Seasonal sources of carbon to the Brazilian upwelling system

Abstract Environmental heterogeneity on coastal areas is an outcome of several hydrodynamic forces. Particularly, wind-driven upwelling is usually associated with alternating periods of water mixing and stratification. In addition, the effects of near shore oceanographic conditions may vary with coastline topography and anthropogenic impacts. Herein we evaluated the seasonal differences in the hydrodynamics of the Cabo Frio upwelling system (Brazil) in function of its local bay. Surface and deep water from 5 contrasting coastal areas were sampled 13 times during the spring/summer and in the fall/winter periods to use two-way Analysis of Variance comparing the measured variables in function of season and depth. Northeastern wind was predominant during both seasons but it was more intense during the spring/summer period when water temperature was colder and concentration of nutrients peaked. Southwestern wind was more common during the fall/winter period and was associated with cold fronts that decreased water salinity inner the bay. Consequently, the concentrations of nutrients, chlorophylls, prokaryotic secondary production (PSP) as well as fluorescent and non-fluorescent particles were significantly higher during the upwelling season while the concentration of particulate organic matter (POM) was highest during the non-upwelling season. Respectively, mean nitrate concentration varied from 2.2 to 0.9 μM, ammonium from 2.7 to 1.0 μM, chlorophyll a from 2.4 to 1.4 mg m −3 , PSP from 1.8 to 1.0 μgC.L −1  h −1 and POM from 2.6 to 6.4 mg L −1 . Contrasting surface and deep waters, mean nitrate concentration ranged from 0.9 to 2.2 μM, POM from 4.2 to 5.2 mg L −1 and PSP from 1.9 to 0.8 μgC.L −1  h −1 . Three scenarios were identified: water stratification, upwelling and water homogenization. The first two scenarios were more common outside the bay during the upwelling season. When upwelling was intense, deep water temperature in the bay dropped to less than 20 °C resulting in the stratification of water column in shallow sites. Water homogenization was common in the whole system through the non-upwelling season and was more pronounced during cold fronts. Our results evidenced a strong seasonal variation of carbon origin which was mainly associated with phytoplankton during the upwelling season and to POM during the non-upwelling season. The seasonal effect of upwelling and cold fronts on the hydrodynamics of the Cabo Frio upwelling system varied at a small spatial scale in function of site depth.