Trans‐oceanic characterization of zooplankton community size structure using an optical plankton counter

Abstract

The Atlantic Meridional Transect (AMT), sampling between 50°N and 50°S, is a trans‐oceanic research programme to characterize plankton community structure, productivity and bio‐optical properties of the upper ocean in relation to ocean provinces and meso‐scale features. As part of a suite of continuous measurements, an optical plankton counter (OPC) has been used on the AMT to characterize the surface distribution of zooplankton based on their abundance and community size structure. Results from the OPC were available in real time, and were subsequently validated against microscope counts of zooplankton sampled concurrently along the AMT. Zooplankton community size structure from two sections of the Transect are presented: from the northern edge of upwelling off West Africa to the UK shelf and between the Falkland Islands and Uruguay, where the Transect traversed a warm core ring. Results showing the zooplankton community size structure together with statistical analysis, using principal component analysis (PCA), are compared with the dynamic oceanographic characteristics along the Transect. The application of the OPC for such basin‐scale studies provides a methodology for obtaining data on zooplankton abundance and community structure at the appropriate spatial scales for global‐scale modelling of the marine ecosystem.

Variability of phytoplankton and mesozooplankton biomass in the subtropical and tropical Atlantic Ocean

“…The results of continuous measurements of zooplankton abundance and biomass by optical plankton counters in a range from 0.25 to 8.0 mm have indicated that minimal values are typical for the oligotrophic South Atlantic Subtropical Gyre and oligotrophic Canary Basin along a transect from 49°N to 46°S (Gallienne & Robins 1998). Values were 20 times higher in the equatorial region and 10 times higher in the waters near the West African upwelling.…”

Use of spherical and spheroidal models to calculate zooplankton biovolume from particle equivalent spherical diameter as measured by an optical plankton counter

“…Such discrepancies are most likely explained by the OPC sensing non-zooplankton particles, e.g., detritus, which can be more numerous than zooplankton (Herman 1992;Heath et al 1999;Zhang et al 2000). It should, nevertheless, be noted that some previous studies have found good agreement between OPC counts and net abundance (Herman 1992;Gallienne and Robins 1998). In our study, the zooplankton assemblage was strongly dominated by a single species, Calanus finmarchicus, making a comparison between the OPC and net data relatively straightforward.…”

Epipelagic mesozooplankton distribution and abundance over the Mascarene Plateau and Basin, south-western Indian Ocean

“…Raymont, 1983). A ¢ve year survey of the Atlantic Ocean along the Atlantic Meridional Transect (AMT; Robins & Aiken, 1996;Aiken & Bale, 2000) sampled the corresponding western part of the South Atlantic, between 4 and 208S, in April/ May of ¢ve consecutive years (Gallienne & Robins, 1998). The average surface mixed layer biomass for the AMT series was 39 mg m À3 .…”

“…The OPC produces a reliable and robust estimate of mesozooplankton numbers and size, in the size range 250 mm^16 mm equivalent spherical diameter (ESD), yielding a size distribution which can be converted into biomass (Gallienne & Robins, 1998;. Mesozooplankton biomass (mg m À3 ) data were derived from OPC biovolume (ml m À3 ), assuming a speci¢c gravity for mesozooplankton of 1.…”

“…In the north-bound data set (Figure 2A), from Mauritius (19.58S) up to 148S, biomass values average 24 mg m À3 (minimum 15, maximum 29 mg m À3 ), typical for sub-tropical, oligotrophic waters (Raymont, 1983;Gallienne & Robins, 1998). From this point on, biomass increased greatly, averaging 263 mg m À3 (minimum 87, maximum 575 mg m À3 ), with two peaks around 11.8^13.88S, and 7.6^9.38S.…”