Limnology and Oceanography

Sinking marine particles, one pathway of the biological carbon pump, transport carbon to the deep ocean from the oceans surface, thereby contributing to atmospheric carbon dioxide modulation and benthic food supply. Few in situ measurements exist of sinking particles in the Northern Gulf of Alaska (NGA); therefore, regional carbon flux prediction is poorly constrained. In this study, we aim to (1) characterize the magnitude and efficiency of the biological carbon pump and (2) identify drivers of carbon flux in the NGA. We deployed drifting sediment traps to simultaneously collect bulk carbon and intact sinking particles in polyacrylamide gels and measured net primary productivity from deck-board incubations. Through deployments during the summer of 2019, we found high carbon flux magnitude, low attenuation with depth, and high export efficiency. We quantitatively attributed carbon flux between ten particle types, including various fecal pellet categories, dense detritus, and aggregates using polyacrylamide gels. The contribution of aggregates to total carbon flux (41 - 93%) and total carbon flux variability (95%) suggests that aggregation processes, not zooplankton repackaging, played a dominant role in carbon export during the summer of 2019 in the NGA. Furthermore, efficient export correlated significantly with the proportion of chlA > 20 {micro}m, total aggregate flux, and proportion aggregate flux. These results suggest that this stratified, small-cell-dominated ecosystem can have sufficient aggregation to allow for a strong and efficient biological carbon pump. These are the first measurements of carbon flux and the first integrative description of the BCP in this region. Significance StatementO_ST_ABSNovelty and significanceC_ST_ABSWe use a comprehensive approach that brings together sediment trap sampling and imaging, optically measured distribution of sinking and suspended particles, and incubations to make the first description of the biological carbon pump in the Northern Gulf of Alaska. We found high carbon flux magnitude, low attenuation with depth, and high export efficiency with a phytoplankton community consisting of mostly pico-and nanoplankton. Notably, just 25% of carbon flux out of the euphotic zone was as recognizable fecal pellets; instead, we demonstrate that aggregation processes were the main driver of carbon flux. Additionally, size-fractionated chlorophyll-a (> 20 {micro}m) strongly correlated with export efficiency across our region. These results lead us to question our expectations about what conditions and processes can create strong and efficient flux events in the Gulf of Alaska. Breadth of InterestThis study is the first description of the biological carbon pump in the Northern Gulf of Alaska and greatly improves biogeochemical constraints on this system. We report observed primary production, carbon flux, export ratio, carbon flux attenuation, and carbon flux by 10 particle types, which can be used to test regional climate models. This study builds on previous studies published in L&O: Strom et al. 2007; Ebersbach & Trull 2008; McDonnell & Buesseler 2010, 2012; and Durkin et al. 2016. Author contribution statementSO, SS, and AM: conceptualization, methodology, and investigation. SS, AM, GH: funding acquisition and project administration. SO, TK, AM: formal analysis. GH, TK, and AM: supervision. SO: visualization, writing-original draft preparation. SO, GH, TK, SS, and AM: writing-reviewing and editing.

The sinking of large particles (i.e., marine snow) has long been recognized as a key pathway for efficient particulate organic carbon (POC) export to the ocean interior during the decline of spring diatom blooms. Recent work has suggested that particles smaller than marine snow can also substantially contribute to POC export. However, a detailed characterization of small and large sinking particles at the end of blooms is missing. Here, we separately collected suspended and small and large sinking particles using Marine Snow Catchers and assessed their biogeochemical composition after the North Atlantic spring bloom in May 2021. During the three weeks of sampling, when four intense storms (maximum wind speeds 37 - 50 kts) created high turbulent energy dissipation rates and deepened the mixed layer, we observed two distinct sedimentation episodes. During the storm periods, sinking particles were dominated by small (diameter < 0.1 mm), slow-sinking (~18 m d-1), silica-rich particles that carried a moderate POC flux (< 6 mmol C m-2 d-1) to 500 m depth. Once the storms ceased, the volume of large (diameter > 0.1 mm), fast-sinking (> 75 m d-1), carbon-rich marine snow aggregates (not fecal pellets) increased exponentially and POC fluxes at 100 m depth were more than fourfold greater (30{+/-}12 mmol C m-2 d-1) than those during the previous event. The aggregates consisted of a mixed post-bloom plankton community. Our data suggest that the intense storms determined the timing, type, and magnitude of POC flux at the end of a spring phytoplankton bloom.

