Marine and Freshwater Research Marine and Freshwater Research Society
Advances in the aquatic sciences

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The effects of autotrophic biomass and composition on photosynthesis, respiration and light utilization efficiency for a tropical savannah river.

Simon Townsend , Ian Webster , Michele Burford , Julia Schult

Abstract

The efficiency of light used for photosynthesis, when standardized for areal chlorophyll a biomass, is summarized by the light utilization efficiency parameter and is dependent on light at the water’s surface, the underwater light climate and autotroph characteristics. We examine the relationships between light, photosynthesis, respiration and autotroph biomass in a tropical savannah river in northern Australia during the dry season when autotroph biomass accumulated following wet season disturbance. The river’s autotrophs comprised mainly benthic microalgae, macroalgae and macrophytes. Total chlorophyll a and dry weight biomasses increased respectively 4- and 27-fold over 5 months, whereas photosynthesis doubled. Photosynthesis was light limited, and when standardized for chlorophyll a and dry weight biomasses declined with increasing biomass, despite increasing incident light through the study period. We surmised this was due to self-shading and autotrophic composition which had variable chlorophyll a content, and resulted in a 10-fold reduction in the light utilization efficiency with increasing light and biomass. Because respiration was tightly coupled to photosynthesis, biomass standardized respiration also decreased with increasing biomass. Autotrophic self-shading and composition can have a significant effect on light utilization efficiency and the biomass-photosynthesis relationship, and warrants consideration when interpreting photosynthesis for river health monitoring.

MF17172  Accepted 23 January 2018

© CSIRO 2018