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4a. Estuarine patterns

In all estuaries studied, chlorinity obviously seems to be the most important structuring element. As chlorinity increases towards the sea, dissolved constituent such as nitrogen species, phosphate and dissolved silica decrease, while dissolved oxygen and pH increase. This could be found from both correlation analyses and multivariate analyses. It appears the mixing effect from sea is dominant in all estuaries. However, this structuring element is of most importance in the Scheldt where it explains 66.5 % of the variation observed (p<0.01). In the Humber and Elbe zonation explains only about 43.6 % and 32.5 % of the variation observed respectively (p<0.01). Furthermore, a correlation of total dissolved inorganic nitrogen with nitrate was found in all estuaries, implying nitrate constitutes the major part of total dissolved inorganic nitrogen. Most estuaries are currently well oxygenated. Hence, this is indeed not very surprisingly (Statham et al. 2011). This might indicate the overall importance of nitrification and mineralization processes over primary production and denitrification processes for all estuaries studied. Another correlation found in most estuaries (Elbe, Weser, Schelde) is that between suspended particulate matter and total phosphorus. Absence of a correlation with phosphate in these estuaries likely indicates an association of suspended particulate matter with organic phosphorus and consequently represents a source of phosphate by mineralization. Even though total phosphorus is not measured in the Humber estuary this correlation could also be existent within this turbid estuary. Although, sink source function of phosphate in literature is rather explained by purely physical adsorption and desorption in function of phosphate, salinity and suspended particulate matter concentration (van Beusekom & Brockmann 1998, van der Zee et al. 2007, Deborde et al. 2007), an association of suspended particulate matter with organic phosphorus could give an explanation for the large source found in the Humber at the border between the meso- and polyhaline zone. Another general aspect to be found is the opposition between winter and summer. Some obvious common winter-summer differences include higher temperatures and salinity in summer, and higher dissolved oxygen and other dissolved constituents in winter. Nevertheless, variation explained by seasonality is lower in general (Scheldt: 18.6 %; Elbe: 29.2 %; Humber: 24.7 %; p<0.01) and seasonal estuarine patterns often differ between estuaries. For all estuaries no significant in between year effect could be found for 2004 to 2006.

In total most variation of the estuarine patterns are explained in the Scheldt, next in the Humber estuary. Thus in the Elbe the estuarine patterns are least explained, mainly due to lack of zonation effects. In fact, where data variability (and thus intensity of ecological processes) is lower in the polyhaline zones of the Scheldt and Humber, this reduced data variability is not observed in the polyhaline zone of the Elbe estuary. This could have several reasons. Baborowski et al. (2011) also found water quality patterns in the Elbe river to be rather explained by seasonal variation (37.5 %). This is likely to be attributed to the much larger and seasonally variable freshwater discharge observed in the Elbe compared to the Scheldt and Humber (645 m3/s on average, fig. 9), continuously disrupting the salinity gradients typically for estuaries. Aside from a sudden deepening of more than 20 m nearby the Hamburg port area in the Elbe estuary (TIDE-km 40), the Elbe still demonstrates a gradient of increasing depth as observed in the other TIDE estuaries. Hence, solely bathymetry does not explain the less conspicuous estuarine patterns observed in the Elbe. Also different patterns in residence time or euphotic depth give no conclusive answer (fig. 10 & fig. 14). It seems that indeed a reduced mixing effect from sea is the main reason for less spatially structured patterns in the Elbe. However, the tidal amplitude has been found largest in the freshwater zone of the Elbe, indicating the tide intrudes up to the freshwater end member. Nonetheless, tidal amplitude is the lowest in the Elbe (3.6 ± 0.04 m), compared to the Scheldt and Humber (5.5 ± 0.1 m and 4.4 ± 0.2 m, resp.) and effects of the sea are likely to be markedly less than in the more narrow and smaller Scheldt and Humber estuaries. Other reasons could be that other processes not included in this multivariate analysis, are more characteristic for the Elbe estuary.

