Ecohydrological research of lake-watershed relations in diversified landscape (Masurian Lakeland, Poland)

The postglacial, hilly-moraine, lowland landscape that dominates in the lakeland region of northeastern Poland (Masurian Lakeland) is highly diversified in terms of land forms (hills, ridges, outwash plains, land depressions, steep slopes), land use and cover (mosaic pattern of different sites, vegetation patches, and arable land), and geological substrate (surface deposits of different permeability in respect to precipitation infiltration). The stream network and wetland patches dispersed among the fields and wetland zones close to lake shores play a crucial role in transport and transformation of nutrient compounds before they eventually reach the lake ecosystems. The river Jorka watershed (area about 65 km(2), length of river about 15 km, discharge between 0.1 and 1.5 m(3) s(-1)) contains a typical fragment of lakeland area, with a highly mosaic structure. Precipitation, discharge in 11-17 small streams, nitrate-nitrogen and total phosphorus loading transported in streams, and changes in their concentration while passing through lakeshore wetland zones were studied over several years and seasons (vernal freshet, summer drought). There was no trend in seasonal and year-to-year changes of TP and N-NO3 concentration while transported with stream waters and no direct relation to discharge. The most frequent range of values of concentration for TP and N-NO3 were 0.08-0.5 and 0.1-0.8 mg l(-1), respectively. The export rates for both nutrients were higher in spring than in summer. The vernal freshet was found to be the most important hydrological event that stimulated the transport of phosphorus and nitrate-nitrogen in surface waters and sustained an adequate moisture condition in wetland zones to remove both nutrients transported with groundwaters. It was found that the concentration of both nutrients in that period significantly decreased towards the lakeshore, particularly in the case of nitrate-nitrogen. In the summer (drought period) there was a significant decrease of dissolved phosphorus but no change in the case of nitrate-nitrogen. The consequences of these phenomena for lake protection zones are emphasised. (C) 2000 Elsevier Science B.V. All rights reserved.