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4b. Different sink and source functions for nutrients

Dissolved inorganic nitrogen

In all estuaries dynamics (both gain and loss) decrease towards the sea. The polyhaline zone is much larger in cross-section and width and is usually marked by an increase in euphotic depth. Nevertheless, the intensity of ecological processes as calculated per kilometer is highest in the upstream parts of the estuary. This can be related to the higher nutrient concentrations in the freshwater zone. Often the upstream part is also shallower. The Elbe appears to be the largest source for dissolved inorganic nitrogen, while the Scheldt can be considered a sink. Furthermore, impact of nitrogen delivery on estuarine functioning of the pelagic is ought to be higher in the Elbe, than nitrogen removal has upon the Scheldt estuary, since concentrations are lower within the Elbe estuary. Also the Humber appears to be a source for dissolved nitrogen species. Differences in source and sink functions of an estuary can be due to (1) differences in input (direct/indirect via organic matter degradation) and (2) differences in estuarine functioning (transformation processes) along the estuarine gradient.

Taking into account freshwater discharge, the input (1) of dissolved inorganic nitrogen is observed to be about four times higher in the Elbe estuary than in the Scheldt estuary. Also the largest part of organic nitrogen input is observed in the Elbe estuary, likely related to decaying algal biomass. (fig. 34). Different input can be attributed to differences in freshwater discharge. The source sink functions for nitrogen can in part be explained by these differences in nitrogen input. As less dissolved inorganic nitrogen is removed by biological activity in winter, inputs at the boundaries are generally higher than in summer explaining the general observation of higher dissolved inorganic nitrogen gain in winter. Furthermore, high nitrogen concentrations in winter are often related to the fact that nitrogen sources are mostly diffuse. Hence, higher freshwater discharge in winter comes along with higher nitrogen input (Sanders et al. 1997, Statham et al. 2011). Input of dissolved nitrogen and freshwater discharge in the Humber estuary are similar to the Scheldt estuary. However, the Humber estuary is rather a limited source than it is a sink for inorganic nitrogen like observed in the Scheldt estuary. By consequence, differences for source and sink function between the Scheldt and Humber will be mainly explained by differences in estuarine processing.

Estuarine processing (2) of dissolved nitrogen within the estuary is mainly driven by organic matter mineralization, nitrification, denitrification and primary production. To identify underlying processes in dissolved inorganic nitrogen dynamics first ammonium and nitrate concentrations (a) are discussed. Next, gain and loss of dissolved inorganic nitrogen species is compared between estuaries (b) and finally, gain and loss of dissolved inorganic nitrogen species is discussed per estuary (c), related to hydrodynamics and morphology (see also ‘3.1.1 Hydrodynamics’ & ‘3.1.2 Morphology’ ).

Both nitrate and ammonium concentrations (a) are observed to be highest in the Scheldt estuary, while clearly lower in the Elbe estuary. The difference is most pronounced for ammonium concentrations, with ammonium concentrations being more than three times larger in the Scheldt than in the Elbe. Compared to the Elbe, the Scheldt is also characterized by very high organic matter concentrations (as approximated by biological oxygen demand, fig. 46). This is likely to be a source for ammonium input and explanation for the higher concentrations observed in the Scheldt estuary (Attachment 2 ). Compared to the TIDE estuaries, concentrations for nitrate and ammonium in the Humber are intermediate and lowest respectively. Also ammonium input in the Humber is observed to be lowest (fig. 35). Both the Trent and Ouse have been reported a sink for ammonium (Sanders et al. 1997), hence explaining this low estuary input. The Rupel on the contrary, is a source for ammonium to the Scheldt (input concentrations are about ten times higher than those observed for the Trent in the Humber, fig. 35). Concentrations in the Weser are in the same order of magnitude than those in the Elbe.

