Land cover/use alterations are widely considered as a dominant stressor for the water quality and freshwater ecosystems status within watersheds [1
]. In particular, human activities and climate change are considered the most important factors that determine the impact of land cover/use changes on the water bodies around the world [2
]. On a catchment scale, land uses associated with human presence can significantly alter natural hydrological processes such as runoff and ground water recharge with subsequent effect on the balance between supply and demand of water resources [1
]. In particular, the hydrology of the freshwater ecosystems, especially those of the dry regions of the world, significantly relies on water level fluctuations, which are closely related to the human pressure from irrigated cultivations [5
]. Moreover, land cover/use changes also affect ecosystem processes like nutrient transportation from watershed sources to lakes [6
]. In addition, land cover/use changes may not only increase the demands for domestic and agricultural use of water but can also affect nutrient loading and nutrient leaching mechanisms.
In the Mediterranean region, climate change is expected to enhance many of these processes not only through changes in the major climate variables (precipitation, air temperature) but also due to predicted land use alterations. Recently, Pulido-Velasquez et al. [7
] showed that the combined climate and land use change scenarios predicted a larger effect on the groundwater recharge in the Mancha Oriental aquifer system than climate change only. Such effects are likely to lower water tables and affect water supply, especially in areas where water abstraction will be intensified in order to meet the growing demand for water [8
]. Other examples from Greece showed that historical land cover/use changes of lakes and wetlands revealed a significant shrinkage of water surface and its replacement with other types of land cover, mainly cultivations [9
]. Consequently, these land use changes combined with climate induced hydrologic alterations are expected to have a drastic effect on the ecology of freshwater lakes, including changes of the biotic communities, eutrophication enhancement, increase of turbidity and a shift to brackish or even saline conditions [12
]. Therefore, understanding the dynamics of land cover/uses in a changing climate and their impacts on freshwater systems is a matter of priority for remote sensing and landscape ecology.
Remote sensing techniques are commonly used throughout the world to monitor land cover/use changes [15
]. Particularly in lake ecology, satellite images and aerial historical photos can provide detailed information on the spatiotemporal changes of land cover/uses and can therefore be used to assess the impacts on various components of ecosystem integrity [15
]. Recently, Pilgrim et al. [15
] applied remote sensing analysis to assess the changes of water quality in a lake of South Carolina (USA) due to related changes of the catchment’s land cover. Other studies investigated the role of climate change for the spatiotemporal changes of lake area [15
], showing that remote sensing techniques have the potential to be an efficient tool for assessing the role of multiple pressures on lake ecosystems.
In this paper, we investigated the spatial and temporal changes of land cover/use and landscape structure of two connected Greek lakes that have exhibited a dramatic loss of water volume during the last four decades. We also examined the potential role of climate as a main driver of the water loss by assessing climate data (precipitation, mean air temperature, evapotranspiration and aridity index) for a thirty-year period and identify climate change trends in the area that might have influenced the pattern of the inter-annual water level fluctuations and land cover/use and landscape changes. Land cover changes were highlighted using remote sensing and spatial analysis techniques and the results were associated with major consequential ecological changes that have been recorded in the case study. Additionally, temporal landscape changes were examined using of commonly applied landscape metrics. Results were examined and discussed within the framework of the overall ecological impact.
Lakes in Mediterranean regions with high seasonality in rainfall have experienced a drastic shift in land cover/use over the last 50 years [14
]. Monitoring of changes in the land cover/use in the catchments of the lakes, and the subsequent environmental responses is essential for water resources management and water quality assessment [4
]. Our results showed that the hydrology, the landscape and the water quality of the studied lakes have changed dramatically due to the combined effects of the land cover/use changes and climate variability. The clear positive correlations among water level reduction, Aridity Index, and annual precipitation appear to imply two things. First, a decreasing trend in precipitation is expected to directly affect the inflow and the hydrological regime of all water bodies in the area. Second, the reduction of precipitation will possibly increase the need for irrigation water, which inevitably will lead to increased water abstraction from the adjacent lakes and standing waters. The above results are supported by the predictions of Nastos et al. [33
], which have shown that the climate is progressively shifting from humid to sub-humid and even semi-arid in several regions of Greece. Particularly, in the region of Macedonia, climate change scenarios have predicted that, in the near future (2021–2050), precipitation will decrease by almost 10%. Such predictions are likely to enhance the frequency and magnitude of drought episodes with adverse effects for the ecology, water level fluctuations, and water quality of the freshwaters in the area.
