Zooplankton biomass, size structure, and associated metabolic fluxes with focus on its roles at the chlorophyll maximum layer during the plankton-contaminant MERITE-HIPPOCAMPE cruise

Microplastics in the maximum chlorophyll layer along a north-south transect in the Mediterranean Sea in comparison with zooplankton concentrations

“…The analysis of zooplankton and MPs at the CML was done from aliquots of samples taken with these net tows, and then fractionated by sieving on board in a dedicated container under clean conditions (fractions 60-200; 200-500; 500-1000; 1000-2000 and > 2000 µm). All fractions were treated for zooplankton studies (see Fierro-González et al, 2023). Fractions above 500 µm and the fraction 300-500 µm, from the zooplankton sample 200-500 µm sieved on 300 µm mesh, were fully examined under a binocular microscope for MPs.…”

“…In addition, Manta nets (300 µm mesh size) were programmed to quantify surface microplastics according to MSFD standards (Hanke et al, 2013). Initially, during our analysis of zooplankton communities from Hydrobios samples (60 µm) collected at maximum chlorophyll of the various stations (Fierro et al, 2023), a few microplastics items were observed in the aliquots studied for zooplankton treatment and identification with both Zooscan and binocular loupe. The idea then occurred to us to attempt to exhaustively quantify the microplastics after fractioning on 300 µm sieve of these samples obtained with the Hydrobios (60 µm mesh) at maximum Chlorophyll, in order to compare them with the microplastics data obtained using Manta net surface tows with a mesh size of 300 µm.…”

“…Zooplankton surface counts were obtained from another Manta net with a 60 µm mesh-size net, towed at a constant speed of 2 knots for 10 min, and the sieved-fractions above 200 µm of this sample were used for determining MPs/zooplankton ratio at the surface. Zooplankton analysis for multinet Hydrobios and Manta nets are fully described in Fierro-González et al (2023).…”

“…Zooplankton sampling was carried out by horizontal tows of nets through filtration of large volumes. This approach provided a unique opportunity to examine the MPs content in samples intended for counting and characterizing the collected zooplankton (Fierro-González et al, 2023). The aims of this study were (1) to count, classify and measure MPs collected at the CML during the MERITE-HIPPOCAMPE cruise, (2) to identify differences in concentrations and size structures between the MPs collected at the CML and those collected at the sea surface, (3) to compare the concentrations of MPs to zooplankton and detritus densities in the same samples, (4) to identify potential differences between the coastal and offshore stations.…”

“…Figure 1: Distribution of detritus (brown bar), zooplankton (blue bar), and microplastics (MPs, orange bar) at the surface and at the chlorophyll maximum layer (CML) at the 10 stations of the MERITE-HIPPOCAMPE campaign (see presentation in the text, and detailed information inFierro et al, 2023). Values in Log10 for detritus and zooplankton, Log10 (x 100) for MPs.…”