Diel, seasonal, and interannual patterns in mesozooplankton abundance in the Sargasso Sea

Abstract

Temporal changes in mesozooplankton abundance affect planktonic food web interactions and biogeochemistry. We enumerated mesozooplankton from monthly day and night tows in the epipelagic zone at the Bermuda Atlantic Time-series Study (BATS) site in the Sargasso Sea (1999–2010). Abundances of each taxon were determined using a ZooScan imaging system and microscopy. Generalized linear models were used to determine environmental parameters that best explained abundance patterns. Taxa with pronounced diel vertical migration included euphausiids, amphipods, Limacina spp. pteropods, and other shelled pteropods. Taxa with a pronounced spring abundance peak included euphausiids, appendicularians, and Limacina spp., while harpacticoid copepods peaked in late summer, and calanoid copepods in late winter/early spring and summer. Many taxa increased in 2003, coincident with a diatom bloom and the largest primary production peak in the time series. Long-term, increasing trends occurred in calanoid and oncaeid copepods, and ostracods, with barnacle nauplii significantly increasing. Sub-decadal-scale climate oscillations and long-term warming may be driving decreases in shelled pteropods and appendicularians. Chaetognath abundance increased in response to increased density of a major prey taxon, calanoid copepods. Calanoid copepods and ostracods increased with increasing water column stratification index and the Atlantic Multidecadal Oscillation index, indicating warmer sea surface temperatures favour these taxa.

Metabarcoding Analyses and Seasonality of the Zooplankton Community at BATS

“…previous works have described certain (or all) components of the community in detail (Deevey, 1971;Brooks, 1971, 1977;Stone and Steinberg, 2014) or organized by main taxonomic groups (Ivory et al, 2018), none of the previous described any shifts in the seasonal patterns at community level. These seasonal regimes were characterized by changes in the relative abundances of both key and secondary players of the community, with only a smaller influence based on the fluctuations of the dominant players of the community.…”

“…For direct comparison with image analyses existing for BATS, clustering and nMDS analyses were carried out grouping the OTUs by main taxa groups to levels commonly used in automated image scanning in systems such as ZooSCAN/Ecotaxa (Gorsky et al, 2010;Picheral et al, 2017), and following the classification used from images carried out from BATS data by Ivory et al (2018).…”

“…Grouping the OTUs by main taxa groups following the classification used from images carried out from BATS data (Ivory et al, 2018) resulted in a lack of ability to detect seasonality. Samples analyzed this way were clustered into three groups, one containing summer (June to September) samples and the other two the rest of them.…”

“…A few other studies were carried out in the Sargasso Sea or their vicinity later on, but in general focused on a single or a handful of groups (Angel, 1979;Wiebe et al, 1992). Most of the studies at community level have since used main taxa groups (Eden et al, 2009;Ivory et al, 2018), focused on a single group (Stone and Steinberg, 2014) or processes and biogeochemical cycling (Steinberg et al, 2000;Madin et al, 2001;Schnetzer and Steinberg, 2002;Stone and Steinberg, 2016;Maas et al, 2018) more than in the community composition.…”

“…Understanding the dynamics of the zooplankton community at BATS, in the boundary of the North Atlantic Subtropical Gyre, is therefore essential to properly address the biogeochemical cycles at this site. For the zooplankton community, studies have previously focused on describing the monthly evolution of single species or taxonomic groups (Deevey, 1971;Deevey and Brooks, 1971;Ivory et al, 2018); a study linking hydrography to shifts in community composition is, however, still missing. From the existing data, the best suited would be those from Deevey studies but, unfortunately, the counts were not available in the original publications, and all efforts done to find the original data from her institution, or at the Bermuda Institute of Ocean Sciences, have been unsuccessful.…”