Tidal time-scale variation in nutrient flux across the sediment-water interface of an estuarine tidal flat

We determined the range of the tidal variations in nutrient flux across the sediment-water interface and elucidated mechanisms of the flux variation in two estuarine intertidal flats (one sand, one mud) in northeastern Japan. Nutrient flux was measured using in situ light and dark chambers. which were incubated for 2 h, 2-6 times per day. Results showed that nutrient concentration in overlying water varied by tide and was also affected by sewage-treated water inflow. The nutrient fluxes responded quickly to the tidal variation in overlying water chemistry and the range of the variation in flux was as large as the seasonal-scale variation reported in previous studies. In the sand flat, salinity increase likely enhanced benthos respiration and led to increases in both O-2 consumption and PO43- regeneration under low illumination, while benthic microalgae were likely to actively generate O-2, uptake PO43- and suppress PO43- release under high illumination (> 900 mu mol photons m(-2) s(-1) Also in the mud flat, PO43- flux was related with O-2 flux, although the range of temporal variation in PO43- flux was small. In both the flats, NH4+ flux was always governed by NH4+ concentration in the overlying waters either an increase in NH4+ uptake or a decrease in NH4+ release was observed as the NH4+ concentration rose due to inflow of river water or input of sewage-treated water. Although NO3- tended to be released in both tidal flats when low NO3- concentration seawater dominated, their relationship was likely to be weakened under conditions of low oxygen consumption and suppressed denitrification. It is likely that tidal variation in nutrient flux is governed more by the nutrient concentration than other factors. such as benthic biological processes, particularly in the case where nutrient concentration in the overlying water is relatively high and with wide amplitude. (c) 2006 Elsevier Ltd. All rights reserved.