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

Abstract

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.

Seasonal Variability and Drivers of Microzooplankton Grazing and Phytoplankton Growth in a Subtropical Estuary

“…Phytoplankton growth and microzooplankton grazing rates varied seasonally in the estuary and were highest in the summer, which is consistent with seasonal trends found in temperate and high latitude waters (Kim et al, 2007;Calbet et al, 2008;Gutiérrez-Rodríguez et al, 2011;Lawrence and Menden-Deuer, 2012). The magnitude of grazing rates presented here (0-2.11 day −1 ) are similar to previous estimates from the region (southeast United States; Murrell et al, 2002;Wetz et al, 2006;Sitta et al, 2018) and support the role of microzooplankton as important consumers in subtropical systems. Seasonal differences were also observed in the ratio of growth to grazing, resulting in either negative or positive accumulation of plankton biomass.…”

“…(PO 4 and SiO 4 ) were either stable over the year or in high enough concentrations to support growth (Verity and Borkman, 2010;Bittar et al, 2016). Furthermore, cases of nitrogen limitation are common in the Skidaway River Estuary (Verity, 2002a) and many other southeastern United States estuaries (Howarth and Marino, 2006;Sitta et al, 2018), which supports the nitrate-limiting system we observed. During the summer, environmental conditions improved (e.g., increased temperature, sunlight, and nutrients), which likely contributed to estimates of positive accumulation rates.…”

Evidence for the Primary Role of Phytoplankton on Nitrogen Cycle in a Subtropical Reservoir: Reflected by the Stable Isotope Ratios of Particulate Nitrogen and Total Dissolved Nitrogen

“…Zooplankton play a mediating role in the food web of aquatic ecosystems, and excess zooplankton puts predation pressure on phytoplankton (Boyce et al, 2010; De Stasio et al, 2018; Sitta et al, 2018). When zooplankton biomass is low, phytoplankton will proliferate rapidly (Paerl et al, 2011; Er et al, 2018).…”

First record of straight-needle pteropodCreseis aciculaRang, 1828 bloom in the Çanakkale Strait (NE Aegean Sea, Turkey)

“…Nitrogen is one of the most important nutrients regulating the primary production of phytoplankton, which are grazed by zooplankton (Bristow et al 2017;Sitta et al 2018). In the study area, nitrate (NO 3 − ), i.e.…”