Lake Markermeer Research Articles

Benthic hotspots in the pelagic zone: Light and phosphate availability alter aggregates of microalgae and suspended particles in a shallow turbid lake

AbstractLimnetic aggregates from a turbid delta lake with low dissolved nutrient availability were studied in relation to light and dissolved nutrient availability. Quick light‐attenuation restricts the euphotic zone to the top surface layer of the water column, whereas mineralization processes in the sediment specifically provide dissolved nutrients near the lakebed. This suggests neither the pelagic nor the benthic zone provides the combination of resources required for microalgal growth. Nutrient mineralization in aggregates could bridge this apparent spatial gap in light and nutrients by providing dissolved nutrients in the euphotic zone, promoting microalgal growth. To explore this, aggregates obtained from turbid and phosphate‐limited lake Markermeer (The Netherlands) were exposed in the laboratory to phosphate‐replete and phosphate depleted conditions, at high‐light and low‐light availability. Confocal microscopy revealed that aggregates exhibited alkaline phosphatase activity and contained microalgae, other microbes, and extracellular polymeric substances. The spatial distribution of the phosphatase activity in aggregates largely matched that of chlorophyll a (Chl a)‐lacking microbes, suggesting that these microbes were responsible for the activity. Colorimetric quantification revealed that aggregates exhibited over 1.9‐fold higher phosphatase activity than surrounding water. Two‐day exposure to different light and phosphate availabilities affected aggregate composition. Phosphate depleted conditions resulted in more Chl a‐lacking microbes and more phosphatase activity than phosphate‐replete conditions. Low‐light intensity resulted in higher abundance of extracellular polymeric substances than high‐light intensity. In contrast to aggregates from deep stratified systems, Markermeer aggregates were not enriched with dissolved phosphorus. These results suggest that P‐cycling in aggregates differs between shallow turbid and deep stratified ecosystems.

Vegetation growth and sediment dynamics in a created freshwater wetland

Understanding how the hydrological regime in relation to sediment type interferes with ecosystem development is important when wetlands are created with soft muddy material. Especially when plants are used as ecological engineers to promote crest stability and soil formation. We carried out a two-year mesocosm experiment with sediments derived from the Dutch lake Markermeer to identify the effects of the hydrological regime and sediment type on ecosystem functioning in terms of nutrient availability. We measured plant productivity, plant nutrient stoichiometry, and concentrations of N, P, and K in shoots and roots of Phragmites australis and Rumex maritimus and monitored how the clay-rich sediment from lake Markermeer changed into a wetland soil. Plants grown on Markermeer sediments tend to be N limited when periodically inundated and P limited when not inundated at all. The P availability was determined by the hydrological regime, while the N availability was determined by both the hydrological regime and the sediment type. Ecosystem development on created wetlands can be manipulated by adequate management of the hydrological regime, as plant species respond differently to changes in nutrient availability. This should be considered in eco-engineering projects where plants are used as ecological engineers to fasten ecosystem development on wetlands that are to be created from clay-rich material.

Plant-specific effects of iron-toxicity in wetlands

Background and aimsUnderstanding the potential effects of iron toxicity on plant development is important when constructing new wetland from iron-rich sediment. We aim to study plant species-specific effects of iron toxicity when grown in the iron-rich sediments of lake Markermeer (the Netherlands).MethodsUsing three sediment sources that varied in total Fe and Fe-P concentrations, we performed a greenhouse experiment to study the development of three wetland species that differ in their tolerance to iron and utilization capacity of Fe-P: Rumex maritimus, Phragmites australis and Eupatorium cannabinum.ResultsPhragmites australis was the only species that developed an epidermis-damaging iron plaque on its roots. Plaque formation mainly depended on the Fe(III) and Fe-P concentration of the sediment, which led to different nutrient imbalances in leaves. All three species showed reduced growth compared to the control substrate, which could not be linked to indirect Fe toxicity. In contrast, direct Fe toxicity following the uptake of Fe could not be excluded as a mechanism potentially explaining our results, and this result warrants further examination in longer-term experiments.ConclusionsOur results highlight the importance of considering the Fe and Fe-P availability in sediments, as these properties may constrain plant performance and delay the development of pioneer ecosystems in wetland construction sites.

