Degree Type


Date of Award


Degree Name

Doctor of Philosophy


Animal Ecology


Transport phenomena in rapidly flushed lakes can uncouple phytoplankton standing crop from nutrient and light limitations. This study examines the effect of algal sinking and riverine transport on phytoplankton abundance in a nutrient-rich impoundment having a 2-3 week retention-time (Red Rock Reservoir, Des Moines River, Iowa). Results suggest that sedimentation of river-borne algae entering the reservoir causes a significant longitudinal reduction in phytoplankton standing crop and also a shift toward smaller planktonic algae with lower sinking speeds;Minimum sinking losses were estimated by mass balance using chlorophyll budgets derived from weekly physiochemical and in situ primary productivity measurements. Significant algal sedimentation was calculated for the headwater section (about 0.2 d('-1) minimum) with negligible sinking losses near the dam (roughly 0.01 d('-1)). Analysis of sedimented phytopigments and diatoms verified this result;Chlorophyll size-fractionation revealed a downstream decline in phytoplankton size. Sedimented diatoms showed a similar pattern. This size shift was linked to sinking losses by settling experiments which used 14C as a tracer of active algal biomass. Upstream samples had a greater portion of total activity in the larger, more rapidly sinking size-fractions;In this system, riverine inputs exceed in situ algal production and are needed to maintain phytoplankton at observed densities. Chlorophyll concentrations are only 10 to 15% of the values predicted by published chlorophyll-phosphorus regression equations.



Digital Repository @ Iowa State University,

Copyright Owner

David M. Søballe



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92 pages