Author(s): Sovannara Uk
Email (s): uk.s.ab@m.titech.ac.jp
Institution or organization of origin: Tokyo Institute of Technology
Country: Japan
Abstract
The flood pulse is responsible for driving the ecosystem dynamics and productivity of a lake. Thus, we hypothesized that sediment and phosphorus (P) dynamics caused by the inter-annual hydrological phases are major factors for determining the distribution of phytoplankton and cyanobacteria. To test the hypothesis, we investigated the spatial-temporal variation in the phytoplankton pigments chlorophyll a (Chl-a) and phycocyanin (BGA-PC, cyanobacterial-specific pigment), and their related factors in Tonle Sap Lake (2017–2019) encompassing four hydrological phases: low-water (LW), rising-water (RW), high-water (HW), and falling-water (FW). Results showed that concentrations of Chl-a and BGA-PC were relatively high in LW and RW. During the survey, we also visually observed the prevalent presence of floating algal scums on the surface of highly turbid water during the low-water period while it was hardly observed during the high-water period. Chl-a and BGA-PC showed their peaks during the low-water period owing to the resuspension-induced superfluous P, when the concentration of soluble reactive P (SRP) increased with total suspended solids (TSS). In contrast, decreasing SRP concentration with increasing water depth during the high-water period limited their biomass in terms of concentration, although the underwater light is more availability in this period. Fluctuation of water level, therefore, drove sediment resuspension that supplements SRP for phytoplankton during the low-water period while sedimentation dwindled the supply of SRP during the high-water period. Our results suggested that the P dynamics, rather than sediment dynamics, is the significant direct driver for the distribution of phytoplankton and cyanobacteria in this lake.