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Microzooplankton grazing is an essential parameter to predict the fate of organic matter production in planktonic food webs. To identify predictors of grazing, we leveraged a 6‐yr time series of coastal plankton growth and grazing rates across contrasting environmental conditions. Phytoplankton size–structure and trophic transfer were seasonally consistent with small phytoplankton cell dominance and low trophic transfer in summer, and large cell dominance and higher trophic transfer in winter. Departures from this pattern during two disruptive events revealed a critical link between phytoplankton size–structure and trophic transfer. An unusual summer bloom of large phytoplankton cells yielded high trophic transfer, and an atypical winter dominance of small phytoplankton resulted in seasonally atypical low trophic transfer. Environmental conditions during these events were neither seasonally atypical nor unique. Thus, phytoplankton size–structure rather than environmental conditions held a key‐role driving trophic transfer. Phytoplankton size–structure is easily measurable and could impart predictive power of food‐web structure and the fate of primary production in coastal ecosystems.

 

Abstract. Because of its temperate location, high dynamic range of environmental conditions, and extensive human activity, the long-term ecological research site in the coastal Northeastern US Shelf (NES) of the northwestern Atlantic Ocean offers an ideal opportunity to understand how productivity shifts in response to changes in planktonic community composition. Ocean production and trophic transfer rates, including net community production (NCP), net primary production (NPP), gross oxygen production (GOP), and microzooplankton grazing rates, are key metrics for understanding marine ecosystem dynamics and associated impacts on biogeochemical cycles. Although small phytoplankton usually dominate phytoplankton community composition and Chl a concentration in the NES waters during the summer, in August 2019, a bloom of the large diatom genus Hemiaulus, with N2-fixing symbionts, was observed in the mid-shelf region. NCP was 2.5 to 9 times higher when Hemiaulus dominated phytoplankton carbon compared to NCP throughout the same geographic area during the summers of 2020–2022. The Hemiaulus bloom in summer 2019 also coincided with higher trophic transfer efficiency from phytoplankton to microzooplankton and higher GOP and NPP than in the summers 2020–2022. This study suggests that the dominance of an atypical phytoplankton community that alters the typical size distribution of primary producers can significantly influence productivity and trophic transfer, highlighting the dynamic nature of the coastal ocean. Notably, summer 2018 NCP levels were also high, although the size distribution of Chl a was typical and an atypical phytoplankton community was not observed. A better understanding of the dynamics of the NES in terms of biological productivity is of primary importance, especially in the context of changing environmental conditions due to climate processes.

 

Phytoplankton growth and microzooplankton grazing rates were measured from incubation experiments using the dilution method in the framework of the Northeast U.S. Shelf Long-Term Ecological Research project. The data set includes plankton population dynamics rates obtained during 12 cruises from winter 2018 (EN608) to summer 2022 (EN687) along a north/south transect from Martha’s Vineyard to the shelf-break. Phytoplankton growth and microzooplankton grazing rates were measured for the total phytoplankton community (chl-a concentrations) and for size fractions (chl-a size fractionation) less than and greater than 10 µm. Phytoplankton growth and microzooplankton grazing rates, the first trophic interaction between primary producers and higher trophic levels, are essential parameters to assess the cycling and export of carbon in the ocean and to better understand marine food webs. 

Size-fractionated chlorophyll a and phaeopigments are measured from discrete bottle samples collected during CTD-rosette casts on Northeast U.S. Shelf Long-Term Ecological Research (NES-LTER) Transect cruises (ongoing since 2017), as a proxy for phytoplankton biomass. Sampling frequency is approximately seasonal. Samples were processed by three lab groups using different methods. Size fractions in addition to whole seawater (>0.7 micron) include >5 , <10, >10, and >20 microns with some samples pre-filtered <200 microns. Pigments were analyzed using fluorometers in which fluorescence was measured versus a blank and a standard, and final concentrations were calculated in micrograms per liter (or mg per cubic meter). Some of the data are from cruises in collaboration with the Ocean Observatories Initiative (OOI). 

The continuous underway fluorescence, induced by in vivo chlorophyll-a (Chl-a), of surface waters of the Northeast U.S. shelf is compared to discrete Chl-a samples for post-calibration, collected ship-board as part of the Northeast U.S. Shelf Long-Term Ecological Research (NES LTER). Chl-a values derived from the manufacturer-calibrated sensors (hereafter, “continuous fluorescence”) and collected continuously are often different from the precise Chl-a concentrations obtained from discrete, extracted samples. Moreover, underway fluorometers and manufacturer calibrations differ per cruise. Thus, post-calibration of the continuous fluorescence signals using discrete Chl-a measurements is essential to standardize and compare the high-resolution underway Chl-a data along cruise tracks. For six cruises aboard the R/V Endeavor between summer 2019 and summer 2021, 12 to 22 discrete samples were collected from the underway system to measure Chl-a concentrations. These discrete Chl-a concentrations were then compared, using simple linear regressions (Model I least square fit), to corresponding continuous fluorescence values recorded by the two independent fluorometers installed with the underway system. For each cruise a preferred fluorometer was identified based on the best fit of the linear regression between discrete Chl-a concentrations and continuous fluorescence values. The slope and the intercept of the linear regression were used to post-calibrate continuous fluorescence values into standardized and intercomparable Chl-a concentration. This data package includes a table for the underway discrete Chl-a values and a table for the 1-min post-calibrated continuous fluorescence values for the preferred underway fluorometer per cruise. 

The pandemic has had innumerable impacts on the oceanographic community, including on summer research internship programs that expose undergraduates to diverse career paths in oceanography while immersed in an active laboratory. For many students, these internships are formative in their career choices. The Summer Undergraduate Research Fellowship in Oceanography (SURFO) at the University of Rhode Island’s Graduate School of Oceanography is one of the Research Experiences for Undergraduates (REU) programs that proceeded remotely during the summer of 2020. Here, we highlight one project that, although remote, maintained a hands-on research experience focused on quantitative skill building. The pandemic forced the REU advisors to identify key learning goals and ensure their safe delivery, given the circumstances. Although all participants agreed that in-person instruction would have been preferable, we were pleased that we did not let a virus halt essential oceanographic research training. 

Herbivorous consumption of primary production is a key transformation in global biogeochemical cycles, directing matter and energy either to higher trophic levels, export production, or remineralization. Grazing by microzooplankton is often poorly constrained, particularly in dynamic coastal systems. Temperate coastal areas are seasonally and spatially variable, which presents both challenges and opportunities to identify patterns and drivers of grazing pressure. Here we report on two winter and one summer week‐long cruises (2018–2019), as part of the new Northeast U.S. Shelf Long‐Term Ecological Research program. During both seasons, coastal waters were colder and fresher, and had higher phytoplankton biomass than waters at the shelf break. The phytoplankton community was dominated by large cells in winter and by small cells in summer. Phytoplankton growth rates ranged from < 0.5 d⁻¹in winter and up to 1.4 d⁻¹in summer and were strongly correlated to temperature, to light availability, and to phytoplankton community size‐structure. Grazing rates were not correlated with total chlorophyll a, which points to other biological drivers, including species composition in predator‐prey interactions at the first trophic level. The percentage of primary production consumed (%PP) indicated higher trophic transfer in winter (%PP > 50%) than during summer (%PP < 20%), highlighting seasonal shifts in planktonic food web structure and function. These results imply that predictable shifts in environmental conditions can be linked to ecosystem shifts in net primary production. Hierarchies of variability, from localized to interannual and long‐term climate driven, can be understood within the context of sustained measurements of ecosystem properties and function.