Short‐term estimates of phytoplankton growth and mortality in a tidal estuary

Abstract

Rates of phytoplankton growth and mortality are poorly defined over short‐time scales (hours to days), despite these scales being relevant over the daily tidal cycle in coastal marine areas. To assess the influence of tidal variability on phytoplankton rates, we performed eight, same‐day dilution experiments at high and low tide (6‐h intervals) in the Skidaway River Estuary, Georgia, measuring phytoplankton growth, microzooplankton grazing, and virus‐induced mortality. Chlorophyll and group‐specific biomass (Synechococcus spp., picoeukaryotes, and nanoeukaryotes) varied between tides and multidimensional scaling clustering of tidal biomass revealed separation in community composition based on tide. Rates also varied significantly over the tidal cycle, especially microzooplankton grazing, which was higher at low tide in most experiments with rates ranging from 0–3.77 d−1 at low tide to 0–1.51 d−1 at high tide. Virus‐induced mortality rates were rarely detected, only being observed in Synechococcus spp. in three experiments (0.3–0.8 d−1) and picoeukaryotes twice (~ 0.36 d−1). The differences in grazing and growth rates observed between the two tides were not explained by temperature, salinity, or sunlight (i.e., time of day) and grazing was only weakly explained by initial chlorophyll concentration (R2 = 0.36), highlighting the influence of community composition on rate measurements. These results suggest that within tidally influenced areas, short‐term sampling over the tidal cycle is essential to accurately characterize daily phytoplankton dynamics and reliably monitor and predict shifts in primary production and coastal ecosystem health.

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

“…Our findings of seasonal variability in growth and grazing contributes to the growing knowledge on the importance of time in structuring phytoplankton rate dynamics, particularly in complex coastal regions (Lawrence and Menden-Deuer, 2012;Anderson et al, 2018). 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.…”

“…Microzooplankton are ecologically important in marine food webs, as their grazing activity can elicit shifts in phytoplankton populations (e.g., biomass and diversity) and influence trophic transfer and carbon cycling (Schmoker et al, 2013;Steinberg and Landry, 2017). Few studies have considered changes in grazing rates over time at a fixed location (Schmoker et al, 2013), despite such resolution being relevant in coastal regions which often experience temporal variability in environmental factors (e.g., temperature and nutrients) and phytoplankton populations (Cloern and Jassby, 2010;Blauw et al, 2012;Anderson et al, 2018). Moreover, recurrent estimates of grazing over a range of biochemical conditions may offer valuable insight into mechanisms that control microzooplankton grazing in natural communities (Strom et al, 2007;Chen et al, 2012).…”

“…Over a full year (February 21, 2017 to February 21, 2018), 41 weekly dilution experiments were conducted using surface water (1 m) from the Skidaway River Estuary, which is a well-mixed and highly productive subtropical system accessible by dock at the Skidaway Institute of Oceanography (31 • 99 N, 81 • 02 W). Dilutions were always performed at high tide, which allowed for temporal comparison between dates and minimized strong tidal effects previously observed in the estuary (see Anderson et al, 2018). Tide height predictions were taken from a nearby station (NOAA ID: TEC3411); semidiurnal tides in the estuary consistently ranged between 2 and 3 m and sampling dates rarely coincided with spring or neap tides.…”

“…Net phytoplankton accumulation rate (r; day −1 ), which describes the relationship between µ and g, was inferred from the above equations and is equal to the value of k 1 . For cases where no grazing was measured, which occurred when apparent growth was not significantly different between the two dilution levels (t-test, p < 0.05) or when the growth in the 100% WSW was higher (e.g., positive slope), grazing rates were recorded as zero (Strom and Fredrickson, 2008;Anderson et al, 2018;Menden-Deuer et al, 2018). Negative grazing rates were observed for Synechococcus spp.…”

“…To explore temporal rate dynamics, we performed 41, weekly to bi-weekly dilution experiments over an annual cycle in the Skidaway River Estuary (GA, United States), measuring rates of phytoplankton growth, microzooplankton grazing, and viral lysis for bulk chlorophyll and specific phytoplankton groups (Synechococcus spp., pico-, and nanoeukaryotes). Recently, these methods were used to measure phytoplankton rates over the tidal cycle in the estuary (see Anderson et al, 2018), though seasonal patterns in rates remain unclear. Previous long-term sampling in the estuary has revealed significant seasonal and interannual variability in many surface-layer properties like temperature, nutrients, dissolved and particulate organic carbon (DOC and POC), chlorophyll, and plankton biomass (Verity, 2002a,b;Verity and Borkman, 2010;Bittar et al, 2016).…”