Long term changes in the eutrophication process in a shallow Mediterranean lake ecosystem of W. Greece: Response after the reduction of external load
Introduction
The state and evolution of freshwater ecosystems are affected by a variety of biotic and abiotic factors, as well as by natural and human-induced processes that may differ both in duration and intensity. Lake ecosystems tend to maintain a state of equilibrium but their chemical resilience depends on certain site-specific factors and processes (Carpenter et al., 1999). In contrast to deep lakes, many shallow lakes can switch quite abruptly between different stable states, representing alternative equilibria; a hypothesis developed and established over the past 20 years and summarized in Scheffer (1998). Lake eutrophication has been a major problem for decades. It involves a change in lake status from a macrophyte-dominated clear water state to a phytoplankton-dominated turbid state, with detrimental effects to the ecosystem (Portielje and Van der Molen, 1999). This “alternative states” model has been intensively investigated, whilst the need for clearer definitions in order to describe these situations has been addressed by Rojo and Alvarez-Cobelas (2003), Naselli-Flores et al. (2003).
Studies on shallow lakes from north temperate climate clearly demonstrate that they alternate between clear and turbid water states in response to some control factors. There is evidence that nutrient control may be of greater priority in eutrophicated warm shallow lakes than in similar lakes at higher latitudes. The influence of nutrients appears to increase southward from the north temperate regions (Nöges et al., 2003), and this was probably manifested through naturally greater annual macrophyte abundance in warmer locations as a consequence of longer plant-growing seasons (Moss et al., 2004). The eutrophication processes, as a response to nutrient loading, consist of the “tools” in understanding the ecosystem processes. In response to a period of globally intensive eutrophication, an increasing interest in freshwater restoration has taken place over the last two or three decades (Villena and Romo, 2003). According to the previously mentioned biostability model, the first step in the recovery of shallow lakes is considered to be the reduction of nutrient loading and specifically the reduction in the concentrations of orthophosphate (Scheffer et al., 1993). Previous studies concerning mostly north temperate lakes have recorded a decline in phytoplankton biomass after a reduction in phosphates (Jeppesen et al., 2000, Jeppesen et al., 1998a), although this relationship is not as clear in shallow Mediterranean lakes (Villena and Romo, 2003). Recently, Gonzalez-Sagrario et al. (2005) argued that the focus should be widened to incorporate the control of both nitrogen (N) and phosphorus (P) in the restoration of eutrophic shallow lakes.
Here we present the changes in eutrophication parameters (nutrients and phytoplankton chlorophyll-a) for the shallow Mediterranean Lake Pamvotis, during three monitoring periods, over the last 20 years. The main questions addressed are:
- (1)
Are there any inter-annually changes in nutrient concentrations and chlorophyll-a as a part of a long-term development?
- (2)
What is the long-term development of eutrophication after a reduction in phosphorus, associated with sewage diversion?
- (3)
What are the most effective restoration measures for the recovery and conservation of lake ecosystems?
Section snippets
Study area
Lake Pamvotis, NW Greece (39° 40′N, 20° 53′E) is a shallow Mediterranean lake with a mean depth of 4.3 m and a maximum depth of 7.5 m. The lake occupies an area of 22.8 km2 (Table 1, Fig. 1) and is situated approximately 470.25 m above the sea level. The lake is located next to the city of Ioannina (150,000 inhabitants) and 40% of the catchment is used for agriculture. It is an ancient lake ecosystem, situated in a topographically diverse landscape on the western flank of the Pindus mountain range (
Results
Morphometric, hydrological, physical and chemical data, as well as the water budget for the three monitoring periods are presented in Table 1.
Phosphorus budget
Many lakes are highly eutrophic because of high nutrient loads from sewage and agricultural activities in their catchments. During the last decades many efforts have been made to reduce and control the input of nutrients, mostly P, through sewage diversion. According to Cullen and Forsberg (1988) there are three responses in lake water quality parameters after the reduction of external nutrient loads. The relationship between the external nutrient loading and in-lake nutrient concentrations
Conclusions
The research on Lake Pamvotis serves as a further example of the valuable insight provided by developing a two-tiered approach to lake classification and assessment. The lake can be considered as a shallow eutrophic Mediterranean lake located at a high altitude and influenced by changes in climate.
The long-term eutrophication of Lake Pamvotis confirms that eutrophic to hypertrophic conditions have existed at least since the 1980s. Lake restoration efforts were traditionally focused on reducing
Acknowledgments
We wish to thank Dr. Graham Wilson for the linguistic revision of the manuscript and the two unknown Reviewers for their valuable comments.
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