The impact of dredging deep pits on organic matter decomposition in sediments

In this study, the results of investigations of organic matter decomposition in natural and dredged areas of the inner Puck Bay (Baltic Sea) are presented. The dredging of relatively deep pits causes environmental problems. Researched post-dredging pits are sediment traps in which 3 times more organic carbon (C-org), 3.5 times more total nitrogen (N-tot), about 1.5 times more organic phosphorus (P-org) and 1.7 times more total phosphorus (P-tot) accumulate as compared to the non-dredged regions; they are also characterized by very intensive decomposition of organic matter. About 42, 44, 95 and 50% of the annual load of, respectively, C-org, N-tot, P-org and P-tot undergo decomposition in the dredged area, whereas the respective values for natural seabed are ca. 11, 44, 41 and 21%. Reduction of nitrogen in the pit occurs mainly through ammonification, while in the natural areas of seabed denitrification prevails. In non-dredged sediments, 84% of the released nitrogen comes from denitrification whereas in the pit, it is only 18%. Organic matter degradation in the pit sediments manifests itself by a 7-fold increase in the phosphate flux into near-bottom water as compared to natural seabed. The observed phosphate flux originated from the organic matter as well as from the decomposition of inorganic phosphorus compounds. Periodically, sulphate reduction in the pit sediments resulted in hydrogen sulfide occurrence.