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Research Technical Report

Microbial Ecology of Elkhorn Slough: Relation to Inorganic/Organic Nutrients

Welschmeyer, N., L. Younan, and K. Yurus (March 2005)

Poster presentation at the 2005 Sanctuary Currents Symposium, Seaside, CA


Weekly sampling over two years has now established a persistent division in the phytoplankton community of Elkhorn Slough. The lower half of the slough is dominated by coastal phytoplankton representative of the source waters in Monterey Bay. The upper half of the slough is uniquely dominated by cryptophytes (up to 75% of the total phytoplankton biomass). Here we present new data showing strong parallels between cryptophytes and the distribution of planktonic/benthic anoxygenic bacteria. The anoxygenic, autotrophic bacteria, tracked by their unique pigment, bacteriochlorophyll a, are almost exclusively limited to the upper half of Elkhorn Slough, matching the distribution of cryptophytes. Surprisingly, sediment-derived samples, fromthe shallow intertidal mudflats, also show a 20-fold decrease in the concentration of bacteriochlorophyll a as one approaches Monterey Bay. We examined trends in inorganic nutrient chemistry to explore possible links to watershed nutrient drainage and cryptophyte/anoxygenic bacteria distributions. Highest concentrations of nitrate (>200um, indicative of agricultural drainage) were limited to the lower section of Elkhorn Slough, near Moss Landing Harbor. In contrast, the upper slough nutrients were characterized by low N/P ratios often <2 (mole/mole), substantially less than expected from traditional Redfield ratios (ca. 16). This suggests high rates of bacterial denitrification in the upper slough. Examination of dissolved organic matter (DOM) by fluorescent excitation/emission mapping showed ten-fold increases in water-column DOM concentrations in the upper slough, relative to lower slough. It appears the pattern in phytoplankton distribution may be linked to strong microbial influences, with dissolved organic nutrients, not inorganic nutrients, providing a principle link.