In both the Scheldt and Elbe gain and loss (b) of nitrogen mainly occur in the upstream part of the estuary, while in the Humber nitrogen dynamics are more spread along the estuarine gradient. Most nitrate is gained in the Elbe freshwater part of the estuary, while most nitrate is lost in the mesohaline zone of the Humber estuary. The Elbe is also characterized by the highest input from upstream. Furthermore an intense zone of nitrification can be observed in the deeper part of the freshwater zone, likely related to algal die off. The latter loss in the Humber seems contradictory to the surfer plot where actually a major zone of nitrate gain can be observed in the oligo- and mesohaline zone. This is because only two sampling stations were averaged in the mesohaline zone and the second sampling station lies at the border with the polyhaline zone, where there is a sudden loss of nitrate observed, masking the increase in the first sampling station within the mesohaline zone. Thus, it is more accurate to state that nitrate loss is highest in the Humber polyhaline zone. This corresponds to what has been shown by Mortimer et al. (1998). Intertidal and subtidal areas generally increase towards the polyhaline zone so that in fact in total nitrate is mostly removed in the polyhaline zone. In the Scheldt nitrate gain and loss dynamics seem to alternate quickly along the estuarine gradient. This results in a limited gain of nitrate, mainly in the freshwater zone. Hence, in the Scheldt estuary nitrate dynamics appear to be more variable. Most ammonium is lost in the oligohaline zone of the Scheldt estuary, nearby the Rupel tributary. Previously, ammonium production was observed in the most upstream parts of the Scheldt estuary. However, from 2000 to 2002 ammonium removal was found almost everywhere along the estuarine gradient (Soetaert et al. 2006). With increasing oxygen levels most ammonium is almost immediately nitrified. Nitrate is subsequently found to be removed, nowadays mainly near the Rupel area, indicating a large area for denitrification (fig. 26). However, with increasing oxygen values also denitrification rates are lowered (Soetaert et al. 2006) and nitrification can be expected to gain in importance in the near future. In the Elbe gain and loss for ammonium are visible in the freshwater zone, but dynamics are limited in general. This is likely to be attributed to the high seasonal dynamics and much lower concentrations in the Elbe. In the Humber estuary most ammonium is gained in the oligo- and mesohaline zone. Peaks in ammonium concentration are found in the turbidity maximum zone, in agreement with earlier observations (Sanders et al. 1997) and could possibly be related to increased mineralization of particulate nitrogen, like observed previously in the Elbe estuary (Schlarbaum et al. 2010). However, considering input concentrations and the whole estuarine gradient, the impact of ammonium removal in the Scheldt is larger than ammonium gain in the Humber estuary. In general, considering the input ammonium concentrations, more ammonium is lost in summer (fig. 29), which can be related to more intense nitrification when temperatures are higher.

Overall, the Elbe (c) can be considered a major exporter of dissolved inorganic nitrogen, mostly attributed to nitrate dynamics. Previously the Elbe has been reported a nitrate sink, and it is only recently that the Elbe changed to a source for nitrate (Dähnke et al. 2008). This could be attributed to loss of sediments because of filling up of the shallow water zones and recent deepening of the Elbe freshwater zone near Hamburg (Kerner 2007, Dähnke et al. 2008). Gain of ammonium and nitrate seems to follow each other successively. Coinciding with peaks in biological oxygen demand and dissolved oxygen loss, it is likely that ammonium initially gained from organic matter mineralization is immediately further nitrified, explaining these consecutive peaks of ammonium and nitrate gain. Although, not an immediate link can be observed with chlorophyll a, the correlation of chlorophyll a with phaepigments and biological oxygen demand suggests chlorophyll a is in a degraded form. The peak in nitrate gain, together with the peak of dissolved oxygen loss both occur near TIDE kilometer 40. In this part of the freshwater zone of the Elbe, the estuary deepens with more than 10 meter. Alexander et al. (2000) found nitrogen to be less retained in deeper channels. Residence time increases, but euphotic depth decreases (fig. 14, fig. 15). By consequence algae might die off. Furthermore, the Elbe is characterized by the largest amount of organic nitrogen input. Accumulation of allochthonous and autochthonous organic matter within this deepened area (with increased residence time) most likely promotes intense nitrification. The study by BfG in TIDE (Schol et al. 2012 ) developed a model that simulated the effect of a lowered algal input at the boundary. This model demonstrated that a significant decrement of algae could enhance oxygen concentrations again along the estuarine gradient (see also Quiel et al. 2011). Thus, after mineralization, ammonium is gained, which is subsequently nitrified. Further in the estuary, ammonium is lost, while nitrate is still gained, probably since ammonium input has stopped while nitrification processes proceed further. Additional nitrate gain could also originate from input from of the Lühe river mouth, although discharge is likely to be too small to lead to a significant contribution. Near Pagensander Nebenelbe (80 TIDE km), a small peak of ammonium gain can be observed together with a peak of nitrate loss. This could indicate a small zone of denitrification, which could be attributed to the shallow water zones in this area. However, in general nitrification seems to be the major process. Together with high nitrogen input this makes that the Elbe is a nitrogen source. However, it has to be noted that total dissolved inorganic nitrogen concentrations in the Elbe are the lowest of all estuaries studied in this report.

