18.5 Distribution of chl a and primary production

18.5 Distribution of chl a and primary production

The rivers draining into the northern Adriatic are the major sources of external nutrient input, especially during stratified periods. The water mass exchange between the northern region and the remainder of the essentially oligotrophic Adriatic, as well as the major influence of Ionian water in the south, has a great influence on the productivity and standing crops of different sub-areas. A terrigenous supply of nutrients in some semi-enclosed bays and channels of the eastern coast and all along the western coast via run-off influences the productivity of a relatively narrow coastal belt, and consequently the biomasses and production rates are spatially very variable. Buljan [61] who made an estimate of the productivity of the Adriatic Sea on the basis of its hydrographic properties, suggested its division into four productivity zones: open waters of the central and southern Adriatic having low production; the shallow northern Adriatic including a narrow coastal belt along western coast, characterized by permanent high production; the area of moderate production occupying the eastern coastal waters and finally the limited zones with high production under strong coastal influences (lagoons and embayments) along the eastern and also western shore.

The northern Adriatic has been recognized for many years as a region of high marine production at several trophic levels from phytoplankton to fish, but recently the external nutrient input is thought by some authors to be the source of the eutrophication problem of this area [123].

A region of high but variable phytoplankton biomass and production was quantified off the delta of the River Po and related to the spreading of its plume, and a marked west to east gradient of the standing crop and production was observed. The mean water column chl a (2.87 mg m^-3) in the western zone under the direct influence of the River Po is about twice as high as in open north Adriatic waters, which in turn have a significantly higher biomass than the eastern North Adriatic zone (mean 0.9 mg m^-3 chl a), which is under the influence of oligotrophic waters of more southern origin. Very high chl a values are found along the western coast of the Italian region of Emilia-Romagna, where close to the coast the annual mean chl a has values over 10 mg m^-3, falling below 8 mg m^-3 at about 2 Km offshore [157].

Similar trends are evident for primary production (PP) data, with maxima over 30 mgC l^-1 h^-1 in the western and below 10 mg C l^-1 h^-1 in the eastern part. Minimal values are similar for both regions <1 mgC l^-1 h^-1. More recently one of the most important results of ELNA Project was the estimation of annual PP rates in the three Northern Adriatic areas, identified by means of their respective fresh water contents: 1 -- the so called dystrophic area, close to the Po River mouth where mean annual PP was equal to 183 gCm^-2y^-1; 2 -- the neighbor eutrophic area with an average PP of 94 gCm^-2y^-1 and 3 -- the open waters where PP was 64 gCm^-2y^-1 [234].

18.5 Distribution of chl a and primary production