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Abstract 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 Picophytoplankton are a ubiquitous component of marine plankton communities and are expected to be favored by global increases in seawater temperature and stratification associated with climate change. Eukaryotic and prokaryotic picophytoplankton have distinct ecology, and global models predict that the two groups will respond differently to future climate scenarios. At a nearshore observatory on the Northeast US Shelf, however, decades of year‐round monitoring have shown these two groups to be highly synchronized in their responses to environmental variability. To reconcile the differences between regional and global predictions for picophytoplankton dynamics, we here investigate the picophytoplankton community across the continental shelf gradient from the nearshore observatory to the continental slope. We analyze flow cytometry data from 22 research cruises, comparing the response of picoeukaryote andSynechococcuscommunities to environmental variability across time and space. We find that the mechanisms controlling picophytoplankton abundance differ across taxa, season, and distance from shore. Like the prokaryote,Synechococcus, picoeukaryote division rates are limited nearshore by low temperatures in winter and spring, and higher temperatures offshore lead to an earlier spring bloom. UnlikeSynechococcus, picoeukaryote concentration in summer decreases dramatically in offshore surface waters and exhibits deeper subsurface maxima. The offshore picoeukaryote community appears to be nutrient limited in the summer and subject to much greater loss rates thanSynechococcus. This work both produces and demonstrates the necessity of taxon‐ and site‐specific knowledge for accurately predicting the responses of picophytoplankton to ongoing environmental change. 

Abstract Spatial population synchrony, defined as spatial covariation in population density fluctuations, exists across different temporal and spatial scales. Determining the degree of spatial synchrony is useful for inferring environmental drivers of population variability in the wake of climate change. In this study, we applied novel statistical methods to detect spatial synchrony patterns ofCalanus finmarchicuson the Northeast U.S. Shelf at multiple spatiotemporal scales using unevenly distributed data. Our results reveal thatC. finmarchicussubpopulations connected by advection are not necessarily in synchrony, indicating that the degree of synchrony is likely influenced by heterogeneity of local habitats. In addition, regionally synchronous environmental conditions (e.g., sea surface temperature) may not play as significant a role in influencing subregional population dynamics as was previously hypothesized. Overlooking the spatial heterogeneity of synchronous patterns at different time scales could lead to erroneous inferences of potential environmental drivers responsible forC. finmarchicusvariability. 

Abstract The Atlantic sea scallop supports one of the most lucrative fisheries on the Northeast U.S. shelf. Understanding the interannual variability of sea scallop size structure and associated drivers is critically important for projecting the response of population dynamics to climate change and designing coherent fishery management strategies. In this study, we constructed time series of sea scallop size structures in three rotationally closed areas in the Mid-Atlantic Bight (MAB) and decomposed their total variances using the variance partitioning method. The results suggested that the interannual variances in sea scallop size structures were associated more with thermal stress in regions shallower than 60 m but more with fishing mortality in regions deeper than 60 m. The percentages of small (large) size groups increased (decreased) with elevated thermal stress and fishing pressure. We adopted a scope for growth model to build a mechanistic link between temperature and sea scallop size. Model results suggested a gradual decrease in maximum shell height and habitat contraction under warming. This study quantified the relative contributions of thermal stress and fishing mortality to the variance of scallop size structure and discussed the need for adaptive management plans to mitigate potential socioeconomic impacts caused by size structure changes. 

Abstract Diatoms are among the most abundant phytoplankton that inhabit coastal ecosystems, forming large blooms that fuel coastal food webs. Although diatoms are often large and morphologically distinct, many are small or morphologically cryptic making it difficult to understand the temporal dynamics of whole diatom communities and the environmental factors that drive them. Here, we investigated diatom diversity and its environmental correlates using 6 yr of monthly surface water samples from the Narragansett Bay Plankton Time Series to investigate the seasonal and annual variability of diatom species occurrence. High‐throughput amplicon sequencing of filtered biomass yielded 658 diatom amplicon sequence variants (ASVs), of which 347 were identified to species. Of the 49 diatom genera in the sequencing dataset, 33% had never been observed in the time series using microscopy (1959–2014). We observed a weak quadratic relationship between ASV richness and chlorophyll‐aconcentrations, suggesting that richness decreases during blooms. There was a significant difference in diatom ASV richness by season and we identified distinct assemblages associated with different seasons. These assemblages were remarkably synchronous, exhibiting a sinewave‐like pattern, over 6 yr with an annual periodicity that correlated significantly with seasonal changes in temperature, light, and dissolved inorganic nitrogen. The annual cycle of diatom assemblages suggests stability in a key component of the estuarine food web known to influence ecosystem resilience and function. Deviations from the annual cycle of recurrence could be used to distinguish between changes in community structure driven by annual fluctuations in the environment and those driven by climate‐change stressors. 

