Exploring Seasonal and Annual Nitrogen Transfer and Ecological Response in River-Coast Continuums Based on Spatially Explicit Models

Perturbed nutrient balances in watersheds may eventually impact the marine ecosystem, but this river-coast coupling is poorly understood. Monthly dissolved inorganic nitrogen (DIN) fluxes from seven major rivers in China were calculated by a global river nutrient model (IMAGE-GNM), and then used in a regional ocean model system (ROMS) to explore changes in surface chlorophyll a (Chla) in the plume area (salinity <33) of the Taiwan Strait (TWS) in 2001–2010. Model results showed that river N input increased surface Chla by a factor of 2.1–2.7, revealing a clear eutrophic response. Without river N input, there was only one Chla peak in fall driven by current upwelling N. In contrast, sufficient river N supply and optimum temperature (above 20°C) likely caused another Chla peak (spring bloom) in the northern TWS (NTWS). The difference in the timing of spring blooms (Chla maxima) between the southern TWS (STWS) and the NTWS (April and May, respectively) may be explained by faster growth of phytoplankton at higher temperatures. Diagnostic analysis suggested that DIN was the main factor controlling interannual variation of Chla in the STWS, but only in the wet season in the NTWS. In the NTWS, reduced Chla in winter was mainly due to mixing by the strong northeast monsoon and lower temperatures. This study implies that the STWS is more sensitive to further increases in riverine N export and highlights the importance of fluvial N inputs in phytoplankton dynamics and the unique phenological features in the TWS.