The Scheldt (c) is observed to be an overall sink for dissolved inorganic nitrogen. Most nitrogen loss occurs near the mouth of the Rupel tributary. Within this area peaks of ammonium gain and nitrate loss coincide with peaks of biological oxygen demand and dissolved oxygen loss. Hence, ammonium gained from organic matter mineralization is further nitrified to nitrate, which is probably immediately further denitrified, because of the low dissolved oxygen concentrations resulting from previous intense organic matter mineralization (fig. 36). Near the border between Belgium and the Netherlands two consecutive peaks of nitrate loss and gain can be observed, coinciding with a peak of dissolved oxygen gain. This could be attributed to the spiraling effect of nutrients and delivery of oxygen from the nearby ‘Land van Saeftinghe’ (Van Damme et al. 2009), first indicating an increased area of denitrification, followed by a peak of nitrification due to increased oxygen input. This intertidal area is however not represented in the graphs for intertidal and subtidal area as calculated in Vandenbruwaene et al. (2012) . Chlorophyll a concentrations do not seem to explain any of the patterns observed in nitrogen dynamics. However, chlorophyll a concentrations seem to be higher in the Scheldt than those observed in the Elbe estuary. This can in part be related to differences in euphotic depth – mixing depth ratios (Underwood & Kromkamp 1999; see further, ‘4.4 Primary production’ ). However, in the most upstream part of the Elbe dissolved oxygen oversaturation is frequently noted and euphotic depth, mixing depth ratio is favorable. Thus, also grazing could be an important controlling factor explaining the lower chlorophyll a values in the Elbe (which is rather a proxy for biomass than for effective primary production). Although, dissolved inorganic nitrogen concentrations are highest in the Scheldt estuary compared to the other TIDE estuaries, lower nitrogen input and denitrification processes combined cause the Scheldt estuary to be a sink for total dissolved inorganic nitrogen. The sink function for dissolved inorganic nitrogen in the Scheldt estuary did not come very surprisingly, as this corresponds well to earlier results (Van Damme et al. 2005, Soetaert et al. 2006). However, with recent improvements of the oxygen state (fig. 43) and increasing importance of nitrification over denitrification near the Rupel, the Scheldt estuary might evolve towards a nitrogen source in the nearby future.

