Causal relationship between alkaline phosphatase activities and phosphorus dynamics in a eutrophic coastal lagoon in Lake Michigan

Abstract

Alkaline phosphatase plays an important role in regulating nutrient dynamics and algal blooms in aquatic environments, but the causal relationship between alkaline phosphatase (AP), nutrient status, and chemical speciation of phosphorus (P) in the water column remains elusive. A year-long time series (weekly to biweekly) sampling was conducted to investigate the causality between alkaline phosphatase dynamics, transformation of P species, and phytoplankton biomass in Veterans Park Lagoon, a closed lagoon with intense seasonal cyanobacterial blooms. As water temperature increased from spring to summer, depletion of dissolved inorganic P (DIP) drove increases in cell-associated AP (APcell, in nmol L−1). The elevated cell-associated alkaline phosphatase activities (APAcell, in nmol L−1 h−1) resulted in the transformation of particulate organic P (POP) into DIP, which enhanced the algal blooms. Not driven by DIP depletion, dissolved AP (APdiss) was spontaneously released during cyanobacterial blooms and the elevated dissolved APA (APAdiss) resulted in increase of DIP during the growing stage. This explains the concurrent high potential APA and high DIP concentrations in the water column observed here and other previous studies. The low-molecular-weight dissolved organic P (LMW-DOP) was found to be independent of both APAcell and APAdiss, consistent with previous results derived from isotope tracers. During the transition stage between fall and winter with lower water temperatures, APAdiss increased as a result of DIP depletion, while DOP was depleted and APAcell increased, and vice versa. The increase in both APAcell and APAdiss further resulted in the increase in concentration of particulate inorganic P through promoting the release of DIP from carbon-phosphate bonds and the adsorption of DIP onto particle surfaces especially under lower water temperature. Our results provide improved understanding of the concurrently high APA and DIP in the water column and provide new insights into the causal relationship between AP and P species in aquatic environments.