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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 Southern Ocean contributes substantially to the global biological carbon pump (BCP). Salps in the Southern Ocean, in particular Salpa thompsoni , are important grazers that produce large, fast-sinking fecal pellets. Here, we quantify the salp bloom impacts on microbial dynamics and the BCP, by contrasting locations differing in salp bloom presence/absence. Salp blooms coincide with phytoplankton dominated by diatoms or prymnesiophytes, depending on water mass characteristics. Their grazing is comparable to microzooplankton during their early bloom, resulting in a decrease of ~1/3 of primary production, and negative phytoplankton rates of change are associated with all salp locations. Particle export in salp waters is always higher, ranging 2- to 8- fold (average 5-fold), compared to non-salp locations, exporting up to 46% of primary production out of the euphotic zone. BCP efficiency increases from 5 to 28% in salp areas, which is among the highest recorded in the global ocean. 

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.

 

A flow cytometry method for enumerating marine heterotrophic bacteria and phytoplankton in a living or preserved sample using a low power solid state near‐ultraviolet laser is described. The method uses Hoechst 34580 to stain DNA in microbial cells in seawater samples. This stain is optimally excited at 375 nm unlike the similar Hoechst 33342, which requires ~ 350 nm excitation only available on more expensive lasers. Phytoplankton abundances from the Hoechst 34580 method are comparable to those of unstained samples and when analyzed by the Hoechst 33342 staining method. With this new method, nonpigmented marine bacteria and phytoplankton abundances are obtained simultaneously in a single sample as the Hoechst emission wavelength (~ 450 nm) is well separated from the emission wavelengths of chlorophyll and phycoerythrin fluorescence. Bacteria abundances are similar between this new method and those obtained with established Hoechst 33342 and SybrGreen I methods. Precision estimates (coefficient of variation) on populations with abundances near ~ 10⁵cells mL⁻¹are 1–3%, increasing to 3–9% at lower cell concentrations of 10³cells mL⁻¹. The Hoechst 34580 method is simple, requiring no heating or pretreatment with RNAse, can be used on unpreserved and formaldehyde‐preserved cells, and is amenable to at‐sea use with portable, compact, low power‐requiring flow cytometers.

 

Western Atlantic bluefin tuna (ABT) undertake long-distance migrations from rich feeding grounds in the North Atlantic to spawn in oligotrophic waters of the Gulf of Mexico (GoM). Stock recruitment is strongly affected by interannual variability in the physical features associated with ABT larvae, but the nutrient sources and food-web structure of preferred habitat, the edges of anticyclonic loop eddies, are unknown. Here, we describe the goals, physical context, design and major findings of an end-to-end process study conducted during peak ABT spawning in May 2017 and 2018. Mesoscale features in the oceanic GoM were surveyed for larvae, and five multi-day Lagrangian experiments measured hydrography and nutrients; plankton biomass and composition from bacteria to zooplankton and fish larvae; phytoplankton nutrient uptake, productivity and taxon-specific growth rates; micro- and mesozooplankton grazing; particle export; and ABT larval feeding and growth rates. We provide a general introduction to the BLOOFINZ-GoM project (Bluefin tuna Larvae in Oligotrophic Ocean Foodwebs, Investigation of Nitrogen to Zooplankton) and highlight the finding, based on backtracking of experimental waters to their positions weeks earlier, that lateral transport from the continental slope region may be more of a key determinant of available habitat utilized by larvae than eddy edges per se.

 

Abstract The availability of nitrogen (N) in ocean surface waters affects rates of photosynthesis and marine ecosystem structure. In spite of low dissolved inorganic N concentrations, export production in oligotrophic waters is comparable to more nutrient replete regions. Prior observations raise the possibility that di-nitrogen (N2) fixation supplies a significant fraction of N supporting export production in the Gulf of Mexico. In this study, geochemical tools were used to quantify the relative and absolute importance of both subsurface nitrate and N2 fixation as sources of new N fueling export production in the oligotrophic Gulf of Mexico in May 2017 and May 2018. Comparing the isotopic composition (“δ15N”) of nitrate with the δ15N of sinking particulate N collected during five sediment trap deployments each lasting two to four days indicates that N2 fixation is typically not detected and that the majority (≥80%) of export production is supported by subsurface nitrate. Moreover, no gradients in upper ocean dissolved organic N and suspended particulate N concentration and/or δ15N were found that would indicate significant N2 fixation fluxes accumulated in these pools, consistent with low Trichodesmium spp. abundance. Finally, comparing the δ15N of sinking particulate N captured within vs. below the euphotic zone indicates that during late spring regenerated N is low in δ15N compared to sinking N. 

Abstract During two cruises in the oligotrophic oceanic Gulf of Mexico, we deployed sediment traps at three depths: center of the euphotic zone (EZ) (60 m), base of the EZ (117–151 m), and in the twilight zone (231 m). Organic carbon export declined with depth from 6.4 to 4.6 to 2.4 mmol C m−2 d−1, suggesting that net particle production was concentrated in the upper EZ. Net primary production varied from 24 to 29 mmol C m−2 d−1, slightly more than half in the upper EZ. Export ratios varied from 11 to 25%. Trap measurements of chlorophyll and phaeopigments allowed us to quantify fluxes of fresh phytoplankton and herbivorous fecal pellets, respectively, which were both minor contributors to total flux, although their contributions varied with depth. Phytoplankton flux was more important from the upper to lower EZ; fecal pellets were more important at the EZ base and below. C:N elemental ratios and 13C and 15N isotope analyses indicated particle transformations within and beneath the EZ. 234Th-238U disequilibrium measurements varied, likely reflecting the mixing of water from multiple regions over the ~month-long time-scale of 234Th. Our results highlight the complexity of the biological carbon pump in oligotrophic regions.