Distribution and biomass of benthic microalgae in Manukau Harbour, New Zealand

Abstract

Benthic microalgal biomass (as sediment chlorophyll a (Chl. a), by spectrophotometry) and taxonomic composition (by HPLC pigments analysis) were investigated at intertidal locations throughout Manukau Harbour, North Island, New Zealand. Benthic microalgal biomass averaged 97.5 mg Chl. a m‐2 for all sediment samples. Benthic microalgal biomass was higher in sediments containing at least some sand than in muddy sediments, in contrast to previous findings from Manukau Harbour. Loading of fine sediments from erosion within the harbour's basin may, therefore, affect the amount and distribution of benthic microalgal biomass in the harbour. Average benthic microalgal biomass for the entire area of the harbour was at least 62.5 mg Chl. a m‐2. The latter value is at least 4 times higher than mean annual, spatially integrated phytoplankton biomass in Manukau Harbour, suggesting that benthic microalgae are a more important food source for estuarine consumers. High fucoxanthin:chlorophyll a ratios indicated that benthic microalgae were primarily diatoms. The physical characteristics of Manukau Harbour and similarity in taxonomic composition of the phytoplankton and benthic microalgae suggest that resuspended benthic microalgae are an important component of the harbour's phytoplankton biomass.