The Humber (c) was found an overall small source for dissolved inorganic nitrogen. Most gain of nitrate and ammonium and dissolved oxygen occurs at the border of the meso- and polyhaline zone. Nitrification appears to be more important than denitrification. As found earlier (in ‘4.1 Estuarine patterns’ ) seasonal dynamics of chlorophyll concentrations are absent in the Humber estuary, attributed to very high concentrations of suspended particulate matter, thus low euphotic depths (see further, ‘4.4 Primary production’ ) (Jickels et al. 2000). Furthermore, chlorophyll concentrations are very low and therefore not further discussed as explanatory factor in nitrogen dynamics. Despite similar nitrogen input in the Humber estuary and Scheldt estuary, the Humber is rather a source than a sink for dissolved inorganic nitrogen. This could be linked to a much smaller input of organic matter and higher dissolved oxygen concentrations as compared to the other TIDE estuaries (Attachment 2 ). Unfortunately no biological oxygen demand data were provided. However, the small nitrogen source function is not in correspondence to findings by Sanders et al. (1997) and Jickels et al. (2000), who found the Humber estuary to be a minor sink (4 % removal according to input). This small difference could well be in the range of errors for the method used. We averaged the gain and loss for every station sampled. However, a better approach is to integrate these expected gain and loss dynamics along the different sampling stations, since the number of sampling stations can strongly influence the outcome (Sanders et al. 1997). By consequence, it is better to interpret the overall surfer plot than using the averaged discrete fluxes per sampling station calculated. Indeed, when we consider the surfer plot for nitrate and ammonium in the Humber (fig. 26 & fig. 28), it could be that removal is slightly more intense than found from overall averaging (more intense red patches for both nitrate and ammonium). Hence, it can be concluded that either source or sink function for dissolved inorganic nitrogen of the Humber estuary is rather limited for the pelagic. Differences found with literature are likely attributed to the variability of these limited nitrogen dynamics and the error of the method.

In the Weser (c) not enough data was provided to find the sink or source function according to the conservative mixing plot methods used in this report (see table 1 & Attachment 1 ). Only limited studies have been performed in the Weser, mainly focusing on sediment dynamics (e.g. Müller et al. 1990, Grabemann & Krause 2001). However, some study reported also high oxygen consuming processes within the upper reaches (nitrification can reach up 50 to 75% of the oxygen consumption observed) (in Cox et al. 2009: Schurchard et al. 1993).

In summary it can be concluded that in general chlorophyll a as indicator for nitrogen uptake by algae contributed only in a minor extent to nitrogen dynamics (in the Elbe estuary likely indirect as organic matter input). Organic matter mineralization, nitrification and more limited denitrification in function of organic matter and nitrogen input appear to be the main regulating processes for an estuary becoming a sink or source, with oxygen concentration as main indicator variable. However, when oxygen production caused by primary production increases it might also become a more important process to consider in nitrogen dynamics. Hence, in order to follow sink or source function for nitrogen in an estuary it is important to consider the most important processes influencing oxygen concentrations. Within both Elbe and Scheldt biological oxygen demand is an important indicator. Unfortunately, in the Weser and Humber this was not measured. Next, it is important to consider the hydro-morphological aspects such as residence time and freshwater discharge, bathymetrical depth, input from tributaries and upstream boundaries and the contribution of intertidal and subtidal area next to the estuary, and their implication oxygen concentrations along the estuarine gradient.

Phosphate

Overall, considering the whole estuarine gradient and all seasons, the Humber estuary can be considered a source, while the Scheldt can be considered a sink for phosphate (fig. 38). In the Humber most phosphate is gained along the oligo- and mesohaline stretch, coinciding with the turbidity maximum zone. In the Scheldt, most phosphate is lost in the oligohaline zone. In the Elbe gain and loss processes alternate each other in the freshwater part of the estuary, however concentrations are about four times lower and dynamics are rather limited. In the Weser phosphate concentrations are in the same order of magnitude than within the Elbe estuary. Considering the initial input concentrations, the Humber can be considered as a source of phosphate more important than the Scheldt can be considered a sink for phosphate for estuarine functioning. Comparing between seasons, in both the Elbe and Scheldt more phosphate is gained in summer than in winter. However in the Humber quite the opposite can be observed (more is lost in summer). Differences between winter and summer concentrations are less clear than for nitrogen. Most difference is observed in the Humber estuary, with clearly higher concentrations in summer. Within the other estuaries differences are very limited, but also for these estuaries concentrations are observed to be higher in summer. This can be attributed to the fact that phosphate sources are mostly point sources, and concentrations increase when discharge is lower in summer (Sanders et al. 1997). This corresponds to what can be expected for phosphate dynamics in the Scheldt and Elbe, with more gain in summer. On the contrary, in the Humber estuary an actual increase in loss in summer can be observed and this cannot be explained by the temporal pattern in phosphate concentrations. Differences in sink source functions between estuaries can be explained by differences in (1) phosphorus input (both inorganic and organic) from the upper boundary and main tributaries and (2) estuarine processing.