In this work a post-comparison classification was applied to detect temporal changes in land cover/uses over a period of 39 years (1972–2011). The issue with such approaches is the difficulty to produce consistent classifications for each image [35
]. There will always be an error in change detection especially when the errors in each classification are not correlated [35
]. Therefore, the accuracy of the change detection is highly dependent on the accuracy of the classification.
Moreover, pixel-based classification procedures are in general limited by the spatial resolution of the images [36
]. This practically means that objects smaller than the size of the pixel is not detected. In our case, the comparison made between land cover maps obtained from images with different spatial resolution (1972 MMS and 1984, 2002 and 2011 TM), contain an extra amount of uncertainty due to such limitations. Therefore, it is not unlikely that some of the observed differences between the land cover/uses are due to misclassification issues. For example, continuous transitions between two classes of land cover/uses when there is not a profound explanation could be attributed to mapping errors. Consequently, it is very possible that the assessment of the landscape changes is affected especially when comparing metrics calculated for classifications based on different spatial resolutions.
Nevertheless, regarding the changes in land cover/use most pronounced changes were the extent of irrigated cultivations, as well as a significant increase of 38.7% from 1972 to 2011 of reed beds, but a more thorough examination of the results reveals that this change mainly occurred over the period between 2002 and 2011 (Table 4
and Table 5
). A small expansion of the reed beds occurred from 1972 to 1983 due the water level drop that enabled the formation of large zones of reed beds in the shallower parts of the lakes. From 1984 to 2002, the reduction of the water level (by almost 16 m) caused a partial replacement of the reed beds by cultivations. While the extent of the surface of Lake Vegoritis increased during 2002–2011, the reed beds expanded again in the southern part of the lake due to flooding of former cultivated areas (Figure 4
and Figure 5
). These changes were confirmed by field observations during 2006–2008 where, apart from the reed bed a diverse submerged macrophytic community was identified [37
The increase of irrigated cultivations within the studied area from 1972 to 2002 by 1092 ha corresponds to almost 186% of its original extent (Table 4
and Table 5
). Our findings indicate that the increase of cultivations combined intensified irrigation due to possibly decreasing precipitation and AI
have contributed further to the water level decline, especially after 1985 when the hydro-electrical demand was minimized [19
]. However, there are studies that have shown that the effect of land use changes on the irrigation water demand is larger than the climate effect at a Pan-European scale [39
]. This practically means that the impact of the socio-economic driver is higher than the impact of the climate change. Moreover, although the climate change will affect the growth conditions of the crops, the impact on the demand for irrigation water depends on the capability of the farmers to adapt their crop production to the changing climate conditions. Nevertheless, in Greece, many lakes that serve as a source of irrigation water [40
] are likely to be subjected to seasonal water level fluctuations or other hydrologic alterations. From 1972 to 2011, the extent of shrubs and trees in the studied area showed a decrease of 37.5% along with significant changes in patch and edge density (Table 5
, Figure 6
), as well as, steppic grasslands and bare lands presented an increased fragmentation and patch density. These findings indicate signs of degradation of semi-natural and natural habitats with negative consequences for the local biodiversity [42
The LPI for the irrigated arable land showed a rising trend from 1972 to 2002, which indicates patch aggregation for the particular class. Similarly, the results for the class of reed beds reveal an increasing trend of LPI from 1972 to 2011, suggesting that the reed beds have expanded and now cover a significant area consisting of larger and larger patches. Regarding the edge complexity of both irrigated and non-irrigated arable land, the ED values increased greatly from 1972 to 2002 (Figure 6
), indicating that the boundaries of agricultural lands have become more complex and irregular over the years. It is very likely that the cultivations have expanded against the open water in a disorderly manner, which would cause an enhancement of the edge complexity. The expansion of arable land in many aquatic ecosystems in Greece is quite common, and particularly for the natural lakes the arable land is the most common habitat with a high fragmentation [41
These changes in the land uses were expected to have a significant impact on the water quality and the ecology of the studied lakes. It is well known that human pressures strongly influence the nutrient input in lakes [4
]. For example, an increase of agricultural land in the watershed will probably lead to higher lake concentrations of total nitrogen, total phosphorus and chlorophyll-a with implications for the aquatic communities [4
]. In our case, the water quality of Lakes Vegoritis and Petron has shown strong signs of deterioration during the last thirty years [12
]. Recently, it has been suggested by us that a shift from mesotrophic to eutrophic conditions might have triggered a decline of charophytes and dominance of nutrient-tolerant angiosperms [38
]. Similarly, a shift of the fish community towards zoobenthivorous dominance has been noted, which also reflects the increased eutrophication conditions [38
]. Although the eutrophication process was not assessed in this work, our results suggested that the increasing trends of conductivity and chloride concentrations could be related to the increased concentrations of total dissolved solids and nutrients of the agricultural runoff. Another important finding was the identification of strong correlations between the water level and the mean annual conductivity and chloride concentration of Lake Vegoritis. These results suggest a possible connection between water regime and water quality changes.
