The monthly variation of the examined parameters is presented in Fig. 2. Monthly rainfall during the studied period ranged between 0 mm in October and 101 mm in November (Fig. 2). Water inflows from Pinios River and from the watershed runoff through a system of constructed ditches as well as through collector channels. The discharge from Inflow 1 contained several peaks coinciding with the precipitation pattern (Fig. 2) while the discharge from Inflow 2 was greater during the months April- May. The lake stored water volume ranged between 29.5 × 106 and 56 × 106 m3, while the lake level at the full supply level (FSL) of 46.35 m was recorded in June. The lake level started to increase from March 2012 just after the first inflow event in February 2012.
Functioning of water bodies depends on water balance, which depends on groundwater and surface water hydrology. The watershed area of Lake Karla is characterized as intensive productivity area, thus, requiring over-pumping of groundwater which has led to deterioration of the aquifer during the past decades. During the winter period, precipitation feeds the lake while, at the moment, the Karla reservoir experiences intra-annual water level fluctuations due to the management of the inflows from Pinios river through a system of constructed ditches.
The water temperature ranged from 7 °C in February to 31.5 °C in August. The dissolved oxygen (D.O.) concentration ranged between 2.24 mg/L (in October) and 18.0 mg/L (in January) (Fig. 2) and it was highly correlated with Secchi depth (r = 0.851; p < 0.01) explaining that water clarity coincides with high D.O. values. The pH values were ranged between 7.43 and 9.12 without any apparent seasonal variation. The water conductivity was relatively high, ranging between 2.11 mS/cm (in March) and 3.68 mS/cm (in September) (Fig. 2) which may be attributed to the geological profile of the whole catchment (Jouni 2011) but also to runoff from the surrounding agricultural area. The values of the in situ measured parameters showed no differences among stations, thus suggesting the absence of any spatial heterogeneity. With regard to nutrient concentrations, Nitrate-N was the most important form in the DIN pool varying between 0.1 and 1.7 mg/L, exhibiting higher values during the summer months. Nitrate-N correlated with water temperature (r = 0.401; p < 0.05), thus, indicating high decomposition rates during the warm period. Ammonia-N ranged from 0.011 to 0.3 mg/L, exhibiting higher values during the spring. Also, during February and April, Ammonia-N concentrations were above the value of 0.2 mg/L, which is the limiting value for fish intoxication, according to the European Directive 2006/44. Total Phosphorus (TP) ranged from 0.05 to 0.461 mg/L, peaking in April (Fig. 2). The values of nutrients showed no differences among stations. Chlorophyll-α ranged between 16.7 and 403.58 mg/L, with higher values during the warmer months indicating a strong hypertrophication (Fig. 2). The seasonal pattern of Chlorophyll-α was also confirmed by correlation analysis, linking it with water temperature (r = 0.549; p < 0.01). Secchi depth varied between 0.19 and 0.5 m with low values persisting during the summer months, thus reflecting the turbid character of the water body.
A reduced water transparency due to organic material and plankton, along with the presence of frequent algal blooms (Anabaena sp., Aphanizomenon sp.) reported by Ananiadis (1956), suggests that the lake has been eutrophic since at least the 1950s. The water analysis through the water column, carried out in 1955, revealed that Karla was relatively rich in dissolved nutrients and low dissolved oxygen concentrations near the bottom, classifying it as eutrophic. According to the present nutrient and chlorophyll-a profile, it becomes clear that Lake Karla is a eutrophicated system, with apparent signals of hypertrophication during the warm period (OECD 1982). The absence of any outlet, and thus of any flushing process, leads to a high water residence time, strengthening the eutrophic conditions. During the study period, the lake reached a maximum depth of 2.5 m, rendering Lake Karla a shallow system, with the typical temperature pattern of the Mediterranean region, i.e., two well-separated hydrological periods. Hydrological balance, in terms of quantity and periodicity of the water resource appear to be a strong control factor on the concentrations of major ions and nutrients (nitrogen and phosphorus) for most shallow lakes and reservoirs of Mediterranean climate (Beklioglu et al. 2007). As a result, sensitivity to hydrological conditions has also significant consequences for the in-lake concentration of nutrients. In shallow Mediterranean lakes, nutrient inputs from the catchment occur mainly in winter–spring due to high precipitation, while in cases of outflow absence, they act as nutrient sinks. The reduction of the residence time by regulating the annual timing of the inflows and outflows could certainly aim at the improvement of water quality in Lake Karla. This is considered as an emergent issue concerning the future management process.
