AME 75:139-153 (2015)  -  DOI: https://doi.org/10.3354/ame01751

ABSTRACT: Phosphorus (P) is an essential element for phytoplankton growth, and in recent years our understanding of P use based on kinetics has been overturned with new knowledge of the complexity of P utilization. However, much of this knowledge is based on culture studies with individual species. Our objective was to measure the effect of dissolved inorganic P (DIP) concentrations on DIP uptake rates by phytoplankton, in the context of seasonal phytoplankton succession in a large monomictic, DIP-depauperate lake. We demonstrated an inverse relationship between surface DIP concentration and DIP uptake rate, with substantially higher uptake rates occurring under thermally stratified, DIP-depauperate conditions. The combination of surface water DIP concentration and water temperature explained 50.3% of the variation in uptake rates. DIP concentration explained the majority of variation, with a concentration of 4.75 µg DIP l^-1 appearing to be a transition between low- and high-affinity uptake. Variability below 4.75 µg DIP l^-1 was further explained by water temperature. High-affinity DIP uptake was most common when Cyanobacteria dominated the phytoplankton assemblage. We validated our field results by conducting a DIP starvation study on an isolate of a dominant cyanobacterium, Cylindrospermopsis raciborskii. High-affinity uptake was demonstrated as the culture became progressively starved of P. Our findings indicate that rapid DIP scavenging via high-affinity uptake is advantageous under DIP-depauperate conditions during the summer-stratification period. It may also contribute to the switch from diatom/cryptophyte/chlorophyte dominance to cyanobacterial dominance in summer. This study also has implications for phytoplankton–nutrient models, which typically do not incorporate high-affinity P uptake.

KEY WORDS: Phosphorus limitation · Phosphorus dynamics · Phosphorus cycling · Cyanobacteria · Cylindrospermopsis raciborskii · Phytoplankton blooms · Seasonal succession