Indeed, besides from general observed estuarine patterns, some characteristic patterns can be distinguished per estuary examined. Where dilution seems to be most characteristic for the Scheldt estuary, mainly physical correlations such as between temperature and dissolved oxygen concentration are to be found in the Humber and Weser estuaries. However, this rather obvious correlation for temperature with oxygen is least pronounced in the Scheldt, reflecting the importance of other more biological oxygen influencing processes. The Scheldt is amongst the estuaries with highest dissolved inorganic nitrogen and organic matter concentrations (van der Zee et al. 2007). When comparing average concentrations (Attachment 2 ), indeed nitrogen, biological oxygen demand and also phosphorus concentrations are observed to be highest within the Scheldt compared to the other TIDE-estuaries. Hence, processes influencing oxygen concentrations can be expected to be most intense within this estuary. Within the Elbe a specific correlation can be found between chlorophyll a, phaeopigments and biological oxygen demand. This might indicate the importance of autochthonous organic matter input (from algae) in the Elbe estuary. This is in agreement with general findings in the BfG contribution in TIDE (Schöl et al. 2012 ). According to Quiel et al. (2011) phytoplankton peaks occur upstream in the Elbe river where they are more susceptible to grazing and are subsequently exported in a degradable form more downstream in function of freshwater discharge (Quiel et al. 2011). Nonetheless, it is not clear why chlorophyll a values are not higher in the Elbe than in the Scheldt. Although freshwater discharge is much higher in the Elbe, residence time is in fact higher than in the Scheldt estuary (fig. 10). Comparing euphotic depth, mixing depth ratios, this could be an explanation (fig. 16). However, more oversaturation of dissolved oxygen is observed in the most upstream part of the Elbe estuary, indicating intense primary production (fig. 44). Thus, it could be that in fact there is more primary production within the Elbe, but biomass is more effectively controlled by grazing. From the decrease in chlorophyll a concentrations observed, most likely algae die off in the deepened part nearby the Hamburg port area, where euphotic depth, mixing depth ratio drops about fivefold (fig. 16). From the multivariate analysis also specific differences in seasonal and spatial patterns per estuary could be discriminated. Most estuarine zones show similar estuarine patterns throughout all seasons, except for the upstream parts of the Scheldt and Humber (freshwater and oligohaline zones). The Scheldt differs from the Elbe in timing of primary production. In the Elbe a peak of primary production can be observed early in spring, while in the Scheldt in autumn. Although when absolute values in chlorophyll a are considered, Scheldt maxima can be rather found in summer than in autumn (fig. 43), corresponding to findings of Arndt et al. (2011). Anyway, the peak of algal bloom is observed earlier in the Elbe estuary. Mostly in the Scheldt, ammonium decreases with increase in temperature and chlorophyll a in autumn and summer, indicating increased primary production and nitrification. Nutrient concentrations are lower in the Elbe. As the largest river drainage basin (148 286 km2, Grabemann & Krause 2001), the Elbe also comprises a much larger volume, both coming from upstream and from tidal exchange, compared to the other estuaries examined (fig. 9 & table 3). Hence, in general nutrients are expected to be more diluted. Furthermore, the effort for water treatment occurred at an earlier stage (80ies) than within the Scheldt (90ies), also contributing to the overall lower nutrient concentrations when both estuaries are compared at present (Soetaert et al. 2006, Schlarbaum et al. 2010). Next, in the Scheldt there are more then twice as many inhabitants per square kilometer observed (Arndt et al. 2011, Quiel et al. 2011). The Humber estuary distinguishes itself by its high suspended matter concentrations in summer and autumn, absence of seasonal chlorophyll (extract) dynamics and low dissolved silica concentrations in summer in the low salinity zones. This is in general agreement with previous literature for the Humber estuary, stating that primary production is to be considered negligible (Jickels et al. 2000) and finding only minor dissolved silica fluxes at the intertidal mudflats (Mortimer et al. 1998). Except for some obvious physical correlations, not many specific estuarine patterns could be found for the Weser. This does not imply a less ecologically relevant estuary, as high dissolved oxygen values observed (fig. 44) and gross primary production values calculated from continuous oxygen data series (fig. 51) in the upstream parts of the Weser estuary suggest. However, more data is needed to reach this conclusion.

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