Quantifying the role of organic phosphorus mineralisation on phytoplankton communities in a warm-monomictic lake

Abstract

ABSTRACTPhosphorus (P) is a key driver of phytoplankton growth in lentic systems. During summer-stratified periods when dissolved inorganic P (DIP) is low, the phytoplankton assemblage often has elevated P uptake rates. Under these conditions, dissolved organic P (DOP) can become an important source of P for phytoplankton and a driver of phytoplankton succession. We examined how DOP mineralisation by alkaline phosphatase (AP) activity (APA), relative to phytoplankton P release (i.e., from exudation and lysis), meets phytoplankton P uptake and how it is associated with phytoplankton community composition changes under thermally stratified DIP-depauperate conditions in a large warm-monomictic lake. We found that when phytoplankton internal P stores were low (<0.65 µg P µg−1 Chl-a), APA increased, resulting in potential DOP mineralisation rates of 88 µg L−1 h−1 P, which is at the upper end of rates in the literature. APA represented up to 89% of the total phytoplankton P uptake and was much higher than the rate of phytoplankton P release. We found that elevated APA coincided with peak phytoplankton biovolume and dominance of the phytoplankton assemblage by cyanobacteria. Further, APA was strongly positively associated with the majority of the 10 most dominant cyanobacteria, including the key toxin-producer Raphidiopsis raciborskii (previously Cylindrospermopsis raciborskii). Our results indicate that under DIP-depauperate conditions, DOP mineralised by APA may be a major source of P for phytoplankton, and the ability to produce AP may be a key factor in the occurrence of bloom-forming cyanobacteria.