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1d. Differences and similarities between Elbe, Scheldt, Humber and Weser

Estuaries are globally considered among the most productive biomes and are known for the delivery of several ecosystem services (Costanza et al 1997). TIDE focuses on estuaries of the Elbe (Germany), Weser (Germany), Humber (United Kingdom) and Scheldt (Belgium-the Netherlands). All these northern temperate estuaries are faced with similar challenges for economy and ecology. On the one hand large ports need to be further developed, while on the other hand European Natura 2000 Directives have to be implemented. TIDE aims to develop a framework for an integrated management and planning in these estuaries. Ecosystem services can only be optimised when ecological and hydro-geomorphological functioning are well understood. In this report we focus upon ecological functioning. Based upon specific management issues and research questions given by the several TIDE partners (Maris et al. 2011 ) and taking into account data availability for the estuaries examined, the following four questions concerning ecological functioning were decided at the Antwerp TIDE meeting (July 2012):

a) What are the important factors controlling ecosystem functioning within the TIDE estuaries?

Each estuary has it specific abiotic and biotic characteristics defining the framework in which ecosystem services can be delivered. In TIDE we focus upon ecological functioning in the water column (pelagic), because of data availability. Nonetheless, intertidal area constitutes a large part of the ecological functioning, providing habitat for plants and animals. When specific patterns per estuary are revealed, monitoring per estuary can be used more efficiently to further increase knowledge for estuarine specific ecosystem services.

b) How can we avoid oxygen deficiency situations in the TIDE estuaries?

Massive bacterial breakdown of organic matter can lead to wide spread oxygen deficiencies. This can be attributed to boundary input and/or estuarine processing. In TIDE mineralization, nitrification, denitrification and primary production are carefully studied. A dissolved oxygen concentration of 5mg/l is the minimum amount of dissolved oxygen required to sustain a healthy ecological functioning system (Holzhauer et al. 2011). Oxygen deficiencies (<5mg/l) appear to be a specific management issue for the Elbe and Scheldt estuary.

c) How do TIDE estuaries function as a filter for nutrients?

Estuaries can be a very important sink for nutrients. They reduce the loads significantly towards more downstream parts of the estuary and finally the coastal sea. This sink function is based on biogeochemical processing and assimilation by primary production in both the pelagic and benthic compartment. In TIDE we focused upon major dissolved nutrients for nitrogen, phosphorus and silica and examined the (expected) effect upon Redfield ratios and hence, primary production.

d) Which variables limit primary production in the TIDE estuaries?

Primary production and allochthonous organic matter is the basis of the estuarine food chain. Understanding the variables limiting primary production are essential to device management measures. If production is too low, the food chain can be hampered. Excessive production can cause oxygen problems impacting the higher trophic levels. Under nutrient limitation, e.g. silica, harmful algal blooms can cause major problems even for human health. However, high levels of primary production are not necessarily bad, since it might compensate for elevated biological oxygen demand (Maris et al. 2011 ). For the TIDE estuaries primary production was mainly studied for the Elbe, Scheldt and Weser estuaries. In the Humber no significant primary production in the water column could be detected.

Lessons learnt from differences and similarities can help to ensure further ecological functioning and hence, to optimize ecosystem services delivered.

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