Precision of mesoplankton sampling: A case study based on three net series in the South Atlantic and in the Black Sea.

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.