Life form dependent impacts of macrophyte vegetation on the ratio of resuspended nutrients

The effects of floating-leaved and submerged macrophytes on sediment resuspension and on the ratio of resuspended nitrogen and phosphorus were studied by sediment traps in the Kirkkojärvi basin in southern Finland. The effect of submerged macrophytes on preventing sediment resuspension was stronger than the effect of floating-leaved plants. On average, among submerged plants the resuspension rate of suspended solids was 43%, and among floating-leaved plants 87% of that in the open water. The floating-leaved Nuphar lutea had a reductive effect on P resuspension but no significant effect on N resuspension. The impact on P resuspension was strong, because root uptake by Nuphar lutea reduced the P content of the sediment. N:P ratio in resuspended nutrients was 6.7 among the plants and 4.1 in the open water. Among suzbmerged plants, sediment N content was strongly increased but P content was not affected due to the pleustophytic life form of the dominant plants (Ceratophyllum demersum, Ranunculus circinatus). The effect of pleustophytes on sediment nutrients was weak, because their nutrient uptake is mostly foliar. The N:P ratio of resuspended nutrients was 7.9 among the submerged plants and 7.0 in the open water. The results suggested that depending on the life form, macrophytes can modify the flux of N and/or P to the water column through their effects on nutrient resuspension and possibly modify phytoplankton communities via their effects on the N:P ratio. If the overall nutrient level is the most important factor for the dominance of cyanobacteria, submerged macrophytes can have stronger effects on phytoplankton community structure than floating-leaved species. If N:P ratio is of importance, the effects of floating-leaved species may be more pronounced.