The marine biological carbon pump is mainly driven by the interplay between aggregate sinking velocity and remineralization. Sinking velocity of natural marine aggregates is not routinely measured but often calculated using Stokes law, which does not consider size-dependent changes in porosity. We analyzed the flow fields around 81 in situ-formed aggregates using Particle Image Velocimetry (PIV) to determine the factors controlling aggregate settling. Using an independently derived scaling of porosity with size, we predicted the sinking velocity of laboratory-formed and in situ-formed aggregates with known densities. Small aggregates (<500 {micro}m) have relatively lower porosities than large aggregates, and their increased compactness and density leads to higher size-specific settling velocities, and generally higher carbon-to-volume ratios. Applying our scaling approach to a global data set of vertical aggregate abundance and size distribution, we found that small aggregates contribute 40-70% to total carbon fluxes in situ. TeaserImproved sinking velocity prediction for marine aggregates highlights the contribution of small aggregates to carbon sequestration.

The efficiency of marine diatom carbon concentrating mechanisms (CCMs) play a critical role in photosynthesis and enable cells to maintain rapid growth rates under a variety of environmental conditions. To assess the plasticity of the model diatom P. tricornutums CCMs, specifically carbonic anhydrase (CA) enzymes and bicarbonate transporters, we measured growth response, bulk CA activity, and corresponding shifts in the proteome under a range of Zn and pCO2 conditions in culture. CA activity increased with Zn availability and decreased with pCO2. A positive growth effect was observed due to Zn addition and increasing pCO2 from 200 to 400 ppm, however growth rate decreased as pCO2 further increased to 750 ppm. Across the six treatments, the protein abundance of ISIP2A, which functions to bring Fe into the cell via a FeCO3 complex and is used as a biomarker for Fe stress, demonstrated an inverse relationship with [CO32-], consistent with its role as a phytotransferrin. Under conditions of Zn limitation ([Zn2+] = 0.3 pM), the cell appeared to allocate this metal away from CA, instead relying on a Mn-CA with a 100-fold lower intrinsic activity than that of the primary Zn-CA, as calculated using paired abundance-activity measurements. We further observed a continued increase in bicarbonate transport protein abundance after CA activity plateaued at 1.2x10-6 (reactions sec- 1cell-1), suggesting any deficit in DIC required to maintain high growth rates is accomplished through HCO3- uptake. We hypothesize that bicarbonate uptake and CO2 diffusion operate in tandem via CA enzymatic activity to supply adequate CO2 for photosynthesis.

The sea-surface microlayer (SML) is a crucial ocean-atmosphere interface involved in gas exchange and nutrient cycling. Slicks, i.e., viscous surface layers, common in coastal regions serve as microbial hotspots. We studied microbial abundance, surfactants, dissolved organic carbon (DOC), and net community production (NCP) of O2 in slick and non-slick SMLs and underlying water (ULW) in the coastal Baltic Sea. Slicks often showed higher surfactant levels compared to the ULW. Microbial respiration often exceeded production, resulting in net O2 consumption, although some ULW sites exhibited net O2 production. The SML was enriched with pico- and nanophytoplankton, with cyanobacteria being negatively correlated with total dissolved nitrogen. In contrast, microphytoplankton accumulated in the ULW, indicating niche separation with depth. Microscopy revealed ciliates and juvenile sporophytes dominating a slicks >100 {micro}m fraction. In eutrophic coastal systems, slicks influence plankton communities and O2 dynamics, supporting their role in surface biogeochemical cycling and climate-driven changes.

