ABSTRACT

Macrophytes play a major role in maintaining the nutrient levels in urban aquatic systems. However their prolific growth result in spread of invasive species such as water hyacinth (Eichhornia crassipes) due to the availability of higher nutrient concentrations. This hinders aerobic functioning of the lake by restricting sunlight penetration and also affecting algal photosynthesis. This also results in anoxic environment due to blockage of air-water interface, influencing oxygen diffusivity. Reduction in DO (0 mg/l) impacts the viability of aquatic biota and result in the disappearance of biodiversity. This communication evaluates the influence of the invasive macrophytes in the functioning of lake across the seasons. Significant seasonal changes in water quality were noticed due to changes in the redox conditions (- 235 mV) and dissolved oxygen levels at various locations depending on the extent and location of macrophyte spread based on the nutrient levels coupled with wind regime prevailing during the season. The analysis of seasonal data reveals that dissolved oxygen concentration and redox condition is dependent on the extent of macrophyte spread. N content in Lemna and Alternanthera species (of 4 g/100 g dry weight) is significant compared to other species (p<0.005). During monsoon, lake functions in the absence of macrophytes, predominantly as aerobic lagoon; and functions as aerobic-anaerobic lagoon (pre-monsoon) and as anaerobic–aerobic system (post-monsoon). Anaerobic conditions are mainly due to the interference of macrophytes in lake functioning and inefficient handling of nutrients in the absence of algae. This necessitates the regular removal of macrophytes from the lake. Provision to allow the growth of primary producers will help in nutrient management.

Keywords: macrophytes; wastewater; nutrients; eutrophication; lagoon; sewage; urban lakes