Controls on Temporal and Spatial Variations of Nutrients and Metabolic States in a River Associated Coastal Sea in Taiwan

English Abstract:

This study focuses on the effects of river delivered nutrients and organic carbon (OC) on temporal and spatial variations of nutrients and OC conditions, metabolic states and air-sea CO₂ fluxes in the tropical Kaoping coastal sea off southwestern Taiwan. River discharge and OC and nutrient exports are three to 12 times higher in the wet season than in the dry season. Thus, river exports affect spatial and temporal distributions of nutrients, dissolved and particulate organic carbon (DOC and POC), which may determine the metabolic state of the Kaoping coastal sea. The spatial variability of depth-integrated gross primary production (IGPP) ranged from 844 to 16230 mg C m^-2 d^-1 in summer and winter seasons. The monthly variability of IGPP ranged from 1389 to 8919 mg C m^-2 d^-1 along the Kaoping Canyon. The IGPP was apparently regulated by temperature and the availability of nutrients and OC. The depth-integrated dark community respiration (IDCR), attributed largely to bacterial respiration (69%-78%), ranged from 861 to 12418 mg C m^-2 d^-1 in the study area, and from 919 to 5848 mg C m^-2 d^-1 in monthly variation along the canyon. Strong positive correlations exist between GPP, nutrients and chlorophyll a (Chl-a) and between DCR and DOC (POC), but strong negative correlations exist between GPP (DCR) and salinity. The autotrophic state (IGPP/ICR > 1) prevails in most stations in summer, but the heterotrophic state (IGPP/ICR < 1) occurs in all but a station near the river mouth in winter. However, the air-sea CO₂ fluxes show a year-round CO₂ sink along the Kaoping Canyon, indicating that the physical focing other than metabolic states may be also important in determining the source and sink of atmospheric CO₂ in this coastal sea.