Rhizaria are a diverse supergroup of large marine protists that are often overlooked due to their fragility, lower abundances, and wide size range relative to other plankton. Despite their global distribution, Rhizaria ecology and biogeography is poorly understood due to a paucity of datasets and use of differing methodologies. Here we present the first characterization of Rhizaria ecology in the northern Gulf of Alaska (NGA), a variable yet productive subarctic ecosystem with important fisheries that is experiencing long-term warming. Seawater samples were collected from CTD-secured Niskin bottles at stations within the NGA Long-Term Ecological Research study area during summer 2023. We report some of the highest Rhizaria abundances (25 cells L-1) from any ocean environment to date and thus suggest a restructuring of the current biogeographical paradigm that posits highest abundances at the equator and decreases at higher latitudes. Acantharia was the most ubiquitous subgroup. Distinct depth niches were also revealed: Foraminifera dominated surface waters, Radiolaria exhibited a cosmopolitan distribution, and Phaeodaria were the deepest living. Prey captures and algal interactions primarily occurred offshore in the upper water column. A wide range of taxa had captured prey while the hosts to presumptively symbiotic algae were mainly Foraminifera and Acantharia. We highlight Rhizaria as key players in NGA food web dynamics as evidenced by their wide depth distributions, taxonomic diversity, and variable nutrition strategies. O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=97 SRC="FIGDIR/small/652060v1_ufig1.gif" ALT="Figure 1"> View larger version (33K): org.highwire.dtl.DTLVardef@1cb948corg.highwire.dtl.DTLVardef@857c74org.highwire.dtl.DTLVardef@1adcaeborg.highwire.dtl.DTLVardef@e5332b_HPS_FORMAT_FIGEXP M_FIG Graphical Abstract.Distribution of Rhizaria subgroups in the northern Gulf of Alaska (left). Proposed revision of the biogeographical distribution of Rhizaria in the Pacific and Southern Oceans (right). C_FIG HighlightsO_LIThe N. Gulf of Alaska contains some of the highest Rhizaria abundances yet reported C_LIO_LIAcantharia was the most abundant taxon C_LIO_LIRhizaria subgroups inhabited distinct depth niches C_LIO_LIA wide range of taxa had captured prey C_LIO_LIForaminifera and Acantharia were the most common hosts to algal cells C_LI

While phytoplankton dynamics in the annual North Atlantic spring bloom have been well characterized, the physiological underpinnings driving these changes and their net impact on the biogeochemistry of the region are less understood. Phytoplankton metabolism is both affected by, and influences the regions nutrient cycling, primary production, and ultimately, the fate of carbon export. Thus, developing an understanding of these processes is critical. Phytoplankton biomass, biological rates, and gene expression data along with associated environmental parameters were measured as part of the NASA EXport Processes in the Ocean from RemoTe Sensing programs campaign to the North Atlantic to evaluate the relationships amongst these processes within the four most dominant phytoplankton groups (diatoms, dinoflagellates, haptophytes, and chlorophytes) during the spring bloom. We observe a transition from a period dominated by active diatom growth (defined as Phase I) to a period dominated by non-diatom phytoplankton groups (Phase II). Silicic acid depletion appears to limit overall production and reduce competition from diatoms, likely leading to enhanced contributions of haptophytes in Phase II. Expression of key protein-encoding genes involved in cell maintenance, photosynthesis, and nitrogen and vitamin metabolisms varied amongst the taxa throughout the observation period. Expression patterns of diatom genes involved in silicon transport suggest an apparent uncoupling between genes involved in nitrate uptake and photosynthesis, resulting in an increase in silicification independent growth. Our analysis demonstrates the utility in combining gene expression with biological rate processes to provide a more holistic view of phytoplankton bloom dynamics and phenology.

