Quantifying measures to limit wind driven resuspension of sediments for improvement of the ecological quality in some shallow Dutch lakes

Although phosphorus loadings are considered the main pressure for most shallow lakes, wind-driven resuspension can cause additional problems for these aquatic ecosystems. We quantified the potential effectiveness of measures to reduce the contribution of resuspended sediments, resulting from wind action, to the overall light attenuation for three comparable shallow peat lakes with poor ecological status in the Netherlands: Loosdrecht, Nieuwkoop, and Reeuwijk (1.8–2.7 m depth, 1.6–2.5 km fetch). These measures are: 1. wave reducing barriers, 2. water level fluctuations, 3. capping of the sediment with sand, and 4. combinations of above. Critical shear stress of the sediments for resuspension (Vcrit), size distribution, and optical properties of the suspended material were quantified in the field (June 2009) and laboratory. Water quality monitoring data (2002–2009) showed that light attenuation by organic suspended matter in all lakes is high. Spatial modeling of the impact of these measures showed that in Lake Loosdrecht limiting wave action can have significant effects (reductions from 6% exceedance to 2% exceedance of Vcrit), whereas in Lake Nieuwkoop and Lake Reeuwijk this is less effective. The depth distribution and shape of Lake Nieuwkoop and Lake Reeuwijk limit the role of wind-driven resuspension in the total suspended matter concentration. Although the lakes are similar in general appearance (origin, size, and depth range) measures suitable to improve their ecological status differ. This calls for care when defining the programme of measures to improve the ecological status of a specific lake based on experience from other lakes.

Restoration of salt marshes in the Netherlands

The conquest of land from the sea has been a long tradition in the Netherlands. When salt marshes were high enough, they were embanked when it was economically feasible, and transformed into intensively exploited agricultural land. This resulted in the transformation of halophytic communities to glycophytic communities. Often as an alternative, a low levee, a summerdike was built, which greatly reduced the flooding frequency of the landward summerpolder, hence creating a sedimentation deficit therein. Such summerpolders now cover 1200 ha in the Netherlands, 2100 ha in NW-Germany and small areas in England. Due to continuous embankments, the present salt-marsh area is relatively small with respect to the tidal basins. Discussions have been started how to increase the salt-marsh area. Two options will be discussed, firstly de-embankment of summerpolders and maintenance of the protective seawall, secondly increase of the effects of saline seepage behind the seawall by top soil removal. Both options include the restoration of salt-marsh communities (target communities) in intensively agriculturally exploited sites that have been salt marshes before. From the few examples abroad and experiments it is discussed (1) to which extent the sedimentation deficit in summerpolders could be compensated for, (2) if the soil seed bank is likely to contribute to re-establishment of salt-marsh communities, (3) if the dispersal of propagules of halophytic plants will be possible by hydrochory when the summerdike is breached, (4) to what extent is dispersal by endozoochory through waterfowl important in case re-establishment in a saline seepage area behind the seawall without open connection to the sea is envisaged. Two case studies of de-embanked summerpolders in the Netherlands revealed that the sedimentation deficit can be counteracted by rapid sedimentation, provided enough transport is possible from the foreshore. Dispersal by incoming tidal water from the nearby salt-marsh source area into the target area is possible for many salt-marsh plant species. The rate of success seems to depend on the relative position of source area and target area. A case study in a saline seepage area after top soil removal in the Netherlands, showed that the number of viable seeds dispersed by droppings from waterfowl is limited. Hence the possibilities for restoration of inland halophytic plant communities seem much lower than after de-embankment of summerpolders