A microcosm experiment comparing nitrogen flow near a seawall and along the sandy seashore per se in Hamana Bay, Japan, using 15N

A flow-through microcosm experiment was conducted using filter-feedering bivalves and ascideans together with intact sediment cores collected from near a seawall and from the natural sandy seashore and an artificial sandy tidal flat in Hamana Bay. The objective of this experiment was to examine individual specimens’ ability to remove particulate organic matter and release nitrogen occurring in the different environments of the seawall and the shore. In consideration of apparent seasonal changes observed in Hamana Bay, summer- and fall- based models were used. Mytilus galloprovincialis (summer seawall model), Molgula manhattensis (fall seawall model), and Ruditapes philippinarum (summer and fall seashore models) served as the filter feeders in the experiments, and 15N-labeled Chaetoceros calcitrans was used as the nitrogen source. In both the seawall and seashore models, 30% of PON was decomposed into NH4+ in the fall, while only a few percent was decomposed in the seawall model in summer due to inactivation of the macrobenthic metabolism by hypoxia. In the latter model, Mytilus galloprovincialis and the sediment both acted as sinks to accumulate most of the nitrogen, thus causing a de-acceleration in the nitrogen cycling rate and, consequently, summer eutrophication. In contrast the nitrogen cycling rate of the seawall model in fall was as high as that of the seashore model. These results suggest that the sandy seashore and tidal flat region may be playing a significant role in eutrophication improvement in Hamana Bay.