3.3. reservoir and lake eutrophication

3.3. Reservoir and Lake Eutrophication

The characteristics of the eutrophication processes in reservoir and lakes, are quite specific in the semi-arid areas of the EEA. Not only do the problems derived from eutrophication affect both the quality of water for irrigation and human consumption, but they also have an adverse effect on the river and reservoir fauna.

An increase in nutrients (nitrogen and phosphorus), and thus eutrophication in semi-arid areas, is enhanced by temperature and light, which are the two predominant factors in biological production. Phosphorus is usually the main nutrient responsible for freshwater eutrophication, whereas nitrogen is the primary nutrient causing eutrophication of coastal areas and seas.

In contrast to the wet areas of Europe, potential evapotranspiration exceeds precipitation in the semi-arid zones. As result, many reservoirs and lakes lose large amounts of water of good quality not replaced by precipitation. The concentration of salts reaches much higher levels than in the wet zones, particularly those salts (phosphorus) which serve as nutrients for algae.

In most cases in semi-arid zones there is a natural tendency for reservoirs to lose their oligotrophic characteristics unless they are much more closely monitored and protected by legislation than in the wet zones. The eutrophic and mesotrophic levels are often high, without this having been induced by major incidences of contamination.

Erosion of the basins is another phenomenon that produces eutrophication, because this brings about an increasing accumulation of sediment in the reservoir which, in turn, gives rise to a greater exchange between the dissolved elements present in the water and the materials lying on the bed.

The fact that the water resources are scarce in semi-arid areas, means that there is a greater degree of water re-use and, therefore, are more susceptible to eutrophication processes than the areas having more abundant supplies.

3.3.1. Spain

The General Directorate of Hydraulic Works has carried out fieldwork to measure and study the limnological state of its reservoirs. Data are available for 411 reservoirs which, although this accounts for over 40% of the total, have been chosen so that they cover 85% of the country’s water storage capacity. By extrapolating the results obtained from this limnological survey, it can be deduced that 31% of the reservoirs, containing 40% of the water volume stored, are eutrophic. Such figures are self-explanatory when comes to assessing the seriousness of the problem (MOPTMA, 1993). Table 3.3-1 summarises the outcome of the study.

Table 3.3.-1 Trophic status of studied Spanish reservoirs.

A total of 113 reservoirs out of the 411 studied ones are located in basins having water scarcity problems. Table 3.3-2 shows the trophic status of reservoirs that are located in semi-arid areas in Spain.

Table 3.3.-2 Trophic status of reservoirs in semi-arid basins

In view of Table 3.3-2, 31% of the reservoirs in semi-arid areas have eutrophication problems. This figure is the same that the percentage calculated for the total studied reservoirs. Therefore the eutrophication is a major problem in southern areas of Spain. To group the reservoirs into the same trophic degree, different criteria have been taken into account, such as CarlsonŽŽŽŽs index, dissolved oxygen distribution, total nutrient content, plankton composition, chlorophyll-a concentration and the probability distribution curves of trophic degree by OECD 1982.

Figure 3.3-1 shows the average annual concentrations of nitrogen and phosphorus in the reservoirs located in semi-arid basins. Data correspond to the average concentration of nitrogen and phosphorus in the sampled reservoirs of each basin, therefore, a mean concentration is obtained for each semi-arid catchment area.

Figure 3.3.-1 Nitrogen and Phosphorus concentration in Spanish reservoirs.

Apart from assessing nitrogen and phosphorus concentration, other indicators for determining whether or not the reservoirs have eutrophication problems are the existence of stratification, the mean oxygen concentration measured close to the bottom

and the mean surface chloride concentration. Table 3.3.-3 shows the figures related to these features.

The Guadalquivir catchment area appears to have reservoirs with major eutrophication problems; 49% of the reservoirs are eutrophic and have the highest surface chloride average concentrations and the lowest value of oxygen measured in deep waters. The average nitrogen concentration is also high, (maximum 8.4 mg/l). On the other hand, the reservoirs located in the Jucar and Sur catchment areas are in a better situation, the latter having high values of oxygen measured at low depth, and low values of surface chloride.

3.3.2. Portugal

The Directorate of Water Resources in Portugal has carried out a classification of the trophic state of Portuguese reservoirs. As a result of these works, 62 reservoirs turn out to be classified as being oligotrophic and only 10 show eutrophication problems.

Data from Portuguese reservoirs are summarised in table 3.3-4 (INAG, 1995b):

Table 3.3.-3 Main characteristics of sampled reservoirs in Spanish semi-arid basins (*).

(*) Notes to the headings of columns:

Basin: Tot. No is the total number of reservoirs in the basin;
Nitrogen: No. is the number of sampled reservoirs; Aver. is the mean average concentration in mg/l; Min. is the minimum average concentration in mg/l; Max. is the maximum average concentration in mg/l.
Phosphorus: The same as for Nitrogen;
Cl: Aver. is the mean average surface Chloride concentration in mg/m³;
Strat: No. is the number of reservoirs having water stratification;
O₂: Aver. is the average of the minimum value of Oxygen concentration in deep waters -in mg/l- obtained from different sampling campaigns.

Table -4 Trophic status of Portuguese reservoirs and lakes.

As can be observed from Table 3.3-2 and Table 3.3-4, the eutrophication problems in Portuguese reservoirs are not as serious as in the Spanish ones.

Figure 3.3-2 shows the data concerning reservoir and lake eutrophication in Portugal (INAG, 1995b).

Figure 3.3.-2 Reservoir and lake eutrophication in Portugal

In Figure 3.3-2 the mean annual concentrations of nitrogen and phosphorus are expressed by lake or reservoir, as well as the mean annual precipitation. Close to the name of each lake or reservoir, the eutrophic degree (E, M or ME) and the region number (1 to 4) to which the reservoir belongs are stated. (See Table 2.2-1).

Reservoirs having major eutrophication problems are located in regions 1 (Alto Douro) and 2 (Sul Tejo). The trophic degree has been assessed according to the chlorophyll-a concentration, the Secchi disk depth and the content of phytoplankton in water.

3.3.3. Italy

In the island of Sardinia, there are four lakes with eutrophication problems. Table 3.3-5 indicates the concentration of phosphorus and nitrogen in mg/m³.

Table -5 Reservoir and lake eutrophication in Sardinia

Notes:

(*) Sampled between 0 and 15 m. depth.
(**) Sampled between 20 and 70 m. depth.

3.3.4. Greece

There are six lakes where eutrophication is detected in Greece. Concentration of nitrogen is expressed as mg/l NH₄-N, ranging from 0.03 to 1.73 mg/l. phosphorus concentration ranges between 0.07 and 0.25 mg/l P, on average since 1992. Figure 3.3-3 shows these data:

Figure 3.3.-3 Lake eutrophication in Greece

Eutrophication problems in lakes 1,2,3 and 4 are due to wastewater discharges. Lake 5 is also receiving wastewater, and is also located nearby an area having water scarcity problems (Central Macedonia). Lake 6, located in the Attiki area, tends to have eutrophication problems, showing seasonal increases in nutrient concentrations.