Abstract
Rates of microzooplankton grazing and phytoplankton growth are seldom measured with respect to time, yet such estimates may better reflect temporal variability in coastal phytoplankton communities and offer insight into mechanisms that control populations. To assess seasonal patterns in rates, we performed 41, weekly dilution experiments over a full year in the Skidaway River Estuary (GA), measuring rates of phytoplankton growth, microzooplankton grazing, and viral lysis based on total chlorophyll and group-specific abundances (Synechococcus spp., picoeukaryotes, and nanoeukaryotes). Seasonal variability in microzooplankton grazing (0-2.11 d-1) and phytoplankton growth rates (0-2.43 d-1) was observed, with highest values typically recorded in summer and lowest in winter. Grazing pressure was strongest in winter-spring, as phytoplankton accumulation rates were often negative (-0.16-0.28 d-1). Rates varied similarly over seasons for chlorophyll, pico-, and nanoeukaryotes, while rates on Synechococcus spp. were rarely significant in dilutions and did not follow seasonal trends. Few experiments (7%) yielded significant rates of viral lysis. Temperature correlated most strongly with phytoplankton growth rates (R2 = 0.46-0.56), but not grazing rates (R2 = 0.11-0.27), which were more likely driven by observed seasonal shifts in plankton community composition (e.g. fall diatom blooms). Establishing temporal rate measurements is critical to identify factors that drive phytoplankton growth and mortality and accurately predict shifts in phytoplankton population dynamics and food web processes within marine systems.
Highlights
The fate of phytoplankton populations largely depends on the balance between growth and mortality, which can determine the productivity of a marine system, as well as the availability and cycling of carbon and nutrients (Steinberg and Landry, 2017)
Over seasonal scales in the Skidaway River Estuary, phytoplankton growth was universally controlled by temperature and to a lesser extent by concentrations of major dissolved nutrients
Though several environmental factors were important in predicting grazing rates, there was no clear linear relationship between temperature and microzooplankton grazing
Summary
The fate of phytoplankton populations largely depends on the balance between growth and mortality, which can determine the productivity of a marine system, as well as the availability and cycling of carbon and nutrients (Steinberg and Landry, 2017). Compared to an extensive collection of spatial grazing rate data (>100 studies from polar to tropical regions; Schmoker et al, 2013), only a handful of studies have consistently measured microzooplankton grazing rates over an annual cycle at monthly (Calbet et al, 2008; Gutiérrez-Rodríguez et al, 2011) to weekly intervals (Kim et al, 2007; Lawrence and Menden-Deuer, 2012) Such efforts have revealed seasonality in microzooplankton grazing (and phytoplankton growth), typically associated with shifts in plankton community composition, though temperature has emerged as a potential driver of grazing rates (Rose and Caron, 2007; Calbet et al, 2008; Lawrence and Menden-Deuer, 2012; Zhou et al, 2015).
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