Implications of Different Nitrogen Input Sources for Primary Production and Carbon Flux Estimates in Coastal Waters

Abstract

The coastal Gulf of Mexico (GOM) and Coastal Sea off Korea (CSK) both suffer from eutrophication and/or hypoxia, both of which are driven mainly by humans. We compared two different regions with different nitrogen input sources to estimate organic carbon fluxes and predict future carbon fluxes based on our model scenarios. This research focuses on how we take advantage of carbon-nitrogen cycling and ecological consequences to estimate the effects of future nutrient inputs. We tested the Rowe and Chapman (RC02) three-zone hypothesis of hypoxia using two different methods. We found that RC02 applied only in certain seasons and that a major nutrient input source is necessary. We used both nutrient/salinity relationships and a N-mass balance model to identify three different zones, each with different productivity and carbon fluxes. We define the brown zone as having a linear nutrient/salinity relationship, where physical forcing (river flow) dominates over local production, and defined the blue zone as having nutrient (N or Si) concentrations < 1 μM. The green zone, with variable nutrient concentrations, occurs between them. Based on our N-mass balance model results, we could set the potential primary production rate in the brown zone of the GOM and CSK, respectively, as over 2 (GOM) and over 1.5 gC m^-2 day^-1 (CSK). In the green zone, production was between 0.1 to 2 (GOM) and 0.3 to 1.5 gC m^-2 day^-1 (CSK) and in the blue zone less than 0.1and 0.3 gC m^-2 day^-1, respectively. From our results, we have estimated the fluxes of nitrogen via the atmosphere, groundwater, and river to the ocean, based on observational and literature data. The coastal Gulf of Mexico receives nitrogen predominantly from the Mississippi and Atchafalaya Rivers and AN-D is only a minor component in this region. However, in the coastal sea off Korea, either groundwater or atmospheric nitrogen deposition is more important controlling factors of our model results. In the future, we need to consider collecting data on both groundwater and AN-D inputs in investigations of chemical cycling in the coastal ocean