تجزیه و تحلیل اثرات فصول خشک و مرطوب در حوضه دریای مدیترانه: پیامدهایی برای آبهای ساحلی و مدیریت کیفیت آن

Rivers play a major role in the delivery of nutrients to coastal ecosystems which are essential for ecosystem productivity. However, the increase of nutrients due to anthropogenic activities can cause eutrophication problems. This study analyzes the seasonal variation of phytoplankton communities in the coastal receiving waters of a Mediterranean river. Two scenarios are compared: the wet and the dry season with distinctive characteristics. During the wet season agricultural runoff and combined sewer overflows (CSO) were responsible for nutrient discharges, while during the dry season partially treated effluent from wastewater was the main nutrient source. In the receiving waters, diatoms typical seasonal cycle was modified by CSO discharges during rain episodes, while dinoflagellate abundance was higher in the dry season due to partially treated effluents discharges and low turbulence. We recommend that the design of the Water Framework Directive monitoring programs should take into account wastewater treatment plants and combined sewer systems located near the coast. Management decisions should take into account that only reductions in CSO and partially treated summer effluent are likely to be efficient in the short term. Analyzing the corrective measures cost through a cost–benefit analysis would help to determine whether the costs are excessive or not.

Estuaries and coastal areas receive high loads of nutrients of terrestrial origin, either from point sources, which flow out at discrete and identifiable locations (e.g. rivers, submarine wastewater outfalls) or non-point sources, which are rather diffuse and highly variable from year to year depending on climate and rainfall (e.g. surface and groundwater runoff) (Paerl, 2006). Among nutrients, nitrogen and phosphorus are both required to support marine productivity and are the key limiting nutrients in most aquatic and terrestrial ecosystems. However, the increase in these nutrients due to anthropogenic activities requires their inputs into coastal marine ecosystems to be reduced in order to minimise eutrophication problems (Paerl et al., 2010). The influence of these terrestrial inputs is especially important in the marine oligotrophic areas, such as the Mediterranean Sea (Romero et al., 2007). Several policies have been adopted to reduce nutrient inputs with varying degrees of success depending on the source (Artioli et al., 2008). While improvements in wastewater treatment have achieved reductions in inputs of phosphorus and nitrogen from point sources, the largest nutrient contribution for coastal marine environments is from diffuse sources, especially those from agricultural land runoff (fertilizers), and reducing this is a difficult and slow process (Carstersen et al., 2006). Then, there is a tendency to focus European Union (EU) policies on non-point sources rather than on point sources, which are supposed to have been successfully addressed in past legislation (Torrecilla et al., 2005). Rivers play a major role in the delivery of nutrients to coastal ecosystems, both from natural (e.g. silicates weathering) and anthropogenic (urban and industrial wastewater, agriculture runoff) sources. Wastewater treatment plants (WWTPs) can discharge treated effluent to rivers or to coastal areas through submarine wastewater outfalls. The objective of the 91/271/EEC Council Directive, concerning urban wastewater treatment, was to protect the environment from the adverse effects of wastewater discharges. Different studies have demonstrated that the submarine outfall discharges of treated effluent which comply with the Directive, have either no significant effects or only minor effects on the quality of receiving waters (Juanes et al., 2005). However, wastewater discharge can cause water quality problems in rivers and this is especially relevant in Mediterranean rivers because of the hydrologic singularities of the Mediterranean climate and fluvial regime (Torrecilla et al., 2005). In Mediterranean-type climate regions (areas surrounding the Mediterranean Sea, parts of western North America, parts of western and southern Australia, the south-west of South Africa, and parts of central Chile), characteristic precipitation is scarce and torrential and has an extremely high spatial and temporal variability. Natural river discharge, driven by precipitation variability, has two distinct seasons: wet and dry (Gasith and Resh, 1999; González-Hidalgo et al., 2005). Thus, wastewater discharges present different problems depending on the season. During the wet season combined sewers overflows (CSO) can reach receiving waters without treatment (Clark et al., 2007). The importance of storm overflows for water quality have been recognized by the 91/271/EEC Directive: “the design, construction and maintenance of collecting systems shall be undertaken in accordance with the best technical knowledge not entailing excessive costs, notably regarding limitation of pollution of receiving waters due to storm water overflows” (Annex I, 91/271/EEC Directive). This importance is also recognized by the USA legislation in the CSO Control Policy which was published on April 19, 1994 by the United States Environmental Protection Agency (USEPA). During the dry season, urban sewage effluents are the main freshwater source in ephemeral streams in arid to semiarid areas of the world (Brooks et al., 2006; García-Pintado et al., 2007) and can account for up to 90% of the total river flow in some Mediterranean rivers such as the Serpis River (Molinos-Senante et al., 2011). In addition, fluctuating winter and summer populations in tourist areas, typical on Mediterranean coasts, provoke significantly variable wastewater loadings that can exceed the capacity of the treatment facilities (Aguilera et al., 2001; García-Pintado et al., 2007). Several studies have addressed the quality problems of Mediterranean rivers receiving wastewater discharges, for instance, on the Mediterranean coast of the Iberian Peninsula, Torrecilla et al. (2005) studied the Ebro River (NE Spain) and Molinos-Senante et al. (2011) studied the Serpis River (Eastern Spain). On the other hand, studies on receiving coastal waters have mainly addressed the water quality in the submarine outfall areas (Aguilera et al., 2001) and not river mouths. Some studies of Californian recreational bathing beaches have addressed the effects of CSO discharges but have focused on fecal indicator bacteria (total coliforms, fecal coliforms and enterococci) and benthic invertebrates (Schiff et al., 2003; Clark et al., 2007). Nutrient inputs into coastal ecosystems can induce eutrophication problems, among these, harmful algal blooms of species responsible for the synthesis of toxins and high-biomass producers that can cause hypoxia and anoxia and indiscriminant mortalities of marine life after reaching dense concentrations (Heisler et al., 2008). As regards marine recreational bathing beaches, these effects can cause beach closures that negatively impact the local economy of tourist areas. Thus, adequate management of nutrient sources is essential because clean water and healthy coastal habitats are clearly fundamental to successful coastal tourism which is characteristic of the Mediterranean climate regions (Hall, 2001). However, to develop appropriate management strategies, it is necessary to fully understand how ecosystems function by first of all establishing the relationships between phytoplankton and nutrient sources and patterns. This study analyzes the seasonal variation of phytoplankton communities in the coastal receiving waters of the Serpis River input, which is a Mediterranean river basin in Spain. A comparison is established with marine waters to assess different patterns between high and low terrestrial influence areas. Two scenarios are analyzed: the wet and the dry season with distinctive characteristics. The aim is to provide information about ecosystem functioning that could help management decisions and could be extrapolated to other Mediterranean climate areas with similar characteristics.

