Spatial variability of chromophoric dissolved organic matter in a large floodplain river: Control factors and relations with phytoplankton during a low water period

Abstract

Chromophoric dissolved organic matter (CDOM) affects ecological processes in freshwater environments. Few studies assessed its spatial variability and relations with phytoplankton in floodplain rivers. Therefore, these topics were examined in a hydrological connectivity gradient in the Middle Paraná system. Absorption coefficients at 250 and 365nm were measured to estimate CDOM concentration and molecular weight (MW), to find their explanatory limnological variables (Redundancy Analysis, RDA), and to assess their contribution to the explanation of phytoplankton structure (Canonical Correspondence Analysis, CCA). Conductivity and turbidity explained CDOM significantly. Increased conductivity from the main channel to floodplain lakes was associated with increased CDOM concentration. The lake indirectly connected to the river showed the highest turbidity associated with high-MW-CDOM, while the isolated lake showed the highest conductivity associated with low-MW-CDOM. Phytoplankton structure was significantly explained by conductivity, turbidity, and high-MW-CDOM (CCA). Environments directly connected to the river (with the lowest conductivity) were associated with diatoms. Phytoflagellates were associated with turbidity and high-MW-CDOM in the lake indirectly connected to the river, whereas Cyanobacteria were associated with conductivity and low-MW-CDOM in the isolated lake. The combined effect of physical factors and CDOM explained the highest fraction of species variation (partial CCA). As a conclusion: (1) CDOM increases as hydrological connectivity with the river decreases; (2) lake sediment resuspension is linked to an increase of high-MW-CDOM; (3) lake isolation from the river corresponds to an increase in low-MW-CDOM; and (4) CDOM explains phytoplankton structure significantly and should be considered as an important variable in studies of microalgal assembly.