Potential influence of landscape structure on water quality and phytoplankton community in highly degraded urban lakes: Implications for restoration

Urban lakes are of particular concern in urban landscapes, as they provide ecological, social and economic benefits, but are under strong human pressure. The current study examined eleven urban lakes that differ in their morphometric and hydrological characteristics and were located across various macroregions of Poland. The aim was to examine potential relationships among landscape structure, water quality, and phytoplankton communities across three buffer zones (10 m, 100 m, and 1000 m), selected to reflect different spatial scales of land use/land cover (LU/LC). The urban lakes were characterised by different phytoplankton biomass, with a maximum of 181.3 mg/l. Most of them were dominated by toxic or potentially toxic cyanobacteria. Land use/cover explained the greatest percentage of variability (24.3%) in phytoplankton composition in the 100 m buffer zone. However, this relationship was only suggestive and did not reach the conventional level of statistical significance. In contrast, LU/LC explained little of the variation in physicochemical parameters across three spatial scales, accounting for up to 0.6% in the 10 m buffer zone, but with no statistically significant relationships. Phytoplankton play a more important role than other water quality parameters and can serve as a more sensitive indicator, providing complementary information for assessing the impact of landscape structure. Despite the large proportion of forest and seminatural areas, the urban lakes were classified as highly degraded, exhibiting poor or bad water quality, which can be more related to historical pollution than to current landscape structure. Our results indicate that a 100 m wide buffer zone may be a useful study-specific spatial scale for considering shoreline and surrounding LU/LC management. They can be prioritised for protection and restoration activities, as well as for planning future spatial development. Restoring natural vegetation within this zone may reduce the nutrient inflows and slow the process of cultural eutrophication. However, further studies with a larger number of lakes are needed to confirm these relationships.