Shoreline and surface area changes induced by floating islands in peatland lakes (Tuchola Pinewood Forest, Poland)
Multitemporal aerial photography and topographic maps were applied to investigate shoreline and surface area changes induced by floating islands in three small (area < 1 ha) peatland lakes in the Tuchola Forest. The observation period covers the last 60 years (1951-2011). Shorelines of peatland lakes are stable, unless parts of them detach as floating mats and become floating islands which could have migrated sporadically in the past. Currently, they are rooted permanently to the mat (Dury I and Dury V Lakes) or float loose (Kozie Lake). In the observation period, movement of the islands and closing of lake bays due to overgrowing were the main reasons of shoreline changes. Due to low resolution of old aerial photographs, other changes are not decipherable and can be treated as negligible. Therefore, no reasonable conclusions can be drawn regarding the actual floating mat encroachment during the last 60 years, and the rate of lake shrinking, based of aerial photographs.
Meliorations of the Łęczna-Włodawa Lakeland have considerably determined its water relations within the last more than 200 years. The analysis of the map by Anton Mayer von Heldensfeld, so far not applied in studies on the drainage network of this lakeland, executed in the years 1801-04, revealed the drainage of the Uściwierskie Lowering already at the turn of the 18th and 19th century. This shifts the commencement of strong anthropogenic impact on this area to a considerably earlier period than was previously assumed. The lakes subject to analysis (Rotcze, Uściwierz, Bikcze, and Lakes Piaseczno and Łukie neighbouring with the Lowering) were incorporated into the drainage system in various periods. Such incorporation of a water body involved water level drawdown. The water level decreased first in Lake Bikcze, and the latest in Lake Rotcze. More detailed information on changes in the water level in the lakes of the Uściwierskie Lowering since 1882 was obtained based on calculation and on comparison with current data of measurements conducted by Rostworowski. The analysis revealed that the water level in Lakes Bikcze and Piaseczno, although fluctuating, has not changed within the last 130 years, whereas the water level in Lake Uściwierz decreased by 33 cm, and in Lake Rotcze by 90 cm.
Based on archival cartographic sources, teledetection materials, and research results in the scope of history and environmental sciences, changes occurring in the catchment of Lake Rotcze and its close vicinity over the last two hundred years were analysed. The area of the catchment is dominated by moderately fertile minerogenic soils on which forest assemblages developed (poor hornbeamoak forest Tilio-Carpinetum and thermophilous oak forest Potentillo albae-Quercetum). The first trace of anthropopressure was a clearing at the southern shore of the lake, for agricultural use, established not later than at the turn of the 17th and 18th century. The forests were almost completely cleared in the first quarter of the 20th century, and transformed into cultivated land of the village of Grabniak. A large fragment of the catchment is occupied by peatlands, constituting the final stage of lake-peatland succession developed in the former bay of fossil Lake Uściwierskie of which Lake Rotcze constitutes the remains. During the 20th century, the peatlands were intensively drained. This resulted in their overdrying, and growing over with bushes and marshy forests. In the 20th century, Grabniak was an agricultural village. Over the last 25 years, it has turned into a recreation resort. The relatively short agricultural and settlement activity in the lake catchment, in comparison with other parts of Poland, makes the lake appropriate for palaeolimnological reconstruction in terms of looking for reference conditions (sensu WFD).
Transport of fine sediments depends mainly on the efficiency of flocculation. Flocculation, understood as the result of simultaneous processes of aggregation of particles and floc break-up, is a common phenomenon in marine environments. It is typical of fine sediments. This study presents a mathematical model of fine sediment transport. A model of flocculation is an important part of this model. Its main assumption is that flocculation is governed by turbulence. The model was qualitatively tested in a simplified theoretical waterbody. Such factors as the wind direction, wind speed, river discharge and concentration of suspension in the river were investigated. The results show that the proposed model describes reasonably well the lithodynamic processes characteristic of fine flocculating sediments. Thus it seems possible to apply it for description of fine sediment transport under real wave–current conditions that occur in many marine waterbodies near river mouths.