The tidal freshwater reach of the Weser estuary: Riverine or estuarine?

Abstract

The tidal freshwater reaches of estuaries have received little attention in ecological research although they are often heavily stressed by environmental impacts. We have thus compiled published and previously unpublished data from the Weser Estuary, Germany. Physical, chemical and biological properties are described within the tidal freshwater reach and compared with the upstream (riverine) and downstream (mixohaline) habitats.The tidal freshwater region is different from riverine sites, mainly due to tidally induced physical processes such as prolonged residence time of the water, oscillating water levels and changing current velocities and directions. Distinctions from the mixohaline situation are mainly due to differences in salinity and particulate suspended matter concentration. It is thus possible to define the tidal freshwater reach of the Weser Estuary as a separate biotope with a specific community structure. The phyto-and zooplankton is a mixture of riverine and autochthonous species whereas marine species are almost completely lacking. In the macroinvertebrate component, as well as in the fish community, marine and riverine species are combined with autochthonous species. This is probably typical for the tidal freshwater reaches of many river dominated coastal plain estuaries. We conclude that the tidal freshwater reaches are an important site of physical, chemical and biological processes which may alter riverine input considerably before it reaches the freshwater-seawater interface. For a better understanding of the ecological functioning of estuaries, it is essential to include these areas within estuarine research programmes.

A macro-tidal freshwater ecosystem recovering from hypereutrophication: the Schelde case study

“…Unpublished oxygen uptake experiments with and without nitrification inhibitors on the freshwater Schelde T. J. S. Cox et al: Recovering from hypereutrophication 2937 for the 1998-2003 period, show that on average nitrification in this part of the estuary accounts for 50% of total oxygen consumption, with maxima of 75%. Schuchardt et al (1993) reported similar values in the freshwater reach of the Weser estuary (Germany). Research on the brackish and saline parts of the Schelde estuary confirms that nitrification rates are highest in the most upstream parts, and show a declining long term trend of relative contribution to total oxygen demand (Billen G, 1975;Somville, 1984;Gazeau et al, 2005).…”

228Ra, 226Ra, 224Ra and 223Ra in potential sources and sinks of land-derived material in the German Bight of the North Sea: implications for the use of radium as a tracer

“…To the south of the Sahlenburg tidal flats, the Weser River estuary is associated with a mean discharge of (Schuchardt et al 1993), representing the second-largest freshwater source for the German Bight, after the Elbe River. The outer estuary is characterised by a funnel-like morphology with extensive open tidal flats.…”

“…The mean tidal range exceeds 3.5 m, but water levels are also influenced by strong winds. Due to high current velocities in the estuary (Schuchardt et al 1993), the water is generally well mixed and highly turbid, with a weakly stratified salt wedge. Highly mobile fluid mud layers >1 m in thickness have been observed, particularly at slack water (Schrottke et al 2006).…”

Correspondence between environmental gradients and summer littoral fish assemblages in low salinity reaches of the Chesapeake Bay, USA

“…The tidal freshwater river reach is different from rlverine habitats, mainly due to tidally induced physical processes such as prolonged residence time of the water, oscillating water levels, and reversing current velocities and directions (Schuchardt et al 1993). There appears to be no species specialized for exclusive existence in tidal freshwater reaches of the lower Chesapeake Bay, though the inland silverside is largely constrained to these environs, probably due to the presence of the Atlantic silverside within the estuary (Bengtson 1984).…”