The Weddell-Scotia Confluence during summer: An ecosystem where ammonium is worth gold

Abstract

The Uptake of nitrogen sources in the Weddell-Scotia Confluence shows different nutritional conditions than other oceanic and neritic sectors. In this ecosystem, ammonium plays a leading role, either as a product of biological activities or as a trigger for them. During the summer of 2012, aboard the R/V Puerto Deseado, the area was visited and a series of stations were sampled. The objective of this work was to study the use of nitrogen sources and complement with associated parameters to link them to biological production. Experiments were carried out on board to estimate the uptake of nitrogen sources, and temperature and salinity were measured in situ. In addition, nitrate (NO3−), ammonium (NH4+), phosphate (PO4 − 3), silicic acid (Si(OH)4) and chlorophyll-a (Chlo-a) concentration was measured in the CESIMAR laboratory. The spatial interpretation was complemented with satellite images of sea surface temperature and chlorophyll-a. The main results were: high allochthonous macronutrients (NO3−: 13.36–25.53 µM; PO4 − 3: 1.51–1.90 µM; Si(OH)4: 34.5–62.8 µM) and low Chlo-a (0.44–0.76 mg.m− 3) concentration; nitrate uptake apparently limited by factors unrelated to its availability; a strong preference for ammonium uptake as a nitrogen source (negatively correlated; r = -0.99); low primary productivity estimated as total nitrogen uptake (TNU: 8.9-14.23 nM.d− 1); and its consequent low export rate of carbon. Due to the importance of this ecosystem for relevant biological species such as krill, and the important role it plays in the formation of deep waters, it is essential to keep a monitoring over time that allow estimating its evolution.