Wetland eco-engineering: measuring and modeling feedbacks of oxidation processes between plants and clay-rich material

Abstract. Interest is growing in using soft sediment as a foundation in eco-engineering projects. Wetland construction in the Dutch lake Markermeer is an example: here, dredging some of the clay-rich lake-bed sediment and using it to construct wetland will soon begin. Natural processes will be utilized during and after construction to accelerate ecosystem development. Knowing that plants can eco-engineer their environment via positive or negative biogeochemical plant–soil feedbacks, we conducted a 6-month greenhouse experiment to identify the key biogeochemical processes in the mud when Phragmites australis is used as an eco-engineering species. We applied inverse biogeochemical modeling to link observed changes in pore water composition to biogeochemical processes. Two months after transplantation we observed reduced plant growth and shriveling and yellowing of foliage. The N : P ratios of the plant tissue were low, and these were affected not by hampered uptake of N but by enhanced uptake of P. Subsequent analyses revealed high Fe concentrations in the leaves and roots. Sulfate concentrations rose drastically in our experiment due to pyrite oxidation; as reduction of sulfate will decouple Fe-P in reducing conditions, we argue that plant-induced iron toxicity hampered plant growth, forming a negative feedback loop, while simultaneously there was a positive feedback loop, as iron toxicity promotes P mobilization as a result of reduced conditions through root death, thereby stimulating plant growth and regeneration. Given these two feedback mechanisms, we propose the use of Fe-tolerant species rather than species that thrive in N-limited conditions. The results presented in this study demonstrate the importance of studying the biogeochemical properties of the situated sediment and the feedback mechanisms between plant and soil prior to finalizing the design of the eco-engineering project.

Greening flood protection through knowledge processes: lessons from the Markermeer dikes project in the Netherlands

Greening flood protection (GFP) is increasingly recognized as an adaptive and flexible approach to water management that is well suited to addressing uncertain futures associated with climate change. In the last decade, GFP knowledge and policies have developed rapidly, but implementation has been less successful and has run into numerous barriers. In this paper, we address the challenge of realizing green flood protection goals by specifically considering knowledge in the decision-making of a Dutch flood protection project in Lake Markermeer. In this project, an ecological knowledge arrangement and a traditional flood protection knowledge arrangement are compared and their interactions analysed. The analysis provides insight into the specific difficulties of implementing GFP measures and identifies ways to realize GFP goals. The primary challenge is twofold: First, a self-reinforcing cycle of knowledge production and decision-making in the flood protection domain inhibits the introduction of innovative and multifunctional approaches such as GFP; second, the distribution of power is severely unbalanced in terms of ecological enhancement and flood protection, favouring the latter. Implementation of GFP requires structural change and the integration of ecological and flood protection knowledge and policy. Potentially rewarding routes towards this integration are the exploration of shared interests in GFP and the creation of mutual dependency between knowledge arrangements. The case study and the insights it provides show that GFP is far from mainstream practice and that implementation requires serious effort and courage to break with historical practices.

Establishing vegetated foreshores to increase dike safety along lake shores

Vegetated foreshores in front of existing dikes can contribute to the overall reduction of wave loads on the dike. In order to test this concept in large shallow lakes a field pilot was constructed along the Houtribdijk in Lake Markermeer (the Netherlands) in 2014 to gain experience with construction, stability, maintenance and governance aspects. A large scale monitoring programme was set up to follow the hydrodynamic forcing, morphological changes and vegetation development on the pilot. The pilot is located on an exposed south-westerly direction, and experiences substantial wave impact. As a result the desired vegetation on the land-water interface has not been able to establish, but a rather dynamic sandy beach is currently the main feature along the waterline of the site. Higher up the slope planted reeds, and a mixture of willows has well established itself in the first growing season. The exposed position of the location makes that hardly any natural pioneer vegetation has settled, only in small sheltered areas some annuals were able to germinate and maintain themselves.

