Efficacy of the application of organic fertilizer to oyster growth.

Hiroshima Bay is the top oyster-producing bay in Japan. However, the bay ecosystem has suffered from oligotrophication due to a 40-year nutrient reduction measure. Poor growth of cultured oysters caused by oligotrophication is a serious problem. The aim of the present study was to evaluate the efficacy of a newly developed organic solid fertilizer on oyster growth. In the field experiments, it was demonstrated that oyster meat weight increased by 34 % with the application of the fertilizer than without. A simulation model revealed that the nutrients eluted from the fertilizer enhanced the growth of oysters through the growth of phytoplankton. The observations implied that the decomposition of dissolved organic matter excreted from phytoplankton and the decomposition of detrital matter accelerated nutrient cycling and the oyster growth. The developed organic solid fertilizer is effective in enhancing oyster growth in an oligotrophic area.

Effects of ulvoid (Ulva spp.) accumulation on the structure and function of eelgrass (Zostera marina L.) bed.

The objective of this study is to clarify the effect of ulvoid (Ulva spp.) accumulation on the structure and function of an eelgrass bed by the coast of Iwakuni, Seto Inland Sea, Japan. We monitored eelgrass shoot density and volume of ulvoid accumulation in the study site and evaluated effects of the accumulated ulvoid canopy on the percent survival, seedling density, growth rates, photosynthetic photon flux density (PPFD) and carbon contents of eelgrass. Eelgrass shoot density decreased by the accumulation of ulvoid. Also, seedling density decreased by the increase in the ulvoid volumes. Shoot density, seedling density and leaf elongation were negatively correlated with ulvoid volume. Carbon contents in eelgrass decreased by the accumulation of ulvoid (canopy height: 25cm). These results suggest that accumulation of ulvoid bloom has significant negative impacts on the structure and function of eelgrass bed, i.e. decreases in vegetative shoot density, seedling density, shoot height and growth rate.

Effects of oil spill on seawater infiltration and macrobenthic community in tidal flats.

Tidal flat ecosystem simulators are used to clarify the effects of stranded fuel oil on tidal flat ecosystems. Results show that oil spills increase the periphyton on sediment by decreasing the predation stress caused by deposit feeders. About a month after an oil spill, the total population density of the macrobenthos recovered. The oxidation-reduction potential in the surface sediment drops to a negative value after the oil spill, and the anaerobic condition throughout the sediment seems to be responsible for the decrease in the population density of the macrobenthos. The infiltration volume of seawater into the oil-stranded sediment decreases to a third of that without the oil spill. The recovery of infiltration volume after about a month coincides with the recovery of the population density of the macrobenthos. This result suggests that the macrobenthic population is highly dependent on the infiltration of seawater.