,
the Chl a max could still occur if phytoplankton are able to show physiological flexibility to
adequately utilize light at varying intensities. If not, Chl a could possibly reduce with increasing
depth.
CONCLUSION
Observations from this study have shown the varying effects of physical water properties
and motion on the formation of benthic Chl a concentrations on the NWFS. May Chl a values
were slightly higher than October values and showed a clear horizontal delineation as compared
to a more vertical delineation in the later period. In most cases, surface Chl a concentrations
ranged between ~0.2 and 0.6 µg l-1 while near-bottom concentrations were between ~0.5 and 1.6
µg l-1. During BenDim I, the highest Chl a max value was observed in tow M3. During BenDim
II cruise, observed Chl a concentrations ranged between 0 and 0.8 µg l-1 over most of the water
column, except in near-bottom maxima where concentrations were 1.0-1.6 µg l-1. Whiles
near-bottom Chl a max occurred more inshore (30.23° N) and at shallower depth (15-20 m) in
BenDim I, they occurred more offshore (30.10° N to 30.15° N) and in deeper waters (25 m and 30
m). Also BenDim II Chl a max were observed in cooler and less saline waters as compared to
cooler but more saline water in BenDim I. Results also show Chl a max occurs near
29
thermohaline fronts and disappears when that structure is broken. Generally, the observed
patterns in distribution of Chl a followed the structure of the water column as influenced by the
impacts of temperature and salinity and the effects of winds that changed those structures. In the
occurrence of thermoclines for example, mixing was almost absent and the distribution of Chl a
in the water column was less homogenous. Higher concentrations were observed closer to shore
than offshore and were attributed to the impact of river runoffs as well as resuspension of benthic
plankton.
I propose that the seasonal cycle of the presence of the Chl a maximum on west Florida
continental shelf may be summarized as follows: during the winter season, the water column on
the WFS is strongly horizontally stratified going off the shelf caused by winter storm convection.
This convection mixed nutrients up into the entire water column, making the entire water column
replete with nutrients and ready for production to start as the water gets seasonally warmed. The
Chl a maxima begins to form in the spring season within the main thermocline as the result of
production in the surface layer when seasonal heating establishes a strongly, vertical stratified
water column and the water above the thermocline warm to the point where production begins.
During the early spring the water column is replete with nutrients. These nutrients are used up in
the euphotic zone in photosynthesis. As the spring and summer progresses, the Chl a maxima
sinks further in the water column below the thermocline as the nutrients continue to drive
photosynthesis at lower light levels. By the time of the fall cruise, the nutrients in the water
column appear to be used up, so in order for production to continue the phytoplankton must find
another source of nutrients, the bottom benthic layer, and the Chl a maximum is found at the
bottom of the water column in very low light conditions. I speculate that during this period the
phytoplankton acquire nutrients within the bottom layer, migrate up into the water column during
30
the day to get the light needed for photosynthesis (production). This condition remains until the
passage of storms over the northern Gulf of Mexico during the late fall. These storms cause
convective mixing, and horizontally stratify the water column going offshore. This convective
mixing brings nutrients back up from the bottom into the water column setting the stage for the
spring bloom, when the water once again becomes warm enough for production to occur above
the thermocline.
Unfortunately, this work only covers the cruise in May at the beginning of this scenario
and in the late fall at the end of this scenario. Further work is being done by the research group
with a cruise aboard the R/V Pelican in early July 2009. This period is the mid-point of the
seasonal cycle when the thermocline and hence vertical stratification should be strongest. Data
from this cruise will hopefully fill in the gap and confirm my hypothesized seasonal development
of the Chl a max on the WFS. Also, this type of seasonal evolution suggests a seasonal progress
in the speciation of the phytoplankton on the WFS that is being investigated by other members of
the group. In addition, the work presented here needs to be integrated with the biological, remote
sensing, drifter (surface and subsurface), nutrient and CTD data collected by other components
of this multi-disciplinary effort in order to relate the seasonal generation and destruction of the
Chl a maximum and its potential relationship to a season progression of the phytoplankton
community structure.
31
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Figure 1. A depiction of MPB, nutrient and light fields (blue) on the continental shelf.
