10.2478/v10060-008-0026-1 Annals of Warsaw University of Life Sciences – SGGW Land Reclamation No 38, 2007: 95–104 (Ann. Warsaw Univ. of Life Sci. – SGGW, Land Reclam. 38, 2007) Possible effects of climate change on ecological functioning of shallow lakes, Lake Loenderveen as a case study SEBASTIAAN A. SCHEP1, GERARD N.J. TER HEERDT2, JAN H. JANSE3, MAARTEN OUBOTER2 1Witteveen+Bos, The Netherlands 2Waternet, The Netherlands 3MNP/NLB, The Netherlands Abstract: Possible effects of climate change on INTRODUCTION ecological functioning of shallow lakes, Lake Loenderveen as a case study. The European The European Water Framework Water Framework Directive (WFD) requires all inland and coastal waters to reach “good Directive (WFD) requires all inland and ecological status” by 2015. The good ecological coastal waters to reach “good ecological status of shallow lakes can be characterised by status” ultimately in 2027. In general, clear water dominated by submerged vegetation. a shallow lake in good ecological The ecological response of shallow lakes on condition can be characterised by clear nutrients largely depends on morphological and hydrological features, such as water depth, water, small amounts of phytoplankton, retention time, water level fl uctuations, bottom vegetation dominated by submerged type, fetch etc. These features determine the vegetation and rich (macro) invertebrate “critical nutrient load” of a lake. When the actual and fi sh communities (Scheffer, 2001). nutrient load of a lake is higher than the critical The actual ecological state (clear nutrient load, the ecological quality of this lake will deteriorate, resulting in a turbid state dominated or turbid) of shallow lakes largely by algae. Climate change might lead to changes depends on the actual nutrient load and in both environmental factors and ecosystem the “critical nutrient load”. The critical response. This certainly will have an effect on the nutrient load is the nutrient load needed to ecological status. As an illustration the results of make a clear lake become turbid and vice a multidiscipline study of a shallow peaty lake versa. Because of hysteresis processes (Loenderveen) are presented, including hydrology, geochemistry and ecology. Ground- and surface two critical nutrient loads (CNL) can be water fl ows, nutrient dynamics and ecosystem defi ned (Scheffer, 2001). The shift from functioning have been studied culminating in an clear to turbid (eutophication, CNLeu) application of the ecological model of the lake often occurs at a much higher nutrient (PCLake). Future scenarios were implemented load than the switch back from turbid through changing precipitation, evaporation and temperature. Climate change will lead to higher to clear (oligotrophication, CNLoligo). nutrient loads and lower critical nutrient loads. As When the nutrient load is below CNLoligo a consequence lakes shift easier from clear water the lake is stably clear, when the load is to a turbid state. above CNLeu the lake is stably turbid (Fig. Key words: shallow lakes, climatic change, 1). In the intermediate range, both states PCLake model, nutrient load. may exist and switches between the two 96 S.A. Schep et al. FIGURE 1. Critical nutrient loads of shallow lakes related to submerged vegetation states are possible (multiple stable state Chemical and bacteriological processes (Scheffer et al., 1993; Scheffer, 2001)). and the growth rates of phytoplankton The critical nutrient load depends on and submerged vegetation increase when morphological features (water depth, soil temperature increases (Mooij et al., 2005; type and lake size), hydrological features Mooij et al., 2007). (ground- and surface water fl ow, retention Each individual lake will be differently time, water level fl uctuations) and climate affected by climate change. In general, conditions (water temperature). The the critical nutrient loads, CNLoligo actual nutrient load depends on climate and CNLeu might likely decrease with conditions (precipitation, evaporation) increasing temperature and precipitation and environmental conditions (water (Mooij et al., 2005). The decrease of quality). CNLeu combined with an increase of Most shallow lakes in the Netherlands the nutrient load might cause a clear are in the turbid, eutrophic state (Gulati lake to become turbid, while a decreased and van Donk, 2002). Because of CNLoligo will hamper the return towards mostly food-web related self-stabilizing the clear state. Therefore, to determine mechanisms (see e.g. Scheffer, 1998) realistic and sustainable goals and restoration measures mainly aimed at measures for the WFD, water quality reducing the nutrient load did not succeed. managers should take climate change Therefore other approaches are needed. into account. Instead of reducing the nutrient load, the The main goal of this study is to critical nutrient load can be increased by relate effects of climate change to changing system characteristics or when the ecological functioning of Lake the nutrient load is in the intermediate Loenderveen. Further a methodology range biomanipulation can be a preferable to study the potential effect of climate measure (Gulati and Van Donk, 2002). change on lake ecosystems is developed Climate change might lead to and tested. changes in both environmental factors Ground- and surface water fl ows, and ecosystem response. Hydraulic and nutrient dynamics and ecosystem nutrient load will increase with increasing functioning have been studied culminating precipitation (Mooij et al., 2005). in an application of the ecological model Possible effects of climate change on ecological functioning... 