During 2015, monthly zooplankton sampling and measurements of surface temperature (SST), salinity, dissolved oxygen and chlorophyll a were conducted. Collections were made during neap tides at four locations between the inner and near the mouth of Magdalena Bay, Mexico. Thirty-three taxonomic groups were identified, and the most abundant taxa were copepods, diplostracans, decapods, ichthyoplankton (fish eggs), and chaetognaths. Zooplankton abundance did not vary significantly over time but did vary between sampling stations. SST and salinity were significantly correlated with the spatial distribution of organisms. Differences were found between the mortality percentages for the sampling stations and also for the taxonomic groups analyzed (Copepods 18%; Decapods 32%; Chaetognaths 33%), which implies the importance of carrying out mortality determination analyses in ecological studies of zooplankton.

Mesoplankton is a key element of pelagic communities representing the largest biome on the planet. Many concepts in marine and freshwater biology are based on quantitative estimates of mesoplankton abundance, whereas precision of mesoplankton sampling remains underexplored and may depend on various factors. We analyzed ten contiguous daytime epipelagic samples in the Black Sea and 13 nighttime mesopelagic samples in the South Atlantic. We used a relative error as a measure of the sampling precision and ran a set of Generalized Linear Mixed Models (GLMMs) to estimate effects of six possible factors: abundance, size, diel migration, movement speed, taxonomic group, and net type. Abundance of taxa was the most powerful factor affecting sampling precision (positive effect) followed by the net type (BR provided better precision than Judey net) and taxonomic group. Conversely, size, movement speed, and diel migrations did not significantly influence sampling precision in all sample sets. We conclude that abundance and biomass of dominant species may be estimated with a satisfactory accuracy (relative error <20% of assessed values), which suggests that recent conceptions based on total mesoplankton abundance and biomass (contributed mainly by dominant taxa) are not greatly biased. Quantitative zooplankton structure and biodiversity assessed on the basis of non-transformed matrices are likely more relevant than those based on the root-transformed or presence/absence data.

Floating plastic debris is a pervasive pollutant in seas and oceans, affecting a wide range of animals. In particular, microplastics (<5 mm in size) increase the possibility that marine species consume plastic and enter the food chain. The present study investigates this potential mistake between plastic debris and zooplankton by calculating the plastic debris to zooplankton ratio over the whole Mediterranean Sea. To this aim, in situ data from the Tara Mediterranean Expedition are combined with environmental and Lagrangian diagnostics in a machine learning approach to produce spatially-explicit maps of plastic debris and zooplankton abundance. We then analyse the plastic to zooplankton ratio in regions with high abundances of pelagic fish. Two of the major hotspots of pelagic fish, located in the Gulf of Gabès and Cilician basin, were associated with high ratio values. Finally, we compare the plastic to zooplankton ratio values in the Pelagos Sanctuary, an important hotspot for marine mammals, with other Geographical Sub-Areas, and find that they were among the larger of the Western Mediterranean Sea. Our results indicate a high potential risk of contamination of marine fauna by plastic and advocate for novel integrated modelling approaches which account for potential trophic transfer within the food chain.