Effect of algae on flocculation of suspended bed sediments in a large shallow lake. Consequences for ecology and sediment transport processes

Lake Markermeer, a large shallow lake in The Netherlands, suffers from turbidity and ecology problems. As part of a study aiming to mitigate these problems, we study flocculation processes in the lake; in particular, the possible mutual flocculation between algae and re-suspended bed sediments. We show that sediment re-suspended from the bed of the lake can flocculate, forming flocs for which size is a function of the turbulence level in the water column. Moreover, we also demonstrate that algae and re-suspended bed sediments can mutually flocculate, yielding organic-inorganic aggregates. These aggregates have different features to those of their individual components, some of which have been measured and characterized in this paper. Furthermore, the characteristics of the resulting organic-inorganic flocs are strongly influenced by the type of algae in the aggregate. We found that, in the case of flocs consisting of bed sediments and filamentous algae, flocculation yields smaller flocs than for bed sediments only, resulting in an increased turbidity in the water column. In the case of flocs consisting of bed sediments and colonial algae, flocs grow faster and become larger than bed sediment flocs, which may result in the depletion of most colonies from the water column.

Laboratory, in situ and full-scale load tests to assess flood embankment stability on peat

The low submerged unit weights of peats usually lead to low effective self-weight stresses, stiffnesses and undrained shear strengths. These features, in combination with high compressibility, a propensity to creep and the uncertain effects of fibrous inclusions, make foundation stability hard to assess reliably. It is usual to apply high safety, or strong material reduction, factors in foundation design. However, over-conservatism can lead to undesirable environmental and financial costs. This paper describes full-scale field tests conducted on peat, with and without pre-loading, at Uitdam on the borders of Lake Markermeer, north of Amsterdam. The experiments investigated the peat layers' consolidation behaviour and their response under loading, including full shear failure. Noting the complex final test geometries and the large displacements developed, simple numerical analyses were undertaken to help interpret the failures within a Tresca and ‘consolidated undrained shear strength' framework. The trials that included modest pre-loading developed large vertical consolidation strains (up to 35%) and significant bearing capacity improvements. The field experiments provide a rich resource for testing advanced numerical techniques. They also allowed a range of practical characterisation techniques to be assessed and calibrated for flood dyke applications.

Connectivity between Migrating and Landlocked Populations of a Diadromous Fish Species Investigated Using Otolith Microchemistry

Smelt Osmerus eperlanus has two different life history strategies in the Netherlands. The migrating population inhabits the Wadden Sea and spawns in freshwater areas. After the closure of the Afsluitdijk in 1932, part of the smelt population became landlocked. The fresh water smelt population has been in severe decline since 1990, and has strongly negatively impacted the numbers of piscivorous water birds relying on smelt as their main prey. The lakes that were formed after the dike closure, IJsselmeer and Markermeer have been assigned as Natura 2000 sites, based on their importance for (among others) piscivorous water birds. Because of the declining fresh water smelt population, the question arose whether this population is still supported by the diadromous population. Opportunities for exchange between fresh water and the sea are however limited to discharge sluices. The relationship between the diadromous and landlocked smelt population was analysed by means of otolith microchemistry. Our interpretation of otolith strontium (88Sr) patterns from smelt specimens collected in the fresh water area of Lake IJsselmeer and Markermeer, compared to those collected in the nearby marine environment, is that there is currently no evidence for a substantial contribution from the diadromous population to the spawning stock of the landlocked population.

