Ramesh D. Gulati

Lakes in the Netherlands, their origin, eutrophication and restoration: state-of-the-art review

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 under-water 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).

Restoring macrophyte diversity in shallow temperate lakes: biotic versus abiotic constraints

Although many lake restoration projects have led to decreased nutrient loads and increased water transparency, the establishment or expansion of macrophytes does not immediately follow the improved abiotic conditions and it is often unclear whether vegetation with high macrophyte diversity will return. We provide an overview of the potential bottlenecks for restoration of submerged macrophyte vegetation with a high biodiversity and focus on the biotic factors, including the availability of propagules, herbivory, plant competition and the role of remnant populations. We found that the potential for restoration in many lakes is large when clear water conditions are met, even though the macrophyte community composition of the early 1900s, the start of human-induced large-scale eutrophication in Northwestern Europe, could not be restored. However, emerging charophytes and species rich vegetation are often lost due to competition with eutrophic species. Disturbances such as herbivory can limit dominance by eutrophic species and improve macrophyte diversity. We conclude that it is imperative to study the role of propagule availability more closely as well as the biotic interactions including herbivory and plant competition. After abiotic conditions are met, these will further determine macrophyte diversity and define what exactly can be restored and what not.

Growth and nutrient uptake by two species of Elodea in experimental conditions and their role in nutrient accumulation in a macrophyte-dominated lake

The capacity of Elodea nuttallii (Planch.) St. John and Elodea canadensis Michx. to remove nitrogen from water was evaluated in laboratory experiment. The growth rate of plants and their effect on the nitrogen level of hypertrophic Lake Zwemlust (the Netherlands) as well as on lake water enriched with nitrogen were investigated. The plants grew best in water enriched with up to 2 mg NH4-Nl−1 and 2 mg NH4-Nl−1 plus 2 mg NO3 Nl−1. During a 14 day experiment, plants absorbed from 75% to 90% of nitrogen. Higher nitrogen concentration than 4 mg l−1 had a negative effect on growth of both species. Elodea nuttallii and E. canadensis prefer NOinf4/p+ over NOinf4/p− when both ions were present in water in equal concentrations.

The dynamics and role of limnetic zooplankton in Loosdrecht lakes (The Netherlands)

The paper summarizes the results of a ten-year (1981–1991) zooplankton research on the Lake Loosdrecht, a highly eutrophic lake. The main cause of the lakes eutrophication and deteriorating water quality was supply up to mid 1984 of water from the River Vecht. This supply was replaced by dephosphorized water from the Amsterdam-Rhine Canal in 1984. The effects of this and other restoration measures on the lakes ecosystem were studied. Despite a reduction in the external P-load from ca. 1.0 g P m−2 y−1 to ca. 0.35 g m−2 y−1 now, the filamentous prokaryotes, including cyanobacteria and Prochlorothrix, continue to dominate the phytoplankton.Among the crustacean plankton Bosmina spp, Chydorus sp. and three species of cyclopoid copepods and their nauplii are quite common. Though there was no major change in the composition of abundant species, Daphnia cucullata, which is the only daphnid in these lakes, became virtually extinct since 1989. Among about 20 genera and 40 species of rotifers the important ones are: Anuraeopsis fissa, Keratella cochlearis, Filinia longiseta and Polyarthra. The rotifers usually peak in mid-summer following the crustacean peak in spring. The mean annual densities of crustaceans decreased during 1988–1991. Whereas seston (< 150 µm) mean mass in the lake increased since 1983 by 20–60%, zooplankton (> 150 µm) mass decreased by 15–35%.The grazing by crustacean community, which was attributable mainly to Bosmina, had mean rates between 10 and 25% d−1. Between 42 and 47% of the food ingested was assimilated. In spring and early summer when both rotifers and crustaceans have their maximal densities the clearance rates of the rotifers were much higher. Based on C/P ratios, the zooplankton (> 150 µm) mass contained 2.5 times more phosphorus than seston (< 150 µm) mass so that the zooplankton comprised 12.5 % of the total-P in total particulate matter in the open water, compared with only 4.5% of the total particulate C. The mean excretion rates of P by zooplankton varied narrowly between 1.5 and 1.8 µg P 1− d−1, which equalled between 14 and 28% d−1 of the P needed for phytoplankton production.The lack of response to restoration measures cannot be ascribed to one single factor. Apparently, the external P-loading is still not low enough and internal P-loading, though low, may be still high enough to sustain high seston levels. Intensive predation by bream is perhaps more important than food quality (high concentrations of filamentous cyanobacteria) in depressing the development of large-bodied zooplankton grazers, e.g. Daphnia. This may also contribute to resistance of the lakes ecosystem to respond to rehabilitation measures.

