Can different mesh sizes affect the results of copepod community studies?

Abstract

Although mesozooplankton are important components of aquatic food webs and biogeochemical cycling, this plankton fraction is inadequately described and quantified. Standardized sampling procedures for its worldwide characterization are lacking as yet, affecting meaningful comparisons between regional situations. Mesh sizes in use vary greatly among studies, raising the possibility that sampling method bias results for size-related phenomena of the plankton, such as abundance and diversity in space and time. We tested the effects of different mesh sizes for sampling on the resulting size-frequency distributions of mesozooplankton by collecting copepod assemblages in the northern South China Sea. Sample retrieval by different mesh-size plankton nets were compared, using the abundance and distribution patterns of mesozooplankton collected with NORPAC zooplankton nets of 333, 200, and 100-μm mesh size. One-way ANOVA results for species number and richness indices of all samples showed significant differences between these three mesh size nets. In contrast, total copepod abundance of the 100-μm net was significantly higher than that from the 200-μm net (p=0.001) and of the 333-μm net (p<0.001). Index values of evenness and diversity were significantly different among the copepod assemblages obtained from the three mesh size nets. The evenness index values of the 100-μm net were significantly lower than those recorded from the 200-μm net (p=0.007) and the 333-μm net (p<0.001). The Shannon–Wiener diversity index of the 333-μm net was significantly higher than for the 200-μm net (p=0.016) and for the 100-μm net (p=0.006). In addition, the dominant species proportion in each copepod assemblage obtained in the three mesh size nets differed significantly. Densities of smaller mesozooplanktons such as copepodites were highly underestimated by larger meshed nets that were commonly used in previous studies in the waters of the North Pacific Ocean. Samples collected with a 100-μm mesh net demonstrated the numerical importance of smaller mesozooplankton in the northern South China Sea. Present results, therefore, suggest a striking selectivity of plankton net mesh sizes that significantly affected qualitative and quantitative aspects of copepod assemblages.