CONCLUSION

Macrophyte population in the lake maintains the nutrient levels in urban aquatic systems. Higher nutrient content (32 t N/d) has contributed to the prolific growth of invasive species. During summer, maximum quantity of nutrients in dissolved form is taken up by the macrophytes that cover almost 85% of the lake surface. These macrophytes form thick mat hindering air-water interface and hamper the aerobic functioning of the lake. It was noticed the prevailing anaerobic condition (-235 mV) below macrophyte mat surface has lowered DO level further resulting in anoxia, which is evident from  higher BOD (180 mg/l) and lower DO levels (0 mg/l). During monsoon in the absence of macrophytes, Varthur lake functions as aerobic lagoon aided by micro-algae in nutrient uptake and hence treatment of the lake. However in the pre monsoon the system behaves as an aerobic-anaerobic lagoon and finally in the post monsoon period it behaves as an anaerobic–aerobic system.