Abstract Hourly, year‐round flow cytometry has made it possible to relate seasonal environmental variability to the population dynamics of the smallest, most abundant phytoplankton on the Northeast US Shelf. To evaluate whether the insights from these data extend toSynechococcusfarther from shore, we analyze flow cytometry measurements made continuously from the underway systems on 21 cruises traveling between the Martha's Vineyard Coastal Observatory (MVCO) and the continental shelf break. We describe how seasonal patterns inSynechococcus, which have been documented in detail at MVCO, occur across the region with subtle variation. We find that the underlying relationship between temperature and division rate is consistent across the shelf and can explain much of the observed spatial variability in concentration. Connecting individual cell properties to annual and regional patterns in environmental conditions, these results demonstrate the value of autonomous monitoring and create an improved picture of picophytoplankton dynamics within an economically important ecosystem. 

Abstract Diatoms are important components of the marine food web and one of the most species‐rich groups of phytoplankton. The diversity and composition of diatoms in eutrophic nearshore habitats have been well documented due to the outsized influence of diatoms on coastal ecosystem functioning. In contrast, patterns of both diatom diversity and community composition in offshore oligotrophic regions where diatom biomass is low have been poorly resolved. To compare the diatom diversity and community composition in oligotrophic and eutrophic waters, diatom communities were sampled along a 1,250 km transect from the oligotrophic Sargasso Sea to the coastal waters of the northeast US shelf. Diatom community composition was determined by amplifying and sequencing the 18S rDNA V4 region. Of the 301 amplicon sequence variants (ASVs) identified along the transect, the majority (70%) were sampled exclusively from oligotrophic waters of the Gulf Stream and Sargasso Sea and included the generaBacteriastrum,Haslea,Hemiaulus,Pseudo‐nitzschia, andNitzschia. Diatom ASV richness did not vary along the transect, indicating that the oligotrophic Sargasso Sea and Gulf Stream are occupied by a diverse diatom community. Although ASV richness was similar between oligotrophic and coastal waters, diatom community composition in these regions differed significantly and was correlated with temperature and phosphate, two environmental variables known to influence diatom metabolism and geographic distribution. In sum, oligotrophic waters of the western North Atlantic harbor diverse diatom assemblages that are distinct from coastal regions, and these open ocean diatoms warrant additional study, as they may play critical roles in oligotrophic ecosystems. 

abstract The marine coastal region makes up just 10% of the total area of the global ocean but contributes nearly 20% of its total primary production and over 80% of fisheries landings. Unicellular phytoplankton dominate primary production. Climate variability has had impacts on various marine ecosystems, but most sites are just approaching the age at which ecological responses to longer term, unidirectional climate trends might be distinguished. All five marine pelagic sites in the US Long Term Ecological Research (LTER) network are experiencing warming trends in surface air temperature. The marine physical system is responding at all sites with increasing mixed layer temperatures and decreasing depth and with declining sea ice cover at the two polar sites. Their ecological responses are more varied. Some sites show multiple population or ecosystem changes, whereas, at others, changes have not been detected, either because more time is needed or because they are not being measured. 

Summary Marine microbes often show a high degree of physiological or ecological diversity below the species level. This microdiversity raises questions about the processes that drive diversification and permit coexistence of diverse yet closely related marine microbes, especially given the theoretical efficiency of competitive exclusion. Here, we provide insight with an 8‐year time series of diversity withinSynechococcus, a widespread and important marine picophytoplankter. The population ofSynechococcuson the Northeast U.S. Shelf is comprised of six main types, each of which displays a distinct and consistent seasonal pattern. With compositional data analysis, we show that these patterns can be reproduced with a simple model that couples differential responses to temperature and light with the seasonal cycle of the physical environment. These observations support the hypothesis that temporal variability in environmental factors can maintain microdiversity in marine microbial populations. We also identify how seasonal diversity patterns directly determine overarchingSynechococcuspopulation abundance features. 

Abstract Northern sand lance (Ammodytes dubius) are among the most critically important forage fish throughout the Northeast US shelf. Despite their ecological importance, little is known about the larval transport of this species. Here, we use otolith microstructure analysis to estimate hatch and settlement dates of sand lance and then use these measurements to parametrize particle tracking experiments to assess the source–sink dynamics of three prominent sand lance habitats in the Gulf of Maine: Stellwagen Bank, the Great South Channel, and Georges Bank. Our results indicate the pelagic larval duration of northern sand lance lasts about 2 months (range: 50–84 days) and exhibit a broad range of hatch and settlement dates. Forward and backward particle tracking experiments show substantial interannual variability, yet suggest transport generally follows the north to south circulation in the Gulf of Maine region. We find that Stellwagen Bank is a major source of larvae for the Great South Channel, while the Great South Channel primarily serves as a sink for larvae from Stellwagen Bank and Georges Bank. Retention is likely the primary source of larvae on Georges Bank. Retention within both Georges Bank and Stellwagen Bank varies interannually in response to changes in local wind events, while the Great South Channel only exhibited notable retention in a single year. Collectively, these results provide a framework to assess population connectivity among these sand lance habitats, which informs the species' recruitment dynamics and impacts its vulnerability to exploitation.