Chiaudani and Premazzi (1986) consider that phosphorus is the key element, even though many factors determine aquatic ecosystem plant biomass. In unaffected lakes and reservoirs, the concentration of phosphorus is below 25 μg/L (EEA 1999). Lake Karla’s phosphorus concentration is very far exceeding this limit value, suggesting strong anthropogenic impact. Furthermore, Jouni (2011) reported phosphorus accumulation in the bottom sediment of the lake, suggesting an internal loading process. Nutrient release from the sediment has been reported for lakes, lagoons and estuaries highlighting that, it might be an important input of nutrients (Beklioglu et al. 2007; Gikas et al. 2006; Markou et al. 2007). Regarding the Mediterranean systems, the low water level will often enhance resuspension and, together with the higher temperature, amplify the sediment release of nutrients, especially of phosphorus (Ozen et al. 2010). Since lake Karla has experienced a long period of dryness accompanied by intense application of fertilizers, the nutrient release from the sediment and the enrichment of the water column should be tested in order to eliminate the internal loading process. Nitrogen plays a secondary role, but can become important at a high level of eutrophication. In Lake Karla, both nutrients are in excess, thus favouring the excess of algal biomass and specifically of the cyanobacteria, whose occurrence and persistence are linked to high eutrophicated conditions (Papadimitriou et al. 2013).
PCA has been a widely used tool to identify relevant groups of water and the most important factors affecting water quality variation. A PCA was performed to evaluate the main parameters influencing water quality in Lake Karla (Fig. 3). The first two components explained 92.7 % of the system’s variability. The first axis accounted for 57.7 % of the total variance and chlorophyll-a, DIN, nitrate and temperature were the main factors contributing to it. The second axis explained 35 % of the data variation and was highly correlated with ammonium-nitrogen, Secchi depth, TP and conductivity. The PCA also shows separation between summer and winter, related to ammonium, Secchi depth, TP and Conductivity. It is characteristic that the clear water phase of the lake, in terms of Secchi depth, is related to the Spring period and further to adequate dissolved oxygen content. In addition, the PCA analysis and, generally, the multiparametric response functions clearly demonstrate their usefulness as “tools” in order to describe the complexity and the variability of the shallow aquatic ecosystems (Gikas et al. 2006; Scheffer et al. 1993, 2001).
The Water Framework Directive (WFD, EC, 2000/60) is potentially the most significant piece of legislation ever enacted in Europe in the interests of conservation of fresh and saline ecosystems. The aim of the WFD is to ensure sustainable management of groundwater, freshwater and marine water in the European Union, so that a minimum “good ecological status” is obtained by year 2015. The characterization of the ecological status of each water body in Greece has become a legal imperative after the approval of the WFD.
Lake Karla, in terms of typology, is considered as an artificial lake, and furthermore, as a highly modified system. Since there are not established pristine reference conditions for this lake type based on biological data, as it is suggested by the WFD, the classification into categories is quite unsafe. Based on the present algal biomass data, comparable data from similar case studies, and based on our experience from our biological data during the same monitoring period (Papadimitriou et al. 2013; Oikonomou et al. 2012), we could argue that the water status of lake Karla falls into ‘poor’ or ‘bad’ category. River Basin Management Plan of Thessaly Water District provides also valuable scientific information about regional water bodies classification (Ministry of Environment 2012) based on the physicochemical parameters. Taking into consideration the boundary values suggested only for Secchi depth, Total Phosphorus and Nitrogen compounds, since Karla reservoir is a shallow, non-stratified water body, we could classify it into ‘moderate” class. Secchi depth is always lower than 0.6 m, the mean total phosphorus value (85 μg/L) exceeds the threshold of 30 μg/L, while the Inorganic Nitrogen fraction frequently exceeds the total nitrogen limit established for the “good” quality. Therefore, we can assume that the possibility to obtain a ‘good’ ecological status by 2015 is extremely weak.