Rob Portielje

Relationships between eutrophication variables: from nutrient loading to transparency

Monitoring data obtained from 231 freshwater lakes and ponds in the Netherlands, covering the period 1980–1996, were used to analyse the relationships between (a) transparency and chlorophyll-a, and the effect of system characteristics on this relationship, (b) chlorophyll-a and nutrient concentrations, and the effect of biological variables and (c) nutrient concentrations and nutrient loading. (a) Chlorophyll-a imposes a maximum on water transparency, but deviations from this maximum can be large. Reducing chlorophyll-a, therefore, does not guarantee a sufficient improvement of transparency. Soil type and the average depth of a lake were shown to influence the relationship between chlorophyll-a and transparency. (b) The maximum ratios of both chlorophyll-a: total-P and chlorophyll-a: total-N were higher in systems dominated by filamentous cyanobacteria than in systems dominated by other algae, indicating the efficiency of the former group with respect to nutrients. In systems with an areal coverage with submersed macrophytes above 5%, concentrations of chlorophyll-a and nutrients were lower than in systems with lower coverages. The ratios between chlorophyll-a and nutrients were lower at coverages larger than 10%. This indicates both bottom-up and top-down control of algae by macrophytes. Grazing pressure by zooplankton was also found to lower the chlorophyll-a: nutrient ratios. (c) System specific linear relationships were found between the average concentrations of total-P and total-N in the incoming water and the summer mean concentration in the lake. This allows the assessment of admissible loads for individual lakes, with narrower confidence limits compared to traditional relationships based on combined data from many lakes. From the analysis, it is concluded that the chain of relationships from nutrient loading to transparency is complex, and depends on biological variables as well as system characteristics.

Defining ecologically relevant water quality targets for lakes in Europe

Summary 1. The implementation of the Water Framework Directive requires EU member states to establish and harmonize ecological status class boundaries for biological quality elements. In this paper, we describe an approach for defining ecological class boundaries that delineates shifts in lake ecosystem functioning and, therefore, provides ecologically meaningful targets for water policy in Europe. 2. We collected an extensive data set of 810 lake-years from nine Central European countries, and we used phytoplankton chlorophyll a, a metric widely used to measure the impact of eutrophication in lakes. Our approach establishes chlorophyll a target values in relation to three significant ecological effects of eutrophication: the decline of aquatic macrophytes, the dominance of potentially harmful cyanobacteria and the major functional switch from a clear water to a turbid state. 3. Ranges of threshold chlorophyll a concentrations are given for the two most common lake types in lowland Central Europe: for moderately deep lakes (mean depth 3–15 m), the greatest ecological shifts occur in the range 10–12 l gL � 1 chlorophyll a, and for shallow lakes (<3 m mean depth), in the range 21–23 l gL � 1 chlorophyll a. 4. Synthesis and applications. Our study provides class boundaries for determining the ecological status of lakes, which have robust ecological consequences for lake functioning and which, therefore, provide strong and objective targets for sustainable water management in Europe. The results have been endorsed by all participant member states and adopted in the European Commission legislation, marking the first attempt in international water policy to move from physico-chemical quality standards to harmonized ecologically based quality targets.

Changes in sediment phosphorus as a result of eutrophication and oligotrophication in Lake Veluwe, The Netherlands

Abstract Since the creation of Lake Veluwe, The Netherlands, in 1956, a period of eutrophication has been followed by a period of oligotrophication. Around 1970, the lake switched from clear to turbid and recently from turbid to clear. The effect of eutrophication and oligotrophication has been studied with a conceptual model, based on the mass balance of total phosphorus in the top layer of the sediment. The model has been applied to the shallow part and the deeper part of Lake Veluwe. Whereas drastic changes in the external phosphorus loading, and concomitantly the phosphorus concentration in the lake water, occurred, the phosphorus pool in the sediment top layer only responds gradually to changed boundary conditions. The phosphorus content in the sediment top layer of the deeper part of Lake Veluwe doubled between 1956 and 1979. After 1979 it decreased. Losses were due to infiltration of dissolved phosphorus and burial of solid phosphorus caused by net sedimentation with material from the shallow part of the lake. Sensitivity analyses revealed that sediment phosphorus is also primary affected by these two factors. Based on the model predictions it is expected that in 2006 the phosphorus content of the sediment will be below the phosphorus content at creation of Lake Veluwe. Changes in flushing of the lake will probably have little effect on sediment phosphorus, while development of macrophytes will cause higher phosphorus contents in the shallow areas. However, with the current boundary conditions further ecological improvement of Lake Veluwe may be expected. The phosphorus pool in the sediment top layer is an important determinant in the long-term development of shallow lake ecosystems and, therefore, the modeling concept may also be applicable to other shallow lakes.

Nitrogen in Dutch freshwater lakes: trends and targets

Abstract A trend-analysis of eutrophication variables revealed that concentrations of total-nitrogen, total-phosphorus and chlorophyll-a decreased over the period 1980–1996 in a large majority of the Dutch freshwater lakes. Relative trends of nutrients compared well with emission reductions, which were much lower for nitrogen compared to phosphorus. Despite the decreasing trends, only 35% of the lakes met the water quality standard for summer mean total-nitrogen of 2.2 mg l−1. Based on protection against ecological deterioration or possibilities for recovery of eutrophic lakes, a promising goal is a summer mean total-nitrogen of 1.35 mg l−1. However, atmospheric dinitrogen fixation by blue-green algae will occur in lakes with a growth limiting nitrogen concentration when sufficient phosphorus, trace metals and light energy are available. Therefore, a combined lake-specific approach of nitrogen and phosphorus emission reduction and top-down control measures may be the best strategy to combat eutrophication.

Multi-lake studies in The Netherlands: trends in eutrophication

The relation between (inter)national programs aiming at nutrient load reduction and changes in eutrophication has been studied for 231 Dutch lakes over the period 1980–1996. Trends in total-phosphorus (P) and total-nitrogen (N) were negative, as determined by analysis of both individual lakes and the complete data set. The relative trends in the nutrient concentrations as well as in the N/P ratio correspond with the significantly reduced P emission and the limited reduction of N emission in The Netherlands since the beginning of the 1980s. Negative trends in chlorophyll-a and positive trends in Secchi-disc transparency may be partly explained by reduced nutrient concentrations. Perspectives for the nearby future are discussed. Lake characteristics had only a minor impact on the trends. The improvement of the water quality was found for all subsets of average depth, surface area, hydraulic retention time and soil type. Furthermore, the effect of restoration measures and meteorological conditions on the trends were studied. Biomanipulation resulted in an additional improvement of several water quality variables compared to lakes that were only subject to (inter)national programs on nutrient load reduction. Specific measures resulting in additional P load reduction resulted only in lowered P concentrations. Severe winters resulted in lower chlorophyll-a concentrations in the following summer and dry conditions in spring were favourable for all eutrophication variables in the following summer.

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.

Biodiversity in European Shallow Lakes: a Multilevel-Multifactorial Field Study

This chapter is based on the premise that the precipitous decline in freshwa- ter wetlands and species can only be arrested through conservation and sus- tainable management at a large scale, based on water (usually river) basins. A number of approaches to large-scale freshwater wetlands conservation are presented and assessed to draw conclusions on future conservation priori- ties.

Macrophyte assessment in European lakes: Diverse approaches but convergent views of ‘good’ ecological status

Graphical abstract