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Surface‐ocean mixing creates dynamic light environments with predictable effects on phytoplankton growth but unknown consequences for predation. We investigated how variations in average mixed‐layer (ML) irradiance shaped plankton trophic dynamics by incubating a Northwest‐Atlantic plankton community for 4 days at high (H) and low (L) light, followed by exposure to either sustained or reversed light intensities. In deep‐ML (sustained L), phytoplankton biomass declined (μ = −0.2 ± 0.08 d−1) and grazing was absent. In shallow‐ML (sustained H), growth exceeded grazing (μ = 0.46 ± 0.07 d−1; g = 0.32 ± 0.04 d−1). In rapidly changing ML‐conditions simulated by switching light‐availability, growth and grazing responded on different timescales. During rapid ML‐shoaling (L to H), μ immediately increased (0.23 ± 0.01 d−1) with no change in grazing. During rapid ML‐deepening (H to L), μ immediately decreased (0.02 ± 0.09 d−1), whereas grazing remained high (g = 0.38 ± 0.05 d−1). Predictable rate responses of phytoplankton growth (rapid) vs. grazing (delayed) to measurable light variability can provide insights into predator‐prey processes and their effects on spatio‐temporal dynamics of phytoplankton biomass.

International audience; The goal of the EXport Processes in the Ocean from RemoTe Sensing (EXPORTS) field campaign is to develop a predictive understanding of the export, fate, and carbon cycle impacts of global ocean net primary production. To accomplish this goal, observations of export flux pathways, plankton community composition, food web processes, and optical, physical, and biogeochemical (BGC) properties are needed over a range of ecosystem states. Here we introduce the first EXPORTS field deployment to Ocean Station Papa in the Northeast Pacific Ocean during summer of 2018, providing context for other papers in this special collection. The experiment was conducted with two ships: a Process Ship, focused on ecological rates, BGC fluxes, temporal changes in food web, and BGC and optical properties, that followed an instrumented Lagrangian float; and a Survey Ship that sampled BGC and optical properties in spatial patterns around the Process Ship. An array of autonomous underwater assets provided measurements over a range of spatial and temporal scales, and partnering programs and remote sensing observations provided additional observational context. The oceanographic setting was typical of late-summer conditions at Ocean Station Papa: a shallow mixed layer, strong vertical and weak horizontal gradients in hydrographic properties, sluggish sub-inertial currents, elevated macronutrient concentrations and low phytoplankton abundances. Although nutrient concentrations were consistent with previous observations, mixed layer chlorophyll was lower than typically observed, resulting in a deeper euphotic zone. Analyses of surface layer temperature and salinity found three distinct surface water types, allowing for diagnosis of whether observed changes were spatial or temporal. The 2018 EXPORTS field deployment is among the most comprehensive biological pump studies ever conducted. A second deployment to the North Atlantic Ocean occurred in spring 2021, which will be followed by focused work on data synthesis and modeling using the entire EXPORTS data set.

WOS:000522382500001; International audience; The North Atlantic phytoplankton spring bloom is the pinnacle in an annual cycle that is driven by physical, chemical, and biological seasonality. Despite its important contributions to the global carbon cycle, transitions in plankton community composition between the winter and spring have been scarcely examined in the North Atlantic. Phytoplankton composition in early winter was compared with latitudinal transects that captured the subsequent spring bloom climax. Amplicon sequence variants (ASVs), imaging flow cytometry, and flow-cytometry provided a synoptic view of phytoplankton diversity. Phytoplankton communities were not uniform across the sites studied, but rather mapped with apparent fidelity onto subpolar- and subtropical-influenced water masses of the North Atlantic. At most stations, cells \textless 20-mu m diameter were the main contributors to phytoplankton biomass. Winter phytoplankton communities were dominated by cyanobacteria and pico-phytoeukaryotes. These transitioned to more diverse and dynamic spring communities in which pico- and nano-phytoeukaryotes, including many prasinophyte algae, dominated. Diatoms, which are often assumed to be the dominant phytoplankton in blooms, were contributors but not the major component of biomass. We show that diverse, small phytoplankton taxa are unexpectedly common in the western North Atlantic and that regional influences play a large role in modulating community transitions during the seasonal progression of blooms.

