Human sources of nutrients Nutrients in seawater present a paradox. Nutrients are, of course, essential for life. In the oligotrophic environment of the Mediterranean, the ecosystems with the most nutrients are gen- erally the most productive and diverse. At the same time, many Mediterranean nearshore areas are threatened by nutrient over- enrichment due to coastal and watershed development. Mu- nicipal sewage is the big offender, followed by fertiliser run-off from agricultural areas, lawns and golf courses. The problem is particularly acute in shallow sub-basins with limited flushing, common features in parts of the Adriatic and along the Mediter- ranean’s southern shore.
Many developed coastal areas suffer particularly from increased influx of dissolved nitrogen and phosphorus. Sources include untreated human sewage, animal waste, transportation, ferti- lisers and industrial discharges. The largest emitters of nitro- gen are urban wastewater treatment (45 %), livestock farming (24 %) and the organic chemical industry (2 %). Ammonia emis- sions from animal manure used as fertiliser also contribute ni- trogen. The main sources of phosphorus are fertiliser manufac- turing (40 %), livestock farming (39 %) and urban wastewater treatment (13 %) (UNEP/MAP/MED POL 2012). Although the overall inputs of nitrogen (about 1,5–4,5 million tonnes per year) and phosphorus (about 0,1–0,4 million tonnes per year) are low compared to some other seas (e.g., Black Sea), these nu- trients are problematic in coastal areas (UNEP/MAP/MED POL 2005). According to National Baseline Budget 2008 data, total nitrogen is mostly emitted by wastewater treatment plants, animal farms, the organic chemical industry in the northern Mediterranean countries and the tanning sector in the south- ern and eastern shores. The fertiliser industry is the main source of total phosphorus, especially in those producer countries like Tunisia, Algeria, Lebanon or Greece. Aquaculture is also report- ed as an important source of nutrients and suspended solids. Although total discharges are not comparable to other sectors, they can lead to localised impacts in the marine environment. Spain, Greece, Turkey, Italy and Croatia are the countries with the largest marine aquaculture development (UNEP/MAP/MED POL 2012).
Eutrophication and the human impact In natural aquatic ecosystems, high concentrations of nutri- ents can result in rapid growth of phytoplankton, a process called eutrophication (UNEP/MAP 2009). Algal blooms are a common natural phenomenon associated with eutrophica- tion. In addition, there is a shift in phytoplankton species composition, with large species favoured over smaller ones. Long-living and slow-growing plants cannot compete with fast-growing algal species. Since the larger plants provide cover and food for fish and substrate for invertebrate or- ganisms, biodiversity suffers. Although eutrophication is a natural aging process of a water body, added nutrients from human activity can greatly speed up the process with result- ing negative impacts on the ecosystem.
HUMAN PRESSURE, STATE AND IMPACTS ON MEDITERRANEAN ECOSYSTEMS 51
Agriculture is the largest non-point source of pollutants in the Mediterranean (UNEP/MAP 2011). Agriculture-related nutrients enter the sea through groundwater, lakes, wetlands, and rivers. Nitrogen consumption per surface unit of arable land is highest in countries of the northern watershed, with the exception of Bos- nia-Herzegovina and Albania. In contrast, point-source release is highest on the eastern coast of the Adriatic. Other point sources of nitrogen are concentrated in the Ebro watershed, the eastern coast of the Levantine Basin and the western coast of Tunisia.
Direct effects of nutrient overenrichment
The most eutrophic areas of the Mediterranean are linked to the mixing of nutrients from deeper waters through intense mesoscale circulation (Alboran Sea), local tidal mixing (Gulf of Gabes), or the input and alongshore redistribution of nutrients from large rivers. In addition, high chlorophyll and productivity levels have been found near large urban areas.
The Mediterranean phytoplankton community is not well de- scribed, but it is believed to be changing, along with the rest of the sea’s ecosystem. Changes in nutrient concentrations and ratios sug- gest a shift in the distribution of nutrients and, therefore, of phyto- plankton species (UNEP/MAP 2009). Macrophytes such as Cystoseira spp., Dictyota spp. and Halymenia spp. are in decline and are being replaced by short-lived algal species (UNEP/MAP/MED POL 2005).
One of the most serious effects of eutrophication comes from al- gal blooms or red tides. At least 57 species of algae are reported to cause algal blooms in the Mediterranean (UNEP/MAP/MED POL 2005). During blooms, algae accumulate quickly, causing a discoloration of the water column. When marine algae occur in significant numbers and produce biotoxins, the result is harmful