Effects of Suspended Sediments on Seston Food Quality for Zebra Mussels in Lake Markermeer, The Netherlands

Until 1992, zebra mussels (Dreissena polymorpha) were an important food source for diving ducks in Lake Markermeer (The Netherlands). After 1993, the mussel biomass sharply declined, and the current population is in poor condition (maximum shell length <15 mm) compared to populations from adjacent Lake IJsselmeer. Lake Markermeer is characterized by high concentrations of total suspended matter (TSM), including 70% inorganic suspended sediments (ISS) affected by wind-driven resuspension, which may influence the food uptake by the mussels. We assessed whether the poor condition of the zebra mussels in Lake Markermeer could be attributed to the high TSM concentrations, and especially the fraction of ISS. In situ measurements of near-bottom TSM concentrations were linked to wind-driven wave action, and experiments were carried out using increasing natural sediment (NS) concentrations combined with a fixed concentration of a green alga (Scenedesmus obliquus) to determine the clearance rates (CR) of adult and juvenile zebra mussels. CR on NS particles decreased with increasing NS concentrations. The CR on S. obliquus, however, increased with an increasing NS concentration up to 0.75 g L−1 for adult mussels, indicating that zebra mussels can tolerate relatively high TSM concentrations and selectively graze on phytoplankton. Zebra mussels eventually reduce their CR at high TSM concentrations; the CR of juvenile mussels on S. obliquus collapsed at NS concentrations >0.5 g L−1. The NS concentrations used in this study are high, but analyses of the near-bottom TSM concentrations show that these values occur frequently and may be partly responsible for the poor mussel status in Lake Markermeer.

Sediment dynamics in shallow Lake Markermeer, The Netherlands: field/laboratory surveys and first results for a 3-D suspended solids model

In 2007/08, a study was undertaken on sediment dynamics in shallow Lake Markermeer, The Netherlands. Firstly, the sediment characteristics median grain size, mud content and loss on ignition showed a spatial as well as water depth related pattern indicating wind-induced sediment transport. Sediment dynamics were investigated in a sediment trap field survey at two stations. Sediment yields, virtually all coming from sediment resuspension, were significantly correlated with wind speeds. Resuspension rates for Lake Markermeer were very high, viz. ca. 1,000 g/m(2)day as an annual average, leading to high suspended solids (SS) contents, due to the large lake area and its shallowness (high 'Dynamic Ratio'). Sediment resuspension behaviour was further investigated in preliminary laboratory experiments using a 'micro-flume', applying increasing water currents onto five Lake Markermeer sediments. Resuspension showed a clear exponential behaviour. Finally, a 3-D model was set up for water quality and SS contents in Lake Markermeer; first results showed a good agreement between modelled and actual SS contents. Construction of artificial islands and dams will reduce wind fetches and may be expected to cause a substantial decrease in lake water turbidity.

Sediment characteristics and wind-induced sediment dynamics in shallow Lake Markermeer, the Netherlands

In 2007/08, a study was undertaken on the sediment dynamics in shallow Lake Markermeer (the Netherlands). Firstly, sediment characteristics were determined at 49 sites in the lake. Parameters such as median grain size and loss on ignition showed a spatial as well as water depth related pattern, indicating wind-induced sediment transport. Highly significant correlations were found between all sediment parameters. Lake Markermeer sediment dynamics were investigated in a sediment trap field survey at two permanent stations in the lake. Sediment yields, virtually all coming from sediment resuspension, were significantly correlated with average wind speeds, though periods of extreme winds also played a role. Sediment resuspension rates for Lake Markermeer were high, viz. on average ca. 1,000 g m-2 day-1. The highly dynamic nature of Lake Markermeer sediments must be due to the overall shallowness of the lake, together with its large surface area (dynamic ratio = [H(area)]/[average depth] = 7.5); wind-induced waves and currents will impact most of the lake’s sediment bed. Indeed, near-bed currents can easily reach values[10 cm/s. Measurements of the thickness of the settled ‘‘mud’’ layer, as well as 137Cs dating, showed that long-term deposition only takes place in the deeper SE area of the lake. Finally, lake sediment dynamics were investigated in preliminary laboratory experiments in a small ‘‘micro-flume’’, applying increasing water currents onto five Lake Markermeer sediments. Sediment resuspension started off at 0.5–0.7 cm/s and showed a strongly exponential behaviour with respect to these currents.

