摘要：The purpose of this research project was to study the annual cycle of basic water properties in a lake covered by ice for about half the year close to the Arctic Circle. Samples were collected from the water column of the lake and the outlets of inflowing rivers with a Ruttner sampler from twelve locations during six visits in 1996–1997, a total of 31 samples each time. The analyses were made in accordance with the Finnish standards and included determinations of organic solids, alkalinity, colour, conductivity, total phosphorus (totP), total nitrogen (totN), ammonia (NH4+) and nitrate (NO3−). Lake Paanajärvi is an east–west oriented tectonic lake (width 0.6–1.3 km, length 24.5 km, max. depth 128 m) surrounded by carbonate rich rocks and taiga forests. With over nine-tenths of the lake inflow into the western end and the outlet at the opposite eastern end of the lake, the topmost water layers of the lake behave much like a through-flow river. Most of these waters come from Finland and have been affected by human activity while all other inflowing waters drain from natural conditions. Seasonally, the winter time is characterized by the lowest of all colour values, the highest of all alkalinity contents, and by settling of phosphorus. All these features are mainly due to minimal discharges, high proportion of groundwater flow in relation to surface flow and calm conditions thanks to the ice-cover. Spring-time starts with violent nival floods in May associated with the maximal colour values, minimal alkalinity contents, and the rise of the spring turnover, temperature stratification and biotic activity which proceed in a wave-like fashion through the lake controlled by the through-flow and warming of river waters entering into the western inlet end of the lake. The uptake of nutrients is best seen in a rapid decrease of nitrate in the epilimnion in association with the increase of ammonia as a result of metabolism of organisms. The autumn turnover in late September and early October is associated with close to equal values of all parameters in the whole water column of the lake. The exhaustion of nitrate in the epilimnion suggests that nitrate may be a limiting factor for the biotic life. Other typical features of the lake are the lower temperatures and nutrient levels in the outlet deep compared to the inlet deep of the lake, and this may have led to adaptation and specialization among the biota within this “mini-Baikal” by Fennoscandian standards.
