Tidal resuspension of microphytobenthic chlorophyll a in a Nanaura mudflat, Saga, Ariake Sea, Japan: flood–ebb and spring–neap variations

Abstract

The resuspension of microphytobenthic chlorophyll a (chl a) and suspended particulate matter (SPM) from the upper intertidal flat was studied at Nanaura, Ariake Sea, Japan, and a spatial comparison of chl a and SPM in the northern Ariake Sea is presented here. A 15 d time series of chl a and SPM records (measured at 15 cm above the bottom) revealed that resuspension was clearly asso- ciated with flood-ebb and spring-neap tidal characteristics, where significant resuspension was found during early flood, particularly in the high-energy period (viz. spring tide). The peaks of resus- pended chl a and SPM during 28 tidal periods were observed to consistently occur shortly after the current velocity reached a maximum and when the threshold current velocity was found to be ca. 15 cm s -1 . Much more highly fluctuating, irregular peaks of chl a and SPM were occasionally observed, these were attributable to high wind speeds, ≥3 m s -1 . Computation of chl a and SPM fluxes showed clear spring-neap variation, and time-integrated fluxes denoted predominant offshore residual transport during the observation period. Benthic chl a measured in the surficial sediment (top 0.5 cm) during 13 exposure periods showed a decreasing trend from spring to neap tide, similar to spring-neap variations of chl a in the water column during the semi-lunar tidal period. The daily mean percentage of resuspended chl a in the water column relative to the amount of benthic chl a was estimated to be ca. 10 to 70% (mean = 33%), implying that a certain portion of the microphyto- benthos is resuspended during high tide and partly contributes to the total biomass in the water column of the upper intertidal flat.