The impacts of grazing by meso- and microzooplankton on phytoplankton primary production (PP) was investigated in the surface layer of the western North Atlantic during spring. Shipboard experiments were performed on a latitudinal transect at three stations that differed in mixed layer depth, temperature, and mesozooplankton taxonomic composition. The mesozooplankton community was numerically dominated byCalanus finmarchicusat the northern and central station, withCalanus hyperboreusalso present at the northern station. The southern station was >10 °C warmer than the other stations and had the most diverse mesozooplankton assemblage, dominated by small copepods includingParacalanusspp. Microzooplankton grazing was detected only at the northern station, where it removed 97% of PP. Estimated clearance rates byC. hyperboreusandC. finmarchicussuggested that at in-situ abundance these mesozooplankton were not likely to have a major impact on phytoplankton abundance, unless locally aggregated. Although mesozooplankton grazing impact on total phytoplankton was minimal, these grazers completely removed the numerically scarce > 10 µm particles, altering the particle-size spectrum. At the southern station, grazing by the whole mesozooplankton assemblage resulted in a removal of 14% of PP, and its effect on net phytoplankton growth rate was similar irrespective of ambient light. In contrast, reduction in light availability had an approximately 3-fold greater impact on net phytoplankton growth rate than mesozooplankton grazing pressure. The low mesozooplankton grazing impact across stations suggests limited mesozooplankton-mediated vertical export of phytoplankton production. The constraints provided here on trophic transfer, as well as quantitative estimates of the relative contribution of light and grazer controls of PP and of grazer-induced shifts in particle size spectra, illuminate food web dynamics and aid in parameterizing modeling-frameworks assessing global elemental fluxes and carbon export.

The abundance and size distribution of plankton in the surface ocean are key metrics to understanding primary and export production, food web dynamics, and the optical properties of the water column. Here we quantified cell size and abundance of phytoplankton species from mono-specific laboratory cultures, using optical, electrical, and image-based benchtop instrumentation. Moreover, we combined particle size distribution and size fractionated chlorophyll a (Chl a) analysis with image analysis to estimate phytoplankton community composition and abundance comparing a high and low biomass station in the North Atlantic during springtime. In laboratory cultures, we found generally good agreement in estimates of both particle concentration and particle size among instruments. Image-based approaches (e.g., microscopy, FlowCam) delivered somewhat lower cell abundance estimates, because image-based instruments distinguish cells from non-target particles. Image-based approaches also measured 10–20% greater cell sizes, because measurements were based only on in-focus images of the target species. Compared to image-based instruments, particle counters delivered indistinguishable estimates of size and abundance with much lower effort and technical expertise required; maximum coefficient of variation for cell abundance and size did not exceed 10 and 15%, respectively. Measurement precision was consistent across instrument type and across a cell size spectrum from 3 to >40 μm equivalent spherical diameter (ESD). For whole plankton community analyses from the North Atlantic, combined estimation of Chl a concentration and image-based diversity assessments resolved the dominant phytoplankton species and spatial differences in the size structure of the plankton community. These results provide strong support for using particle counters, that can rapidly process large volumes of samples, to quantify particle size and abundance, including rarer, larger particles. Species identities and community composition can be revealed by supplemental, image-based approaches. Application of this coupled approach can help identify fundamental ecosystem characteristics such as particle size spectra that affect primary production, trophic transfer, and export. Ultimately, the tremendous species diversity of plankton can be leveraged as particle tracking and identification keys, such as near-real time identification of coherent water masses.

The dilution-method has been key in establishing the role of protistan-grazing in marine foodwebs. Yet its laborious application limits the sampling-resolution achieved. We assessed the reliability of an abbreviated method known as the 2-point by analyzing 77 dilution-experiments performed using 4–5 dilutions in diverse biotic and abiotic conditions. Our aim was to inform practitioners on how experimental design and nonlinear feeding behaviors affect the accuracy of 2-point rate-estimates. We found good agreement between rate-estimates of both phytoplankton growth (μ) and grazer-induced mortality (g) from either method, even though the comparison included experiments with nonlinear feeding-responses. The accuracy of 2-point estimates was similar to the inherent standard deviation of dilution-series estimates (± ∼ 0.1 d−1). Nonlinear feeding-responses did not alter overall rate-estimates, negating the need for more than two dilution-levels. Decreasing dilution resulting in an increase in biomass from 10% to 40% resulting in increased biomass increased the median difference between 2-point and dilution-series estimates ∼ 3-fold and increased 2-point estimates' variance ∼ 2-fold, both for μ and g. Recognition of these biomass vs. accuracy tradeoffs enables practitioners to choose whether to procure more biomass at the expense of constraining estimate variance. Using duplicate bottles at each dilution level doubled average accuracy of 2-point estimates. The reduction in effort and water-needs afforded by the 2-point design facilitates acquisition of higher-resolution data of predation-rates across seasons, latitudes, and in response to diverse environmental conditions in the ocean, which is critically needed to decipher abiotic and biotic drivers of protistan-grazing and to parameterize protistan herbivory in global biogeochemical models.

