De Levende Natuur nummer 3 van 2013 (English summary)

Towards a sustainable management of the Scheldt estuary

T. Maris, L. Coen, W. Dauwe, W. Mertens, E. Van den Bergh & P. Meire

The Scheldt estuary, still having the full salinity gradient, from salt over brackish to freshwater habitats, is facing enormous ecological pressure. Major human induced morphological changes have altered the tidal wave. In the past, no integrated management existed: measures like land reclamation, dredging, flood protection or habitat restoration were taken independently, without looking at the entire functioning of the system. Now, an integrated vision has been worked out, with ecosystem functioning as a starting point. This new approach has been elaborated in the Actualized Sigmaplan for the Zeeschelde.

Water-level fluctuations determine the occurrence of macrophytes in the main channel and permanently connected side-arms along the river Rhine in The Netherlands

G. van Geest & S. Teurlincx

Over the past 60 years, there has been a strong decline in the presence and abundance of aquatic vegetation in the main channel (and permanent connected side-arms) of the river Rhine in The Netherlands. The goal of this study was to examine if this decrease could be explained by changes in water level regime. In a first step, the tolerance limits of submerged and aquatic vegetation to water depth and amplitude of water level fluctuations were determined. Subsequently, a GIS-model was constructed for the river Rhine which calculated the suitability for submerged and floating-leaved vegetation. The results of the GIS-model were coupled to the abundance of aquatic vegetation. The stability of the habitat during subsequent years was the most important factor for explaining the abundance of aquatic vegetation in the main channel. High abundance of aquatic vegetation was only recorded, when the tolerance limits for macrophyte growth were not exceeded during 8 years out of 10 subsequent years. The results indicate that - during the last decades - the water level during the growing season in the river Rhine has become less stable, and may be a good explanation for the strong decline of aquatic vegetation in the main channel and associated side-arms that are permanently connected to the river. 

Developments of the aquatic macro invertebrates in the Dutch rivers and opportunities for ecological restoration

A.G. Klink, M.M. Schoor, H.D. van Rheede & P.P. Duijn

Historical developments and paleo-ecological studies indicate that aquatic macrofauna changed significantly in the rivers over the past 300 years. The diversity has strongly diminished and groups like mayflies, stoneflies, caddisflies and elmid beetles are almost disappeared. Many species can recolonize from remnant populations upstream. Also the food and biotopes have changed completely. In 1700, 67 % of the insects was living on submerged trees. In the current rivers trees are absent and 77 % of the river fauna lives on the stones armouring the shores. After a strong improvement of water quality in the 70's some species returned, but many of them disappeared again after 1992 when the Danube-Main-Rhine Canal was put into operation and the ‘killer shrimp’ and other invasive exotics mass invaded the rivers. Two discussed nature development projects appear to lag behind the expectations. The ecological restoration causes are:

The rivers are fully adapted for shipping and wave action causes a strong disturbance.

Biotope diversity is low due to the lack of trees.

In secondary channels and in dammed areas of the rivers the flow is too low and changes too fast

Invasive alien species outcompete the native inhabitants.

The solution is sought in increasing the habitat diversity by anchoring trees into the river bottom, adjusting weir programs and re-dimension side channels so that the flow increases. Finally experiments will take place which hopefully reduce the effects of shipping.

The nursery function of newly constructed and restored floodplain habitats for juvenile riverine fish

