Downwelling persistence modifies nitrogen sources available for benthic food webs

In highly productive coastal ecosystems, new nitrogen inputs by coastal upwelling can be overridden at times by those from other sources. Determination of these sources is important to understand changes in food web structure and productivity. In this study, the relative importance of upwelling dynamics and continental waters as nitrogen sources for coastal food webs at annual time scales was analyzed in Galicia (NW Spain). Stable nitrogen isotopes of bulk tissue in macroalgae and of amino acids in mussels, as proxies for the preferred source for growth, were compared with a time series of nutrient and oceanographic observations. When upwelling dominated during all seasons, the low isotopic enrichment in organisms was consistent with the use of oceanic nitrate from Eastern North Atlantic Central Waters (ENACW) as the main source of new nitrogen in shelf waters. In contrast, when downwelling dominated in autumn and winter, the large fraction of coastal nitrate unrelated to ENACW and the high isotopic enrichment in organisms suggested the assimilation of additional nitrogen sources. Incomplete nitrate uptake and denitrification in sediments were identified as potential sources of isotopically enriched nitrate in the downwelling-dominated period. Other nitrate sources, such as strong winter mixing or continental inputs related to precipitation appeared to play a minor role because the precipitation regimes and surface nitrate concentrations were similar in both periods. This study pointed out the importance of persistent and intense autumn and winter downwelling for nutrient retention in coastal waters. The proposed mechanisms, while speculative, would require further attention in future studies.