Considering differences in phosphorus input (1), it is clear that when solely input of phosphate is compared, most input is observed in the Humber estuary (fig. 39). However, when also the organic phosphorus fraction is considered, by comparing total phosphorus input, the Elbe is observed to have a larger phosphorus input. Nonetheless, total phosphorus is not measured within the Humber estuary and hence, it could be that the overall input is still largest within the Humber indeed. Furthermore, the Humber estuary over which the phosphate load is distributed, is about three times smaller than the Elbe estuary. Least input is observed in the Scheldt estuary, with slightly higher inputs near the Rupel tributary. Also in the Humber, input is clearly higher near the tributaries (Aire, Don and Trent). Hence, differences in input do in part explain differences in source and sink functions of the Humber and Scheldt respectively. Input from tributaries also explains the area of gain within the Humber, with increased phosphorus input from the Aire, Don and Trent along the oligo- and mesohaline stretch. In the Scheldt however, increased input near the Rupel is not reflected in the phosphate dynamics, since most phosphate is actually lost within the oligohaline zone of the Scheldt estuary. Considering seasonal differences, input is higher in winter than in summer except for the Humber, in which input is higher in summer. For none of the estuaries this corresponds to the observed seasonal patterns of gain and loss in winter and summer. In the Scheldt and Elbe more is gained in summer, while in the Humber more is lost in summer. Although differences in phosphorus input appear to explain major difference in sink and source function between estuaries, it does not explain differences at a local scale within the estuary. From the difference in input and phosphate dynamics between winter and summer for Scheldt and Elbe as opposed to the Humber, it seems that estuarine dynamics in phosphate, both upstream and within the estuary, are regulated by different ecological processes. In the next paragraph estuarine differences in estuarine processing will be further discussed.

Estuarine processing (2) of phosphate within the estuary is mainly driven by suspended particulate matter dynamics, primary production and burial. To identify underlying processes in estuarine processing, gain and loss of phosphate is discussed, taking into account phosphate concentrations and hydro-geomorphology (see also ‘3.1.1 Hydrodynamics’ & ‘3.1.2 Morphology’) . Both in the Elbe and Scheldt phosphate is alternately gained and lost, with dynamics abruptly decreasing from the mid-mesohaline zone towards the sea. However, in the Elbe concentrations are very low and phosphate dynamics are limited (fig. 40).

In the Elbe no clear correlation could be found with neither dissolved oxygen dynamics, biological oxygen demand, suspended matter concentration, nor chlorophyll a concentrations. Then again, the Elbe is only a minor source for phosphate, dynamics are very limited and concentrations of phosphate are very low compared to the other estuaries examined. The low phosphate concentrations in the Elbe were in agreement with earlier studies (van Beusekom & Brockmann 1998). However, van Beusekom & Brockmann (1998) found the Elbe to be a sink, while in this report the Elbe is found to be a minor source for phosphate. Anyway, phosphate adsorption to suspended particulate matter concentrations is only considered a temporal sink (Jickels et al. 2000). When oxygen concentrations are low and iron-oxy-hydroxides are reduced, phosphate is again released. Furthermore, as the suspended matter concentration reaches the sea, other anions compete for adsorption and phosphate is desorbed again (cf. bell-shaped theory in Sanders et al. 1997, van Beusekom & Brockmann 1998, van der Zee et al. 2007 & Deborde et al. 2007).

