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The influences of nitrogen form and zooplankton grazing on phytoplankton assemblages in two coastal southeastern systems

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” (removals). This study investigated phytoplankton biomass and assemblage regulation by both N‐form (ammonium, nitrate, urea) and zooplankton at two southeastern United States estuaries (the urban Charleston Harbor and less‐developed Ashepoo‐Combahee‐Edisto Basin) 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 during spring. By comparison, summer chlorophyll a (Chl a) increases were significant (p < 0.05) in response to all provided N forms, and diatoms contributed most to total biovolume with biovolume increases higher at the more developed site per estuary. Phytoplankton Chl a and biovolume were generally greatest when all nutrients were added in combination, but nitrate added alone frequently elicited the lowest response. Ciliate abundances were high at most sites with lower mesozooplankton abundances. Accordingly, microzooplankton grazing rates were highest for < 5 μm and < 20 μm Chl a fractions, often corresponding with the highest phytoplankton growth rates. Ciliate abundances tracked Chl a during bioassays, highlighting their role as important 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.