Long-term variability in larval recruitment rates of a callianassid shrimp population on an intertidal sandflat in an estuary-to-coastal ocean interface area in relation to river discharge and shelf water movement, western Kyushu, Japan

Food availability is believed to affect macro-invertebrate larval survival at sea, which was poorly demonstrated. On the coast immediately outside an estuary (Ariake Sound, Kyushu, Japan), a population of the callianassid shrimp, Neotrypaea harmandi, with diatom-feeding larvae, lies on an intertidal sandflat. It is likely from previous research that the shrimp population is self-recruited by larvae in 20-50-m water depths of an interface area influenced by estuary and inner shelf and growth of a dominant diatom species in the estuary is nitrogen-limited. Apparent shrimp self-recruitment rates were monitored each year during 1989-2013. Public databases were used about the largest river's flow and dissolved inorganic nitrogen (DIN) in the estuary. A high positive correlation was detected between shrimp recruitment rate and DIN concentration which was in turn correlated with river flow rate. Hydrographic surveys were conducted from inner shelf to estuary in 2011-2013. Chlorophyll-fluorescence was positively related with DIN concentration in the near-surface for estuary and in the lower subsurface for inner shelf. Based on a public reanalysis model database for temperature in the western North Pacific were examined yearly variations in the degree of western Kyushu-coastward movement by potentially nutrient-rich, cold bottom waters from offshore. The contributions of DIN from estuary and from inner shelf to shrimp recruitment rate were assessed by multiple regression, with, as explanatory variables, estuarine DIN concentration and lower temperature anomaly in inner-shelf water (surrogate for the higher DIN concentration). DIN of estuary and 'DIN' of inner shelf explained recruitment-rate variations by 77% and 23%, respectively.