Influences of urbanization on algal community and water clarity in lakes in typical international Wetland cities: Evidence from Wuhan, Central China

Rapid urbanization has extensively modified the biogeochemical cycling of phosphorus and nitrogen which caused eutrophication and harmful algal blooms (HABs) in freshwater lakes. The purpose of balancing urbanization with lake ecosystem protection has promoted the initiative of International Wetland Cities. In order to explore how urbanization has modified key limnological properties (i.e., algal community and water clarity) of lakes for the sustainable development of wetland cities, we used a "top-bottom" approach by comparing sedimentary pigment remains in contemporary period (surface sediments) with those before the 1850s (bottom sediments) in 33 lakes along the urban to rural gradient in Wuhan (one of international wetland cities). The results showed that the overall algal production significantly increased in the lakes after the 1970s, with a prevalence of cyanobacteria (including N2-fixing taxa) in the algal community. As a consequence, water clarity severely deteriorated, suggested by the decreases in ultraviolet radiation absorbing pigments. Compared with urban lakes (inside the 3rd ring road of Wuhan), the magnitudes of decreases in contemporary water clarity and increases in contemporary algal production from reference conditions are more pronounced in lakes in the rural areas (outside the 3rd ring road of Wuhan). This might be attributed to the longer history of urbanization and hence more deteriorated reference conditions in lakes in the city center, and the lags in wastewater treatment in the rural areas. Our study reveals that the rapid social-economic development has the potential of increasing the risk of HABs and water clarity deterioration in wetland cities similar to Wuhan, especially the rural areas. Therefore, priorities should be set to lakes located in rural areas as urbanization often expands outward from city center for the sustainable development of wetland cities.