Long-term hydrobiological monitoring importance for sustainable management of a shallow coastal macrophyte lake in the Baltic Ecoregion

This literature review analyzes the current state, challenges, and knowledge gaps for an adaptive management of the Engure Long-Term Socio-Ecological Research (LTSER) platform: Lake Engure and its catchment in Latvia. Lake Engure catchment area (644 km 2 ) includes shallow lake (a remnant of the Litorina sea); part of the territory is a nature park and Ramsar site. Most of the catchment area is covered by the pine forests, large areas of marshlands, meadows, deciduous forests, dunes and agricultural lands. It is a biodiversity hotspot: avifauna - 186, vascular plants – 876 species; 44 species of birds, 5 fish species, 3 plant species and 28 habitats are protected at EU level (e.g., Springe et al. (2011), LatViaNature (2024)). It is the largest area of a good quality freshwater habitat 3140 Hard oligo-mesotrophic waters with benthic vegetation of Chara spp. in Latvia. The investigation of the lake flora started at the beginning of 20th century (Kupffer 1907) and continued with similar intensity (Zviedre and Grinberga 2012). Long-term hydrobiological monitoring has been carried out in Lake Engure since 1995; including research of benthic invertebrates, phytoplankton, macrophytes, physical and chemical variables, etc. The phytoplankton community characteristic with low biomass (0.13-0.39 mg/l); taxonomic composition fluctuated without an evident trend (recorded >150 species). The long-term development of the hydroecosystem was related to the lake’s geographical location, basin and morphology, as well as the lake’s history and relationship of biotic and abiotic factors (Springe et al. 2011). The ecosystems of the area have been changed by different human activities historically. The agriculture and main industry (fishing and fish processing) has sharply declined. Nowadays, the highest number of employees is in the service sector, setting a new kind of pressure on the ecosystems of the region (Melecis et al. 2014). A major hydromorphological impact was the excavation of Mērsrags Canal (from lake to the sea) in 1842, which lowered the water level by 1.5 m, halved the catchment area, reduced the open water surface, and increased the lake’s vulnerability to eutrophication. Eutrophication of the lake is expressed by an increase in macrophyte biomass, non typical species abundance un unvegetated areas. Over the last 50 years the overgrowing of the littoral zone with macrophytes, mainly by reeds and bulrush, has been stimulated due to an increased eutrophication until the 90-ies of the 20th century and the cessation of once traditional land-use activities (Springe et al. 2007). In the last decades the lake Engure has also served as a pilot territory for various management measures for both, especially protected aquatic and terrestrial habitats, and species. During several LIFE programme projects since 2001, expansive reed Phragmites australis overgrowth was mowed, fragmented, a mosaic of watercourses and marshy places created, as well as shoreline grasslands restored (e.g., by cattle grazing) (LatViaNature 2024). Even though the lake and its basin are one of the most studied areas in Latvia, a comprehensive monitoring program is lacking. This is particularly important as the lake faces significant challenges, including not only eutrophication, but also the complex and unpredictable impacts of climate change. Enhancing the LTER infrastructure will facilitate comprehensive long-term ecosystem and socio-ecological research at a European scale, providing essential knowledge to support informed decision-making and the implementation of adaptive management strategies.