Crustacean zooplankton species richness and productivity: to what extent do the conclusions depend upon the choice of metrics

The form of the relationship between productivity and diversity is the subject of much debate. In studies that compare among site variations in species richness, the relationship varies with taxonomic grouping, spatial scale, productivity range, predation pressure and community assembly. However, direct measurements of productivity are often lacking, necessitating the use of proxies. Nutrient concentrations and algal biomass have both been used as such productivity metrics in studies of freshwaters, but no studies have compared explicitly the form of the richness-productivity relationship when different proxies are used in the same study. Furthermore, different studies use species richness data collected over different time periods, ranging from a single day to many years. In this study the relationship between crustacean zooplankton species richness and productivity is examined, using four metrics of productivity and three temporal scales. The productivity metrics differed in their ability to statistically explain variations in species richness, with concentrations of soluble reactive phosphorus and chlorophyll a being the most powerful predictors. However, the form of the productivity-richness relationship differed in each case, being linear in the former analysis and quadratic in the latter. Comparison of analyses based on species data from different time periods, revealed that the magnitude of the productivity effect was greatest for long-term species richness and smallest for the richness determined on a single sampling occasion. The proportion of the total species pool that was present on any one sampling date was itself dependent on productivity. Differences in the definition of both productivity and species richness can cause variability in the form of the relationship between the two. This may then confound comparisons between studies that use different methodologies.