Nitrogen cycling in an estuarine upwelling system, the Ría de Arousa (NW Spain). I. Short-time-scale patterns of hydrodynamic and biogeochemical circulation

Abstract

Nitrogen species distributions in the Ría de Arousa, NW Spain, during the upwelling season are strongly affected by the influence of shelf wind stress on water circulation. Intermittent northerly winds forced large volumes of nitrate-rich Eastern North Atlantic Central Water into the ría with a fortnightly periodicity and led to net community production maxima supported by external nitrate. Net community production in the central segment of the estuary was high (average 251 mg m-2 d-1 of nitrogen). However, only 20% of the total nutrient input was net taken up due to rapid water exchange. An intensive culture of hanging mussels favoured dissolved organic nitrogen formation (58% of net community production) and particulate organic nitrogen sedimentation (21% of net community production, or 50% of net particulate organic nitrogen formation). Consequently, more than 75% of the net dissolved+particulate organic nitrogen production was exported from the central segment, with a dissolved/particulate ratio of ~3. In addition, 40% of phytoplankton gross primary production was internally recycled by pelagic+benthic ammonification in the central segment, mainly during prolonged upwelling relaxations. Low rates of particulate organic nitrogen sedimentation and low levels of conversion to dissolved organic nitrogen were observed during the development of a red tide assemblage. This appears to be field evidence of limited filtration by mussels in the presence of these organisms. A strong reversal in the residual circulation occurred in mid-October, during the transition from northerly to southerly winds. Ammonium released from the estuarine sediments, at a high average net rate of 162 mg N m-2 d-1, was exported towards the shelf.