During NAAMES May 2016 field campaign, we examined the response of plankton population dynamics to rapid re-stratification of surface waters following a deep mixing event in the subpolar North Atlantic. Over the 4-day occupation of a station, we measured phytoplankton growth and grazer/virus-induced mortality rates in dilution assays conducted across a range of light intensities. Rates were estimated from changes in chlorophyll a and the abundance of three phytoplankton groups identified by flow cytometry. Initially, biological and physical water column properties were homogeneous down to >200 m, followed by rapid shoaling of the mixed layer to 20-30 m. Initial in situ chlorophyll a concentration was 0.4 µg L-1, and phytoplankton biomass accumulated at an average rate of 0.4 d-1 over the next two days. When mixed layer depth was maximal, there were no mortality losses and phytoplankton growth rates increased with increased light intensity. After shoaling, grazing rates increased, but never matched the magnitude of phytoplankton growth rates. When the mixed layer was shallowest, growth rates exceeded >1 d-1 at all non-dark light intensities. Chlorophyll a based grazing rates were consistent across light levels (~0.3 d-1) and were highest on Synechococcus (0.3-0.6 d-1) and lowest on pico-eukaryotes (~0.2 d-1). The delay with which grazing resumed resulted in growth exceeding losses and consumption of an average of 30% of primary production. Virus-induced mortality rates were minimal across all mixing profiles and light intensities. Overall, both chlorophyll a and group-specific phytoplankton accumulation rates predicted from the shipboard experiments matched those in situ, suggesting that incubation conditions faithfully captured the growth and loss processes governing in situ population dynamics. The observations made here indicate that transient deepening of the mixed layer followed by rapid stratification provided conditions under which phytoplankton escape protistan grazing and accumulate biomass. During the physically dynamic springtime in the North Atlantic, frequent ephemeral fluctuations in the depth of mixing may represent an important mechanism governing the formation and the magnitude of the North Atlantic spring bloom.

The North Atlantic Aerosols and Marine Ecosystems Study (NAAMES) is an interdisciplinary investigation to improve understanding of Earth's ocean ecosystem-aerosol-cloud system. Specific overarching science objectives for NAAMES are to (1) characterize plankton ecosystem properties during primary phases of the annual cycle and their dependence on environmental forcings, (2) determine how these phases interact to recreate each year the conditions for an annual plankton bloom, and (3) resolve how remote marine aerosols and boundary layer clouds are influenced by plankton ecosystems. Four NAAMES field campaigns were conducted in the western subarctic Atlantic between November 2015 and April 2018, with each campaign targeting specific seasonal events in the annual plankton cycle. A broad diversity of measurements were collected during each campaign, including ship, aircraft, autonomous float and drifter, and satellite observations. Here, we present an overview of NAAMES science motives, experimental design, and measurements. We then briefly describe conditions and accomplishments during each of the four field campaigns and provide information on how to access NAAMES data. The intent of this manuscript is to familiarize the broad scientific community with NAAMES and to provide a common reference overview of the project for upcoming publications.

We measured phytoplankton-growth (μ) and herbivorous-protist grazing (g) rates in relation to mixed-layer-depth (MLD) during the March/April 2012 EuroBasin cruise in the subpolar North Atlantic. We performed 15 dilution experiments at two open-ocean (∼ 1300 m) and one shelf (160 m) station. Of the two open-ocean stations one was deeply mixed (476 m), the other stratified (46 m). At the shelf station, MLD reached the bottom. Initial chlorophyll a (Chl a) varied from 0.2–1.9 μg L−1 and increased up to 2.7 μg L−1 at the shelf station. In 80% of experiments, regardless of MLD, growth-rates exceeded grazing-mortality rates. At the open-ocean stations, the deep ML coincided with μ and g that varied over the same range (≤ 0–0.6 d−1), whereas stratification corresponded to μ and g that ranged from 0.14–0.41 d−1 to 0.11–0.34 d−1, respectively. At the stratified station, the balance between μ and g explained 98% of in situ variations in Chl a, whereas at the deep-ML station, rate estimates had no explanatory power. The consistent relationship between μ and g, which corresponded to a grazing-removal of 64% of primary production, suggests that g might be predictable if μ is known, and that a coefficient of 0.64 may be a useful parameter for subarctic carbon models. Composition and persistence of the plankton assemblages differed at the stations and may have been a significant driver of grazing-pressure. Overall, these results showed no association of MLD with grazing-pressure and highlight the need to assess to what extent MLD represents the depth of active-mixing to understand the effects of protistan-grazing on the development of the North Atlantic spring bloom.

SUMMARY American lobsters (Homarus americanus) will on rare occasions produce sounds by vibrating their dorsal carapace. Although this behavior can be elicited in the laboratory by handling lobsters, the stimulus that triggers the production of sounds in the lobster's natural habitat is not known. We investigated the influence of two fish that are known to prey on lobsters, cod (Gadus morhua) and striped bass (Morone saxatilis), on the production of sounds by American lobsters. In addition, we examined the response of the same fish to the sounds the lobsters produced. Although solitary lobsters spontaneously produced sounds at a low rate of 1.2±0.23 sound events per 30 min, the presence of a single cod or striped bass led to an increase in the rate of sound production (cod: 51.1±13.1 events per 30 min; striped bass: 17.0±7.0 events per 30 min). Most (74.6±6.6%) of the 292 sound events recorded occurred when a fish came within 0.5 m of a lobster, but a fish did not have to come into contact with a lobster to elicit sounds. Immediately following the production of a sound by a lobster, fish turned and swam away significantly faster than when they encountered a lobster that did not make a sound. Moreover, after striped bass (but not cod) experienced a number of these sound events, they subsequently tended to avoid swimming close to the lobsters. These data, taken together, suggest that sound production by American lobsters may serve to deter potential fish predators.