M. Dorenbosch. N. van Kessel, J. Kranenbarg, F. Spikmans, W.C.E.P. Verberk & R.S.E.W. Leuven

Over the centuries, large rivers in the Netherlands have been heavily modified as a result of river regulation and habitat deterioration. Together with a poor water quality, this has led to an impoverished fish community. Towards the end of the twentieth century, the river water quality was greatly improved, and many habitat rehabilitation projects were carried out, i.e. (re-)creation of secondary channels, and restoration of the connection between the main channel and disconnected oxbow lakes and (periodically) isolated water bodies. The significance of these newly created riverine habitats for juvenile riverine fish was compared with existing habitats in the main channel of the rivers Rhine and Meuse. All types of newly created and restored floodplain habitats displayed a higher fish species diversity and density than existing habitats in the main channel. The newly constructed and restored habitats in floodplains display an important nursery function for riverine fish. Water bodies connected to a main river channel were found to be dominated by the rheophilous Ide (Leuciscus idus), the eurotopic species Perch (Perca fluviatilis) and Roach (Rutilus rutilus), and the non-native rheophilous species Asp (Leuciscus aspius). Isolated water bodies were dominated by the eurotopic species Bream (Abramis brama), Three-spined stickleback (Gasterosteus aculeatus) and the non-native Pike-perch (Sander lucioperca), and the limnophilous species Rudd (Scardinius erythrophthalmus) and Belica (Leucaspius delineatus). For future rehabilitation projects we advise creating a high degree of habitat heterogeneity within newly created floodplain habitats in addition to further preservation of low dynamic and isolated water bodies to ensure further recovery of both rheophilous and limnophilous riverine fish communities.

Potential for further recovery of river floodplain fauna

M. de Lange, J. Noordijk & M. Nijssen

Characteristic river floodplain habitats as well as characteristic river fauna have disappeared in Dutch river systems. We identified the causes for species to be absent or underperforming by matching their ecological traits and requirements to the current landscape. Advice is given on the possible measures to solve these bottlenecks.

Colonisation of Dutch large rivers by invasive non-native gobiids and their impact on native Bullhead

N. van Kessel, M. Dorenbosch, J. Kranenbarg , G. van der Velde & R.S.E.W. Leuven

Over the last ten years, non-native gobiids have displayed a rapid invasive dispersal pattern in The Netherlands.  High densities of these species have been recorded within the Dutch large rivers. However, until these non-native gobiid species colonised the River Meuse, their impact on native species had not been recorded. Here, Tubenose goby (Proterorhinus semilunaris), Round goby (Neogobius melanostomus) and Bighead goby (Ponticola kessleri) occupy the same habitat as native Bullhead (Cottus perifretum), i.e. stony substrate which is used to prevent the erosion of river banks. The colonisation of the River Meuse by gobiids coincided with a rapid decline in Bullhead, most likely due to competition mechanisms, such as competition for shelter and/or food. In the near future, it is not unlikely that this same negative impact will occur on other native species. Objectives relating to the European Habitats Directive and the European Water Framework Directive may become impossible to achieve as a result of the potential loss of species in certain areas.

Seasonal inundated floodplains as missing link

G. Kurstjens, G. van Geest, B.W.E. Peters & T.B.M. Wijers

Due to large scale nature restoration projects along the Dutch rivers during the past 20 years many populations of plants and animals grew or even returned. However, species adapted to less dynamic parts of the floodplain did not profit. 

In 2008 and 2009 a seasonal inundated floodplain was created in a river foreland near Nijmegen. This article describes the ecological results of this quite unique experiment. The return of typical marshland breeding birds was most distinctive. Research on aquatic fauna showed that fry, newts and several macro fauna species formed the source of food for these birds. 

It seems that this type of habitat forms an important ecological missing link for fish eating birds within the Dutch river system. The possibilities to develop this habitat at other places along the forelands of the river Rhine are described.

Keep dry levee grasslands short and environmental dynamics low
Dry levee grasslands declined considerably and only few well-developed locations remain. It is assigned as a Natura2000 priority habitat. The ecology and distribution of dry levee grassland and of its characteristic species is discussed and related to management and habitat requirements.

The characteristic dry levee species consist of short, light demanding, thermophilous herbs, and occur in The Netherlands at the fringe of their Central European to (sub) Mediterranean distribution. Dry levee grassland vegetation is assigned to the Medicagini-Avenetum, Sedo-Thymetum and Festuco-Thymetum. 