In the Scheldt dynamics could be related to primary production (if you consider chlorophyll a here as a good proxy). Peaks of phosphate loss seem to decrease together with chlorophyll a concentrations from the freshwater towards the sea. It is observed that the Scheldt estuary shifted from a heterotrophic hyper-eutrophied system to a more autotrophic, eutrophied system (Cox et al. 2009). Hence, it could be that nowadays primary production is indeed the main regulating process in phosphate dynamics. Another explanation could be the gradual desorption of phosphate from suspended particulate matter, according to the bell-shaped theory. However, no clear correlation with suspended particulate matter is found (fig. 40). A small peak in phosphate gain near the Rupel could be associated with organic matter input and subsequent mineralization, as indicated by a peak in biological oxygen demand. Nevertheless, overall a loss of phosphate can be observed in the freshwater and oligohaline zone of the Scheldt estuary. A peak of gain followed again by phosphate loss near the border between Belgium and the Netherlands could be respectively attributed to the effects of release of phosphate in the high salinity zone and the spiraling effect with the ‘Land van Saefthinge’ serving as a major storage zone (uptake, burial). It has to be noted that the phosphate concentrations are highest in the Scheldt. However, the sink function for phosphate in the Scheldt is not in agreement with previous findings by Soetaert et al. (2006) who stated that the Scheldt estuary evolved from an overall sink to source for phosphate since 1995. At that time, this was also found to be rather unexpected. Oxygen concentrations increased and by consequence adsorptive removal by suspended particulate matter was expected to increase as well (Soetaert et al. 2006). Thus, contrary to earlier findings, it could be that the Scheldt did rather evolve to a more efficient sink and that corresponding to findings by Deborde et al. (2007) desorption is not increased by higher phosphate concentrations.

In the Humber phosphate dynamics can be related to a peak of oxygen gain and high suspended matter concentrations at the limit between the meso- and polyhaline zone. Again, dynamics in the Humber estuary seem to be mainly regulated by suspended matter dynamics. Here it could indicate that phosphate highly associated with suspended particulate matter being released as it comes in the high salinity zone. This is rather unexpected, since according to the bell-shaped theory phosphate would be removed by adsorption in the freshwater part of the estuary and desorbed again in the low salinity region (cf. fig.4, in the oligo- to mesohaline zone; van Beusekom & Brockmann 1998, van der Zee et al. 2007 & Deborde et al. 2007). Indeed, phosphate dynamics observed in this report are not in agreement with previous results. In previous studies the Humber is actually considered a major sink associated with the adsorption to suspended matter particles in the turbidity maximum zone, as also observed in the very turbid Gironde estuary (Sanders et al. 1997, Jickels et al. 2000, Deborde et al. 2007). This could be due to the combined effect of increased adsorptive removal efficiency in the Humber, even in the lower salinity reaches, because of the very high suspended particulate matter concentrations and of a recent shift of the turbidity maximum zone towards the sea (fig. 13) giving rise to a major area of phosphate desorption (Sanders et al. 1997, Jickels et al. 2000, Deborde et al. 2007). It could also be consequence of less sampling stations within the low salinity reach in the Humber estuary, masking true dynamics. Chlorophyll concentrations are not considered in the Humber estuary, because of absence of seasonal dynamics and very low concentrations compared to the other two estuaries.

In summary, the Humber is a major source, both because of very high phosphorus input from upstream and likely because of desorption from the very high suspended matter concentrations in the high salinity reach. The Scheldt can be considered rather a sink for phosphate, likely related to increased oxygen state and primary production in the upper reaches of the estuary. Although, considering concentrations observed within the estuaries, the Scheldt being a sink affects the estuary less than the Humber does being a source. The Elbe is only a minor source and concentrations within this estuary are very low. Based on similarities of phosphate concentrations found in the Weser with concentrations observed in the Elbe, and input of phosphorus from the upper boundary being in the same range as input of phosphorus in the Scheldt, the Weser will also be characterized by low and rather variable and more unpredictable phosphate dynamics.

