TOP-DOWN AND BOTTOM-UP CONTROLS ON PHYTOPLANKTON ASSEMBLAGES IN TWO CONTRASTING ESTUARIES
Sitta, Kimberly Ann
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Coastal environments are vulnerable to land development due to human population growth that has increased nitrogen and phosphorus loading into receiving estuaries. As a result, phytoplankton blooms have proliferated as ‘bottom-up’ (resource) processes outweigh ‘top-down’ (removal) processes. This study investigated phytoplankton biomass and assemblage regulation by both N-form (ammonium, nitrate, urea) and zooplankton at two southeastern US estuaries with contrasting land cover patterns through in situ nutrient addition bioassays (spring and summer, 2015-2016) and dilution experiments (summer 2016). Phytoplankton responses to N exhibited no clear patterns in spring, likely due to physical factors (light, flushing), but most summer biomass increases were significant (p < 0.05) at each site to all provided N-forms, with diatoms comprising the greatest portion of total biovolume. The influence of land cover on phytoplankton was most pronounced during summer as biovolume increased to a greater extent at the more developed site per estuary. Phytoplankton biomass and biovolume was generally greatest when all nutrients in combination were added, while nitrate frequently elicited the lowest response. Microzooplankton grazing rates were highest for < 5 µm and < 20 µm Chl a fractions, often corresponding with the highest phytoplankton growth rates, and ciliate abundances tracked biomass during bioassays. Conversely, mesozooplankton grazing rates were generally lower and often non-significant, indicative of saturated feeding or trophic cascades, highlighting the role of microzooplankton as primary biomass controls in these estuaries. Results can be used to inform nutrient management decisions, as continued development will increase nutrient loading to developed and developing systems.