A review of the past and present status of anadromous fish species in the Netherlands: is restocking the Rhine feasible?

Abstract

This article starts with a brief description of the origin and eutrophication of shallow Dutch lakes, followed by a review of the various lake restoration techniques in use and the results obtained. Most freshwater lakes in the Netherlands are very shallow (<2 m), and owe their origins to large-scale dredging and removal of peat during the early 17th century. They vary in area from a few hectares to a few thousand hectares, and are generally found in the northern and western part of the country. Most of them lie in the catchment areas of the major rivers: the Rhine, the Meuse and the Schelde. Because of their natural and aesthetic value, these lakes fulfil a recreational function. The lakes are important to the hydrology, water balance and agriculture in the surrounding polder country. The external input to the lakes of phosphorus (P) and nitrogen (N) and of polluted waters from the rivers and canals have been the major cause of eutrophication, which began during the 1950s. In addition, more recently climate changes, habitat fragmentation and biotic exploitation of many of these waters have probably led to loss of resilience and thus to accelerated eutrophication. Lake eutrophication is manifested essentially in the poor underwater light climate with high turbidity (Secchi-disc, 20-40 cm) caused usually by cyanobacterial blooms (e.g. Oscillatoria sp.), and loss of littoral vegetation. Despite recent perceptible reductions in external P inputs, non-point sources, especially of N from agriculture, still remain high and constitute a major challenge to the lake restorers. Lake recovery is also invariably afflicted by in-lake nutrient sources. These include P loading from the P-rich sediments, mineralization in the water and release by the foraging and metabolic activities of the abundant benthivorous and planktivorous fish, mainly bream (Abramis brama). A variety of restoration techniques have been employed in the Dutch lakes: hydrological management, reduction of P in the external loads, in-lake reduction or immobilisation of P, and complementary ecological management. This last involves biomanipulation, or the top-down control of the food web. Hydrological management has resulted in an improvement in the lake water quality only in a few cases. The failure of lake restoration measures (e.g. in the Loosdrecht lakes, described as a case study) has led water managers to use biomanipulation in other lakes under restoration. Lake biomanipulation principally involves reducing the existing planktivore population, bream in most cases, and introducing piscivores such as northern pike (Esox lucius). Lake Zwemlust is discussed as a case study, with brief mention of some other small lakes which have been biomanipulated. The restoration studies reveal that decrease of P to low levels is no guarantee that cyanobacterial populations will also follow suit. This is because cyanobacteria can withstand great variation in their P content and thus in their C:P ratios. Thus, for a unit weight of P, the Cyanobacteria can yield relatively more biomass and cause greater turbidity than, for example, green algae, which have relatively lower C:P ratios. This is possibly an explanation for the success of these filamentous Cyanobacteria in many Dutch lakes, and the failure of restoration endeavours. In addition, a falling trend in chlorophyll-a content in these shallow lakes does not set off an immediate increase in * NIOO Publication no. 2991. 74 lake transparency because of resuspension of seston and inorganic suspended matter from the lake bottom by both wind-induced waves and fish foraging activity. The zooplankton-grazing peak in spring, caused usually by large-bodied grazers, Daphnia spp., is invariably the first step in bringing about a clear-water phase. Subsequently, summer light conditions trigger optimal growth conditions for macrophytes, which then maintain the high water clarity by competing successfully with phytoplankton for nutrients, especially N. The 'return' of macrophytes, especially stoneworts(Chara spp.) in some lakes, has contributed to the sustaining of improved light conditions and success of the restoration measures. In addition to competing with phytoplankton for nutrients, the macrophytes exert their positive influence in manifold ways. They act as a major nutrient sink, provide refuges for zooplankton and young pike and reduce wind-and fish-induced bioturbation of sediment. Most restoration accomplishments in recent years have been attributed to the success of aquatic macrovegetation. In general, the achievements of restoration work in the Dutch lakes, especially those using biomanipulation measures, are questionable: there are probably more examples of failures than of successes. The failures are generally linked not only to insufficient or no decrease at all in the autochthonous or in-lake nutrient loadings, but also to rapid increase of the planktivorous fish in the years following their reduction. A 75% reduction in the existing planktivore population has often been used as an arbitrary yardstick for effective reduction, but may not be sufficient. However, fish stock reductions to <50 kg FW ha −1 and maintenance at that level might have a greater chance of success, though maintaining the existing fish population at preconceived levels is difficult since for reasons not yet fully understood, piscivores, pike in particular, fail to develop sizeable populations. Studies so far have helped us recognise that for sustainability of the positive effects on water quality, 'natural develop-ment' should be central to future lake restoration programmes. Future restoration plans typically visualise lakes as integral parts of their landscape, and envisage their 'nature development'. Such thinking aims at reinforcing the lakes' shoreline vegetation to prevent erosion and improve the subtlety of the land-water transition (e.g. Volkerak Zoommeer lake system). Where in-lake P stocks have retarded the pace of lake recovery (e.g. Loosdrecht Lakes), excavation of 20-30 m deep pits in shallower lake areas to allow wind-induced shifting of the nutrient-rich upper sediment layers and burial in the pits in order to hinder P releases from the sediments is now under way. For some lakes the creation of artificial islands to reduce the wind fetch factor and erosion has been planned; in other cases, more natural development of the quasi-aquatic ecosystems by water-level management in order to encourage the shoreline macrovegetation to develop has been planned. Such plans also have the provision of extending the upper and lower limits for permissible annual water-level fluctuations and exploring the effects of transient draw-downs. Ideally, near-natural water levels, unlike the current levels, are under consideration as possibly being the best option, also bearing climate change in mind. However, the consequences of flooding and recessions on the ecosystems and other water uses by man still need to be thoroughly investigated. In short, the experiences acquired from the failures and some successes of the last two decades should pave the way to development of more enduring strategies for sustainable restoration of our lake ecosystems.