Silicon and nitrogen cycling in the upwelling area off Peru: A dual isotope approach

We present a comparison of the dissolved stable isotope composition of silicate (Si-30(OH)(4)) and nitrate ((15) NO3-) to investigate the biogeochemical processes controlling nutrient cycling in the upwelling area off Peru, where one of the globally largest Oxygen Minimum Zones (OMZs) is located. Besides strong upwelling of nutrient rich waters mainly favoring diatom growth, an anticyclonic eddy influenced the study area. We observe a tight coupling between the silicon (Si) and nitrogen (N) cycles in the study area. Waters on the shelf showed high Si(OH)(4) concentrations accompanied by diminished NO3- concentration as a consequence of intense remineralization, high Si fluxes from the shelf sediments, and N-loss processes such as anammox/denitrification within the OMZ. Correspondingly, the surface waters show low Si-30(OH)(4) values (+2 parts per thousand) due to low Si utilization but relatively high (15) NO3- (+13 parts per thousand) values due to upwelling of waters influenced by N-loss processes. In contrast, as a consequence of the deepening of the thermocline in the eddy center, a pronounced Si(OH)(4) depletion led to the highest Si-30(OH)(4) values (+3.7 parts per thousand) accompanied by high (15) NO3- values (+16 parts per thousand). In the eddy center, high NO3-: Si(OH)(4) ratios favored the growth of non-siliceous organisms (Synechococcus). Our data show that upwelling processes and the presence of eddies play important roles controlling the nutrient cycles and therefore also exert a major influence on the phytoplankton communities in the Peruvian Upwelling. Our findings also show that the combined approach of Si-30(OH)(4) and (15) NO3- can improve our understanding of paleo records as it can help to disentangle utilization and N-loss processes.