Biological state of the Northwest Pacific and coastal zones as revealed by ocean color
Chuanmin Hu and Frank E. Muller-Karger, Institute for Marine Remote Sensing, College of
Marine Science, University of South Florida, 140 Seventh Ave. S., St. Petersburg, FL 33701,
hu@seas.marine.usf.edu
Introduction
The color of the ocean is determined by various constituents in both particulate and dissolved
forms. Other environmental factors, such as wind (for waves and whitecaps), also play a role in
modulating the color. The optically active constituents include phytoplankton and its associated
debris, dissolved matter, and suspended particles. In coastal waters, terrestrial runoff, sediment
resuspension, and shallow bottom may all play important roles in affecting the color. In contrast,
the open ocean is generally classified as Case I (Morel and Prieur, 1977), where the color is
dominated by phytoplankton and its direct degradation products (mainly detrital particles).
Therefore, the concentration of the latter can be inferred from the color of the ocean with the
radiative transfer theory – the theory to study how light interacts with water and its constituents.
Knowledge of the distribution of phytoplankton (through its chlorophyll-a pigment) in the
surface ocean is of fundamental importance for several reasons. Photosynthesis of chlorophyll
(i.e., the so called primary production) modulates the CO2 (one of the greenhouse gases) content
in both the ocean and the atmosphere, therefore is related to climate changes (e.g., Benhrefeld et
al., 2001). Chlorophyll distribution also reflects the biological state of the ocean, since
phytoplankton is on the bottom of the food chain and affects all other higher trophic levels.
Chlorophyll also absorbs light, limiting the light intensity at depth where various marine species
live.
Obtaining synoptic chlorophyll distribution in the global ocean is only possible with satellite
ocean color sensors. From the late seventies, the National Aeronautics and Space Administration
(NASA) of the United States has launched a series of satellite sensors for such purpose, from the
Coastal Zone Color Scanner (CZCS, 1978-1986), the Sea-viewing Wide Field-of-view Sensor
(SeaWiFS, 1997-present), to the Moderate Resolution Imaging Spectroradiometer (MODIS,
2001-present). SeaWiFS and MODIS provide one to two day coverage of the entire earth,
therefore serve as operational sensors for ocean color studies in regional to global scales. The
primary data product from the sensors is the surface chlorophyll concentration (in mg m-3).
Combined with the sea surface temperature (SST) data obtained from the Advanced Very High
Resolution Radiometer (AVHRR) onboard the polar orbiters of the National Oceanic and
Atmospheric Administration (NOAA) or from MODIS, primary production can also be
estimated from empirical models.
In this study, using ocean color and temperature data, we document the biological state of the
Northwest Pacific and coastal zones.
Method
Monthly chlorophyll data for the region bounded by 12-72oN and 180-100oW between
September 1997 and June 2002 were obtained from the NASA Distributed Active Archive
Center (DAAC, http://daac.gsfc.nasa.gov). These data were generated at NASA using the
SeaWiFS Data Analysis System (SeaDAS version 4) software. Briefly, atmospheric effects were
removed using several correction schemes (Gordon and Wang, 1994; Ding and Gordon, 1995;
Arnone et al., 1998), and chlorophyll concentration was estimated using a band-ratio bio-optical
algorithm (O’Reilly et al., 2000). Validation results from thousands of field measurements show
that the uncertainties in these global estimates are generally within 35%. Data were remapped
to cylindrical equal-distance projection with 9-km resolution. To estimate primary production we
also obtained the monthly Photosynthetically Available Radiation (PAR) data from SeaWiFS
(Frouine et al., 2000). Monthly SST data were obtained from NASA’s Jet Propulsion Laboratory
(JPL, http://podaac.jpl.nasa.gov/sst).
With the three parameters (chlorophyll, PAR, and SST), we used the Benhrefeld and
Falkowski (1997) model to estimate the net primary production (NPP, in g C m-2 month-1). The
model takes into account of the depth-dependent chlorophyll and light profile, and estimates the
primary production per unit chlorophyll from SST, using an empirical relationship.
