Effects of sediment and nutrient enrichment on water quality in the Archipelago Sea, northern Baltic: An enclosure experiment in shallow water

The effects of bottom sediment and nutrient enrichment on water quality were studied in an enclosure experiment in the Archipelago Sea, northern Baltic. The three-week experiment was conducted in a small and shallow bay, where the organic content of the sediment is low. The enclosures were large (diameter 3.6 m, depth 3.5 m), and reached from the surface to the bottom. Some of the enclosures included the natural sediment, some had a plastic bottom without contact with the sediment. Concentrations of nutrients and chlorophyll a and physical variables in the water column, concentrations of inorganic nutrients in the pore water of the sediment, and numbers and biomasses of the benthic macrofauna were measured. Both the presence of sediment and nutrient enrichment significantly increased the concentrations of total nitrogen, total phosphorus and chlorophyll a in the water column. The concentration of chlorophyll a doubled in the sediment-bottomed enclosures without nutrient enrichment; the increase was similar to that in the plastic-bottomed enclosures with nutrient enrichment (7.2 μM NH4+ and 0.46 μM PO43− during the three-week study). The concentration of silica doubled or tripled in the sediment-bottomed enclosures. No shortage of oxygen was found in the water column during the experiment. The results show that sediment with a low content of organic matter may serve as an important source of nutrients in shallow and littoral oxic waters and may be important in sustaining their eutrophic state during the productive season. It is suggested that an important part of the nutrients released from this erosion bottom had originated from the surface layer of the sediment, which had been sedimented and transported to the area quite recently. The results indicate that it is important to include sediment in mesocosm studies dealing with nutrient dynamics, especially in shallow waters.