The Mississippi River (MR) is the largest source of freshwater and nutrients to the Gulf of Mexico (GoM), strongly influencing stratification, primary production, and plankton organization. The interaction between buoyant plume waters and denser shelf waters in the northern Gulf of Mexico (nGoM) generates sharp density gradients that can promote fine-scale biological aggregation. We investigated how hydrographic structure associated with the MR plume controls the vertical distribution of plankton during May 2017 using an integrated instrumentation suite that included an in situ digital holographic imaging system (HOLOCAM) coupled with CTD and optical sensors. Phytoplankton thin layers were repeatedly detected at plume-edge stations within or immediately above a compressed pycnocline formed by bottom-trapped saline wedges. These layers were 1.2-3.5 m thick and exhibited chlorophyll-a concentrations up to threefold higher than background levels. The assemblage was dominated by chain-forming diatoms, particularly Chaetoceros debilis and C. socialis, whose local abundance maxima coincided with chlorophyll peaks. In contrast, copepods, appendicularians, and other zooplankton were broadly distributed throughout the upper water column and rarely aggregated within the layers. Redundancy analysis indicated that chlorophyll concentration and stratification intensity were primary drivers of community structure across stations. Satellite imagery revealed rapid short-term variability in plume extent, helping explain differences in stratification and thin layer development among sampling days. Our results demonstrate that salt-wedge dynamics at the plume-shelf interface constitute a key physical mechanism governing transient phytoplankton thin layer formation in the nGoM, while zooplankton responses remain weakly coupled at the temporal scales resolved here.

The second field campaign of the NASA EXport Processes in the Ocean from RemoTe Sensing (EXPORTS) program was conducted in the late spring of 2021 within the vicinity of the Porcupine Abyssal Plain (49.0{degrees}N, 16.5{degrees}W) in the North Atlantic Ocean. Observations from EXPORTS support previous characterizations of this system as highly productive and organic matter rich, with the majority of primary production occurring in large cells ([&ge;] 5 {micro}m) such as diatoms that are primarily utilizing nitrate. Rates of total euphotic zone depth-integrated net primary production ranged from 36.4 to 146.6 mmol C m- 2 d-1, with an observational period average f-ratio of 0.74, indicating predominantly new production. Substantial variability in the contribution of small (<5 {micro}m) and large cells occurred over the observation period, coinciding with the end of the annual spring phytoplankton bloom. Physical changes associated with storms appear to have impacted the integrated production rates substantially, enhancing rates by [~]10%. These disturbances altered the balance between contributions of the different phytoplankton size fractions, thus highlighting the important role of mixed layer variability in nutrient entrainment into the upper water column and production dynamics. In diatoms, inputs of silicic acid related to deepening of the mixed layer increased silicic acid uptake rates yet concomitant increases in NPP in large cells was not observed. This campaign serves as the high productivity endmember within the EXPORTS program and as such, elucidates how nutrient concentrations and size class play key roles in both low and high productivity systems, but in differing ways.

Eutrophication in the Baltic Sea has caused an imbalance in the inorganic nitrogen (N) to phosphorus (P) ratio, leaving excess phosphate (PO4) after the phytoplankton spring bloom that terminates after N-depletion. Using monitoring data, we demonstrated that the PO4 concentration has continued to increase in the outermost Gulf of Finland during past decades. We further investigated the fate of such excess PO4 in a two-week mesocosm (1.2 m3) experiment. The starting concentration of PO4 was 0.66 {micro}M, and treatments included a non-treated control (control), nitrate addition (N-add; 3.6 {micro}M), glucose addition (C-add; 25 {micro}M) and combined nitrate and glucose addition (N+C-add). The addition of N both in N-add and N+C-add treatments stimulated nano- and microphytoplankton, while the picophytoplankton abundance increased only after N-depletion. Also, the copepod biomass was positively affected by the N-addition. N2-fixing cyanobacteria were present but in low abundance. Carbon addition did not enhance heterotrophic bacterial uptake of PO4 contrary to our expectations, nor did it affect the phyto- or zooplankton community composition. The PO4 concentration was reduced to [~]0.4 {micro}M in the control and C-add treatments and to 0.16 {micro}M in the two N-amended treatments, with an inorganic N:P uptake ratio of 6.7. These results underscore the role of picophytoplankton in reducing the excess PO4 pool after the spring bloom, a function traditionally ascribed to bloom-forming diazotrophic cyanobacteria in the Baltic Sea.

