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Ecological and specific biological characteristics of the Adriatic Sea

The Adriatic Sea is the northernmost extension of the Mediterranean Sea, and highly influenced by the European continent. There are the following six specific characteristics of the Adriatic Sea. 1) There is very strong influence of northern winds in winter, which transform the Surface Northern Adriatic Water (SNAW) into the Northern Adriatic Dense Water (NASW), initiating general Adriatic and Eastern Mediterranean (Levantine) circulation. The river Po discharge is forcing the general cyclonic circulation, which is composed of the northwesterly ingoing Eastern Adriatic Current (EAC), and southeasterly outgoing Western Adriatic Current (WAC). 2) The river Po is the most important source of nutrients and eutrophication, in the northern Adriatic. On the other hand, EAC is bringing the Eastern Mediterranean oligotrophic water from the Eastern Mediterranean (Ionian) Sea, along the eastern coast. Annual maximum of the river Po discharge is correlated with the maximum appearance of phytoplankton biomass. 3) There is an across-shelf and longitudinal trophic gradient in the Adriatic, resulting in the asymmetric distribution of biota. The contribution of smaller phytoplankton cell size fractions in total primary biomass is greater in the southern than in the northern Adriatic. Contribution of bacterial biomass in total microbial biomass is greater in the southern Adriatic than in the northern Adriatic. 4) The Adriatic is influenced by a small tidal range. 5) Karstic hydrogeology and small karstic rivers discharging along the eastern coast form specific highly stratified estuaries, where subsurface marine phytoplankton blooms may sometimes occur (bellow the halocline). The karstic rivers bring a small content of nutrients, resulting in the oligotrophic character of the eastern coastal Adriatic Sea. Primary production is phosphorus limited. 6) The two Adriatic depressions, Middle Adriatic (Jabuka) Pit and South Adriatic Pit play important role in the biology of the Adriatic Sea. Microorganisms and microbial loop processes participate in recycling of non-living organic matter and ecosystem functioning during summer oligotrophic conditions. Among zooplankton, the greatest metabolic activity is provided by a small zooplankton. The two Adriatic pits are characterized by appearance of specific open sea zooplankton, composed by euphausiids – large zooplankton, known as a krill in oceans. There we can also find mesopelagic and deep zooplankton. Specific environmental conditions in the eastern coastal Adriatic Sea provide specific assemblages of plankton, benthos and nekton. 7) Anchialine (submerged) caves provide ecosystems where endemic copepods have been described. The biological diversity in the Adriatic Sea is great, providing about 800 phytoplankton species, as well as 900 holoplanktonic and 20 times more meroplanktonic species of zooplankton (zoobenthic and fish larvae).

Plankton; distribution; circulation; hydrography; Adriatic Sea

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