Effects of Benthic Flux on Short Term Variations of Nutrients in Aburatsubo Bay

Abstract

In order to investigate effects of benthic flux on the short-term variations in the distribution of nutrients in coastal waters, the concentrations of nutrients (PO4 3-, NH4 + NO3 -, NO2 - and H4SiO4) and other oceanographic parameters were measured every three hours over a 24-hour period at four fixed stations in the water column of Aburatsubo Bay, a shallow semi-enclosed inlet. Sediment cores were also taken from a fixed station once in each season over one year to quantitatively determine their benthic flux. Consistent linear negative correlations were found between their concentrations and salinity in the surface layers. This result suggests that fresh water was the main source of these nutrients and a physical mixing was the major process controlling their distribution. Monthly variations of PO4 3- and NH4 + monitored for 18 months in the bay also indicate that the high surf concentration of these nutrients was associated with the appearance of low salinity waters. On the other hand, in the bottom layers, a linear correlation between the concentration of the nutrients and salinity became weak, especially for NH4 + and PO4 3-. Their concentrations were higher than the predicted value from the conservative mixing between the fresh water and seawater, indicating the possibility of another source in the bottom layers. Benthic flux is suggested as a possible source. Pore water profiles of NH4 + and PO4 3- indicate their flux towards the overlying seawater, which is quantitatively consistent with their water column distributions.