Mesozooplankton biomass, grazing and trophic structure in the bluefin tuna spawning area of the oceanic Gulf of Mexico

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.

Lateral advection supports nitrogen export in the oligotrophic open-ocean Gulf of Mexico

“…These lateral fluxes likely have implications for the entire GoM food web. Organismal carbon budgets suggest that carnivorous metazooplankton in the GoM may rely on subsidies of prey advected from the coast/shelf break region 37 . An individual-based model developed for Atlantic bluefin tuna larvae suggests that shelf break regions with strong offshore flow may be particularly important spawning locations that allow first-feeding larvae to find sufficient prey while transporting older larvae to low-predator regions 38 .…”

Surface chlorophyll concentration as a mesoplankton biomass assessment tool in the Southern Ocean region

“…In fact, retrieved trophic structure of plankton communities depends on size fractions we sample. For example, within the same community, carnivores may dominate the biomass of larger (>1‐mm) zooplankton (60%–78%) and represent only 13% of smaller zooplankton (Landry & Swalethorp, 2021). Larger fractions such as krill and salps may distort proportions here, especially in the coastal areas (e.g., Atkinson et al., 2009; Voronina, 1998).…”

“…In fact, retrieved trophic structure of plankton communities depends on size fractions we sample. For example, within the same community, carnivores may dominate the biomass of larger (>1-mm) zooplankton (60%-78%) and represent only 13% of smaller zooplankton (Landry & Swalethorp, 2021).…”

Quantifying biological carbon pump pathways with a data-constrained mechanistic model ensemble approach

“…We assimilated a broad suite of standing stock and rate measurements across multiple trophic levels that included: 466 measurements of NO 3 - concentration and 423 measurements of NH 4 + concentration (Knapp et al 2021); 422 measurements each of silicic acid and 84 measurements of biogenic silica (Krause et al 2015; Krause et al 2016); 455 chlorophyll a measurements (Goericke 2011); 193 measurements of small phytoplankton biomass by a combination of epifluorescence microscopy and flow cytometry (Selph et al 2021; Taylor et al 2012); 193 measurements of diatom biomass by epifluorescence microscopy (Taylor et al 2012; Taylor et al 2016); 193 measurements of protistan zooplankton biomass by epifluorescence microscopy and/or light microscopy of Lugol’s stained samples (Freibott et al 2016); 44 measurements each of vertically-integrated <1- and >1-mm epipelagic-resident mesozooplankton biomass; 43 measurements each of vertically-integrated <1- and >1-mm diel-vertically-migrating mesozooplankton biomass; 413 measurements of particulate organic nitrogen and 28 measurements of dissolved organic nitrogen (Stephens et al 2018); 342 measurements of net primary productivity by either H 13 CO 3 - or H 14 CO 3 - uptake methods (Morrow et al 2018; Yingling et al 2021); 149 measurements of nitrate uptake by incorporation of 15 NO 3 - (Kranz et al 2020; Stukel et al 2016); 50 measurements of silicic acid uptake by incorporation of 32 Si (Krause et al 2015); 248 measurements each of whole phytoplankton community growth rates and whole phytoplankton community mortality rates due to protistan grazing determined by chlorophyll analyses of microzooplankton dilution experiments (Landry et al 2009; Landry et al 2021); 53 measurements each of small phytoplankton growth rates and small phytoplankton mortality rates due to protistan grazing determined by high-pressure liquid chromatography pigment analyses of microzooplankton dilution experiments combined with flow cytometry and epifluorescence microscopy (Landry et al 2016b; Landry et al 2021); 53 measurements each of diatom growth rates and diatom mortality rates due to protistan grazing determined by high-pressure liquid chromatography pigment analyses of microzooplankton dilution experiments combined with flow cytometry and epifluorescence microscopy (Landry et al 2016b; Landry et al 2021); 41 measurements each of vertically-integrated <1-mm and >1-mm nighttime mesozooplankton grazing rates by the gut pigment method (Décima et al 2016; Landry and Swalethorp 2021); 41 measurements each of vertically-integrated <1-mm and >1-mm daytime mesozooplankton grazing rates by the gut pigment method (Décima et al 2016; Landry and Swalethorp 2021); 37 measurements of sinking nitrogen using sediment traps (Stukel et al 2019a; Stukel et al 2021); 19 measurements of sinking biogenic silica using sediment traps (Krause et al 2016; Stukel et al 2019a); and 475 measurements of photosynthetically-active radiation. Each of the above measurements was typically the mean of measurements taken at a specific depth (or vertically-integrated) on multiple days of the Lagrangian experiment, thus allowing us to also quantify uncertainties for all measurement types.…”