The impact of free-floating plant cover on phytoplankton assemblages of oxbow lakes (The Bug River Valley, Poland)

In the present study we focused on the impact of macrophyte cover (composed mainly of the Lemna genus) on phytoplankton taxonomic and functional diversity. Some important environmental parameters, mainly light (Kd_PAR), and the chemical conditions (pH, dissolved oxygen, ammonium, soluble and total forms of phosphorus) were closely related to the pleustophyte cover. Among them, the key factor in the phytoplankton ecology of the studied oxbow lakes was the dense macrophyte cover which strongly reduced the illumination of water. Neither differences in the mean nutrient concentrations between the lakes with FFP (Free Floating Plants) absence and those with FFP dominance nor significant relationships between nutrients and the phytoplankton structure were observed. The species composition of phytoplankton and the functional (FG) and morpho-functional (MFG) groups reflected the differences between the habitats connected with hydromacrophytes. The free-floating macrophyte cover favours mixotrophic and heterotrophic species, mainly Euglenophyta (coda W1 and W2) and chrysophytes (codon Ws) as well as shade-adapted cyanobacteria with the high tolerance of the low oxygen content (codon K). In lakes with FFP absence — taxa from Chlorophyta and Bacillariophyceae (associations X1, J, and D), or filamentous cyanobacteria (codon S1) dominated the phytoplankton. MFG were less related to the oxbow type and exhibited greater similarity between lakes independently of the presence or absence of FFP. Only unicellular Cyanoprokaryota which created MFG 4 and colonial chroococcales (MFG 5b and 5c) reached a greater percentage share, especially in oxbow lakes with FFP dominance.