150 years of eutrophication in the northern Adriatic Sea: Evidence from a benthic foraminiferal record

The vertical distribution of benthic foraminifera in a sediment core in front of the Po delta has been studied in detail. According to our age model, based on 210Pb and 137Cs analyses of another core from exactly the same locality, the studied core spans the past 160 years. The radio-isotope profiles further show that sediment mixing is largely restricted to the top centimeter, suggesting that the core should provide an extremely detailed record of the youngest history of the northern Adriatic Sea. Benthic foraminiferal patterns and grain-size analyses indicate a number of substantial changes in sedimentation rate and food/oxygen availability in the benthic ecosystem. Changes occurring at about 1840 and 1880 can be attributed to man-induced changes in the main outflow canals of the Po river. The first one led to an important reduction of the marine vegetation cover which probably was present up to that date. The second change resulted in the present-day situation in which the Po outflow is passing the studied core locality close by. The local benthic foraminiferal associations indicate a steadily increasing nutrient load from 1900 AD onwards. This trend is interpreted as the effect of anthropogenic eutrophication due to agriculture and waste water disposal, although the faunal record as discussed here only gives a limited impression of the basin-wide development. A marked faunal transition around 1930 indicates intensification of the eutrophication; around 1960 the first signs of an increasing importance of anoxic events can be recognized. The faunal changes in the last decade, which are ascribed to changes in preservation potential, indicate that more intense or more prolonged anoxia started about 10 years ago, and that the ecological health of this part of the northern Adriatic probably is still in decline.