Prokaryotes represent a major fraction of marine biomass and play a key role in the global carbon cycle. We studied the vertical profiles (from surface down to the bathypelagic realm) of abundance, cytometric signatures, and activity of prokaryotic communities along a productivity gradient in the subtropical and tropical Atlantic to assess whether there is a vertical linkage between surface productivity regimes and deep ocean prokaryotic communities. We found that latitudinal changes in the vertical patterns of cytometric variables were coupled with surface productivity: higher prokaryotic abundances and viabilities, and smaller cell sizes were observed below highly productive surface waters, an effect reaching down to the bathypelagic layer. On the contrary, leucine uptake rates in deep waters showed no clear relationship with surface productivity. Changes in resource and energy allocation to growth vs. maintenance in hostile environments, cell-size-dependent metabolic requirements and variability in leucine to carbon conversion may all be part of the array of factors involved in controlling prokaryotic activity patterns that were measured. Our work adds to the recent findings that highlight the importance of vertical connectivity for prokaryotic communities in the dark ocean.

Marine particle forensics frequently uncover information on composition, age, size, and ecological history. Zooplankton fecal pellets are also studied for process-related data, such as grazing rates and carbon sequestration potential. Here, flow cytometric analyses of fecal pellet contents revealed intact phytoplankton with photophysiological characteristics mirroring those of free-living cells. Mapping the cytometrically derived properties of cells inside fecal pellets onto vertical profiles from free-living cells revealed the potential to estimate depth specific grazing by individual zooplankton. An experiment conducted at sea confirmed that the photophysiological characteristics of free-living phytoplankton from multiple depths, consumed by zooplankton, and excreted within fecal pellets are retained for at least 24 hours after grazing is initiated. These results have implications for high resolution modeling of individual or group specific zooplankton grazing dynamics that are critical for accurately linking zooplankton grazing in the surface ocean with the mesopelagic and deep ocean food webs and carbon export. Scientific Significance StatementFecal pellet forensics have provided significant contributions to the study of zooplankton grazing and the marine carbon cycle. Gaps in knowledge about these processes remain, and continued investigations into fecal pellet contents and their fate are important for assessing connections between the surface and deeper ocean ecosystems. We describe a study conducted in the North Atlantic in the Spring of 2021 using flow cytometry to investigate fecal pellets contents. Observations that intact phytoplankton within the pellets had similar photophysiological properties to the free-living community led to a series of sample collections and experiments which provided a path forward for determining depth specific grazing by zooplankton community members. Phytoplankton survival after passing through zooplankton guts and being packaged into fecal pellets, with their potential for release far below the surface mixed layer, support prior observations of healthy phytoplankton communities at depth and validate this mechanism for the rapid transport of freshly fixed carbon to deep ocean systems. The results should be of interest to plankton ecologists and carbon cycle scientists connecting surface and deep ocean ecosystems as application of this approach at a broader scale will provide opportunities for high resolution modeling of individual and group specific zooplankton behaviors.

