Plankton net mesh size influences the resultant diversity and abundance estimates of copepods in tropical oligotrophic ecosystems

Abstract

Quantitative assessment of planktonic organisms is a key issue in understanding biodiversity, biomass, and carbon fluxes in marine ecosystems during the ongoing Anthropocene. However, the implications of the choice of plankton sampling equipment in tropical marine ecosystems have not been fully addressed. The goal of this study was to investigate the abundance and diversity of copepods derived from two different mesh sizes, 120 and 300 μm, to determine differences that may change our perception of the actual role of the key planktonic organisms in tropical marine ecosystems, due to the fact that missing information may be a real problem in trophic estimations (e.g. benthic-pelagic coupling processes in coastal areas). Samples were collected along 650 km of coastline in the Equatorial Atlantic. The average abundance of copepods calculated using the 120 μm net was five times higher than that of the 300 μm net. However, species richness was higher when using the 300 μm net compared to that of the 120 μm. Using the 300 μm net, the number of exclusive taxa (not found in the 120 μm mesh sampling) was higher. The Venn diagram showed that 10% of the copepod taxa were recorded exclusively in 120 μm net, whereas only 30% occurred in the 300 μm net. To improve our understanding of the structure and functioning of tropical marine ecosystems, plankton nets with smaller mesh openings should always be used to estimate abundance because of the dominance of small organisms in the nutrient-poor food webs, giving a new perspective on the available energy in water column and benthic processes of suspension feeding organisms. The absence of these smaller nets will produce an inaccurate picture of the plankton communities and their contributions to other trophic levels, including the blue carbon budget estimates worldwide.