Selectivity of plankton nets over mesozooplankton taxa: implications for abundance, biomass and diversity estimation

The importance of the smaller copepod species is often underestimated as a result of the use of standard 200 ¼m mesh size nets, even though the small copepods probably represent the dominant component of the mesozooplankton community. Although the inadequacy of relatively coarse nets in providing reliable descriptions of the mesozooplankton assemblage is increasingly welldocumented, such nets continue to be commonly used. A major reason for this unwillingness to break with tradition is the belief that samples remain comparable even though the absolute values are biased. A one-year study of the abundance and size distribution of zooplankton collected in the Venice Lagoon using an 80 ¼m mesh size net showed an overwhelming abundance of small taxa. These data were used to derive estimates of the mesh selection effects of standard WP-2 nets on zooplankton abundance and biomass. Only 11% of numbers and 54% of biomass of lagoon zooplankton are likely to be caught with standard WP-2 nets. A comparison of seasonal changes in diversity, estimated from the fine and coarse datasets, confirmed that retention efficiency is seasonally dependent, which results in serious implications when extrapolating temporal patterns in community structure from WP-2 mesozooplankton counts.