How does mesh-size selection reshape the description of zooplankton community structure in coastal lakes?

To provide evidence of the inadequacy of using conventional sampling methods to study mesozooplankton community structure in confined environments, samples from a Mediterranean meromictic coastal lake were taken using 80-μm and 200-μm mesh nets towed at three different depths (0–18 m). Mesh size significantly affected the description of the community structure of the collected zooplankton. The 80-μm-mesh catch revealed for the first time in such an environment the overwhelming abundance of the copepod species Oithona brevicornis, as two orders of magnitude greater than recorded for the 200-μm-mesh catch. The other dominant species were Paracartia latisetosa and Pseudodiaptomus marinus, which were more efficiently sampled with the 200-μm mesh. These showed a summer abundance peak for P. latisetosa near the surface layer, and for P. marinus in the deepest stratum, close to the anoxic layer. Copepod nauplii and bivalve larvae were more efficiently caught with the 80-μm-mesh, and accounted for most of the zooplankton in winter, and showed the highest loss percentage of abundance between the two mesh-size catches. The differences in the assemblages reflect different diversity patterns, with peaks in summer and winter for the communities collected with the 80-μm-mesh and 200-μm-mesh, respectively. These findings imply the need for the development of a commonly used sampling method with paired nets, to correctly take into account both the large and small fractions of the mesozooplankton in the study of closed and semi-enclosed coastal environments, and to obtain data that can be better compared across studies.