Changes in the C, N, P burial rates in some Baltic Sea sediments over the last 150 years - relevance to P regeneration rates and the phosphorus cycle

Three major depositional basins (the Gdansk, Bornholm and Eastern Gotland b asins) of the Baltic Proper, which together account for >50% of the deposit ional areas in the entire Baltic Sea, have accumulated increased amounts of sediment and organic carbon during the last 50 years, as is shown in Pb-21 0-dated sediment cores. The shallow Arkona Basin has had constant accumulat ion rates and rate increases of bulk sediment, organic carbon and aluminium in the Bornholm Basin are parallel and are interpreted to reflect increase d material input from land or from erosion of shallow-water areas. In the G dansk and Eastern Gotland basins, the accumulation rates of mineral matter have risen at lower rates than those of organic carbon and point to prefere ntial enrichment of organic matter. At increasing rates of burial, the mate rial in the Gdansk and Eastern Gotland basins appears to have been depleted in phosphorus relative to nitrogen and organic carbon. Using C:N:P ratios from a sediment trap mooring in the Gotland Basin and from fluffy layer mat erial in the Pomeranian Eight as characteristic ratios for sedimenting mate rial, we find evidence for significant increases in the ratios of C to P in sediments younger than 15-25 years in cores from the Eastern Gotland basin , coincident with H2S build-up in deep waters. Paired oxygen and phosphorus concentration data from the sub-halocline water column of the Gotland Basi n covering the time since 1970 suggest that the phosphate reflux is fed fro m two sources: At the transition from oxic to anoxic conditions, iron-bound phosphate is suddenly liberated at the sediment-water interface and result s in a concentration jump of approximately 2 mu mol dm(-3) phosphate in dee p water. During anoxic periods with H2S in bottom waters, phosphate diffuse s out of the sediment and adds to the dissolved phosphate pool. Our data im ply that the sediment contributed approximately 14,000 t a(-1) of phosphoru s to the water column of the deep Gotland Basin since the early 1970s. Comp ared to total P input to the Baltic Sea (1993: 39,000 t), the reflux of P f rom anoxic sediments in the Gotland Basin thus is a major source of P for t he Baltic Sea. On time scales of years and decades, the reflux of phosphoru s from the sediments may be the reason for discrepancies between the ratios of nitrogen and phosphorus in the Baltic Sea and for a lack of effects of declining phosphate discharge into this large ecosystem. (C) 2000 Elsevier Science B.V. All rights reserved.