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4d. Primary production

Chlorophyll a concentrations (fig. 49) are used as a proxy for algal biomass. However, this does not necessarily correspond to actual production (Underwood & Kromkamp 1999). Therefore, to see for difference in magnitude of primary production, also estimates of gross primary production based on continuous oxygen series were performed (fig. 51). Highest chlorophyll a concentrations are observed in the Scheldt estuary, next in the Elbe estuary (fig. 49, Attachment 2 ). Comparing gross primary production estimates between Scheldt and Elbe, production in the Elbe is slightly higher than in the Scheldt, contrarily to findings from the chlorophyll a concentrations. However, this rather seems to be an overestimation according to limitations of the method. The method is normally developed for macro-tidal well-mixed estuaries. The Elbe is in fact at the border between a meso- and macro-tidal estuary and is rather partially mixed (Kerner 2007). In the Weser no chlorophyll a measurements were done. Based on estimates for gross primary production from continuous oxygen series, it appears that production in the Weser could be highest of all estuaries studied. Hence, in the future it would be nice to also have an idea about the distribution of chlorophyll a in both time and space in the Weser. In the Humber, chlorophyll values measured were very low. Based on estimates from continuous dissolved oxygen series, in the Humber actually no primary production could be detected. Both findings correspond to earlier results from the multivariate analyses, in which the Humber was distinguished by its absence of seasonal chlorophyll dynamics. In addition the Humber could be distinguished from the other estuaries by its high suspended matter concentrations. Hence, primary production in the pelagic of the Humber estuary clearly seems to be impeded by light limitation.

In summary, primary production is absent in the Humber estuary and higher in the Scheldt than in the Elbe estuary. For both latter estuaries, highest concentrations of chlorophyll a are observed in the freshwater zone. Concentrations in the Scheldt freshwater zone are twice as high as those in the Elbe freshwater zone, despite the fact that in the most upstream part of the Elbe freshwater zone most dissolved oxygen oversaturation events occurred compared to the other estuaries. So, which parameters are limiting primary production in the Elbe and Scheldt and why is production twice as high in the Scheldt than in the Elbe? Light climate, nutrients and residence time are examined as possible factors limiting algal growth.

In the Scheldt, euphotic depth in the freshwater decreases from 0.8 m to 0.4 m, while in the Elbe it decreases from 1.1 m to about 0.25 m. Despite euphotic depth decreasing more in the Elbe, these differences do not seem large enough to explain differences in primary production. However, when also bathymetrical depth (as proxy for mixing depth in macro-tidal estuaries) is considered, it is clear that in the Elbe from TIDE kilometer 40 the increase in depth causes a decrease in the euphotic depth-mixing depth ratio, detrimental to algal growth. Hence, light climate does seem to be an important factor contributing to a lower algal growth in the Elbe estuary.

Residence time when summed over the length for each zone, appears to be higher in the freshwater zone of the Elbe than in the freshwater zone of the Scheldt. Even when corrected for distance per zone (in days/km), residence times is higher in the Elbe than in the Scheldt estuary. Hence, residence time does not provide any clear explanation.

Limitation by nutrients is examined by comparing nutrient ratios with the ‘ideal’ Redfield ratio, along the estuarine gradient. Nitrogen is clearly not limited in any of the estuaries studied within this report (fig. 51, fig. 52 & fig. 53). When nitrogen silica and phosphorus silica ratios are considered, it is clear that most limitation for silica occurs in the freshwater zone of both Scheldt and Elbe estuaries. However, in the Elbe estuary this is most pronounced in the most downstream part of the freshwater zone, after the area of increased depth observed in this estuary. Although relative ratios might be limiting according to the Redfield ratio, it are the absolute concentrations that will effectively prevent algal growth. Most dissolved silica concentrations lower than 0.3 mg/l, are clearly observed in the freshwater zone of the Elbe estuary (fig. 32). This might be attributed to sinking of diatoms to the deeper layers within the freshwater part of the Elbe estuary and more limited recycling to the upper layers of the water column. In the Scheldt, basically no absolute dissolved silica limitations are observed.

Since chlorophyll a is mainly a proxy for algal biomass, grazing (by zooplankton) is another factor that might explain the discrepancy of the lower chlorophyll a values in the shallower, most upstream part of the Elbe and the regular observed oversaturation events in the most upstream part of the Elbe estuary indicating primary production. Unfortunately no data were provided on this. However, grazing is also reported to be an important controlling factor in the Elbe by Quiel et al. (2011).

In summary, it can be concluded primary production is limited by light climate in the Humber estuary, and by a combined effect of dissolved silica limitation, light climate and possibly grazing within the Elbe estuary. Primary production in the Scheldt has been considered to be limited by light. However, recently chlorophyll a values seem to increase again in the freshwater zone, indicating another limitation must have played previously. Hypotheses suggest ammonium and oxygen could have had inhibitory effects upon algal growth, when oxygen deficits were more prevalent in the Scheldt estuary (Cox et al. 2009).

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