Nitrogen dynamics in a Western Boundary Upwelling System (Cabo Frio, Brazil) based on δ15N-nitrate and δ15N of sinking particle signals

Abstract:

To improve the efficiency of upwelling to control nitrogen dynamic in the ocean, better understanding of the occurring processes is necessary. This research explores δ¹⁵N of nitrate and sinking particles on a western boundary upwelling System (Cabo Frio, Brazil). The Continental Shelf of southeastern Brazil is dominated by the oligotrophic Brazil Current, whose instabilities promote the coastal upwelling of South Atlantic Central Water (SACW), and consequently increases of primary productivity. The coastal upwelling system plays an important role in the nitrogen dynamics on the Cabo Frio Upwelling System (CFUS). However, the interactions between biological induced processes, including biological N-fixation and nitrate inputs from upwelled waters in CFUS still have not been well explored. Then, this study aims clarify N-dynamics on CFUS based on a cross-shelf approach. δ¹⁵N-nitrate was characterized for each water masses present on the shelf (South Atlantic Central Water, Tropical Water and Coastal Water) and associated with physicochemical parameters (T/S, nutrients), as well as the δ¹⁵N of sinking particles at different depths (from surface to the bottom water). Samples were collected in a time interval of 1 month during ~4 years (2011 to 2014). Cross-shelf gradients of nitrogen species concentration (ammonium + nitrite + nitrate) and stable isotopes were observed. The δ¹⁵N of nitrate and sinking particles were interpreted according to the prevailing processes of the N-transformations. Considering the region as N-limited (N:P < 16), processes as biological N-fixation seems to be dominant on oligotrophic Tropical Waters. Coastal upwelled SACW showed δ¹⁵N-nitrate signature within the global average of deep ocean (5-6‰) characterizing the inner and mid-shelf conditions, where the input of new nitrate from upwelling is rapidly used by organisms in the euphotic zone without any fractionation. On the other hands, the dominance of N-limited Tropical Waters on the outer shelf provide a δ¹⁵N-nitrate and δ¹⁵N-sinking particles signals (-2.0 to 3.0‰) lower than the global average of deep ocean range indicating the importance of N-biological fixation as a dominant source of nitrogen.