Abstract
The role of the heterotrophic dinoflagellate Noctiluca scintillans in affecting the nutrient-environment and aquatic ecosystem was investigated in the neritic area of Sagami Bay, Kanagawa, Japan, from January 2002 to December 2006, based on abundance, intracellular nutrient content, excretion rate and response of phytoplankton (diatoms) to enrichment of nutrients extracted from N. scintillans cells. Seasonal variations in abundance and vertical distribution of N. scintillans were significantly related to the physical structure of the water column, water temperature, chlorophyll a and primary productivity. Intracellular nutrient contents, except for Si(OH)4-Si, revealed clear seasonal fluctuations, which were significantly correlated to cell size variations. Thalassiosira rotula increased to higher cell abundances at higher concentrations of nutrients, which were extracted from N. scintillans cells. NH4+-N and PO43--P excretion rates were much higher during the first 1–3 h, and decreased rapidly with time. Daily NH4+-N and PO43--P supply by N. scintillans excretion was estimated to be on average 34.8% and 55.3%, especially 50.6–85.4% and 80.5–135.8% in April–July (monthly mean), of the daily N and P requirement for primary production, respectively. The large amounts of nutrients regenerated and released by N. scintillans excretion can increase the N and P concentrations in ambient seawater, especially in the upper layer in spring–summer, and consequently affect phytoplankton (diatom) abundance. These may exacerbate eutrophication in these waters by a mutually supportive relationship between phytoplankton and N. scintillans: bottom-up control (phytoplankton–N. scintillans) and nutrient supply by N. scintillans to phytoplankton through excretion.
