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Picophytoplankton populations [Prochlorococcus,Synechococcus(SYN), and picoeukaryotes] are dominant primary producers in the open ocean and projected to become more important with climate change. Their fates can vary, however, with microbial food web complexities. In the California Current Ecosystem, picophytoplankton biomass and abundance peak in waters of intermediate productivity and decrease at higher production. Using experimental data from eight cruises crossing the pronounced CCE trophic gradient, we tested the hypothesis that these declines are driven by intensified grazing on heterotrophic bacteria (HBAC) passed to similarly sized picophytoplankton via shared predators. Results confirm previously observed distributions as well as significant increases in bacterial abundance, cell growth, and grazing mortality with primary production. Mortalities of picophytoplankton, however, diverge from the bacterial mortality trend such that relative grazing rates on SYN compared to HBAC decline by 12-fold between low and high productivity waters. The large shifts in mortality rate ratios for coexisting populations are not explained by size variability but rather suggest high selectivity of grazer assemblages or tightly coupled tradeoffs in microbial growth advantages and grazing vulnerabilities. These findings challenge the long-held view that protistan grazing mainly determines overall biomass of microbial communities while viruses uniquely regulate diversity by “killing the winners”.

 

Abstract. The ability to constrain the mechanisms that transport organiccarbon into the deep ocean is complicated by the multiple physical,chemical, and ecological processes that intersect to create, transform, andtransport particles in the ocean. In this paper we develop andparameterize a data-assimilative model of the multiple pathways of thebiological carbon pump (NEMUROBCP). The mechanistic model is designedto represent sinking particle flux, active transport by vertically migratingzooplankton, and passive transport by subduction and vertical mixing, whilealso explicitly representing multiple biological and chemical propertiesmeasured directly in the field (including nutrients, phytoplankton andzooplankton taxa, carbon dioxide and oxygen, nitrogen isotopes, and234Thorium). Using 30 different data types (including standing stockand rate measurements related to nutrients, phytoplankton, zooplankton, andnon-living organic matter) from Lagrangian experiments conducted on 11cruises from four ocean regions, we conduct an objective statisticalparameterization of the model and generate 1 million different potentialparameter sets that are used for ensemble model simulations. The modelsimulates in situ parameters that were assimilated (net primary productionand gravitational particle flux) and parameters that were withheld(234Thorium and nitrogen isotopes) with reasonable accuracy. Modelresults show that gravitational flux of sinking particles and verticalmixing of organic matter from the euphotic zone are more importantbiological pump pathways than active transport by vertically migratingzooplankton. However, these processes are regionally variable, with sinkingparticles most important in oligotrophic areas of the Gulf of Mexico andCalifornia Current, sinking particles and vertical mixing roughly equivalentin productive coastal upwelling regions and the subtropical front in theSouthern Ocean, and active transport an important contributor in the easterntropical Pacific. We further find that mortality at depth is an importantcomponent of active transport when mesozooplankton biomass is high, but itis negligible in regions with low mesozooplankton biomass. Our results alsohighlight the high degree of uncertainty, particularly amongstmesozooplankton functional groups, that is derived from uncertainty in modelparameters. Indeed, variability in BCP pathways between simulations for aspecific location using different parameter sets (all with approximatelyequal misfit relative to observations) is comparable to variability in BCPpathways between regions. We discuss the implications of these results forother data-assimilation approaches and for studies that rely on non-ensemblemodel outputs. 

The uptake of³H‐labeled leucine into proteins, a widely used method for estimating bacterial carbon production (BCP), is suggested to underestimate or overestimate bacterial growth in the open ocean by a factor of 40 uncertainty. Meanwhile, an alternative BCP approach, by the dilution method, has untested concerns about potential overestimation of bacterial growth from dissolved substrates released by filtration. We compared BCP_Diland BCP_Leuestimates from three cruises across a broad trophic gradient, from offshore oligotrophy to coastal upwelling, in the California Current Ecosystem. Our initial analyses based on midday microscopical estimates of bacterial size and a priori assumptions of conversions relationships revealed a mean two‐fold difference in BCP estimates (BCP_Dilhigher), but no systematic bias between low and high productivity stations. BCP_Diland BCP_Leuboth demonstrated strong relationships with bacteria cell abundance. Reanalysis of results, involving a different cell carbon‐biovolume relationship and informed by forward angle light scatter from flow cytometry as a relative cell size index, demonstrated that BCP_Diland BCP_Leuare fully compatible, with a 1 : 1 fit for bacteria of 5 fg C cell⁻¹. Based on these results and considering different strengths of the methods, the combined use of³H‐labeled leucine and dilution techniques provide strong mutually supportive constraints on bacterial biomass and production.

 

Abstract Larval abundances of Atlantic bluefin tuna (ABT) in the Gulf of Mexico are currently utilized to inform future recruitment by providing a proxy for the spawning potential of western ABT stock. Inclusion of interannual variations in larval growth is a key advance needed to translate larval abundance to recruitment success. However, little is known about the drivers of growth variations during the first weeks of life. We sampled patches of western ABT larvae in 3–4 day Lagrangian experiments in May 2017 and 2018, and assessed age and growth rates from sagittal otoliths relative to size categories of zooplankton biomass and larval feeding behaviors from stomach contents. Growth rates were similar, on average, between patches (0.37 versus 0.39 mm d−1) but differed significantly through ontogeny and were correlated with a food limitation index, highlighting the importance of prey availability. Otolith increment widths were larger for postflexion stages in 2018, coincident with high feeding on preferred prey (mainly cladocerans) and presumably higher biomass of more favorable prey type. Faster growth reflected in the otolith microstructures may improve survival during the highly vulnerable larval stages of ABT, with direct implications for recruitment processes. 

Abstract We investigated size-fractioned biomass, isotopes and grazing of mesozooplankton communities in the larval habitat of Atlantic bluefin tuna (ABT) in the oceanic Gulf of Mexico (GoM) during the peak spawning month of May. Euphotic-zone biomass ranged from 101 to 513 mg C m−2 during the day and 216 to 798 mg C m−2 at night. Grazing varied from 0.1 to 1.0 mg Chla m−2 d−1, averaging 1–3% of phytoplankton Chla consumed d−1. Carnivorous taxa dominated the biomass of > 1-mm zooplankton (78% day; 60% night), while only 13% of smaller zooplankton were carnivores. δ15N enrichment between small and large sizes indicates a 0.5–0.6 trophic-step difference. Although characteristics of GoM zooplankton are generally similar to those of remote oligotrophic subtropical regions, zooplankton stocks in the ABT larval habitat are disproportionately high relative to primary production, compared with HOT and BATS averages. Growth-grazing balances for phytoplankton were resolved with a statistically insignificant residual, and trophic fluxes from local productivity were sufficient to satisfy C demand of suspension feeding mesozooplankton. While carnivore C demand was met by local processes in the central GoM, experiments closer to the coastal margin suggest the need for a lateral subsidy of zooplankton biomass to the oceanic region.