Water mass subduction & eddy effects on phytoplankton distributions in the Santa Barbara Channel, California
Libe Washburn1, Mark Brzezinski2, Nick Dellaripa3, and Chris Gotschalk3
1Marine
Science Institute & Dept. of Geography, UC Santa Barbara, washburn@eri.ucsb.edu, 2Dept. of Ecology Evolution & Marine Biology, mark.brzezinski@lifesci.ucsb.edu,
3nick@physics.ucsb.edu, Marine Science Institute, UC Santa Barbara, 4Marine Science Institute, UC Santa Barbara, gots@lifesci.ucsb.edu
Abstract: Observations using towed, undulating vehicles show that phytoplankton layers occur well below euphotic zone depths in
the Santa Barbara Channel (SBC). The deep layers are typically found where density surfaces slope steeply suggesting isopycnal
mixing and advection cause downward transport of the phytoplankton. Significant correlation between salinity and chlorophyll along
isopycnals supports this interpretation. The observations also indicate that eddy processes cause downward transport of
phytoplankton. Data were obtained during 16 cruises conducted three times per year from 2001 to 2006 as part of the Santa
Barbara Coastal Long Term Ecological Research project (SBC-LTER). Other sampling during the cruises reveals that wind-driven
upwelling and cyclonic eddies largely control the spatial patterns of phytoplankton primary productivity. Cyclonic eddies influence
productivity by uplifting isopycnal surfaces and by occasionally entraining phytoplankton and nutrients from water upwelled near
Point Conception. Eddy-enhanced productivity changes were superimposed on variable levels of channel-wide productivity caused
by wind-driven upwelling. The combined effects of upwelling and cyclonic circulation enhance phytoplankton biomass and
productivity in the SBC compared to elsewhere in the Southern California Bight.
Cyclonic eddy & domoic acid in a spring bloom: May 2003
High spring productivity: upwelling conditions
Line C
Study site and methods
Line C
Line H
19 May 2003 2003 1600 – 2000
mg m-3
line C
line H
CTD
Grid
Line C
Line H
3 May 2003
10 May
24 May
17 May
Anderson et al. (2006)
Sampling during SBC-LTER cruises:
Strongly upwelling-favorable wind before & during cruise 7: May 2003
- 16 cruises (spring, summer & fall, winter), 2002-2006 on RV Pt Sur
- Upper right: Distribution of wind stress during LTER cruise 7 (solid
arrows) and averaged the 2 weeks preceding the cruise.
- Upper left: Primary productivity distribution during LTER cruise
- 25 CTD/rosette stations in grid & 7 stations on PnB line from mainland to Santa Rosa Island
- phytoplankton primary productivity at 5 m on grid & PAR sensor for euphotic zone (1% light)
- Upper left: primary productivity distribution during LTER cruise 7. Strongly upwelling
favorable spring winds upwelled nutrient rich waters before & during the cruise.
- Upper right: surface currents and surface relative vorticity. Circular current pattern with
high vorticity was associated with high high productivity & high chlorophyll concentration.
- Lower left: Satellite chlorophyll distribution show large phytoplankton bloom (red shades).
- Lower right: Distribution of domoic acid produced by Pseudo-nitzchia australis.
- towed profiler surveys (Scanfish & Triaxus); this poster focuses on lines C and H (red lines)
Approach: Use fluorescence-derived chlorophyll as dye to infer downward transport from near-surface.
Summary and future directions
Three mechanisms for downward transport of phytoplankton
1. Wind-driven downwelling of phytoplankton
2. Eddy-driven downwelling of phytoplankton
3. Isopycnal subduction & mixing of phytoplankton
- Phytoplankton primary productivity in the SB Channel driven by
upwelling winds and cyclonic eddies.
- Large fractions of chlorophyll biomass (cruises: 8 - 42%) are
found below 1% light depth.
1%
1%
1%
46054
40
0
20 30 0
-40
- Three vertical transport mechanisms identified:
1. wind-driven downwelling
2. eddy-driven downwelling
3. subduction & mixing along sloping isopycnals.
-20
11 m/s
Line C
Line C
Santa Cruz I.
Santa Rosa
I.
- Red arrow shows average wind speed over 48 hrs
preceding Line C at buoy 46054.
wind-driven
downwelling of
phytoplankton
10 km
- Winds are downwelling favorable winds along north
side of Channel Islands.
Santa Cruz I.
30
Santa Rosa
I.
20
-30
eddy-driven
downwelling of
phytoplankton
10 km
Surface currents show cyclonic (counterclockwise) eddy.
Core of eddy is in solid body rotation that extends
below 300 m depth (vorticity = z ≈ 0.5f ).
seafloor near
profiler track
Santa Barbara Coastal
Long Term Ecological Research
eastward
currents
(cm s-1)
Acknowledgements:
Thanks to many graduate & undergraduate students, technicians,
and RV Pt. Sur crew members who participated in the cruises.
Anacapa Island
eastward current speed (cm s-1)
- Chlorophyll distribution along cruise Line C during downwelling-favorable winds
- Chlorophyll below 1% light depth (yellow dashed line) along Santa Rosa Island is
consistent with wind-driven downwelling of phytoplankton
- Cyclonic rotation uplifts isopycnals by at least 40 m
- Elevated chlorophyll concentrations occur above region of isopycnal uplift
suggest nutrient injection into euphotic zone
- Speculations for future research:
1. Vertical transport of phytoplankton out of the euphotic zone
important contributor to the demise of blooms.
2. Mechanisms are significant transport pathways for organic
carbon to depth.
-30
- Deep phytoplankton in core of eddy indicate
downward transport out of euphotic zone.
- Highest chlorophyll concentration above eddy
is found above center.
- Current velocity contours on Line H show high chlorophyll, low salinity water mass
advecting eastward & low chlorophyll, higher salinity water mass advecting westward. Funding:
- Santa Barbara Coastal Long Term Ecological Research Project
Isopycnal distribution of salinity & chlorophyll
- NOAA/IOOS & the Southern California Coastal Ocean
Observing System
s = 25.7 kg m
- State of California’s Coastal Ocean Current Monitoring Program
s = 25.6 kg m
s = 25.5 kg m
q
-3
q
-3
q
-3
- Chlorophyll below euphotic zone (red dots) is linearly correlated with salinity.
- Linear correlation indicates chlorophyll concentration behaves as conservative tracer.
- Consistent with downward mixing of phytoplankton by isopycal mixing and advection