Heterocapsa rotundata is a mixotrophic dinoflagellate that can ingest picoplankton, including bacteria, and is known to form large blooms in temperate estuaries during wet winters, particularly when grazing pressure on phytoplankton is low. We hypothesized that phagotrophy gives H. rotundata an advantage over other phytoplankton species during low light conditions. We used laboratory and field experiments to investigate changes in phagotrophy by H. rotundata in response to changes in light availability. Prey removal experiments with a non-axenic culture of H. rotundata were used to determine changes in H. rotundata's ingestion rates in response to changes in irradiance. Fluorescent microspheres were used to measure in situ ingestion rates of H. rotundata collected on 20 different occasions from the Choptank River during the winter of 2016. In situ H. rotundata ingestion rates were tested for correlation with inorganic nutrient concentrations and irradiance levels. Ingestion rates measured with cultured and in situ H. rotundata followed similar patterns and ingestion rates increased as irradiance decreased. H. rotundata has the potential to obtain nutrients from multiple nutrient sources, switching from phototrophy to partial heterotrophy as irradiance decreases. This response may allow H. rotundata to survive and to potentially form blooms when growth rates of most other estuarine phytoplankton species is low.Heterocapsa is a genus of dinoflagellates that contains numerous bloom forming and toxic species (Salas et al. 2014). One particular species, Heterocapsa rotundata (Lohmann) Loeblich (Hansen 1995), is ubiquitous and occasionally forms large blooms. H. rotundata has been reported in a range of environments all over the world including Chesapeake Bay, U. . H. rotundata tends to either dominate or be a prominent part of the phytoplankton community for at least part of the year in some of these areas (Seong et al. 2006;Balzano et al. 2015;Millette et al. 2015). Yet, relatively few studies have focused on the ecology of H. rotundata.Some studies have focused on the formation and decline of H. rotundata winter blooms in Chesapeake Bay tributaries (Cohen 1985; Sellner et al. 1991;Millette et al. 2015). These researchers found abundant H. rotundata blooms in wet, cold winters when salinity is low (Cohen 1985) and when there is a release in grazing pressure from microzooplankton and copepods (Millette et al. 2015). Although wet, cold winters and a release in grazing pressure are factors that impact every phytoplankton species, it is unknown how H. rotundata can take advantage of these conditions over other species to bloom. One possibility is that H. rotundata may use their capacity as a mixotroph to overcome light limitation. This would give H. rotundata an advantage over other phytoplankton in the winter. H. rotundata has been shown to consume heterotrophic bacteria, cyanobacteria such as Synechococcus, and small diatoms (Seong et al. 2006;Jeong et al. 2010). Their bacterial ingestion rate is known to incr...

Winter dinoflagellate blooms in Chesapeake Bay tributaries can account for over 50% of a system's annual primary production, potentially more than the spring diatom bloom. Research on winter blooms has focused on environmental conditions that result in blooms, but little focus has been given to the potential importance of zooplankton grazers. We investigated the impact of microzooplankton and mesozooplankton (copepods) grazing on the population of winter phytoflagellates in the Choptank River, MD, in 2012 to 2013 and 2013 to 2014. We estimated community microzooplankton and copepod grazing rates on the dominant phytoflagellate species, and measured daily gross primary production (GPP) rates. The chlorophyll a concentration and the abundance of the dinoflagellate Heterocapsa rotundata were significantly higher in 2013 to 2014 compared with 2012 to 2013, but average daily GPP was similar between the 2 yr. However, the average percentage of daily GPP removed by grazers in 2013 to 2014 was lower than in 2012 to 2013, despite average environmental conditions and nutrient concentrations not differing between years. We hypothesize that the observed release from grazing pressure is one of the main factors controlling winter dinoflagellate bloom formation in these and other coastal temperate systems.

