Water masses seasonality and meteorological patterns drive the biogeochemical processes of a subtropical and urbanized watershed-bay-shelf continuum.

Understanding the different scales of temporal variability is crucial to improve the knowledge of the biogeochemical processes in the land-ocean interface. In this study, we evaluated the role of continental runoff and intrusion of oceanic water masses in the trophic state of the Bay of Santa Catarina Island (BSCI) over the last three decades (1993-2019) by using multiple biogeochemical and eutrophication assessment tools. The sub-watersheds of BSCI showed high concentrations of nutrients, fecal coliform and chlorophyll-a, directly correlated to the number of inhabitants. Worst-case scenarios were found in summer and fall seasons due to sewage inputs caused by mass tourism and the inefficiency or even absence of treatment systems, boosted by strong rainfall. The intrusion of the South Atlantic Central Water and the Plata Plume Water into the BSCI favored autotrophy in the summer and heterotrophy in the winter, coupled with low and high residence time, respectively. El Niño events enhanced rainfall and continental runoff, exporting elevated nutrients and phytoplankton biomass loads from the eutrophic rivers to the continental shelf. The pattern reverses during La Niña, when chlorophyll and nutrient peaks were detected inside the bay. Eutrophication evaluation indicated that the trophic state oscillated from moderate to high and that these conditions tend to remain the same in future scenarios due to the moderate residence time of the water, anthropogenic pressures, periodic algal blooms and the intrusion of nutrient-rich oceanic water masses. Management actions, such as the improvement of the wastewater treatment system and wetlands restoration, are needed in order to mitigate eutrophication and the loss of ecosystem services and functions.