Development of roughage and tall forb communities due to insufficient grazing pressure is detrimental to the levee grasslands. In heterogeneous nature reserves the grazing horses and cattle tend to selectively graze on nutrient rich grasslands and tend to neglect the more nutrient poor levee grasslands resulting in dominance of Calamagrostis epigejos, Eryngium campestre, Tanacetum vulgare and Rubus. As Calamagrostis epigejos and Rubus easily outgrow a 20 cm or even a 50 cm sand layer the process of sand deposition by the river is insufficient to counteract the dominance of these species. Moreover, deposition of 20 cm of sand will suffocate most of the low growing characteristic levee species. 

Ruderalization of these grasslands can only be prevented if grazing intensity is sufficient to preserve short swards. Managers should either direct the herds or make sure that in the growing season the levees are sufficiently grazed because of an overall high grazing intensity in the area. Apart from the right abiotical conditions like a dry, calcium containing, relatively nutrient poor soil, a short vegetation structure due to sufficient grazing intensity or mowing and a low dynamic environment are essential.

Expanding hardwood floodplain forests by spontaneous development

P.W.F.M. Hommel, R.J. Bijlsma, H.G.J.M. Koop, G.J. Maas, R.W. de Waal & E.J. Weeda

In The Netherlands two main types of floodplain forest are present: softwood- and hardwood  floodplain forest (SFF and HFF).  SFF is generally dominated by various Salix species and occasionally by Populus nigra. It is a common forest type, but restricted to the lower floodplain zones. It hardly contains any forest species. The main tree species of the HFF are Quercus robur, Fraxinus excelsior and Ulmus minor. Species diversity in well-developed stands is high, both inside the forest and in the forest edges. Various forest species are present, including ancient woodland species. HFF is a highly valued forest type, but actually very rare. In The Netherlands it is currently restricted to the higher floodplain zones, and represented by small stands, mostly young and poor in species. Valuable, old HHF is restricted to a few (former) coppice woodlands. 

From a view-point of water management and safety, expansion of floodplain forest is unwanted and mostly prohibited. By contrast, from a view-point of nature conservation and Natura 2000 obligations, expansion of the HFF area is urgently needed. Planting of HFF trees seems to be the most efficient method to do so. However, there are various reasons why spontaneous development is to be preferred. First, there are no good references for HFF in lower river reaches, as present in The Netherlands; second, the prospects of the three traditional dominant tree species are unfavourable, due to elm disease, oak mildew and ash dieback; third, the natural zonation of forest types along the inundation gradient in forest plains is highly uncertain. Our research suggests that HFF can develop all along the flooding gradient, both directly in short vegetation and indirectly by succession from existing SFF. Moreover, recent results of restoration projects in floodplains, combining spontaneous development with extensive grazing, are most promising.

Ecological recovery large Dutch rivers

F.W.B. van den Brink, M.C.C. de Graaf, P.C. Schipper, M.M. Schoor & H.L. Schimmel-ten Kate

In this special issue we look back at the ecological results of 25 years of river restoration and adapted ecological management of the large river-floodplain habitats in The Netherlands. After water quality improvement since the eighties of last century, much effort has been undertaken to restore characteristic riverine habitats, such as secondary channels and natural river banks. Following water quality improvement and development of fish passages along weirs, characteristic riverine fish and macroinvertebrates returned. The development of secondary channels, needed for the return of characteristic riverine fish and macroinvertebrates, however, is still in progress. Attention is needed for continuous flow in these channels. Return of characteristic macroinvertebrates and some fish species is hindered by invasive exotic species. In the river forelands agricultural fields have been transformed into natural areas. Management was based upon natural riverine processes together with extensive grazing by introduced cattle. Especially brushwood species and waterfowl responded successfully to this. Restoration of hardwood floodplain forest is urgently needed, despite its consequences for hydraulic resistance. In selected areas, outside the area most needed for water discharge, the combination of spontaneous development with extensive grazing is very promising.