Dissolved silica

Dissolved silica concentrations are highest in the Scheldt estuary, next within the Elbe estuary (Attachment 2 ). In both the Humber and Weser dissolved silica is only rarely measured in time and space (Attachment 1 ). In the Weser very low concentrations are observed. However, measurements in the Weser were restricted to the polyhaline zone, in which dissolved silica concentrations are expected to be lower in general (cf. the other TIDE estuaries). In the Elbe dissolved silica concentrations were often limiting in the freshwater zone. In the Scheldt freshwater zone, dissolved silica limitation only occurred once in 2004. Next, dissolved silica limitations are regularly found in the polyhaline zone of both the Elbe and Scheldt estuary. However, in the Scheldt polyhaline zone limitations are less pronounced and even disappeared after 2007. In general, concentrations are higher in winter than in summer and despite limitations, dissolved silica concentrations are usually higher in the freshwater zone. Also primary production, and thus silica uptake by diatoms, is mainly observed in the upper part of the estuary and will be most intense during the warmer seasons (Carbonnel et al. 2009). Dissolved silica dynamics could only be considered in the Elbe and Scheldt estuary, due to sampling frequencies. For the Humber this is presumably not an issue, since seasonal dynamics in chlorophyll concentrations were absent anyway (see earlier analyses and discussions). However, in the Weser it would have been interesting to have a more clear view upon the spatial and temporal distribution of dissolved silica concentrations and dynamics.

Comparing dynamics between the Elbe and Scheldt, it shows that the Elbe is an overall source while the Scheldt is an overall sink for dissolved silica. In the Elbe, taking into account the input concentration within the estuary, most is gained along the oligo- to mesohaline stretch. In the Scheldt overall averaging seems to give an overall loss in the oligohaline zone. Most limitations for dissolved silica (< 0.3 mg/l) were observed in the Elbe estuary. Nevertheless, averaged over the whole gradient, dissolved silica removal (loss) seems to be larger in the Scheldt estuary. Considering gain and removal efficiency per zone, dissolved silica gain in the Elbe appears to have a larger impact upon estuarine functioning than loss has upon estuarine functioning in the Scheldt estuary. Differences in sink and source function once again can be explained by (1) differences in dissolved silica input and (2) estuarine processing.

Input (1) of dissolved silica is highest in the Elbe estuary, in part explaining source function of the Elbe estuary. In general input is largest in winter and lowest in summer corresponding with silica gain in winter and loss in summer. In the Scheldt an increased input of dissolved silica can be observed near the Rupel tributary. Although overall loss was observed when calculated as average in the oligohaline zone, green patches of gain were indeed observed near the Rupel (fig. 33). Also at the tidal limit (kilometer 0) green patches of gain are observed in the Scheldt, reflecting gain by input from the upper boundary (Bovenschelde). Carbonnel et al. (2009) found the Scheldt to be a source between Gent (TIDE km 0) and Dendermonde (TIDE km 39) and an overall sink between Dendermonde and Hemiksem (TIDE km 60). On the contrary, our results suggest a sink in the upstream part and source near the Rupel tributary. This is likely corresponding to loss within the upper reaches related to primary production, and gain near the Rupel related to tributary input (see further). Nonetheless, considering the whole estuarine gradient, the Scheldt estuary is observed to be a sink for dissolved silica like also observed by Carbonnel et al. (2009). Also in the Elbe green patches appear to be located near the river mouths of the Lühe and Öste, however contrary to the Scheldt not near the tidal limit. In general it can be concluded that in both estuaries input from boundaries and tributaries are most contributing to gain of dissolved silica observed in the estuaries. Although there is more input from the upper boundary in the Elbe, this is not reflected at the tidal limit like it is in the Scheldt estuary. This could be due to differences in estuarine processing.

Estuarine processing (2) of dissolved silica is mainly regulated by diatom dissolved silica uptake and biogenic silica (diatom frustule) dissolution. The dissolved silica input at the upper boundary of the Elbe could by consequence be masked by an increase in primary production in the upper part of the estuary. However, since chlorophyll a concentrations are actually lower in the Elbe than in the Scheldt, this could not be the reason why input from the upper boundary is outbalanced. It might be because in the Elbe dissolved silica concentrations are less frequently measured in winter and by consequence the input from upstream which is mostly found in winter, is attenuated. In general there seems to be no clear relation of dissolved silica dynamics with neither chlorophyll a, nor suspended matter concentrations. It might be elucidating to also consider biogenic silica concentrations to interpret these dynamics. Unfortunately, this is not measured in either of the estuaries for the time period considered in TIDE.

