Project part-financed by the European Union (European Regional Development Fund)

The Interreg IVB North Sea Region Programme

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An interestuarine comparison for ecology in TIDE

2b. Physics

Hydrodynamics and morphology are both important physical controlling aspects in primary production and biogeochemical processing.


The exchange of water masses between adjacent open sea and freshwater, driven by tide, wind and precipitation processes, creates salinity gradients and induces transport of organic and inorganic matter (suspended solids, nutrients, silt…) (Duarte et al. 2009). Hydrodynamics are represented by freshwater discharge and residence times.

Freshwater discharge

The different TIDE partners for all four estuaries in TIDE collected freshwater discharge data. Monthly discharge data for the Elbe, Weser and Scheldt were available for the time period studied within the ecology report. For the Humber only yearly discharges were available. For the Scheldt and Humber not only freshwater discharges from the most upstream boundaries, but also those from the main tributaries are considered.

Residence time

Residence times are well descriptors of time and scale of physical transport processes. This allows comparison with time and scale of biological and chemical processes. E.g. when residence time is smaller than algal cell doubling time, algal blooms will be inhibited (Duarte et al 2009). Residence times were calculated according to the freshwater fractal method as described in (Vandenbruwaene et al. 2012 ).


Morphology is described by intertidal areas, tidal amplitude, channel width, wet cross section, averaged depth (calculated as cross section divided by channel width) and bathymetrical depth.

Intertidal and subtidal areas

Intertidal and subtidal areas define the scale at which different processes can occur and can serve as an explanatory factor for water quality and ecology. To allow a minimum comparison between estuaries, intertidal and subtidal areas were calculated based on high and low water levels according to the method described in (Vandenbruwaene et al. 2012 ).


Tidal amplitude, channel width, wet cross-section, averaged depth and bathymetrical depth can be additional explanatory factors for water quality and ecological processes. E.g. differences in tidal amplitude can play an important role in tidal pumping and localization of the turbidity maximum in estuaries (Uncles et al. 2006). Tidal amplitude, channel width, wet cross section and bathymetrical depth are all calculated from the cubature as described in the (Vandenbruwaene et al. 2012 ) for (at least) mean low and high tide. Within this report the average for mean high and low tides are each time represented.

The averaged depth is calculated as the wet cross section divided by the channel width. By consequence lower depth corresponds to a more shallow estuarine area with an increased contact surface between the pelagic and benthic compartment (fig. 5).

If only the absolute bathymetrical depth was considered, there would be no discrimination between estuarine areas with similar bathymetrical depth, one being narrower than the other compartment. By using averaged depths, less deep areas indeed imply increased biological and chemical processes (e.g. more primary production, mineralization, nitrification, denitrification…); potentially more re-suspension of sediments (which can influence the light climate and primary production) and more space for shallow water habitats.

Nevertheless, bathymetrical depths are also given, because they represent mixing depth in a well-mixed system, such as the macro-tidal estuaries examined in this report. Bathymetrical depth is the thalweg depth relative to the low water level at the mouthgeo as defined in (Vandenbruwaene et al. 2012 ) and therefore could reach positive levels at the upstream boundary as defined in TIDE (Geerts et al. 2012 ).

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