Characteristics and sources of the settling particulate organic matter in the South Adriatic basin

Abstract

In the South Adriatic Pit a mooring equipped with two time-series sediment traps and two Aanderaa RCM7 current meters was deployed in the frame of EEC-MAST II Mediterranean Targeted Project 1, Subproject Euromarge-AS. The first trap was located 35 m above the bottom (mab) and the second 500 mab. Sediment deposition fluxes were recorded biweekly over 18 months (1 April 1994 to 31 October 1995) in the deeper sediment trap, and over 12 months (15 November 1994 to 31 October 1995) in the 500 mab trap. The settling material and surficial sediment have been analysed for inorganic and organic carbon, δ 13C composition, total nitrogen, and biogenic silica, in order to infer the sources of material. The total mass fluxes at both depths were characterized by a high variability and did not present a clear seasonal trend. Values obtained in the 500 mab trap were generally 2–3 times lower but occasionally similar. The fluctuations were similar in both traps. Based on analyses of inorganic and organic constituents it appears that the fresh organic matter produced in the surface layer rapidly sank through the water column. Strong seasonal variations of biogenic constituents occurred. During spring–early summer an input of fresh organic matter prevailed due to the blooms of mainly siliceous phytoplankton. Late summer and winter samples were characterized by more degraded matter originating from calcareous phytoplankton. A significant imprint of terrigenous organic matter was observed in summer and winter 1994, most likely because an unusually high Po River inflow was the most important fresh water source for the Adriatic. A vertical budget of organic carbon has been calculated; only 2.7% of mean annual primary production reaches the 500 mab trap, suggesting that remineralization processes take place mainly in the upper part of the water column. Comparison between 35 mab trap particle fluxes and bottom rain (calculated as sum of remineralization fluxes and burial) indicates that a strong near-bottom input of material between the deepest trap and the surficial sediment exists due to the lateral advection.