The structuring role of submerged macrophytes in a large subtropical shallow lake : Clear effects on water chemistry and phytoplankton structure community along a vegetated-pelagic gradient

It is well known that submerged macrophytes exert positive feedback effects that enhance the water transparency, stabilizing the clear-water state in shallow temperate lakes. However, the structuring effect of macrophytes on the food web of subtropical and tropical ecosystems is still poorly understood. In this study we investigated the influence of dense submerged vegetation beds on the water chemistry and phytoplankton structure along a littoral-pelagic gradient of large subtropical shallow lake in southern Brazil. Seasonal monitoring was carried throughout one year following along a submerged vegetated-pelagic transect in order to analyze the effects of macrophyte's coverage (percentage of volume infested- PVI) on the water chemistry and phytoplankton community structure. Clear variations on nutrient concentration and phytoplankton biomass/composition could be observed permanently along the transect. Nutrients as orto-phosphate (PO4 −) and bicarbonate increased linearly towards the pelagic zone, whereas dissolved organic carbon and humic substances decreased linearly as PVI decreased. Concomitantly, a significant increase in the phytoplankton biomass was observed outwards from the submerged vegetation bed. In the vegetated area, small species (C-R strategists), unicellular flagellates were selected; whereas in the pelagic zone, larger (K-selected) species of cyanobacteria occurred, especially representatives of the functional groups M, LO, SN, S1 and K. Such results indicate that the macrophytes and inherent metabolism, such as potential excretion of dissolved organic compounds with allelochemicals and nutrient uptake from water column influence the structure of the phytoplankton community reducing also significantly the biomass of cyanobacteria within the dense submerged vegetated zone. Because of the continuous growth of macrophytes over the year in low latitude systems, their feed-back effect pattern tends to also dictate a different role in ecosystem dynamics and structure of the food web. These findings contribute to the management and conservation of subtropical and tropical lakes