Carbon flows in the Westerschelde estuary (The Netherlands) evaluated by means of an ecosystem model (MOSES)

Abstract

The autotrophic production and heterotrophic consumption of organic matter in the Westerschelde, a highly turbid and eutrophic estuary in the Southwest Netherlands is examined by means of a dynamic simulation model . The model describes the ecologically relevant processes in thirteen spatial compartments and adequately fits most observed data.Three autotrophic processes are included in the model . Net pelagic photosynthetic production is relatively low (average 41 gC m -2 yr -1 ) and three spatial compartments near the turbidity maximum zone are respiratory sinks of phytoplankton biomass . According to the model, net phytobenthic primary production is more important than pelagic primary production in the upstream half of the Westerschelde. On the scale of the entire estuary, benthic primary production amounts to about 60% of pelagic primary production . Water-column nitrification, which is very important in the nitrogen cycle, is most pronounced near the turbidity zone where it accounts for the major autotrophic fixation of carbon (up to 27 g C m -2 yr -' ) . Viewed on the scale of the total estuary, however, the process is not very important .Less than 20% of total organic carbon input to the estuary is primary produced, the remainder is imported from waste discharges and from the river .The degree of heterotrophy of the Westerschelde estuary proved to be one of the highest yet reported . On average 380 g carbon per square metre is net lost per year (range 200-1200 gC m -2 yr -1 ) . The yearly community respiration (bacterial mineralization, respiration of higher trophic levels and sedimentation) is 4 to 35 times (estuarine mean of 6) higher than the net production . This degree of heterotrophy is highest near the turbidity maximum and generally decreases from the freshwater to the seaward boundary. About 75% of all carbon losses can be ascribed to pelagic heterotrophic processes ; the sediment is only locally important .Mineralisation rates are highest in the turbidity region, but as only a fraction of total carbon resides here, less than 20% of all organic carbon is lost in this part of the estuary . This result is in contradiction with a previous budget of the estuary, based on data of the early seventies, where more than 80% of all carbon was estimated to be lost in the turbidity zone . Part of this discrepancy is probably caused by changes that have occurred in the estuary since that time .Due to the high heterotrophic activity, nearly all imported and in situ produced carbon is lost in the estuary itself and the Westerschelde is an insignificant source of organic matter to the coastal zone .The model estuary acts as a trap for reactive organic matter, both from the land, from the sea or in situ produced . Internal cycling, mainly in the water column, results in the removal of most of the carbon while the more refractory part is exported to the sea . 247

Spatial and temporal patterns of water quality along the estuarine salinity gradient of the Scheldt estuary (Belgium and The Netherlands): results of an integrated monitoring approach

“…Several carbon balances have been published for the Scheldt estuary in the past (Wollast, 1978;Soetaert & Herman, 1995a;Frankignoulle et al, 1996). They all show that the major part of the carbon input in the estuary does not reach the sea and that most of the carbon load is processed within the estuary itself.…”

“…They all show that the major part of the carbon input in the estuary does not reach the sea and that most of the carbon load is processed within the estuary itself. Only the most refractory fraction reaches the coastal waters (Soetaert & Herman, 1995a). Bacterial production in the Scheldt estuary, especially in the brackish and freshwater part, showed highest values amongst other European estuaries .…”

“…Bacterial respiration consumes a major amount of the available oxygen. As the main source of oxygenation in the estuary is aeration (Soetaert & Herman, 1995a), creation of areas with high surface/depth ratios is advised in the zone where oxygen concentrations are minimal.…”

Carbon and nitrogen cycling in intertidal sediments near Doel, Scheldt Estuary

“…Similar calculations for carbon indicate that intertidal sediments remove 12500 ton C yr 1 by burial and 17100 ton C yr -1 by remineralization to carbon dioxide and methane . Intertidal sediments therefore represent a net sink of about 29700 ton C yr -1 , which is about 30% of the total estimated estuarine retention (98000 ton C yr-1 ; Soetaert & Herman, 1995b) . These very simple calculations provide …”

Estuarine Dissolved Organic Carbon Flux From Space: With Application to Chesapeake and Delaware Bays

“…Linked hydrodynamic-biogeochemical mechanistic models used to estimate estuary-ocean exchange include explicit representation of the key processes that affect carbon in estuaries, such as advection, gas exchange, photosynthesis, and respiration. These models range in hydrodynamic complexity from 1-D, tidally averaged, advection-diffusion models (Hofmann et al, 2008;Laruelle et al, 2017;Soetaert & Herman, 1995;Volta et al, 2016) to 3-D tidally resolving models (Cerco & Cole, 1993;Feng et al, 2015;Kemp et al, 1997). Biogeochemical complexity can be measured by the number of state variables, which ranges from six (Vanderborght et al, 2007) to 20 (Cerco & Cole, 1993;Soetaert & Herman, 1995).…”