In the Mediterranean region, the most common measures used to achieve the Water Framework Directive (WFD) quality targets for rivers and reservoirs involve increasing the quality of effluent produced from WWTPs in accordance with the 91/271/EEC Directive. Such measures have been recommended by the CHJ for the WWTPs discharging upstream the Beniarrés reservoir in the Serpis River Basin. However, in coastal WWTPs, which discharge effluent through submarine outfall, the quality of treated effluent is not the main problem. The main disturbances are localized near the coastline due to 1) CSO discharges, directly near the river mouths and subsequently into receiving coastal waters during rain episodes, and 2) partially treated effluent discharges due to the summer population increase. The importance of such discharges is such that despite the different magnitude of average annual flow of the Serpis River (3 m3 s−1) as compared with the Ebro River (384 m3 s−1) the nutrient concentration in the receiving coastal waters is comparable. The diatom seasonal cycle was modified by CSO discharges during rain episodes, while dinoflagellate abundance was higher in the dry season when partially treated effluents were the main source of freshwater and turbulence was low. The predicted increase in water residence time due to Gandia Harbour enlargement, as described in Sebastiá and Rodilla (2012) could be an additional factor that may trigger dinoflagellate blooms in this area. The relevance of this problem is recognized in Annex I of the 91/271/EEC Directive, but the adoption of preventive measures is conditioned by economic considerations. Before implementing any management measures, the use of simulation models would be recommended to ensure that these measures have the anticipated effects on eutrophication problems. Taking into account that both agricultural land use and WWTP discharges are important sources of nutrients, different scenarios should be explored: e.g. different percentages of reductions in agricultural nutrient inputs and different percentages of reduction in CSO. In addition, different locations (depth and distance from the coast) should be simulated considering an increase in the submarine outfall flow; to make sure that the disturbances will not be relocated from the river mouth to the submarine outfall area. After selecting the best management option, analyzing the cost through a cost–benefit analysis would help to determine whether the costs are excessive or not. This may avoid that management measures will not be implemented due to economic considerations. Management decisions should take into account that only reductions in CSO and partially treated summer effluent are likely to be efficient in the short term, while a reduction in diffuse agricultural sources, especially nitrogen, is only likely to give results in the long term due to the aquifer higher residence time.