Food webs trace the flow of organic matter and energy among producers and consumers; for pelagic marine food webs, network complexity directly influences the amount and form of carbon exported to the deep ocean via the biological pump. Here we present a synoptic view of mixed layer food web dynamics observed during the late summer 2018 EXport Processes in the Ocean from Remote Sensing (EXPORTS) field campaign in the subarctic Northeast Pacific at the long-running time-series site, Ocean Station Papa. Carbon biomass reservoirs of phytoplankton, microzooplankton, and bacterioplankton, were approximately equal while mesozooplankton biomass was 70% lower. Live organisms composed [~]40% of the total particulate organic carbon within the mixed layer: the remainder was attributed to detritus. Rates of carbon transfer among reservoirs indicated production and assimilation rates were well balanced by losses, leaving little organic carbon available for export. The slight positive net community production rate generated organic carbon that was exported from the system in the form of food web byproducts, such as large fecal pellets generated by mesozooplankton. This characteristically regenerative food web had relatively slow turnover times with small-magnitude transfers of carbon relative to standing stocks that occurred amidst a high background concentration of detrital particles and dissolved organic matter. The concurrent estimation of food web components and rates revealed that separated processes dominated the transfer of carbon within the food web compared to those that contributed to export. Plain Language SummaryThe biological carbon pump drives a downward flux of organic matter from the sunlit surface ocean to the vast ocean interior. Ecological interactions in the surface ocean directly affect the amount and type of carbon that is exported to the deep ocean. In this study, we present a synthesis of the late summer mixed layer food web in the Northeast Pacific that was extensively characterized during the 2018 EXport Processes in the Ocean from Remote Sensing (EXPORTS) field campaign. We found the majority of carbon was recycled within the mixed layer by microbes through multiple transfers between producers and consumers. Larger organisms, mesozooplankton and salps, only consumed a small amount of carbon but through the formation of sinking fecal pellets were the main mechanism of transporting carbon out of the system. The study highlights the need to concurrently study microbial and large organism dynamics to develop a predictive understanding of the fate of organic carbon in the oceans. Key PointsO_LIThe microbial loop dominated carbon flow in the late summer mixed layer food web of the North Pacific, most net production was respired leaving little carbon available for export. C_LIO_LIActive production and consumption of organic carbon occurred amid a high background of detrital particulate organic carbon (58% of total) with slow turnover time, 66 d. C_LIO_LIMesozooplankton which had relatively minor carbon consumption rates created the majority of export production due to efficient repackaging of consumed material. C_LI

Gravitational particle sinking is the main mechanism for carbon export in the biological carbon pump. However, the export dynamics of the particle-associated protist community are not fully understood. We used 18S rRNA gene metabarcoding to characterise the exported protist community within sinking particles and bathypelagic surficial sediments in oligotrophic subtropical and high-nutrient, low-chlorophyll subantarctic waters. Sinking particles were collected with formalin-fixed and preservative-free particle interceptor traps (fixed and live traps, respectively) to identify the community involved in particle export (fixed) and protist loss from remineralisation (live). We paired this with community analysis of the upper and lower water column (mixed layer and below mixed layer to mesopelagic, respectively) to compare the relative sources of exported protists. Amplicon sequences variants (ASVs) from upper water column samples accounted for 2 to 4-fold higher proportion of reads and ASV rich-ness compared to lower water column samples in fixed trap and sediment samples, suggesting low influence of the suspended protist community from the lower water column on export. We further traced the export patterns of upper water column protist taxa by analysing the change in taxa relative abundance across the mixed layer to mesopelagic depths. Export patterns differed between taxa, which is similarly suggested by taxa-specific loss of ASV richness between fixed and live traps, but remained the same across biogeochemically-contrasting water masses. This could imply that the drivers for protist loss during export are related to characteristics consistent across environmental conditions, such as specific microbial interactions or inherent cell properties.

Diatom blooms dominate nutrient-rich ecosystems. Less is known about the ecology and bloom dynamics of diatom populations in oligotrophic ecosystems. Here, we investigated seasonal succession of planktonic diatoms in the Gulf of Aqaba (GoA) at the northern Red Sea. The GoA is a subtropical ecosystem alternating between stratified, oligotrophic profiles during summer, and deeply mixed, mesotrophic during winter. Diatom density and diversity were lower during the stratified season, dominated by pennate species, and increased at mid-winter as nitrate exceeded [~]0.5 {micro}mol L-1. Diatom density lagged after total phytoplankton and entailed a transition to centric-diatom dominance, suggesting both higher nutrient requirements for diatom growth and ecophysiological differences between morphotypes. Ephemeral blooms were detected at the mixing-to-stratification transition. Under milder conditions, mixing was shallow and diatoms reached [~]98 individuals. mL-1. Small-centric Thalassiosiraceae and several pennates dominated. However, during the following colder year, mixing depth reached [~]700 m. Consequently, nutrient concentrations were higher and diatoms reached [~]390 individuals. mL-1. This enabled emergence of chain-forming species (namely Chaetoceros and Leptocylindrus) along small-centric and pennates, and high spore abundance was detected. Restratification led to rapid bloom decline. These results illustrate diatom community succession and bloom development as a function of nutrient availability in subtropical ecosystems.

