Dynamics of emergent macrophytes for 50 years in the coastal Lake Engure, Latvia

Coastal Lake Engure is a Ramsar site — a wetland of great importance due to its biological diversity and unique properties, especially its coverage of macrophytes, shallowness and size. This study assesses the dynamics of emergent macrophytes in the lake based on analysis of historical aerial photographs and digital orthophoto maps. The area of emergent macrophytes was shown to have increased by 781 ha (15 ha/year) in the period 1956 to 2007. The expansion rate differed during this period. Expansion was much faster (31 ha/year) from 1956 to 1972, but declined to 6 ha/year from 1972 to 1994 and increased again to 11 ha/year from 1994 to 2007. The greatest rates of expansion of emergent macrophytes from 1956 to 1972 can be partly explained by ceased grazing and hay harvesting activities. As the area covered by emergent macrophytes changes seasonally, only remote sensing materials acquired during the same season should be compared. The open water territory in 2007 was only 22.9 km², compared to 30.7 km² in 1956. Emergent vegetation has been mostly limited to areas where the depth does not exceed 0.5 m. Expansion of emergent vegetation was found to differ in various parts of the lake. The main factors affecting the rates of its development were wave disturbances, hydrological regime and ceased agricultural activities in the lake.