Abstract: We investigated the effects of external nitrogen and phosphorus inputs on submerged macrophytes and epiphytic cyanobacteria in eutrophic lakes, with Vallisneria denseserrulata and Leptolyngbya sp. as model species. We measured changes in water physicochemical parameters, growth responses of the two species, and total phosphorus in water and sediments under low (4.00 mg·L^-1), medium (8.00 and 12.00 mg·L^-1), and high (16.00 mg·L^-1) nitrogen loads, as well as low (0.05 mg·L^-1), medium (0.15 and 1.50 mg·L^-1), and high (7.00 mg·L^-1) phosphorus loads. The results showed that V. denseserrulata adapted to nitrogen and phosphorus variations by adjusting growth strategy. Under low nitrogen load, plant height increased by 3.47%, while the number of ramets increased significantly by 16.44%, 34.25%, and 82.19% under medium and high nitrogen loading, respectively. Plant height was negatively correlated with total ramet number under high nitrogen load (r=-0.285, P<0.05). However, plant height was suppressed under high phosphorus loading. Chlorophyll a content of V. denseserrulata under phosphorus loading was significantly higher than that under nitrogen loading (P<0.05). The biomass of Leptolyngbya sp. did not vary under different nitrogen loading levels. Under low and medium phosphorus loads, the periphytic biofilm biomass of Leptolyngbya sp. was significantly higher than that of the control during the first 30 days (P<0.05). Under high phosphorus load, Leptolyngbya sp. was transformed from periphytic biofilm to planktonic hormogonia within the first 20 days of the experiment, resulting in algal bloom formation. These results suggest that phosphorus loading may be a key environmental factor driving shifts in growth dominance between V. denseserrulata and epiphytic cyanobacteria.

Key words: nitrogen, phosphorus,  sediment, submerged macrophyte, epiphytic cyanobacteria