Microeukaryotes are important components of sinking particles contributing to carbon export from the surface to deep oceans. The knowledge of the sinking microeukaryotic communities and their dynamics is limited. We applied 18S rDNA metabarcoding method to investigate the microeukaryotic communities in sinking and suspended particles distinguished by marine snow catchers (MSC) during spring in the Oyashio region. Sinking particles displayed distinct communities and lower diversity than did suspended particles. The community compositions of the sinking particles varied with depth, suggesting that microeukaryotes were selectively removed through disaggregation or decomposition during settling. Prymnesiophyceae and diatoms were effectively removed, as indicated by their decreased abundance in the sinking particles at increasing depths. Conversely, phototrophic dinoflagellates maintained a higher abundance in the sinking particles across depths, indicating resistance to disaggregation and decomposition. Heterotrophic dinoflagellates and Spirotrichea were enriched in sinking particles and marine stramenopiles (MAST) groups were enriched in suspended particles. Sinking heterotrophic protist communities in the deep layers were similar to those in the surface layers, whereas they differed from the suspended ones in the same layer. Therefore, heterotrophic protists in surface layers were transported to deeper layers. Overall, our results demonstrate the functional differences among microeukaryotes in the biological carbon pump.

AbstractPhytoplankton community composition in estuaries is tightly linked to freshwater input. While regular freshwater input typically delivers nutrients, fueling phytoplankton growth, the impact of extreme events is less certain. Several observational studies have documented increases in phytoplankton biomass following large precipitation events but cannot to adequately identify the mechanism driving this increase. This paper advances two hypotheses about what drives phytoplankton biomass change following extreme precipitation events. First, the Resident Response Hypothesis (RRH), which suggests local estuarine phytoplankton grow in response to favorable conditions. Alternatively, the Production Introduction Hypothesis (PIH) indicates that large rainfall events introduce new phytoplankton taxa to estuaries during run-off events associated with large storms. These hypotheses were tested at North Inlet Estuary (South Carolina, USA) through a novel experimental design, which utilized multiple treatments to mimic the distinct impacts of large rainfall events on natural phytoplankton communities. Experimental samples were analyzed using photopigments and flow-through imaging microscopy. This allowed a wide assessment of phytoplankton community change both through measuring chlorophyll-a and biomass concentration. Ultimately, there was a strong increase in phytoplankton growth in the storm treatments, primarily identified by diatom pigment increases in response to run-off delivered nutrients, providing support for the RRH. However, biomass concentration analysis of select diatom taxa revealed the introduction of new diatom taxa in run-off water communities. This supports the PIH, suggesting that initial growth from nutrient additions may be due to small cell growth as well as introduction of new taxa.

Eastern Boundary Systems support major fisheries whose early life stages depend on upwelling production. Upwelling can be highly variable at the regional scale, with substantial repercussions for new productivity and microbial loop activity. A holistic assessment of plankton community structure is challenging due to the range in body forms and sizes of the taxa. Thus, studies that integrate the classic trophic web based on new production with the microbial loop are rare. Underwater imaging can overcome this limitation, and together with machine learning, enables fine resolution studies spanning large spatial scales. We used the In-situ Ichthyoplankton Imaging System (ISIIS) to investigate the drivers of plankton community structure in the northern California Current, sampled along the Newport Hydrographic (NH) and Trinidad Head (TR) lines, in OR and CA, respectively. The non-invasive imaging of particles and plankton (250m -15cm) over 1644km (30 transects) in the winters and summers of 2018 and 2019 yielded 1.194 billion classified plankton images. The imaged plankton community ranged from protists, crustaceans, and gelatinous taxa to larval fishes. To assess community structure, >2000 single-taxon distribution profiles were analyzed using high resolution spatial correlations. Co-occurrences on the NH line were consistently significantly higher off-shelf while those at TR tended to be highest on-shelf. Taxa co-occurrences at TR increased significantly with upwelling strength and in 2019 TR summer co-occurrences were similar to those on the NH line. Random Forests models identified the concentrations of microbial loop taxa such as protists, Oithona copepods, and appendicularians as important drivers of co-occurrences at NH line, while at TR, cumulative upwelling and chlorophyll a were of the highest importance. Our results indicate that the microbial loop is actively driving plankton community structure in intermittent upwelling systems such as the NH line and may induce temporal stability. Where upwelling is more continuous such as at TR, primary production may dominate patterns of community structure, obscuring the underlying role of the microbial loop. Future changes in upwelling strength are likely to disproportionately affect plankton community structure in continuous upwelling regions, while high microbial loop activity enhances community structure resilience.

