Correspondence between zooplankton assemblages and the Estuary Environment Classification system

•Zooplankton assemblages reflect the 'Estuary Environment Classification' categories.•Zooplankton richness varies among estuaries in a manner predicted by Remane's curve.•Hydrology more important than basin morphometry in determining species composition.•Salinity is the main driver of zooplankton composition and diversity among estuaries.

We tested whether variability in zooplankton assemblages was consistent with the categories of estuarine environments proposed by the 'Estuary Environment Classification' system (EEC) (Hume et al., 2007) across a variety of North Island, New Zealand, estuaries. The EEC classifies estuaries in to eight categories (A to F) based primarily on a combination of three abiotic controlling factors: ocean forcing, river forcing and basin morphometry. Additionally, we tested whether Remane's curve, which predicts higher diversities of benthic macrofauna and high and low salinities, can be applied to zooplankton assemblages. We focused on three of the eight EEC categories (B, D and F), which covered the range of estuaries with river inputs dominating (B) to ocean influence dominating (F). Additionally, we included samples from river (FW) and sea (MW) to encompass the entire salinity range. Zooplankton assemblages varied across the categories examined in accordance with a salinity gradient predicted by the EEC. Three groups of zooplankton were distinguishable: the first formed by the most freshwater categories, FW and B, and dominated by rotifers (primarily Bdelloidea) and estuarine copepods (Gladioferans pectinatus), a second group formed by categories D and F, of intermediate salinity, dominated by copepods (Euterpina acutifrons), and a final group including the purely marine category MW and dominated also by E. acutifrons along with other marine taxa. Zooplankton diversity responded to the salinity gradient in a manner expected from Remane's curve. The results of this study support others which have shown salinity to be the main factor driving zooplankton community composition and diversity.