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

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 day −1) and phytoplankton growth rates (−0.3-2.43 day −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 day −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. While temperature was an important predictor of phytoplankton rates via PLS analysis, temperature exhibited stronger linearity with growth rates (R 2 = 0.46-0.56) compared to grazing rates (R 2 = 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.

Temporal Variability and Ecological Interactions of Parasitic Marine Syndiniales in Coastal Protist Communities

“…Given their importance within the protist community and broad host range, parasitism by Syndiniales is expected to have significant consequences for carbon cycling and ecosystem functioning in marine systems (8,11). In the Skidaway River Estuary, recent work has identified microzooplankton grazing as the primary source of primary production loss (37), though grazing rates were seasonal and at times did not control plankton biomass (positive accumulation in summer), indicating that other forms of mortality that were unaccounted for in the dilution experiments, such as parasitism, may have contributed to phytoplankton loss. Determining the contribution of parasitism to plankton mortality will be important, especially in relation to other forms of mortality (e.g., predation or viral lysis), which have contrasting effects on carbon and nutrient cycling in marine food webs.…”

“…Due to high particulate content in the estuary (see reference 52) and to avoid filter clogging, smaller volumes (250 to 1,000 ml) were filtered onto triplicate 47-mm 0.2-m polycarbonate filters (Millipore) and stored at Ϫ80°C until DNA extraction. Environmental factors, including temperature, salinity, dissolved oxygen, chlorophyll, particulate organic carbon and nitrogen, and nutrients were measured in a previous temporal study of plankton mortality rates in the estuary (37) and included here to explore correlations with protists (raw data available in Table S1 in the supplemental material). Nutrients and organic carbon were not measured on 6 September 2017, and so this day was removed prior to correlation analyses.…”

“…To further assess the importance of environmental factors in predicting class-level relative abundances, a partial least-squares (PLS) regression was applied using the R package pls (V2.7 [82]). PLS was included to mitigate collinearity of environmental factors, as a strong correlation between temperature and silicate was observed in the estuary (Spearman r Ͼ 0.8 [37]). Separate models were run using the predictor variables (environmental factors), and class-level relative abundances were averaged for each major protist group.…”

Assessing the Temporal Variability and Drivers of Transparent Exopolymer Particle Concentrations and Production Rates in a Subtropical Estuary

“…Muted seasonal TEP dynamics we observed in the Skidaway River Estuary may reflect a lack of repetitive sampling over multiple years or the greater importance of physical features (e.g., strong tides or coastal runoff) that may structure TEP in this system. Estuaries, in general, have been shown to experience temporal variability in a suite of parameters including temperature, nutrients, and phytoplankton community composition (Cloern and Jassby 2010;Blauw et al 2012;Anderson and Harvey 2019). Thus, changes in the physical and chemical characteristics of TEP are likely amplified in coastal environments due to dynamically shifting physical parameters (Azetsu-Scott and Passow 2004).…”

“…A collection of environmental and biological measurements were taken concurrently with weekly and tidal TEP samples from the estuary and have been published elsewhere (Anderson et al 2018;Anderson and Harvey 2019). These measurements include particulate organic carbon and nitrogen (POC and PON), dissolved inorganic nutrients, bulk chlorophyll, plankton community composition (via flow cytometry and FlowCam), and phytoplankton growth and mortality rates measured via two-point dilution experiments.…”