Higher species richness and abundance of fish and benthic invertebrates around submarine groundwater discharge in Obama Bay, Japan

Study focus. There have been far more studies on how the variability in surface water discharge affects production of animal communities in aquatic ecosystems while less information has been accumulated on the mechanisms of how the groundwater supply works.

Study region. Physical and biological surveys were conducted to test the hypothesis that high level of submarine ground water discharge enhances species richness, abundance and biomass of fishes and invertebrates in coastal waters of Obama Bay, Japan, where a high contribution of nutrients (ca. 65% of phosphorus) to total provided through all freshwater has been reported. Survey for horizontal distribution of radon-222 (²²²Rn) concentration showed high levels of submarine groundwater discharge in the west part of survey area. Fish and invertebrate communities were compared within a relatively small spatial scale (ca. 100 m) in relation to level of submarine groundwater discharge.

New hydrological insights. Species richness, abundance and biomass of fishes and abundance and biomass of turban snail and hermit crab were significantly higher in the area with high ²²²Rn concentration. Abundance of gammarids, the most major prey item of the fishes, was 18 times higher in the area with high ²²²Rn concentration. Since the turban snail, hermit crab and gammarids feed on producers (phytoplankton and benthic microalgae), submarine groundwater are concluded to increase species richness and production of fishes and invertebrates through providing nutrients and enhancing primary production.