A stable isotopic approach to investigate nitrogen pathways in a coastal aquaculture area

Coastal eutrophication and consequent environmental deterioration have been problematic in many parts of the world. However, oligotrophication and resulting reduction in coastal fisheries and aquaculture productivity have also been a recognized problem in some parts of Japan. Some studies argue that declining fisheries production of coastal resources and unfed aquaculture production is partially attributable to the reduced nutrient concentrations in the coastal waters in recent years. Eutrophication mitigation efforts have reduced the terrestrial nutrient load to the coastal environment over the past forty years in Japan. Excessive oligotrophication has allegedly reduced primary productivity and hence carrying capacity of coastal ecosystems supporting coastal fisheries and aquaculture. Fisheries production of the Manila clam, Ruditapes philippinarum, has decreased by 95% over the past three decades. Insufficient food supply is one amongst many speculated factors causing the reduction. However, complex nutrient flow in coastal environments has not necessarily been elucidated. Our stable isotopic (δ13C and δ15N) studies have suggested that there are large and small scale variations in the nutrient pathways from inorganic nutrients to the clam. Clams inhabiting the same tidal flat only 10 m apart from each other can have a different food environment as indicated by δ13C. Pervasive effects of terrestrial nutrient load on the food availability of the clam may be ascertained by δ15N. Stable isotopic methods may also be used to determine nutrient flow within an integrated multi-trophic aquaculture (IMTA) system, which has potential to enhance unfed aquaculture production in an oligotrophic environment.