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Plugging the Gaps
A gap in the science is an invitation to question any management decision proposed to enhance the health of the Bay-Delta ecosystem, and three years ago the S.F. Estuary Project decided to close some of the most critical gaps by funding six studies costing a total of $242,000. The studies include the first look at how pollution affects the Bay's marine mammals (see cover). Here is a brief overview of the five others (see Now in Print).
CORD GRASSES COMPARED
This study compared the wetland functions of a smooth cord grass introduced to the Bay Area from the Atlantic coast in the 1970s (Spartina alterniflora) with those of a Pacific coast native of the same species class (Spartina foliosa). Both grow in the same intertidal range, and field work focused on two locations - one a sandy spot off Alameda and the second mudflats near San Francisco airport. Wetland functions examined included sedimentation rates, shoreline erosion control, abundance of bottom-dwelling organisms, plant debris and use by shorebirds.
Researchers found that the Atlantic species spread 2-3 times faster, grew more densely and colonized barren mud zones at lower tidal elevations more successfully than its native counterpart. The greater stem density of the introduced species enabled it to trap more sediment, prevent more erosion and thus more effectively control the loss of high marsh to wave action (see chart). Biological differences were less pronounced. No strong trends emerged to suggest that one species had greater or more diverse populations of bottom-dwelling organisms or visiting shorebirds than the other. Grown side by side, the Atlantic species spread into and eventually replaced the Pacific species, suggesting that continuing South Bay invasion by the Atlantic interloper could physically modify the intertidal environment, colonize now barren mudflats and thus reduce foraging area for shorebirds.
ASIAN CLAM FILTRATION RATES
This research delved into the dietary habits of the introduced Asian clam Potamorcorbula amurensis, whose appearance coincides with a dramatic reduction in phytoplankton abundance. The study, still in draft form, measured the clam's filtration rate (bivalves filter water for phytoplankton and other food) under a variety of cross-flow velocities in a laboratory flume. Preliminary results showed that a maximum filtration rate of 3-4 liters per clam per day occurred at cross-flow velocities of 6 and 24 centimeters per second. Rates at mid-flow velocities diminished by 50 percent.
Researchers found the clam capable of filtering the water column once a day (assuming a well-mixed water column, conservative clam densities and percent of animals feeding, and a low individual filtration rate). Once a day far exceeds estimated phytoplankton doubling time of 2.9 - 27 days. Thus, report authors believe Potamorcorbula is capable of limiting the biomass accumulation of phytoplankton in Suisun Bay.
BIOMARKERS OF CONTAMINATION
Researchers wanted to find out whether the response of certain biomarkers - markers of sublethal physical, biochemical or genetic change in an organism - could be clearly linked to contaminant exposure. So they exposed sanddabs (a bottom-dwelling fish) in the laboratory to contaminated sediments from San Francisco Bay's Castro Cove - site of an old oil refinery outfall - and two reference sites. By comparing responses of the fish to amphipod bioassays (lab tests on small crustaceans) using the same sediment, this study also sought to determine if amphipods were responding to pollutants or other sediment characteristics, such as grain size.
Researchers chose four biomarkers. The first was P450 proteins produced by the fish to metabolize contaminants. The three other biomarkers tracked abnormalities in kidney, liver and gill tissues; the production of certain stress proteins (which help repair damaged cells); and increases in abnormalities in the nuclei of red blood cells associated with genetic damage.
Though the latter two biomarkers didn't yield clear results, the sanddabs exposed to contaminants showed significant increases of P450 proteins in their gills, livers and kidneys relative to the controls after 60 days. More importantly, researchers discovered clear correlations between this higher protein production and tissue abnormalities, confirming the usefulness of the P450 biomarker in contaminant studies. The elevated P450 production also correlated highly with amphipod mortality in the companion tests. Because scientists know P450 production is not influenced by sediment grain size, this result supports the use of amphipod sediment bioassays as indicators of pollutant effects.
SEDIMENT DYNAMICS
This report marks the first in a series on the nature of suspended sediment variability in the San Francisco Estuary. It reviews the broad geologic history of the Bay (including the advance and retreat of shorelines), examines how sediments released by hydraulic gold mining in the 1800s overwhelmed the effects of sea level rise, touches on the role of agriculture in annual sediment concentrations and explores how dams built in the middle of this century became new sinks for sediments, reducing riverine emissions into the Bay. It also presents more recent sediment suspension data for 1979 (low riverine emissions) and 1980 (high riverine emissions). The brief history and data sets will provide an information basis for sediment dynamics analysis in the rest of the reports in the series.
PARTICULATE ORGANIC MATTER
This study explores the abundance, origin, composition and nutritional quality of particulate organic matter (POM) in the Bay and Delta. Sampling took place throughout the Estuary - from the Sacramento River to the South Bay - and dates were chosen to represent a range of hydrologic and hydrographic conditions, including periods of low river flow, winter and spring floods, phytoplankton blooms and enhanced resuspension.
Researchers looked at biochemical indicators (such as carbon and nitrogen isotopes and lipids) in both suspended particulate matter and in the tissues of the clam Potamorcorbula amurensis so that both the nature of the matter and its incorporation into the food web (via the clam) could be assessed. They discovered that while phytoplankton sources of POM are important throughout San Francisco Bay, the North Bay receives additional inputs from bacterial and terrestrial sources. Clam tissues indicated that phytoplankton supply a large fraction of the consumable carbon in S.F. Bay clams, and that freshwater algae (probably from Delta rivers) may be more important in the North Bay. The study suggests that consumer organisms with widespread distribution, like the Asian clam, might be exploited as biological indicators of fluctuations in metabolizable POM within ecosystems receiving multiple inputs.
Contact: Cord Grass, Michael Josselyn (415)454-8868; Biomarkers, Bob Spies (510)373-7142; Clams, Janet Thompson (415)354-3219; Organic Matter, Liz Canuel (415)354-3354; Sediments, David Peterson (415)354-3366
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