Restoration and Recovery of Shallow Eutrophic Lake Ecosystems in The Netherlands

Preface. Part One: Aspects of Water Quality Research in Loosdrecht Lakes. Part Two: Eutrophication Research in the Netherlands, with Emphasis on Additional Measures. Part Three: Eutrophication Control in the Netherlands.

Working group Water Quality Research Loosdrecht Lakes: its history, structure, research programme, and some results

In 1984 the external phosphorus load to the Loosdrecht lakes ecosystem was decreased substantially. A working group, Water Quality research Loosdrecht lakes (WQL), was formed to study the consequences for recovery of the lakes, and to evaluate the water management measures taken. Its history from the start in 1979 is described. The working group had an interdisciplinary character; its organization structure (project management) is depicted. From March 1983 a coordinate multidisciplinary research programme was started, carried out uninterrupted up to December 1990. The programme paid special attention to the flow of phosphorus through the lake ecosystems. The results were presented at several international conferences. The WQL was also involved in the international working group ERiFER (Ecosystem Research in Freshwater Environment Recovery). Important financial support was given by Dutch ministries, the Commission of European Communities, regional water authorities, and private funds.The measures taken to counteract eutrophication were not successful. It has been the task of the WQL to explain the ecosystem resilience.

The fate of cyanobacterial detritus in the food web of Lake Taihu: a mesocosm study using 13C and 15N labeling

The ecosystem of the highly eutrophic Lake Taihu (China) is seriously affected by recurrent cyanobacterial blooms, but little is known about the contribution made by cyanobacteria to the food web. In this study, we investigated the fate of detritus of the cyanobacterium Microcystis in the food web of Lake Taihu through a 19-day mesocosm experiment using stable-isotopic tracers of carbon (13C) and nitrogen (15N). 13C- and 15N-labeled Microcystis detritus was added to the mesocosm tanks and tracked through different elements of the food web. We found clear enrichment with both 13C and 15N in some zooplankton species, including Daphnia, Diaphanosoma, and Sinocalanus, which suggests that these zooplankters can utilize cyanobacterial detritus as a food source. Benthic animals, chironomid larvae and Limnodrilus, also showed pronounced increases in 13C and 15N, but the isotope increase was relatively smaller in the gastropods, Radix sp. and Bellamya sp., implying that they either exploited this food source differently or responded slower than the zooplankton, which apparently grew faster than the snails. Our study suggests that cyanobacterial detritus, originating almost wholly from the bloom-forming Microcystis, is an important food source for both planktonic and benthic food webs in eutrophic lakes such as Lake Taihu.