In general, hydrological regime alterations, such as a huge water level decrease, are known to affect the water quality, including conductivity, of the freshwater ecosystems in arid and semi-arid areas of the world [45
]. However, in our case, apart from the water level drop, we consider an additional driver as responsible for the observed changes. It is likely that the increase of irrigated arable land could have contributed to the rise of chloride and conductivity levels through extensive irrigation via groundwater wells (Figure 7
). Thus, through leaching and drainage, salts would eventually move from soils into rivers and lakes, affecting the water quality [46
], which would account for the high recorded values of chloride and conductivity. The increase in conductivity, chloride and nutrients and partially the hydrological alteration are disturbances related to the replacement of a significant part of the lakes’ surface by agricultural land and probably the subsequent intensification of irrigation. Water abstraction from surface water bodies enhances water level fluctuations, while the use of ground water for irrigation affects the conductivity levels, causing salinity problems over a prolonged period. At the same time, it is possible that excessive use of fertilizers increased the nutrient loading, shifting the lakes from a trophic to a eutrophic state.
Finally, the results from the remote sensing analysis suggested that the water level alterations are important for the composition of the littoral macrophyte communities. Not only the extent of the water level fluctuations [14
] but also the morphometry and the bathymetry of the lake determine the extent of the littoral zone [47
]. This is obvious for Lake Vegoritis where the expansion of the reed beds was limited only to the shallower southern part (Figure 4
). Lake Petron, on the other hand, is a shallow lake, allowing progressive expansion of the reed beds over the whole littoral area as shown by the results of the remote sensing analysis.
5. Conclusions—Implications for Restoration
The results showed that the expansion of the reed beds against the lakes’ open water is associated with the intensification of the agricultural activity that occurred the last decades. Moreover, apart from the increase in phosphorus and nitrogen loading, the agriculture activity is likely to have affected the hydrology and water chemistry of surface water bodies through the application of non-sustainable irrigation practices. The possible use of groundwater as irrigation water, without any kind of limitation or control, is very likely to ultimately cause water logging and water quality problems and, eventually, affect the aquatic communities [46
]. The climate, however, appears to also play a key role in controlling land cover/use dynamics and hydrological processes. The findings of this work suggest that the climate trend of precipitation within the studied area may have affected the hydrology and indirectly the water quality of the lake ecosystems.
As these are ongoing procedures strongly related to the spatio-temporal dynamics of the land cover/use, the development of an effective management plan is vital for reducing further the adverse implications for the ecosystem. Any restoration effort should be made within the context of a land management strategy in accordance with environmental policies. Furthermore, the management and restoration measures should be implemented taking into consideration the future impacts of the climate change. Clearly, the increase in agricultural land use poses a significant threat to the water quality and ecological balance of the studied lakes. Therefore, water management schemes should implement practices that are consistent with sustainable irrigation [48
]. Monitoring and controlling the amount of applied water, both from surface and ground water, should be considered as a main priority for public agencies. Irrigation water pricing and/or allocation are practices that can motivate the farmers to use water efficiently [49
]. Additionally, the extensive use of agricultural drainage water should be avoided or at least the water should be reused for irrigation of salt tolerant crops when this is possible.
All the above recommendations combined with an effective reduction of nutrient loading can provide a baseline strategy for mitigating the impacts of eutrophication, salinization and water level fluctuations on the aquatic communities. The results of this study underline the importance of the implementation of a water management plan including specific actions targeted on the development and implementation of sustainable agriculture policies.