Abstract:
A turbid lowland river in Australia was studied to describe factors influencing the light conditions for phytoplankton growth. Vertical attenuation coefficients correlated with nepholometric turbidity enabling estimation of euphotic depths (zeu) from long term turbidity monitoring. Light conditions were assessed from the ratio of zeu to the maximum water depth (zm). Predominantly zeu/zm ratios were below 0.2, a value indicating the minimum light conditions required to support phytoplankton growth. A transitional state with zeu/zm between 0.2 and 0.35 occurred 15% of the time, while light sufficiency occurred for 30% of the time. Peaks in eukaryotic phytoplankton biomass developed when zeu/zm was at or above transitional values. Large increases in cyanobacterial numbers (Anabaena sp.) only occurred when zeu/zm exceeded 0.35. Turbidity increased quickly with elevated flows but did not decline substantially as flows reduced and light limiting conditions extended into low flow periods otherwise conducive to phytoplankton growth.
