Influence of salinity on light conditions and phytoplankton growth in a turbid river

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Journal Article
River Research and Applications, 2010, 26 (7), pp. 894 - 903
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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. However, during extended periods of reduced flows conductivity increased causing a substantial reduction in turbidity with concomitant improvements in light penetration. A turbidity of ca. 100 NTU marked the transition to light sufficiency at the study site and occurred at a conductivity of ca. 300μScm-1 demonstrating that small changes in salinity can have major effects on light penetration. These results show that flow, salinity and turbidity all play a part in determining the growth conditions for phytoplankton in turbid rivers. © 2009 John Wiley & Sons, Ltd.
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