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1. Bluefin Larvae in Oligotrophic Ocean Foodwebs, investigations of nutrients to zooplankton: overview of the BLOOFINZ-Gulf of Mexico program

   https://doi.org/10.1093/plankt/fbac038  

   Gerard, Trika ; Lamkin, John T. ; Kelly, Thomas B. ; Knapp, Angela N. ; Laiz-Carrión, RaÚl ; Malca, Estrella ; Selph, Karen E. ; Shiroza, Akihiro ; Shropshire, Taylor A. ; Stukel, Michael R. ; et al ( July 2022 , Journal of Plankton Research)

   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.

    

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2. Microbial food web dynamics in the oceanic Gulf of Mexico

   https://doi.org/10.1093/plankt/fbab021  

   Landry, Michael R ; Selph, Karen E ; Stukel, Michael R ; Swalethorp, Rasmus ; Kelly, Thomas B ; Beatty, Jennifer L ; Quackenbush, Cameron R ( April 2021 , Journal of Plankton Research)

   null (Ed.)

   Abstract Phytoplankton growth and microzooplankton grazing rates were measured in repeated profiles of dilution experiments incubated in situ on a drift array in order to assess microbial production and food web characteristics in the oligotrophic bluefin tuna spawning habitat of the Gulf of Mexico (May peak spawning seasons, 2017–2018). Grazing often exceeded growth with the processes more balanced overall in the surface mixed layer, but biomass accumulated in the mid-euphotic zone. Community production estimates (260–500 mg C m−2 day−1) were low compared to similar open-ocean studies in the Pacific Ocean. Prochlorococcus was a consistent major contributor (113–204 mg C m−2 day−1) to productivity, while diatoms and dinoflagellates (2–10 and 4–13 mg C m−2 day−1, respectively) were consistently low. Prymnesiophytes, the most dynamic component (34–134 mg C m−2 day−1), co-dominated in 2017 experiments. Unexpected imbalances in grazing relative to production were observed for all picoplankton populations (Prochlorococcus, Synechococcus and heterotrophic bacteria), suggesting a trophic cascade in the absence of mesozooplankton predation on large microzooplankton. Study sites with abundant larval tuna had the shallowest deep chlorophyll maxima and significant net positive phytoplankton growth below the mixed layer. 

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3. Mesozooplankton biomass and temperature-enhanced grazing along a 110°E transect in the eastern Indian Ocean

   https://doi.org/10.3354/meps13444  

   Landry, MR ; Hood, RR ; Davies, CH ( September 2020 , Marine Ecology Progress Series)

   null (Ed.)

   Low-latitude waters of the Indian Ocean are warming faster than other major oceans. Most models predict a zooplankton decline due to lower productivity, enhanced metabolism and phytoplankton size shifts that reduce trophic transfer efficiency. In May-June 2019, we investigated mesozooplankton biomass and grazing along the historic 110°E transect line from the International Indian Ocean Expedition (IIOE) of the 1960s. Twenty sampling stations from 39.5 to 11.5°S spanned latitudinal variability from temperate to tropical waters and a pronounced 14°C gradient in mean euphotic zone temperature. Although mesozooplankton size structure was similar along the transect, with smaller (<2 mm) size classes dominant, total biomass increased 3-fold (400 to 1500 mg dry weight m -2 ) from high to low latitude. More dramatically, gut-fluorescence estimates of grazing (total ingestion or % euphotic zone chl a consumed d -1 ) were 14- and 20-fold higher, respectively, in the low-latitude warmer waters. Biomass-normalized grazing rates varied more than 6-fold over the transect, showing a strong temperature relationship (r 2 = 0.85) that exceeded the temperature effects on gut turnover and metabolic rates. Herbivory contributed more to satisfying zooplankton energetic requirements in low-chl a tropical waters than chl a -rich waters at higher latitude. Our unexpected results are inconsistent with trophic amplification of warming effects on phytoplankton to zooplankton, but might be explained by enhanced coupling efficiency via mixotrophy. Additional implications for selective herbivory and top-down grazing control underscore the need for rigorous field studies to understand relationships and validate assumptions about climate change effects on the food webs of tropical oceans. 

