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5d. Which variables limit primary production in the TIDE estuaries?
From chlorophyll a concentrations, gross primary production estimates and multivariate analysis combined, it can be concluded that primary production in the Humber is impeded by light limitation, due to the very high suspended particluate matter concentrations. In the Weser estimates for gross primary production from continuous oxygen data series suggest that primary production is highest within this estuary. However, in the Weser too few data is available to come to further conclusions. Chlorophyll a values seem to show higher primary production in the Scheldt estuary. However, in the most upstream part of the Elbe estuary dissolved oxygen oversaturation (>100%) is frequently observed. When light climate, residence time and absolute dissolved silica limitation are considered, it could be that primary production is higher in the Scheldt because of more beneficial light climate and less absolute dissolved silica limitation. Another explanation could be that primary production is effectively higher within the Elbe estuary, but not found by difference in chlorophyll a concentration. Indeed, chlorophyll a is a proxy for algal biomass and not for effective primary production. Unfortunately no data for grazing were available for any of the estuaries examined. In the Scheldt it is generally accepted that primary production is further limited by light. However, recently observed increase in chlorophyll a values suggest other limitations might have been important previously.