36
Figure 2. Schematic of a stratified shelf sea showing the profile of temperature, nutrient, PAR
and the trapping of chlorophyll. The spiral sizes depict the varying turbulent intensity within the
water column (From Ross and Sharples 2007).
37
Figure 3. A map showing the Gulf of Mexico with schematic cartoons showing the Loop
Current, and eddy that has been shed from the Loop current and the Gulf Stream. (source:
http://ocean.tamu.edu/Quarterdeck/QD6.1Graphics/eddy-map.gif)
38
Figure 4. Figure showing the northeastern Gulf of Mexico region and AcrobatTM tracks for the
two cruises, BenDim I (red lines) and BenDim II (yellow lines). (source: Map modified after
http://pubs.usgs.gov/of/2000/of00-019/htmldocs/images/browse/bathyc.gif)
39
BenDim I (Pages 40-49)
Figure 5. Plots of tow M1 variables measured with AcrobatTM at time indicated.
Arrow shows the direction of tow relative to shore.
40
Figure 6. Plots of tow M2 variables measured with AcrobatTM at time indicated.
Arrow shows the direction of tow relative to shore.
41
Figure 7. Plots of tow M3 variables measured with Acrobat TM at time indicated. Arrow shows
the direction of tow relative to shore. There were no valid data on sections indicated as tow was
done in the night.
42
Figure 8. Wind field in Pensacola between 5/12/2008-5/25/2008
43
Figure 9. 05/15/2008 QuikScat wind vector plot for NW Florida.
44
Figure 10. 05/16/2008 QuikScat wind vector plot for NW Florida.
45
Figure 11. 05/18/2008 QuikScat wind vector plot for NW Florida.
46
Figure 12. 05/19/2008 QuikScat wind vector plot for NW Florida.
47
Figure 13. 05/22/2008 QuikScat wind vector plot for NW Florida.
48
Figure 14. 05/15/2008-05/22/2008 averaged QuikScat wind vector plot for Gulf of
Mexico.
49
BenDim II (Pages 50-66)
Figure 15. Plots of tow O1 variables measured with AcrobatTM at time indicated.
Arrow shows the direction of tow relative to shore.
50
Figure 16. Plots of tow O2 variables measured with AcrobatTM at time indicated. Arrow shows
the direction of tow relative to shore. There were no valid data on sections indicated as tow was
done in the night.
51
Figure 17. Plots of tow O3 variables measured with AcrobatTM at time indicated.
Arrow shows the direction of tow relative to shore.
52
Figure 18. Plots of tow O4 variables measured with AcrobatTM at time indicated.
Arrow shows the direction of tow relative to shore.
53
Figure 19. Plots of tow O5 variables measured with AcrobatTM at time indicated.
Arrow shows the direction of tow relative to shore.
54
(a)
(b)
Figure 20. Plots showing the extinction of corrected PAR (CPAR) at depth for (a)
10/17/2008 and (b) 10/20/2008
55
Figure 21. Wind field in Panama City between 10/12/2008-10/25/2008
56
Figure 22. Wind field in Pensacola between 10/12/2008-10/25/2008
57
Figure 23. 10/15/2008 QuikScat wind vector plot for NW Florida.
58
Figure 24. 10/16/2008 QuikScat wind vector plot for NW Florida.
59
Figure 25. 10/17/2008 QuikScat wind vector plot for NW Florida.
60
Figure 26. 10/19/2008 QuikScat wind vector plot for NW Florida.
61
Figure 27. 10/20/2008 QuikScat wind vector plot for NW Florida.
62
Figure 28. 10/21/2008 QuikScat wind vector plot for NW Florida.
63
Figure 29. 10/15/2008-10/22/2008 averaged QuikScat wind vector plot for Gulf of
Mexico.
64
(a)
(b)
(c)
Figure 30. WinRiver II screen plots of ADCP 1 data showing (a) current magnitude (m s-1),
(b) current direction (°) and (c) 125° earth projected current velocity (m s-1). Plots are oriented
with reference to the direction of the survey; offshore (deeper water) is on the west and inshore
(shallower water) is on the east of the plots.
65
(a)
(b)
(c)
Figure 31. WinRiver II screen plots of ADCP 2 data showing (a) current magnitude (m s-1),
(b) current direction (°) and (c) 125° earth projected current velocity (m s-1)
66