Based on the NPP monthly results, for each location we estimated the number of occurrence
(frequency) in a year when NPP is exceeding a predefined number, 10 g C m-2 month-1. The
number was chosen according to visual examination of the difference between oligotrophic and
productive waters, but somehow arbitrarily. The result serves as an index to describe how long
an enhanced productivity exists.
Results and Discussion
While all data and imagery can be accessed at http://imars.usf.edu/~hu/scratch/b2b, below we
present a few examples to show how to interpret them.
Figures 1 and 2 show the monthly chlorophyll and NPP distributions for October 1997 and
February 1998, respectively.
The distributions patterns are mainly determined by the physical and biological processes in
the region, where the eutrophic, high-chlorophyll sub-artic waters meet the oligotrohpic, lowchlorophyll subtropical waters at the Transition Zone Chlorophyll Front (TZCF, Polovina et al.,
2001). The TZCF can be visualized in the sample images. It is relatively east west, migrating
between about 40oN in summer and 28oN in winter. It separates two types of water masses where
the bloom dynamics are considerably different – the sub-arctic water is controlled by wintertime
mixing, stratification, and light limitation, while the subtropical water is controlled by
thermocline changes (Glover et al., 1994). A full description of the seasonal transition of the
TZCF can be found in Polovina et al. (2001).
Several phenomena have been observed from these results.
First, there is an episodic bloom near 30oN 150oW, which can be up to 40,000 km2 and can
last for several months from late summer (Wilson, in press). For the period examined (Sept. 1997
to June 2002), these late summer blooms were found for 1997, 1999, and 2000, and were
speculated to result from summer Trichodesmium blooms (Wilson, in press), which were not
observed from the same datasets due to detection limit of the sensor and/or lack of appropriate
algorithm. The episodic, isolated chlorophyll blooms may have significant ecosystem impacts in
the region, which are currently not well understood.
Second, it is clear that most of the coastal zones from Baja California to the Bering region
remain productive throughout the year. Chlorophyll concentrations of > 0.5 mg m-3 can typically
extend to > 100 km offshore, and some feather-like blooms from California to Oregon can reach
up to >400 km offshore.
Finally, the NPP distribution does not always coincides with chlorophyll, as demonstrated by
the February 1998 image set. NPP is primarily a function of both chlorophyll and light. As a
result of light limitation, high NPP is restricted to a narrow band bounded by about 27o to 37oN.
The size and location of this band change with time. This effect is manifested in the NPP
monthly frequency (or density) image, as shown in Figure 3.
As revealed by Fig. 3, enhanced primary production persisted throughout the year (frequency
> 11 months per year) for the narrow band between about 35o to 40oN as well as for the coastal
zones, where the enhanced productivity can reach 500 km offshore for Baja California and 1000
km offshore for Oregon. The enhanced productivity plays important roles in regulating the
coastal ecosystem in the economic zone. In particular, anomaly events observed with ocean color
data may have positive or negative impact on the local ecosystem (e.g., Hu et al., 2003).
Acknowledgement
We thank the US SeaWiFS Project (Code 970.2), the Distributed Active Archive Center
(Code 902), and OrbImage for the production and distribution of the satellite data. We thank Mr.
Zhiqiang Chen for the assistance in estimating the global NPP distributions.
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Figure 1. Top: Chlorophyll distribution for October 1997, obtained from SeaWiFS; Bottom: Net Primary Production
(NPP) for the same month, estimated from SeaWiFS and AVHRR. Complete results can be found at
http://imars.usf.edu/~hu/scratch/b2b/.
Figure 2. Top: Chlorophyll distribution for February 1998, obtained from SeaWiFS; Bottom: Net Primary
Production (NPP) for the same month, estimated from SeaWiFS and AVHRR. Complete results can be found at
http://imars.usf.edu/~hu/scratch/b2b/.
Figure 3. Number of months (from 0 to 12) during a year when NPP is > 10 g C m -2 month-1.