Utilization of Photosynthetically Produced Organic Particles by Dense Patches of Suspension Feeding Bivalves on the Sand Flat of Midori River Estuary, Kyushu, Japan

Abstract

Midori River estuary faces Ariake Bay, and has the largest sandy tidal flat (approximately 2,100ha) in Japan. We investigated the spatial distribution of particle size composition of the sediment, microphytobenthos on the surface sediment and macrobenthic animals at the stations set along a cross transect line in Midori River estuary. The Chl-a concentrations of the surface sediment that reflected the biomass of the microphytobenthos ranged between 19.3 and 113mg/m2 at the stations on the tidal flat, and tended to decrease from the upper part to the lower part of the tidal flat and the offshore area. However, 92.0% of the total biomass of the macrobenthic animals collected at the sampling stations on the tidal flat was concentrated at the stations on the middle to lower part of the tidal flat. The highest biomass reached 3,760g WW/m2, mainly due to the occurrence of dense patches of two suspension feeding bivalves, Ruditapes philippinarum and Mactra veneriformis. These two bivalves alone occupied 93.2% of the biomass of all the benthic animals collected at all sampling stations. The consumption of the primarily produced organic matter estimated from the secondary production of only a single species of Ruditapes apparently exceeded the potential primary production at the same place. The results of this study suggest that the re-suspension of primarily produced organic particles by the microphytobenthos on "the upper part" of the tidal flat and its lateral transportation to "the middle to lower part" of the tidal flat is essential to sustain the high secondary production of the suspension feeding benthic bivalves on the middle to lower part of this tidal flat. (author abst.)