Zooplankton as a compound mineralising and synthesizing system: Phosphorus excretion

Data on phosphate excretion rates of zooplankton are based on measurements using the pelagic crustacean zooplankton of Lake Vechten and laboratory-cultured Daphnia galeata. In case of Daphnia sp we measured the effects of feeding on P-rich algae and P-poor algae (Scenedesmus) as food on the P-excretion rates at 20°C. The excretion rates of the natural zooplankton community, irrespective of the influence of the factors mentioned, varied by an order of magnitude: 0.025–0.275µg PO4-Pmg−1C in zooplankton (Czp) h−1. The temperature accounted for about half the observed variation in excretion rates. The mean excretion rates in the lake, computed for 20°C, varied between 0.141 and 0.260 µg Pmg−1Czph−1. Based on data of zooplankton biomass in the lake the P-regeneration rates by zooplankton covered between 22 and 239% of the P-demand of phytoplankton during the different months of the study period.In D. galeata, whereas the C/P ratios of the Scenedesmus used as food differed by a factor 5 in the experiments, the excretion rates differed by factor 3 only. Despite the higher P-excretion rates (0.258± 0.022 µg PO4-P mg−1 C h−1) of the daphnids fed with P-rich food than those fed with P-poor food (0.105 ± 0.047 µg PO4-P mg−1 C hp−1), both the categories of the animals were apparently conserving P. A survey of the literature on zooplankton excretion shows that in Daphnia the excretion rates vary by a factor 30, irrespective of the species and size of animals and method of estimation and temperature used.About two-thirds of this variation can be explained by size and temperature. A major problem of comparability of studies on P-regeneration by zooplankton relates to the existing techniques of P determination, which necessitates concentrating the animals several times above the in situ concentration (crowding) and prolonged experimental duration (starving), both of which manifest in marked changes that probably lead to underestimation of the ‘real’ rates.

Clearance rates of bacteria by the rotifer Filinia longiseta (Ehrb.) measured using three tracers

The clearance rates (CRs) of bacteria by Filinia longiseta were measured at 19°C, both in situ in Lake Loosdrecht and in the laboratory during summer. The tracer particles used in the field were: (1) 0.51 µm fluorescent microspheres, and (2) fluorescently labelled bacteria (FLB). A third type of tracer particle, natural [methyl-3H]-thymidine-labelled bacteria (< 1.2 µm), were used as a radiotracer in a laboratory experiment. The uptake of the first two tracer-particle types was measured by microscopic examination of the rotifer guts. In the third case, the uptake of radioactivity was determined by liquid scintillation counting. The rate of uptake of the microspheres decreased 10 min after the start of the experiment, probably because the gut passage time was exceeded. Using a 5 min feeding time, the rate of uptake of microspheres was higher than that of the FLB, though the variation in the uptake in both cases was high. The ingestion rates and CRs of bacteria by F. longiseta based on the fluorescent tracers were: microspheres, 5115 bact.ind−1 h−1 and 0.368 µl ind−1 h−1; FLB, 2252 bact.ind−1 h−1 and 0.162 µl ind−1 h−1. The mean CR using the thymidine-labelled natural bacteria and a 10 min feeding time was 0.179 µl ind−1 h−1. Thus, the CR based on the microsphere method was twice as high as for the other two methods.

The influence of Myriophyllum verticillatum and artificial plants on some life history parameters of Daphnia magna

Submerged macrophytes are known to serve as refuge for zooplankton but also seem to suppress the zooplankton growth. Thus, there is a conflict between the positive and negative role of macrophytes for zooplankton. We tested the influence of physical structure using artificial macrophytes, chemicals released by a macrophyte (Myriophyllum verticillatum), as well as the combined effects of these two factors on the life history of Daphnia magna. Daphnids matured at a smaller size and produced fewer eggs but larger individual offspring in the presence of artificial and real plants. Thus, under conditions with plants, we observed a trade off between number of eggs produced and the individual size of the offspring. Daphnids grown in the presence of exudates without plants were larger at maturity and showed no reduction in clutch size as compared with the control. We suggest that the macrophytes (real and artificial) negatively affected the daphnids in two ways: (1) food particles were settling down faster on the plant structures and this reduced the available food for Daphnia, (2) the plant structures were obstacles for the daphnids causing them to spent more energy during swimming. Both effects resulted in a reduced somatic growth, whereas the increased individual offspring size was probably a response to the reduced food level. Daphnia was not negatively affected by Myriophyllum exudates.