Fine sediment dynamics in a shallow lake and implication for design of hydraulic works

Lake Markermeer is a large (680 km2), shallow body of water in the middle of the Netherlands, with a mean water depth of 3.6 m. One of the major problems in the lake is its decreasing ecological value which is, among other reasons, caused by a gradual increase of suspended sediment concentration and associated increase of light attenuation in the water column. A thorough understanding of fine sediment dynamics in the lake is a prerequisite for solving this problem. This paper addresses the 3D nature of near-bed sediment dynamics in Lake Markermeer, based on data sampled from a 1-month field experiment in autumn 2007. The campaign involved the collection of 71 bed samples across the lake. At each location, dual-frequency echo soundings were carried out to assess the thickness of the silt layer, and sediment concentration throughout the water column was measured with an Optical Backscatter Sensor (OBS). Moreover, 2-week time series of wave height, water level, current velocities, and near-bed sediment concentration were collected at a single location. The time series of sediment concentration were measured with a regular OBS and an Argus Surface Meter IV (ASM). During the measurement period, flow velocities ranged between 2 and 15 cm/s, wave heights up to 1.2 m were observed and turbidity levels varied between 40 mg/l to more than 300 mg/l. The ASM data generally showed uniform concentration profiles. However, profiles with steep concentration gradients near the bed were found for wave heights above 0.5 m. The field experiments further revealed pronounced 3D structures near the bed during discrete storms. The results are generalized for a wider range of conditions and across the full water depth through application of a 1DV point model, using a two-fraction representation of the grain size distribution. The fine and coarse fractions are found to resuspend rapidly for wind speeds above 5 m/s and 10–12 m/s, respectively, forming a uniform concentration profile if these wind conditions persists. High-concentration (?1 g/l) layers near the bed, containing the coarse sediment fraction, only occur at the onset and towards the end of a storm, when wind speed changes rapidly. It is under these conditions that horizontal gradients in layer density or thickness can transport considerable fine sediment. This transport provides an additional mechanism for the infill of, for instance, silt traps and navigation channels.

SEDIMENTS AND NUTRIENT DYNAMICS IN THE LAKE MARKERMEER, THE NETHERLANDS

Lake Markermeer is a large fresh water lake in the centre of the Netherlands. Since Lake Markermeer is categorised as a shallow lake, it is very susceptible for sediment resuspension. The general objective of this study is to investigate sediment and nutrient dynamics in Lake Markermeer sediments. This research was carried out in a 5 months period from November 2007 until March 2008 using sediment trap methods. Sampling sites for sediment traps are located at the two fixed sites in which each sampling site had duplicates bottom and half-depth traps. The collection of the sediment traps samples was conducted every two weeks. The parameter measured in sediment traps were: sediment trap yields, Loss on ignition (LOI), total N and total P. Statistical t test analysis was utilised to compare the value of each parameter between two sampling stations as well as between bottom and half depth at the same stations.The results for the sediment trap activities showed that at the two fixed sampling stations, the sediment trap yields in the bottom part tend to be higher than half depth. This difference between bottom and half-way traps is statistically significant. Also, a significant difference was found between STA and STB, both for bottom and half-depth traps. A similar result was found for LOI. For total N and P the results showed that there were no significant differences between bottom and half-depth at the two fixed sampling stations. Keywords: Lake Markermeer, resuspension, sediment trap

Climate change induced salinisation of artificial lakes in the Netherlands and consequences for drinking water production

