Internal nutrient load in a recovering urban lake: a laboratory microcosm study

摘要

Lake Jyvaesjaervi (surface area 3.4km~2, volume 24x10~6 m~3, max depth 27m, MQ 3.3m~3 s~(-1)) is situated in Central Finland, surrounded by the town of Jyvaeskylae (Fig. 1). The lake received an extremely heavy load of untreated municipal and paper mill wastewater until the mid-1970s and became unsuitable for recreational purposes. Since 1977, all municipal wastewater has been directed to a treatment plant, and the high organic load of the paper mill (ca. 5000 kg O_2 d~(-1) as BOD_7) has markedly decreased, being today ca. 120 kg O_2 d~(-1). The external nutrient load, mainly from the catchment area, is at present ca. 10 kg for total phosphorus (TP) d~(-1) and ca. 250 kg for total nitrogen (TN) d~(-1). In the early 1970s, the corresponding figures were about 200 kg and > 1000 kg d~(-1). The ecological status of the lake has recovered from bad to moderate, and the recreational value is satisfactory (Merilaeinen et al. 2003, Salonen et al. 2005). Despite the high reduction of external loading and continuous aeration of the main basin since 1979, the lake is still eutrophic and suffers from hy-polimnetic oxygen deficit, especially in late summer, and the biological quality of the profundal sediment is poor. The management and restoration project of Lake Jyvaesjaervi started in 2001. In the present study the magnitude of nutrient load from sediment in relation to external load was determined in a laboratory microcosm (for details of the microcosm experimental system, see Liikanen et al. 2002).