Production-consumption budget in an estuarine bay: How anoxia is prevented in a forced system

Althought the generation of anoxic situations is a well known fact in estuarine bays, in the southem embayment of the Ebro delta (NW Mediterranean), the Alfacs Bay, oxygen depletion is a rare event. In this paper we examine the processes preventing anoxia in the system through a consumption/production budget. From calculations based upon annual mean values, we conclude that the bay is slightly heterotrophic, that is, that total consumption exceeds autochtonous production by only 10%. But heterogeneity in both space and time results in a high variance around the mean used in our budget. We have examined the main components of this variance (stratification of the water column, horizontal heterogeneity, phytoplankton production in pulses, etc.) and some of them have been incorporated in a simple box-flux model. From this model, we conclude that only the deep water layer is strongly heterotrophic and that its sources of oxygen are vertical mixing and renovation (50%), diapycnal eddy diffusion (25%) and local production (25%).

The generation of anoxia must be driven by the co-occurrence of events associated with oxygen depletion and/or limitation of oxygen transport (i.e., calm weather period after a phytoplankton pulse; lack of estuarine circulation during a long calm weather period, etc.), what we call the covariance between processes. Such covariance is in general low, and in consequence anoxia must be considered a low probability event. Comparing Alfacs Bay with other estuaries in the framework of the proposed conceptual model, three features emerge that may be relevant in order to prevent anoxia: low organic loading (produced plus imported); active estuarine circulation forced by high wind frequency; degradation of the organic matter mainly by benthic consumers.