Fluorescent whitening agents in Tokyo Bay sediments: molecular evidence of lateral transport of land-derived particulate matter

摘要

Two kinds of stilbene-type fluorescent whitening agents (FWAs:i.e., DSBP and DAS1), which are minor components of laundry detergents, were analyzed in water column and surface sediments of Tokyo Bay and adjacent rivers to evaluate their utility as anthropogenic molecular markers for estimating the transport processes of sediment particles in the coastal zone. DSBP and DAS1 were widely distributed in Tokyo Bay sediments at concentrations ranging from 0.01 to 1.85 μg/g and 0.02 to 0.37 μg/g, respectively. The concentration ratio of DSBP to DAS1 (the DSBP/DAS1 ratio) of suspended particles in surface layers decreased from estuaries (～3) to Tokyo Bay (<1) due to selective photodegradation of DSBP in the euphotic zone. On the other hand, a higher DSBP/DAS1 ratio (3-5) was observed in inshore Tokyo Bay sediments. Using these differences in DSBP/DAS1 ratios as an indicator of photodegradation of FWAs, we propose the importance of lateral transport of riverine sediments during floods to the bay through aphotic benthic layers where there is less chance of photodegradation occurring. Relative depletion of DSBP to DAS1 in marine sediments was observed from the head to the mouth of the bay, i.e., with increasing distance from the shore. This effect can be ascribed to an increase in the relative contribution of particles vertically transported from the euphotic zone, where DSBP could be preferentially photodegraded or selective desorption of DSBP from the sediment particles during their offshore transport.