The eutrophic Loosdrecht Lakes: Current ecological research and restoration perspectives

Abstract

Features of the Loosdrecht Lakes, with emphasis on the main lake, are discussed with reference to restoration.Characteristics of the present situation are : (1) very low water transparency -Secchi-disc readings around 0.3 m occur in all seasons; (2) relatively small seasonal changes in sestonic matter; (3) important input of resuspended particles into the seston; (4) predominance of filamentous blue-green algae for most of the year; (5) relative scarcity of crustacean zooplankton, while rotifers are abundant; (6) poor development of littoral communities, and absence of benthic producers. The blue-green algae maintain high population density at very low growth rates : rates of loss are low. The zooplankton grazing rate is low due to inefficient filtering, but predation of larger crustaceans by fish may also be important. Studies on epipelon indicated that loss by deposition may be largely compensated by resuspension.Starting in 1984, the external phosphorus loading was markedly reduced. Results for 1984 and 1985 indicate that complementary measures are needed in order to improve water quality. Action should be directed towards increasing the phytoplankton turnover rates. Accelerated specific growth rate can be expected to accompany lower biomass, more successful competition by other algal groups, and enhancement of grazing pressure.Considering the shallowness of the system, promotion of littoral development and return of submerged vegetation may be important in establishing a new equilibrium of the system.

Zooplankton structure in the Loosdrecht lakes in relation to trophic status and recent restoration measures

“…Because of the low mean depth (Z : < 2 .0 m) a thermal heterogeneity is lacking in summer . Wind-induced resuspension, high concentrations of algae and cyanobacteria in the water column and the colour of water have resulted in a poor light climate so that Secchi-disc, transparency values of > 50 cm are exceptional (Gons et al ., 1986).…”

Coupling of phytoplankton and detritus in a shallow, eutrophic lake (Lake Loosdrecht, The Netherlands)

“…Lake Loosdrecht exhibits only a weak seasonality in the concentrations of chlorophyll and seston dry weight, and their dynamics during summer can be characterized as an oscillating steady state (Gons et al, 1986a;). The validity of applying this concept is illustrated on a basis of a time series of chlorophyll a and seston dry weight concentrations in summer 1988.…”

Restoration and resilience to recovery of the Lake Loosdrecht ecosystem in relation to its phosphorus flow

“…Detritus is partly refractive and therefore, eutrophication builds recalcitrant debris in sediments. However, their density allows an easy resuspension in the water column by wind action (Gons et al, 1986), and they may play the role of sorbate in sorbing and desorbing the available phosphorus for phytoplankton (Rijkeboer et al, 1991). -The largest flow was from the soluble phosphorus pool to the seston compartment at 18mgPm-*d-'.Thisdailyflowisthemost important process in the internal phosphorus cycle of the lake ecosystem.…”

“…It predicted with a reasonable ment. Calculated growth rates were different from accuracy the outcome of further reduction of exthe estimated values based on primary producternal phosphorus load over a short term, pointtion measurements (Van Liere et al, 1986). The ing out at the same time additional measurements calculated phosphorus flow from zooplankton to be taken to reduce primary production (Janse deviated only a little from the measurements.…”

“…Therefore, it is unlikely that phosphorus release will be reduced by removing the top layer of sediments. In fact Van Liere et al, (1986) reported only 30 % reduction in phosphorus release rate from Lake Loosdrecht sediments following the removal of its top layer. With the present high external load of phosphorus, organic matter in the top layer will be replenished rapidly.…”