97 PCLake. A nutrient balance is used to on the water balance of the adjacent lake predict effects of climate change on the Terra Nova, Terra Nova (water surface nutrient load. PCLake is used to predict 89.5 ha, average depth 1.43 m, residence effects of climate change on the critical time 0.5 year) is included in the water and nutrient load. nutrient balance. The studied period is from 1970 to 2004, because of important historical nutrient loads. METHODS The hydrology of both lakes is simple. Precipitation and evaporation dominate This study is focused on the isolated and the water balance, because of the isolated mainly rainwater fed Lake Loenderveen, position of the lakes. Groundwater part of the Loosdrecht Lakes system infl uence is almost negligible. During in the Netherlands (Fig. 2). The lake is water shortage of Lake Loenderveen or oval shaped. Some characteristics: the Terra Nova, water is supplied from the size is 237 ha, the fetch is 1540 m, the adjacent Lake Loosdrecht via an inlet mean depth is 2.31 m and the residence culvert in the south. When the water time is > 1.5 years. The bottom is peaty; level is high in either Lake Loenderveen with 10% dry matter, containing 58% or Terra Nova, water is pumped to the organic matter and 12% lutum. Organic river Vecht through a pumping station. matter is mainly detritus (90%). Until Since 1973 Lake Loenderveen and Terra 1984 the water was clear and large areas Nova are separated through a weir. Water were covered with submerged vegetation passes the weir during high water levels (Best et al., 1994). In 1987 the water in Lake Loenderveen or water shortage had become turbid (Dekker et al., 1992) in Terra Nova. and all submerged vegetation had been Precipitation and evaporation gone. data were collected from the Royal The ecological status of the lake is Netherlands Meteorological Institute, predicted by the ecological lake model weather station De Bilt. Water levels of PCLake. The framework of the model is Lake Loenderveen, Terra Nova and its defi ned by an accurate water and nutrient adjacent lakes were measured. Seepage balance. Because inlet and outlet of water and infi ltration in both Terra Nova and in Lake Loenderveen is partly depended Lake Loenderveen were modelled with Terra Nova (outlet) weir pumping station Lake Loenderveen inlet culvert FIGURE 2. Lake Loenderveen (Google Earth 2005) 98 S.A. Schep et al. the Multi Layer Analytical Element estimated as 0.02 mg·l–1 and 0.1 mg·l–1 Model MLAEM (de Lange, 1991) and respectively. PCLake calculates P release appeared to be a linear function of the from the sediment. Nitrogen fl uxes are difference between the water levels of less important in Lake Loenderveen. the adjacent lakes and Lake Loenderveen Based on measurements, the NP-ratio in and Terra Nova. Discharges through PCLake was set to 34. weir, culvert and outlet were estimated PCLake is a mathematical, deter- as the water surplus and water shortage ministic, zero dimensional, process respectively, and compared to the based, integrated ecosystem lake model measured in and outlet. (Fig. 3). The model has been calibrated The water balance was validated on more than 40 mainly Dutch lakes and with the chloride balance. Chloride a systematic sensitivity and uncertainty concentrations are measured in inlet analysis has been performed (Aldenberg water and lake water and estimated in et al., 1995; Janse, 2005). precipitation and seepage water as 6 The aim of PCLake is to analyze mg·l–1 and 25 mg·l–1 respectively. the probability of a transition from a The nutrient balance is mainly clear water (dominated by submerged focused on phosphorus (P), as on average vegetation) to a turbid water state P is the limiting nutrient for algae and (dominated by phytoplankton), or vice water plant production. Concentrations versa, as a function of the external of total P in inlet water and lake water nutrient loading and other factors (Janse, were measured. Concentrations of P in 2005). With PCLake the critical nutrient precipitation and seepage water were loads, CNLeu and CNLoligo, are estimated, FIGURE 3. PCLake Model Structure Possible effects of climate change on ecological functioning... 99 depending on among others water depth, both a macrophyte dominated clear and hydraulic loading rate, fetch, sediment a phytoplankton dominated turbid state. type, marsh size and water temperature A clear state is defi ned as chlorophyll-a (which is related to climate scenarios).