Striped bass (Morone saxatilis) are anadromous fish that support an important fishery along the east coast of North America. In Chesapeake Bay, strong juvenile recruitment of striped bass can occur when larvae overlap with high concentrations of their zooplankton prey, but the mechanisms fostering the temporal overlap are unknown. Here, the influence of winter temperature on the peak abundances of a key prey, Eurytemora carolleeae, was estimated with a temperature-dependent developmental model. The role of these peaks in regulating striped bass recruitment was explored in three nursery areas: upper Chesapeake Bay, Choptank River, and Patuxent River. Model results indicated that cold winters delay the timing and increase the size of peak E. carolleeae spring abundance. When the model output was used in regression relationships with striped bass juvenile recruitment and freshwater discharge, the regression models explained up to 78% of annual recruitment variability. Results suggests that cold, wet winters could increase the chance of a match between striped bass larvae and high concentrations of their prey. This mechanistic link between winter temperatures and striped bass production, acting through prey dynamics, could further understanding of fish recruitment variability and indicates that warmer winters could negatively affect some striped bass populations.

Anthropogenic eutrophication threatens numerous aquatic ecosystems across the globe. Proactive management that prevents a system from becoming eutrophied is more effective and cheaper than restoring a eutrophic system, but detecting early warning signs and problematic nutrient sources in a relatively healthy system can be difficult. The goal of this study was to investigate if rates of change in chlorophyll a and nutrient concentrations at individual stations can be used to identify specific areas that need to be targeted for management. Biscayne Bay is a coastal embayment in southeast Florida with primarily adequate water quality that has experienced rapid human population growth over the last century. Water quality data collected at 48 stations throughout Biscayne Bay over a 20-year period (1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014) were examined to identify any water quality trends associated with eutrophication. Chlorophyll a and phosphate concentrations have increased throughout Biscayne Bay, which is a primary indicator of eutrophication. Moreover, chlorophyll a concentrations throughout the northern area, where circulation is restricted, and in nearshore areas of central Biscayne Bay are increasing at a higher rate compared to the rest of the Bay. This suggests increases in chlorophyll a are due to local nutrient sources from the watershed. These areas are also where recent seagrass die-offs have occurred, suggesting an urgent need for management intervention. This is in contrast with the state of Florida listing of Biscayne Bay as a medium priority impaired body of water.

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Phago-mixotrophy, the combination of photoautotrophy and phagotrophy in mixoplankton, organisms that can combine both trophic strategies, have gained increasing attention over the past decade. It is now recognized that a substantial number of protistan plankton species engage in phago-mixotrophy to obtain nutrients for growth and reproduction under a range of environmental conditions. Unfortunately, our current understanding of mixoplankton in aquatic systems significantly lags behind our understanding of zooplankton and phytoplankton, limiting our ability to fully comprehend the role of mixoplankton (and phago-mixotrophy) in the plankton food web and biogeochemical cycling. Here, we put forward five research directions that we believe will lead to major advancement in the field: (i) evolution: understanding mixotrophy in the context of the evolutionary transition from phagotrophy to photoautotrophy; (ii) traits and trade-offs: identifying the key traits and trade-offs constraining mixotrophic metabolisms; (iii) biogeography: large-scale patterns of mixoplankton distribution; (iv) biogeochemistry and trophic transfer: understanding mixoplankton as conduits of nutrients and energy; and (v) in situ methods: improving the identification of in situ mixoplankton and their phago-mixotrophic activity.

Here, we present a range of interactions, which we term "cryptic interactions." These are interactions that occur throughout the marine planktonic foodweb but are currently largely overlooked by established methods, which mean large-scale data collection for these interactions is limited. Despite this, current evidence suggests some of these interactions may have perceptible impacts on foodweb dynamics and model results. Incorporation of cryptic interactions into models is especially important for those interactions involving the transport of nutrients or energy. Our aim is to highlight a range of cryptic interactions across the plankton foodweb, where they exist, and models that have taken steps to incorporate these interactions. Additionally, it is discussed where additional research and effort is required to continue advancing our understanding of these cryptic interactions. We call for more collaboration between ecologists and modelers in order to incorporate cryptic interactions into biogeochemical and foodweb models. Scientific Significance StatementGeneralizations in foodweb modeling have helped scientists explain and study interactions within the aquatic environment. Established methods focus on quantifying and qualifying these generalizations, but rapid technological advancements in recent years have revealed that our generalizations of interactions may obscure significant processes within the plankton community. We explore a range of interactions within the planktonic foodweb that are "cryptic" because existing methods are biased against them with the intention of highlighting a range of cryptic interactions and the potential impact of overlooking them in future research. We discuss how including these interactions in biogeochemical and foodweb models alters our understanding of the transfer of carbon and other materials from one species/functional group to another and highlight examples of recent models that have incorporated cryptic interactions.1

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