In the North Pacific Subtropical Gyre, the daily pulse of photosynthetic carbon (C) fixation is closely balanced by losses. This concert of growth and loss is driven by a diverse assemblage of plankton, including the diazotroph Crocosphaera sp. While primary production is relatively well characterized in this ecosystem, the extent of C transfer to secondary producers is poorly constrained. Here, we use automated imaging flow cytometry and population modeling to study the coupling of C production by Crocosphaera and subsequent grazing by nanoplanktonic protists. Crocosphaera cells represent on average 30% of the nanoplankton detected by the Imaging FlowCytoBot in the surface layer of mesoscale eddies during summertime. The size spectra show a maximum in the frequency of Crocosphaera doublet cells just prior to mitotic division at midday, with an average estimated growth rate of 0.8 AE 0.5 d −1. We also identified potential predators by fitting a Lotka-Volterra model to plankton time series observations. Significant predators include the dinoflagellates Protoperidinium and Dinophysis as well as the ciliate Strombidium, which were all imaged with Crocosphaera in food vacuoles. The estimated C demand of the main grazers fluctuated between 25% and 250% of Crocosphaera new production in an anticyclonic eddy where we observed the onset of a Crocosphaera-driven bloom. Heterotrophic Protoperidinium drove most of the estimated C demand, with grazing rates nearly equivalent to Crocosphaera growth rates (0.6 AE 0.4 d −1 on average), but saturating at high prey concentrations. Our novel results demonstrate tight coupling between specific protistan predators and a diazotrophic prey.

The net balance between photosynthesis and respiration in the surface ocean is a key regulator of ocean-atmosphere carbon dioxide (CO 2) partitioning, and by extension, Earth's climate. The slight excess of photosynthesis over community respiration in sunlit waters, known as net community production (NCP), sets the upper bound on the sequestration of carbon via biologically mediated export. Prevailing paradigms suggest a high/low binary where net primary production (NPP), NCP, and export are highest in ecosystems characterized by microplankton (>20 μm) and lowest in ecosystems dominated by picoplankton (<2 μm). This bifurcation model neglects the potential importance of nanoplankton (2-20 μm)-i.e., the "middle" size class-toward global biological pump functioning. Here, we show a relationship between the biomass of nanoplankton and oxygen-based estimates of NCP across natural ecological gradients in the North Pacific Ocean. Using a suite of high-resolution optical imaging approaches including SeaFlow, Imaging FlowCytobot, and laser-based scattering, nanoplankton dynamics are observed to dominate the particle size distribution throughout an~1,000 km transition between the subtropical and subpolar North Pacific, where NCP rates are threefold to fivefold higher than subtropical values. Based on ecological theory applied to the Darwin size-based ecosystem model, we hypothesize that intermediate size class organisms are capable of high rates of production via an optimization of bottom-up and top-down control inherent to the "middle class." More broadly, the model indicates the global importance of nanoplankton for ocean biological production.

The deep chlorophyll maximum (DCM) layer is an ecologically important feature of the open ocean. The DCM cannot be observed using aerial or satellite remote sensing; thus, in situ observations are essential. Further, understanding the responses of microbes to the environmental processes driving their metabolism and interactions requires observing in a reference frame that moves with a plankton population drifting in ocean currents, i.e., Lagrangian. Here, we report the development and application of a system of coordinated robots for studying planktonic biological communities drifting within the ocean. The presented Lagrangian system uses three coordinated autonomous robotic platforms. The focal platform consists of an autonomous underwater vehicle (AUV) fitted with a robotic water sampler. This platform localizes and drifts within a DCM community, periodically acquiring samples while continuously monitoring the local environment. The second platform is an AUV equipped with environmental sensing and acoustic tracking capabilities. This platform characterizes environmental conditions by tracking the focal platform and vertically profiling in its vicinity. The third platform is an autonomous surface vehicle equipped with satellite communications and subsea acoustic tracking capabilities. While also acoustically tracking the focal platform, this vehicle serves as a communication relay that connects the subsea robot to human operators, thereby providing situational awareness and enabling intervention if needed. Deployed in the North Pacific Ocean within the core of a cyclonic eddy, this coordinated system autonomously captured fundamental characteristics of the in situ DCM microbial community in a manner not possible previously.

Satellite observations of chlorophyll in coastal waters are often described in terms of changes in productivity in response to regional upwelling processes while optical backscattering coefficients are more often linked to episodic inputs of suspended sediments from storm runoff. Here we show that the surface gravity wave resuspension of sediments has a larger role in controlling backscatter than previously considered. Almost 18 years of SeaWiFS, MODIS, MERIS, and VIIRS satellite imagery of the Santa Barbara Channel, California and its surrounding waters spectrally merged with the Garver‐Siegel‐Maritorena bio‐optical model were used to assess the controls on suspended particle distributions. Analysis revealed that chlorophyll blooms in the warmer portions of the domain occur in phase with SST minima, usually in early spring, while blooms in the cooler regions lag SST minima and occur simultaneously to the strongest equatorward winds every year, often in the summer. Tight coupling between the optical variables was seen in offshore areas, as expected for productive waters. However, values of backscatter near the coast were primarily modulated by surface waves. This relationship holds throughout all seasons and is stronger within the 100 m isobath, but often extends tens of kilometers offshore. This forcing of particle resuspension by surface waves is likely a feature ubiquitous in all coastal oceans characterized by fine sediments. The implication of surface wave processes determining suspended particle loads far beyond the surf zone has large consequences for the interpretation of satellite ocean color signals in coastal waters and potentially redefines the extent of the littoral zone.

The coastal margin of the Pacific Northwest of the United States is a highly dynamic and productive region. Here, we use satellite, high-frequency mooring, and glider estimates of biologically relevant physical and optical variables to characterize seasonal patterns and latitudinal and cross-shore gradients in particle concentrations between the Washington and Oregon shelves. Consistent with prior research, we find that the Columbia River exerts a strong seasonal influence on the Washington shelf, but smaller coastal rivers and resuspension processes also appear important in determining particle distributions nearshore during winter across the full study region. We find fluorescencebased measurements of chlorophyll to be similar in magnitude across the two shelves over the time period examined, although the much weaker wind stresses off Washington indicate that processes other than upwelling are important determinants of chlorophyll changes in those areas, as previously suggested. These in situ observations contrast with the overall differences observed from satellite data, which consistently show higher chlorophyll concentrations off the Washington coast. This research suggests that latitudinal differences in chromophoric dissolved organic matter may be a partial explanation for perceived trends in satellite-derived chlorophyll. The observations presented are nascent; maturation of temporal and spatial coverage of OOI data sets will be necessary to more conclusively link physical forcing and biogeochemical responses.