S. Van Damme

Spatial and temporal patterns of water quality along the estuarine salinity gradient of the Scheldt estuary (Belgium and The Netherlands): results of an integrated monitoring approach

This paper presents the results of 7 years of integrated monitoring along the Scheldt estuary. The combination of two datasets resulted in a full description of the estuaries water quality parameters from the mouth to the upper boundary, including an extended fresh water tidal part. A synthesis of the monitoring results and all relevant ecological knowledge on the Scheldt allowed to identify opportunities to optimize its management. The results show that the effect of discharge on salinity has a distinct maximum in the polyhaline to mesohaline transition area. Oxygen conditions, nitrogen removal and phytoplankton regulation can be enhanced and improved through management measures within the estuary. To lower carbon and phosphorous loads however measures should be taken within the catchment. To restore most of its ecological functions the estuary needs more space. Optimal locations to address specific functions can be derived from the monitoring results.

Tuning the tide: creating ecological conditions for tidal marsh development in a flood control area

The Schelde estuary, characterised as a turbid, polluted and eutrophic system, has nowadays reached a turning point in the restoration of its water quality. During the past century, human activities have reduced the intertidal areas, essential in the estuarine ecosystem for nutrient cycling and the self-cleaning capacity. Today, in combination with a master plan to protect the population from storm surges, an opportunity rises to restore areas with a tidal influence. One specific option of combining safety and ecology is the creation of flood control areas (FCA) under the influence of a controlled reduced tide (CRT). These specific areas will differ in many ways from fully tidal areas. However, these areas can fulfill important ecological functions with effects on aeration, nitrification, denitrification, sedimentation and primary production in the estuary. Opportunities for ecological development within a CRT have been investigated for a specific case. The ecology within a CRT showed to be very case specific, depending e.g. on the morphology of the area, the sluice design and the local water quality. Depending on the sluice design, water quality can be improved and sedimentation can be influenced. Possible measures to design a CRT with a rich habitat variation are discussed.

δ15N and δ13C dynamics of suspended organic matter in freshwater and brackish waters of the Scheldt estuary

Abstract Suspended particulate organic matter was sampled monthly between June 1999 and April 2000 in the Scheldt river and estuary to investigate the seasonal and spatial patterns of δ 13 C and δ 15 N signatures. δ 15 N of suspended matter showed large seasonal variation. Minimum values ranged from −0.5‰ in the freshwater zone (spring situation) to +2.3‰ in the mesohaline zone (winter situation). Maximum values (summer situation) ranged from +8.8‰ in the freshwater zone to +12.9‰ in the mesohaline zone. δ 13 C showed less seasonal variation and ranged overall from −31.1‰ in the freshwater zone to −23.7‰ in the mesohaline zone. During the growth season, decrease of δ 13 C and increase of δ 15 N of suspended matter were due to local phytoplanktonic and bacterial biomass. There is strong evidence that the 15 N enrichment of suspended matter during the growth season reflects the 15 N enrichment of the ambient NH 4 + pool induced by nitrification and NH 4 + uptake. Zooplankton in the mesohaline section of the river was consistently enriched in 15 N relative to suspended matter but followed its seasonal trend. During summer and autumn the isotopic offset between zooplankton and the suspended particulate organic matter was consistent with a pattern of selective feeding on phytoplankton. During summer, δ 15 N of zooplankton reached a value as high as +25.5‰, the highest value observed during this study. During spring, present-day δ 15 N of suspended matter in the oligohaline and mesohaline section increased compared to the 1970s, probably because today nitrification, which enriches the NH 4 + pool in 15 N, starts earlier in the season. For summer, the discrepancy between present-day suspended matter δ 15 N values and those observed in the 1970s was even larger, especially in the oligohaline and freshwater reaches, probably as a result of improved O 2 conditions now favouring nitrification. Likewise, the present decreased input of 15 N-depleted sewage will enhance 15 N enrichment of suspended matter during the growth season.

Wetlands in the Tidal Freshwater Zone

Tidal freshwater wetlands occur in the upper part of estuaries in Europe and North America (and likely elsewhere) and experience tides of up to several meters in amplitude twice a day. They occur at the interface between the brackish zone in the estuary and the river; and where brackish and fresh water mix is an area of maximum suspended matter (i.e., the maximum turbidity zone). The tidal freshwater zone within the estuary plays an important role in overall patterns of nutrient cycling for the whole estuary and the pattern appears to differ in the brackish and saline sections. Although tidal freshwater wetlands do not include many endemic or restricted species, they are characterized by high species and habitat diversity. There is distinct zonation in flora and fauna species, responding to the relationship between surface elevation and tidal amplitude. The dominant species are different between Europe and North America, but the structure of the system and the life strategy of the species are fully comparable. The tidal freshwater wetlands in Europe and North America also have a common history of being highly influenced by human activities, resulting in altered hydrology, losses in wetland area, and high levels of sediment and nutrient input on both sides of the Atlantic Ocean. In recent years, restoration and preservation activities have also been initiated on both sides of the Atlantic and there is hope that tidal freshwater wetlands will increasingly become important elements of estuarine systems that provide many free ecological services to man and nature.

Zooplankton in the Schelde estuary, Belgium and the Netherlands: long-term trends in spring populations

A compilation of available data in between 1967 and 2002 on spring zooplankton abundance was made for the brackish and the freshwater zone of the Schelde estuary. The general picture is a significant increase of 1–2 orders of magnitude in abundance for Rotifera, Copepoda and Branchiopoda (mainly Cladocera) in the freshwater zone, while zooplankton abundance in the brackishwater zone remained more constant. Possible natural and management related causes for this increase in zooplankton abundance are briefly discussed.