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4. Food-web fluxes support high rates of mesozooplankton respiration and production in the equatorial Pacific

   https://doi.org/10.3354/meps13479  

   Landry, MR ; Stukel, MR ; Décima, M ( October 2020 , Marine Ecology Progress Series)

   null (Ed.)

   We investigated how the network of food-web flows in open-ocean systems might support high rates of mesozooplankton respiration and production by comparing predicted rates from empirical relationships to independently determined solutions from an inverse model based on tightly constrained field-measured rates for the equatorial Pacific. Model results were consistent with estimates of gross:net primary production (GPP:NPP), bacterial production:NPP, sinking particulate export, and total export for the equatorial Pacific, as well as general literature values for growth efficiencies of bacteria, protozooplankton, and metazooplankton. Mean rate estimates from the model compared favorably with the respiration predictions from Ikeda (1985; Mar Biol 85:1-11 ) (146 vs. 144 mg C m -2 d -1 , respectively) and with production estimates from the growth rate equation of Hirst & Sheader (1997; Mar Ecol Prog Ser 154:155-165 ) (153 vs. 144 mg C m -2 d -1 ). Metazooplankton nutritional requirements are met with a mixed diet of protozooplankton (39%), phytoplankton (36%), detritus (15%), and carnivory (10%). Within the food-web network, NPP of 896 mg C m -2 d -1 supports a total heterotrophic carbon demand from bacteria, protozoa, and metazooplankton that is 2.5 times higher. Scaling our results to primary production and zooplankton biomass at Stn ALOHA suggests that zooplankton nutritional requirements for high growth might similarly be met in oligotrophic subtropical waters through a less efficient trophic structure. Metazooplankton production available to higher-level consumers is a significant contributor to the total export needed for an overall biogeochemical balance of the region and to export requirements to meet carbon demand in the mesopelagic depth range. 

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5. Nutrients associated with terrestrial dissolved organic matter drive changes in zooplankton:phytoplankton biomass ratios in an alpine lake

   https://doi.org/10.1111/fwb.12847  

   Kissman, Carrie E. H. ; Williamson, Craig E. ; Rose, Kevin C. ; Saros, Jasmine E. ( November 2016 , Freshwater Biology)

   Dissolved organic matter (DOM) is increasing in many lakes due to climate change and other environmental forcing. A 21‐day microcosm experiment that manipulated terrestrialDOMwas used to determine the effect ofDOMon zooplankton:phytoplankton biomass ratios (z:p). We predicted that ifDOMadditions increase the amount of fixed carbon available for higher trophic levels through stimulation of the microbial loop and hence zooplankton, the z:p will increase. However, ifDOMadditions increase other nutrients besides fixed carbon, we predict stable or decreasing z:p due to nutrient stimulation of phytoplankton that subsequently enhances zooplankton.

   The effects of experimental additions of terrestrially derivedDOMon zooplankton, phytoplankton, z:p and zooplankton net grazing were assessed in microcosms (sealed bags) incubated in the epilimnion (shallow; 1.5 m) and hypolimnion (deep; 8.0 m) strata of an alpine lake.

   DOMaddition treatments (DOM+) had a 6.0‐ to 7.5‐fold increase in phytoplankton biomass relative to controls, but only a 1.3‐ to 1.5‐fold increase in zooplankton biomass, on day 21 of the experiment. The z:p was, thus, lower in theDOM+ treatments (ratios: 2.3 deep and 4.4 shallow) than in the control treatments (ratios: 13.4 deep and 17.5 shallow), providing evidence thatDOMadditions provide nutrient subsidies besides fixed carbon that stimulate phytoplankton biomass accumulation.

   The increase in zooplankton biomass during the experiment was similar in magnitude to the total amount of dissolved organic carbon (DOC) in theDOMadded in the sealed bags at the beginning of the experiment, which suggests zooplankton biomass stimulation due to increased phytoplankton biomass, and not fromDOMthrough the microbial loop, which would have greater trophic transfer losses. The consumer net grazing effect in theDOM+ treatments was reduced by 2.8‐fold in the shallow stratum and by 2.9‐fold in the deep stratum relative to the control treatments, indicating that zooplankton were unable to exert strong top–down control on the primary producers.

   The role of nutrients needs to be considered when examining the response of pelagic ecosystems to inputs of terrestrialDOM, especially in lakes with lowerDOCconcentrations.

    

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