In this paper we present a modelling study to investigate the impacts of climate change on the chloride concentration and salinisation processes in two man-made freshwater lakes in the Netherlands, Lake IJsselmeer and Lake Markermeer. We used a transient compartmental chloride and water balance model to elucidate the salinisation processes occurring under present conditions and assess future salinisation under two climate forcing scenarios. The model results showed that the Rhine River is the dominant determinant for the chloride concentration in both lakes, followed by drainage of brackish groundwater from the surrounding polders. The results further show that especially during dry years, seawater intrusion through the tidal closure dam is an important source of chloride to Lake IJsselmeer. The results from the climatic forcing scenarios show that Lake IJsselmeer is especially vulnerable to climate-induced salinisation whereas effects on Lake Markermeer are relatively small. Peak chloride concentrations at the raw water intake of the Andijk drinking water facility on Lake IJsselmeer are projected to increase to values above 250 mg/l in the most far-reaching climate change scenario W+ in 2050 for dry years. This is well above the maximum allowable concentration of 150 mg/l for chloride in drinking water. Modelling showed that climate change impacts the chloride concentrations in a variety of ways: 1) an increasing occurrence of low river flows from summer to autumn reduces the dilution of the chloride that is emitted to the Rhine with a constant load thereby increasing its concentration; 2) increased open water evaporation and reduced rainfall during summer periods and droughts increases the chloride concentration in the water; and 3) rises in sea level increase seawater intrusion through the tidal closure dam of Lake IJsselmeer. The processes described here are likely to affect many other tidal rivers or lakes and should be considered when planning future raw water intake stations for drinking water production or agricultural water supply.

Combining a conceptual framework and a spatial analysis tool, HABITAT, to support the implementation of river basin management plans

Since the Ramsar meeting in 1971 many plans and agreements are developed to conserve and improve the quality of wetlands throughout the world. Within the European Union for example legislation has been developed in four Directives which require that water managers test activities on ecological effects. Moreover, in most areas measures have to be taken and included in river basin management plans in order to achieve ecological objectives in terms of occurrence of species and habitats set by EU Directives. To improve quality and conserve areas it is needed to assess the effects of current land use and autonomous developments and to identify the effectiveness of measures. Different studies have already addressed the need for models to quantify these effects. We developed a conceptual framework to analyse the cause‐effect chain from management to new environmental conditions and to the potential occurrence and quality of species and habitats using response curves. To our knowledge, this is the first framework presented that has the ecological assessments embedded within the structure, which allows for ecological assessments within the development of management plans rather than at the end of the development. For this purpose a spatial analysis tool is used (HABITAT), which is developed for a broad spectrum of users, from ecologists to managers, and purposes, from ecological assessments to flood risk assessments. The framework is applicable on an (inter) national scale and can promote the dialog between politicians, managers and ecologists. In this paper we demonstrate that the conceptual HABITAT framework for ecological impact assessment is able to support the development and implementation of ecologically sound management plans and illustrate this with a case study for the lake Markermeer in The Netherlands.

Towards ecological goals for the heavily modified lakes in the IJsselmeer area, The Netherlands

The European Water Framework Directive (WFD) obliges EU member states to define ecological goals for water bodies and, if necessary, to take measures to achieve these goals by 2015. The goals and measures for the water bodies in the IJsselmeer area of The Netherlands are elaborated in this study, following an approach described by Irmer & Pollard (2006, Alternative methodology for defining Good Ecological Potential (GEP) for Heavily Modified Water Bodies (HMWB) and Artificial Water Bodies (AWB). ECOSTAT). With the closure of the Afsluitdijk in 1932, the former Southern Sea estuary was transformed into the freshwater Lake IJsselmeer. Subsequently, a string of so-called border lakes and Lake Markermeer were created by land reclamation projects and the construction of dams. These alterations serve safety, provide drinking water supplies and created agricultural land. Owing to the change in category, the lakes are by (WFD) definition heavily modified. A natural lake rather than an estuary will be the starting point for determining ecological potential. However, damming and fixing the water table prevented the development of emergent vegetation and caused steep water-land gradients. The Maximum Ecological Potential includes the effect of these hydromorphological changes after all mitigation measures have been considered. Other pressures on the lakes are high nutrient loads, which cause phytoplankton blooms, the disappearance of aquatic macrophytes and intensive fishery, which overexploites the pikeperch and eel populations and causes indirect negative effects on water quality. Good Ecological Potential for these lakes is derived by estimating the effects of all effective hydromorphological measures that have no significant negative impact on existing functions or the wider environment, and the effects of all other measures. The suggested main measures are: construction of fish passages, adaptation of shore-lines, wind sheltered areas, reduction of nutrient load, reduction of fishery pressure and reduction of the bream stock. The effects of these measures are calculated from multivariate analyses that establish the relationships between (a) nutrients and chlorophyll-a, (b) chlorophyll-a and water transparency, (c) water transparency, depth distribution and vegetation coverage and (d) vegetation coverage and fish community. The proposed goals will be refined using new scientific insights and further discussions with stakeholders, and will finally be reported in the River Basin Management Plan 2009. It is expected that exemptions will be used for a phased achievement of the objectives.

