Results and status, including trends (CI13)

The trophic status of the Mediterranean Sea is controlled by the highly populated coastal zone and the riverine input from a draining area of 1.5 million km2 (Ludwig et al. 2009) that induce eutrophic trends in coastal areas. The blue offshore waters of the Mediterranean have been characterized as extremely oligotrophic with an increasing tendency for oligotrophy eastwards (Turley 1999). Eutrophication and oligotrophy in the Mediterranean is illustrated as chlorophyll a distribution in remote sensing imagery (Figure 1). It is observed that the Eastern Mediterranean Sea (EMS) is still the most oligotrophic area of the whole Mediterranean basin. This is due to the low nutrient content of EMS; the maximum concentrations recorded for nitrate were about 6 μmol L-1, for phosphate 0.25 μmol L-1, and for silicate 10–12 μmol L-1, with the nitrate to phosphate ratio (N/P) >20 and in deep waters about 28:1, the EMS has been characterized as the largest phosphorus-limited body of water in the global ocean.

The coastal area of the southeastern part of the Mediterranean shows clearly eutrophic trends. Although the River Nile is the major water resource in the area, its freshwater fluxes are becoming limited because of the Aswan Dam and increasing trends in anthropogenic water use in the lower Nile. Eutrophic conditions in the area are mainly induced by the sewage effluents of Cairo and Alexandria. The Northern Aegean shows mesotrophic to eutrophic trends. This can be explained by the river inputs from northern Greece and the water inflow from the nutrient rich Black Sea.

The nutrient regime and primary productivity in the Western Mediterranean Sea (WMS) are relatively higher compared to the EMS. There is limited nutrient supply through the Strait of Gibraltar due to different nutrient concentrations between the Atlantic and Mediterranean waters. The surface water entering from the Atlantic carries nutrients directly available for photosynthesis (EEA 1999) but at low concentrations. The phosphorus (phosphate) concentrations in the inflowing waters ranges from 0.05 to 0.20 μmol L-1, the nitrogen (nitrate) concentrations being about 1–4 μmol L-1, and the silicon (silicate) concentration is about 1.2 μmol L-1 (Coste et al. 1988). The nutrients of the surface layer are reduced as they propagate eastwards due to mixing with poor basin water and nutrient use by phytoplankton. However, the primary productivity of the main WMS, away from the coastal areas and influenced by rivers and urban agglomerations, is still higher than the primary productivity in the EMS.

The main coastal areas in the Mediterranean which are historically known to be influenced by natural and anthropogenic inputs of nutrients are the Gulf of Lions, the Gulf of Gabès, the Adriatic, Northern Aegean and the SE Mediterranean (Nile–Levantine). A recent work on nutrient and phytoplankton distribution along a large-scale longitudinal east–west transect (3 188 km) of the Mediterranean Sea extended over nine stations was published by Ignatiades et al. (2009). The results confirmed the oligotrophic character of the area and the nutrient and chlorophyll gradient characterized by decreasing concentrations from Gibraltar to the sea of Levantine. Phosphate maxima ranged from 0.05 to 0.26 μmol L-1, nitrate from 4.04 to 1.87 μmol L-1, chlorophyll a (chla) from 0.96 to 0.39 µg L−1.

The results of assessment and status of the key nutrients concentration in the water column are presented on Figs 3-5 showing a rather limited figure of the Mediterranean region. The main reason is the data availability and quality. On the Figure 2 are clearly visible that for the great part of the region data are missing. The implementation of water type criteria for the purpose of IMAP are also limited. Even a rather weak criteria (10 samples in 10 years in surface layer - <= 10 m) were adopted the data availability for assessment were low.

Figure 2
Figure 2: Stations in the Mediterranean region for which nutrient concentrations were sampled. Also are shown the water types (applicable for phytoplankton; IMAP, 2017) were minimal statistical requirements were satisfied (10 samples in the last 10 years and in the surface layer, <= 10 m)

Figure 2 presents the stations in the Mediterranean region for which nutrient concentrations were sampled used for the assessment. On Figures 3-5 data for the Adriatic and Aegean-Levantine sub regions for dissolved inorganic nitrogen (DIN) and total phosphorus (TP) were presented. DIN and TP concentration show a characteristic variability for both coastal sea (Adriatic and Aegean-Levantine Sea) indicating that no hotspot is present for DIN and TP.

Figure 3
Figure 3: Box and whisker plot for dissolved inorganic nitrogen (DIN) concentration (µmol L-1) in the Adriatic Sea sub region (water type IIA) for Croatia (CRO) and Slovenia (SLO)
Figure 4
Figure 4: Box and whisker plot for Total Phosphorous (TP) concentration (µmol L-1) in the Adriatic Sea sub region (water type IIA) for Croatia (CRO) and Slovenia (SLO)
Figure 5
Figure 5: Box and whisker plot for dissolved inorganic nitrogen (DIN) concentration (µmol L-1) in in the Aegean-Levantine Sea subregion (water type IIIE) for Cyprus (CYP) and Israel (ISR).

The available data shows that in areas where assessment is possible the key nutrient concentrations are in ranges characteristic for coastal areas and in line with the main processes undergoing in the interested area. The result also confirms the validity of this indicator in assessing eutrophication.