Selenium in San Francisco Bay Zooplankton

David G. Purkerson¹, Martina A. Doblin³, Christian E. Schlekat²,
Samuel N. Luoma², Stephen M. Bollens¹, and Gregory A. Cutter³

¹ Romberg Tiburon Center for Environmental Studies, San Francisco State University  ² U.S. Geological Survey, Menlo Park
³ Department of Ocean, Earth and Atmospheric Science, Old Dominion University
 
 

Abstract

The potential toxicity of elevated selenium (Se) concentrations in San Francisco Bay has stimulated efforts to measure selenium concentrations in benthos, nekton and waterfowl. In August 1998, we initiated a combined field and laboratory study to determine the concentration of selenium in San Francisco Bay zooplankton. Our specific goals were to determine effects of flow regime (high vs. low freshwater flow) and proximity to sources (e.g. oil refineries and agricultural run-off) on spatial and temporal variations in zooplankton selenium concentrations in North San Francisco Bay. Monthly vertical plankton tows were collected at several stations using a 73 mm mesh ring net. Based on eight months of samples analyzed thus far, concentrations ranged from 0.45 mg Se. g-1 up to 6.55 mg Se. g-1 dry weight. Selenium concentrations in the smallest (73-250 mm) zooplankton group appeared to be higher than those in the larger (>250 mm) groups. To address this difference, we compared the efficiency with which small (73-250 mm) and large (250-500 mm) zooplankton groups assimilated Se from a phytoplankton food source using pulse-chase techniques. Results showed that Se assimilation efficiencies varied within and between size classes.
 
 

Selenium has been a trace element of concern in San Francisco Bay for the past twenty years, with agricultural runoff and oil refineries in the North Bay being the predominant sources.  Elevated Se concentrations have been found in SF Bay clams which are believed to be the source for elevated Se concentrations in some nekton and aquatic fowl such as sturgeon and scoters (Luoma 1997). There are other animals in the Bay, such as striped bass, with low selenium levels, which are thought to be a result of the level of selenium found in their food, predominately zooplankton. Selenium concentration data has been collected for many organisms throughout the Bay, but Se concentrations in the zooplankton community have remained unknown. It is of great importance that we measure Se concentrations of zooplankton in SF Bay in order to understand the trophic transfer of Se throughout the food web.
 

Purpose

   Obtain baseline data for San Francisco Bay Zooplankton.

   Look for any spatial trends found in Se concentrations of SF Bay Zooplankton.

   Look for any temporal, or seasonal (high freshwater vs. low freshwater flow
     periods), trends in Se concentrations.

    Look for any variation of Se concentrations between operationally defined size
      classes (functional groups?)
 
 

From Aug. 1998 until Dec. 1999 zooplankton were sampled once a month at six stations throughout SF Bay on the USGS R/V Polaris. Zooplankton were sampled using a 73 mm mesh ring net. Two vertical tows were conducted at each station; one for Se analysis and the other for species identification. Live zooplankton samples were separated by sieves (73, 250, 500, 2000, and 4000 mm).  Selenium analyses were conducted by atomic absorption spectroscopy at Old Dominion University. Due to the amount of biomass required for Se analyses, the zooplankton samples were combined into size ranges 73-2000 mm and >2000 mm. In February 1999, six stations were sampled in the North Bay to obtain enough biomass at each individual size class for Se analyses.

Assimilation efficiency (AE) experiments were conducted using standard pulse-chase feeding techniques. The diatom, Pheodactylum triconutum, was cultured in the presence of radiolabeled selenium (⁷⁵Se) and fed to copepods. Radioactivity was measured after 30 minutes followed by a cold feed and subsequent radiation measurements over the next 72 hours. The amount of radiolabeled selenium that was absorbed into tissue, or assimilation efficiency, was calculated by dividing the radiation after the cold feed by the radiation after the hot feed. The mysid, Acanthomysis bowmani, was used as a representative of the 500-2000 mm size class.  Since A. bowmani are omnivorous, they were fed radiolabeled copepods.
 
 

1. Baseline Data
From July 1998 to March 1999, selenium concentrations ranged from 0.45 mg Se.g-1 up to 6.55 mg Se.g-1 dry weight. The dispersed data is most likely due to the many variables present, such as selenium load, zooplankton species present, feeding behavior and selectivity, etc.

2. Spatial/Temporal Trends
There were no significant spatial trends as distance from the source (North Bay) increased.  It is believed that Suisun Bay had much lower Se concentrations due to the large amount of detritus that was impossible to separate from the zooplankton for analysis.  There were also no significant seasonal differences in Se concentrations between high FW flow and low FW flow periods.  Selenium concentrations during high FW flow appear to be slightly elevated compared to concentrations during low FW flow, but still are not significantly different.
 
 

3. Size Class Comparison/San Pablo Bay Sampling
     A. Species composition and richness was different between size classes.

3. Size Class Comparison/San Pablo Bay Sampling
     B. The smaller zooplankton had significantly higher Se concentrations than larger
          zooplankton.
 

4. Assimilation Efficiency
Copepods in the 250-500 mm size class, Acartia spp. and Tortanus dextrilobatus, had significantly different assimilation efficiencies, 40% and 15% respectively.  Copepods in the 73-250 mm size class, Oithona spp. and Limnoithona spp., had a high assimilation efficiency (80%).  The mysid, Acanthomysis bowmani, (>500 mm) had a 60% assimilation efficiency.
 

Bioaccumulation appears to be occurring since Se concentrations of zooplankton are slightly higher than that found in phytoplankton of the Bay. There were no significant spatial or temporal (seasonal) trends. Trends could be overlooked due to the vast differences in zooplankton between samples and how Se assimilation efficiencies vary among zooplankton.  Smaller zooplankton had significantly higher Se concentrations than larger zooplankton.  A majority of the small zooplankton are herbivorous, while the larger zooplankton are predominately omnivorous and carnivorous.  Selenium concentrations could be a result of zooplankton feeding behavior and/or physiological differences that allow for varying assimilation efficiencies of Se. Samples from the next eight months will be analyzed and should help support these findings.

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Financial support for this project was provided by the Calfed Bay-Delta program. Many thanks to the crew and scientists onboard the R/V Polaris for their assistance in sampling, espescially Francis Parcheso and Scott Conard for their skilled wench operation techniques.  Special thanks to Sean Avent for his assistance and knowledge in species identification.