In summary, it is clear that the Elbe is an overall source and the Scheldt is an overall sink for dissolved silica. From dynamics in dissolved silica it is shown that this difference in sink and source function is mainly regulated by differences in dissolved silica input from upper boundaries and tributaries. Furthermore, an area of dissolved silica loss in the upper part of the Scheldt estuary can be associated with diatom uptake. Diatom uptake was not very clear within the Elbe estuary, not even within the most upper more shallow part of the estuary. It seems diatom uptake in the Elbe is more limited, while absolute dissolved silica limitation is more quickly reached. Hence, also difference in estuarine processing, suggesting more algal activity within the Scheldt estuary, further promotes the sink function of the Scheldt versus the source function of the Elbe.

Nutrient ratios and expected effects of sink and source function

When considering six yearly average nutrient ratios (N/P, N/Si, P/Si), most ratios generally approach the Redfield ratio heading towards the sea, except for the phosphate dissolved silica ratio in the Elbe estuary (fig. 52, 53 & 54).

It is clear nitrogen is never limiting in any of the estuaries examined in TIDE. However, when compared between estuaries nitrogen is relatively most in excess in the Elbe estuary and least in excess, approaching the Redfield ratio in the Scheldt estuary. Nonetheless, absolute concentrations are highest in the Scheldt estuary for both dissolved inorganic nitrogen and phosphate. Comparing silica limitation relative to nitrogen, dissolved silica is more limiting in the Scheldt than in the Elbe estuary. However, absolute dissolved silica limitations are almost never occurring in the Scheldt estuary, implicating other factors are limiting primary production here (see ‘4.4 Primary production’) . In the Elbe however, dissolved silica limitation is frequently reached, although chlorophyll a values are clearly lower than in the Scheldt. Because of the positioning of the zones of dissolved silica limitation near the freshwater deeper part of the Elbe estuary, these limitations might be related to sinking of diatom frustules and subsequent limited recycling from the deeper parts in the estuary (the Elbe is known to be partially mixed).

When silica limitation relative to phosphorus is compared between estuaries, it can be found that in general silica is 10 times more limiting than in the Elbe estuary. However, dissolved silica is never limiting in winter. In the Elbe mesohaline and most upstream freshwater zone phosphorus can become even limiting over dissolved silica concentration.

For the Humber these ratios are not considered relevant since no significant biological activity could be found (see ‘4.1 Estuarine patterns’ and ‘4.4 Primary production’ ). In the Weser not enough data was provided for this discussion, although high oxygen values in the upper reaches and high gross primary production estimates (see further, ‘4.4 Primary production’ ) indicate that it might be important to examine in this estuary.

Table 7 Source (+) and sink (-) function and six-yearly average estuarine concentrations (between brackets, in mg/l) for total dissolved inorganic nitrogen (TDIN), phosphate (PO4) and dissolved silica (DSi). In the Weser dissolved silica concentration is not given, since it is only measured in the polyhaline zone and therefore not representative as average for the whole estuarine gradient. In the Humber there is no significant biological activity and therefore not relevant (n.a. = not applicable).

	Elbe	Humber	Scheldt	Weser
TDIN	++ (2.96)	+ (3.8)	(-) (4.21)	? (3.33)
PO4	+ (0.06)	++ (0.18)	(-) (0.26)	? (0.07)
DSi	++ (2.26)	n.a. (2.09)	-. (3.43)	n.a.

Taking into account findings about the patterns observed for nitrogen, phosphorus and silica concentrations and the sink and source functions of the TIDE estuaries (table), the following trends might be expected:

• Excess in nitrogen relative to phosphorus will increase further in both the Elbe and Humber, while in the Scheldt it might still approach the Redfield ratio for a while. Nonetheless, it is expected to evolve to nitrogen excess in the Scheldt as well since the estuary is becoming more oxygenated.
• Patterns of silica limitation relative to nitrogen are expected to stay similar for the Elbe and Scheldt.
• Patterns of silica limitation relative to phosphorus are expected to decrease slightly for the Elbe and stay similar for the Scheldt estuary.

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