Atmospheric correction of SeaWiFS imagery for turbid coastal and inland waters

The standard SeaWiFS atmospheric correction algorithm, designed for open ocean water, has been extended for use over turbid coastal and inland waters. Failure of the standard algorithm over turbid waters can be attributed to invalid assumptions of zero water-leaving radiance for the near-infrared bands at 765 and 865 nm. In the present study these assumptions are replaced by the assumptions of spatial homogeneity of the 765:865-nm ratios for aerosol reflectance and for water-leaving reflectance. These two ratios are imposed as calibration parameters after inspection of the Rayleigh-corrected reflectance scatterplot. The performance of the new algorithm is demonstrated for imagery of Belgian coastal waters and yields physically realistic water-leaving radiance spectra. A preliminary comparison with in situ radiance spectra for the Dutch Lake Markermeer shows significant improvement over the standard atmospheric correction algorithm. An analysis is made of the sensitivity of results to the choice of calibration parameters, and perspectives for application of the method to other sensors are briefly discussed.

Ecotoxicity of 3,4-dichloroaniline in enclosed freshwater plankton communities at different nutrient levels

The sensitivity of Daphnia magna to 3,4-dichloroaniline (DCA) was studied in a multispecies mesocosm study and its response was compared to those of naturally occurring zooplankton species. A series of DCA concentrations was tested during two experiments conducted in 1.2 m3 plastic enclosures, which contained water and natural plankton from Lake Markermeer (The Netherlands) and laboratory-cultured D. magna. Low concentrations of 2.7 and 27 mu g l-1 were tested during the first experiment and additional nutrients were supplied to half of the enclosures. Cladocerans were found to be the zooplankton group most sensitive to DCA. The addition of nutrients resulted in a more marked reduction of cladoceran populations at 27 mu g l-1. In the second experiment, additional DCA concentrations (27, 90 and 270 mu g l-1) were tested under nutrient-enriched conditions only. The EC50 for D. magna, calculated from the integrated density over the first 21 days, was 3.2 (1.9--5.5) mu g l-1 DCA when the results of the two experiments were combined. Two naturally occurring dominant cladoceran species were more sensitive to DCA than was D. magna. Less sensitive rotifer and copepod species profited from the reduced competition. The opportunisitic response of the less sensitive species could not prevent an increase in phytoplankton densities at high nutrient supply. The results show that both the effects of DCA on zooplankton and the diminished control exerted by zooplankton on phytoplankton are intensified under nutrient-enriched conditions

Distribution and aspects of population dynamics of the zebra mussel, Dreissena polymorpha (Pallas, 1771), in the lake IJsselmeer area (The Netherlands).

The distribution, settling and growth of the zebra mussel, Dreissena polymorpha Pallas, in Lake IJsselmeer and Lake Markermeer in The Netherlands were studied from 1980 to 1985. In these lakes D. polymorpha is the most important food source for wintering diving ducks. The study was part of an investigation into the carrying capacity of the lakes for these birds.