AO_SCPLOWBSTRACTC_SCPLOWIn the Mediterranean Sea, the demosponge Chondrilla nucula can occur in close association with the native seagrass Posidonia oceanica. C. nucula harbors a diverse and abundant microbial community, including potential nitrifiers. Thus, the sponge may contribute to the nitrogen (N) demand of the seagrass holobiont. In this study, we investigated potential nitrification rates (PNR) and inorganic N fluxes within this association at a site where C. nucula covered 18 {+/-} 3 % of the seagrass meadow area, during plant growth (spring) and senescence (autumn). Using incubation experiments with 15N-labeled ammonium, we measured PNR and inorganic N of the seagrass-sponge association, and of sponge and seagrass independently, under light and dark conditions. We supplemented these experiments with 16s rRNA gene amplicon sequencing to characterize the microbial community of the sponge. PNR was exclusively measured when the sponge was present (alone or in association with the seagrass). PNR was highest in the dark and when C. nucula was associated with the seagrass, ranging from 21 {+/-} 7 to 267 {+/-} 33 nmol N g DW-1 h-1 in spring and autumn, respectively. Sponge-mediated PNR can support 8% of the N demand of the P. oceanica holobiont during growth and 47 % during senescence. We identified key nitrifying bacterial and archaeal groups as members of the sponges microbial community. While C. nucula released inorganic N, potentially sustaining the seagrass, it benefitted from dissolved organic carbon released by P. oceanica. These results suggest that the interaction between C. nucula and P. oceanica is mutually beneficial, ultimately supporting and stabilizing the seagrass ecosystem.

Microbial degradation of suspended and sinking organic carbon regulates long-term oceanic carbon storage by controlling the efficiency of the biological pump. Yet microbial controls on carbon export and remineralization remain poorly constrained, limiting predictions of how ocean carbon cycling will respond to climate change. Here, we combined in situ sampling with ship-based incubations to quantify prokaryote-driven removal rates of suspended and sinking total organic carbon (TOC). Samples were collected below the mixed layer during three stages of a spring Phaeocystis pouchetii bloom in the Labrador Sea. Phaeocystis blooms can dominate regional phytoplankton biomass and are expected to increase under future climate. Removal rates were used as a proxy for carbon lability and combined with 16S rRNA metabarcoding and carbon composition analyses to link microbial community structure with substrate characteristics. Removal rates of sinking particles (0.02-0.06 d-1) were an order of magnitude higher than those of suspended TOC (0.002 d-1) during bloom-decline and non-bloom. In contrast, during late-bloom, suspended carbon exhibited rates of 0.01 d-1, comparable to sinking particles, and was enriched in exopolymer-rich colonies. Prokaryotic community composition varied primarily among bloom stages rather than carbon fractions, indicating that bloom stage-- and thus particle origin and composition--was the dominant control on bacterial degradation and assembly. Bacterial diversity peaked where carbon was refractory and originated from mixed phytoplankton. Together, these results demonstrate that suspended Phaeocystis-derived carbon can be rapidly remineralized when blooms produce exopolymer-rich colonies and highlight bloom stage as key regulator of microbial carbon processing and biological pump efficiency.