Within-day and seasonal patterns of microphytobenthos biomass determined by co-measurement of sediment and water column chlorophylls in the intertidal mudflat of Nanaura, Saga, Ariake Sea, Japan

The microphytobenthos biomass was examined at a fixed site on the Nanaura mudflat, Ariake Sea over 9 months at intervals of 2–3 weeks. During the course of 16 surveys, samples were collected in daytime at intervals of <1 h for measurement of chlorophylls (Chl a and pheopigments) in sediment (during exposure; n = 98) and water column (during ebb, n = 86; flood, n = 95). We found a clear temporal pattern in within-day sediment Chl a indicating net increase of microphytobenthos biomass in surface sediment during a single-daytime exposure. The significant biomass increase was clearly observed during the winter period, but weakened during the rest of the periods. Pheopigments in sediment showed much greater within-day variation compared to corresponding Chl a, with relatively great fluctuation from summer to early fall. However, during fall–winter periods, the sediment pheopigments showed within-day variation, similar to corresponding Chl a, indicating a biomass-dependent degradation approaching winter season. Overall, winter peak of Chl a (176 mg m−2) and summer peak of pheopigments (596 mg m−2) in sediment were characteristic in the Nanaura mudflat, Ariake Sea. In general, seasonal patterns for water column chlorophylls were similar to those for sediment chlorophylls, with winter peak of Chl a (174 μg L−1) and summer maximum of pheopigments (206 μg L−1), respectively. Typically, monthly mean water Chl a (during flood) was significantly correlated with monthly mean sediment Chl a (during exposure), indicating an entrainment of sediment Chl a to the water column during flood by tidal resuspension. Calculations indicated that ca. 66% of Chl a in the water column could be benthic-derived from the surface sediment, on average (n = 181), supporting a significant contribution of microphytobenthos biomass and production in intertidal mudflat ecosystem.