5b. How do TIDE estuaries function as a filter for nutrients?
Dissolved inorganic nitrogen:
The Elbe can be considered a major source, while the Scheldt can be considered a minor sink. This sink or source function is defined by both input and estuarine processing. The input is highest in the Elbe, while lowest in the Scheldt. It is only since recent the Elbe evaluated to a source of dissolved inorganic nitrogen (Dähnke et al. 2008). This is thought to be related to loss of shallow water zones and/or recent deepening (Kerner 2007). Dissolved inorganic nitrogen removal decreased in the Scheldt estuary. This is believed to be associated with increased oxygen concentrations and decreased ammonium input (Van Damme et al. 2005, Soetaert et al. 2006). Hence, finally the Scheldt could also evolve to a source of dissolved inorganic nitrogen. Nevertheless, compared to the Elbe the Scheldt is still a sink today. Ammonium and nitrate concentrations, as indirect and direct substrate for denitrification respectively, are both higher in the Scheldt than in the Elbe estuary. The Humber can be considered a source in which intermediate dissolved inorganic nitrogen concentrations are observed. While in the Scheldt and Elbe nitrogen dynamics are mainly concentrated in the upstream part, nitrogen dynamics in the the Humber are found along the oligo- and mesohaline stretch. In the Elbe and Scheldt nitrogen dynamics are mainly regulated by organic matter dynamics (as indicated by biological oxygen demand), morphology and dissolved inorganic nitrogen input. In the Humber more particularly, the interaction with suspended particulate matter is of utmost importance. Unfortunately in the Humber biological oxygen demand as proxy for organic matter was not measured. For the Weser insufficient data were provided along the estuarine gradient to see any patterns and find the eventual sink or source function.
The Humber can be considered a source, while the Scheldt can be considered a sink for phosphate. The effect of phosphate delivery is higher in the Humber estuary than removal is in the Scheldt estuary, since concentrations are about two times lower in the Humber. In the Elbe very low phosphate concentrations and limited phosphate dynamics are observed. Phosphate concentrations in the Weser are in the same range of the Elbe. Unfortunately, not enough data were available to calculate gain and loss along the estuarine gradient. Difference in phosphate dynamics can be explained by differences in input and estuarine processing. Input of phosphate in the Humber is larger than in the Scheldt. However, input in winter and summer do not explain different source or sink function in neither the Humber, nor the Scheldt estuary. Hence, a closer look at estuarine processing is required. The observation of a phosphate sink is not in agreement with earlier findings of the Scheldt evolving to a phosphate source since 1995 (Soetaert et al. 2006). However, at that time this change was also found to be rather unexpected. With increasing oxygen concentrations, also adsorption of phosphate to suspended particulate matter is expected to increase. Thus, it could be that the estuary is rather a phosphate sink and that high phosphate concentrations do not increase desorption. Or it could be that primary production is nowadays a more regulating factor in phosphate dynamics, as the estuary evolved from a heterotrophic, hyper-eutrophied to a more autotrophic, eutrophied system (Cox et al. 2009). Phosphate dynamics in the Humber are again coupled to suspended matter dynamics. However, rather than a sink because of phosphate adsorption in the turbidity maximum, here a major source could be detected. This could be attributed to a downstream shift of the turbidity maximum zone. When the phosphate-enriched suspended particulate matter arrives in the high salinity zones, phosphate is released again because of competition with stronger anions, such as sulfate. The effect of initial phosphate loss in the upstream part of the estuary could have been overlooked, because of less sampling stations in this area. Or it could be that the particulate suspended matter associated with organic phosphorus is a source of phosphate by mineralization.
The Elbe is a source, while the Scheldt is a sink for dissolved silica. The concentrations are slightly higher in the Scheldt estuary. Most absolute limitations (DSI < 0.3 mg/l, Holzhauer et al. 2011) are observed in the freshwater zone of the Elbe estuary. Concentrations are only limitedly measured in the Humber and Weser estuaries. Therefore, only dynamics in the Elbe and Scheldt could be considered. For the Humber this is likely to be not important, since dissolved silica is mainly biologically regulated and primary production appeared to be insignificant in this estuary. In the Elbe most is gained along the oligo- to mesohaline stretch, suggesting large biogenic silica dissolution and/or tributary input. In the Scheldt estuary many patches of loss are observed in the freshwater zone, while patches of gain can be observed near upper boundary and the mouth of the Rupel tributary. Dissolved silica input is higher in the Elbe than in the Scheldt. Gain is mostly observed near the tidal limit and tributaries, except at the boundary for the Elbe estuary. Likely this difference can be attributed to different sampling campaigns. In the Elbe dissolved silica is less frequently measured in winter, masking the input from upstream and thus gain in calculation of dissolved silica dynamics. No clear relation could be found with chlorophyll or suspended particulate matter. In conclusion, dynamics are possibly most affected by boundary and tributary input, rather than by differences in estuarine processing.
Effect for nutrient ratios:
nitrogen is never limiting in any of the estuaries examined. Taking into account different sink and source functions found, nitrogen excess relative to phosphorus and silica will further increase in both the Elbe and Humber estuaries. In the Scheldt nitrogen availability relative to phosphorus and silica stays more or less similar, approaching the Redfield ratio. In the Humber silica ratios are not relevant, since no significant biological activity was found. Also in the Weser silica ratios are not considered, since only limited measurements in the polyhaline zone are available. Comparing the Scheldt and Elbe, silica is relatively more limiting in the Scheldt estuary. Phosphorus limitation can be found in the most upstream and in the mesohaline part of the Elbe estuary. However, absolute dissolved silica limitation (<0.3 mg/l) mostly occurs in the Elbe freshwater zone. Taking into account sink and source functions, patterns for relative silica and phosphorus limitation do not change in any of the estuaries examined.
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