Transport and Fate of Phosphorus in the Nearshore Zone of Lake Michigan

Abstract

Bioavailable phosphorus loads exported to Lake Michigan from the Milwaukee and Sheboygan River Watersheds appear to have increased in the last 40 years despite meeting total phosphorus (TP) loading goals set by the Great Lakes Water Quality Agreement (GLWQA).Overall, bioavailability of P delivered from the Milwaukee and Sheboygan Rivers was highest during the warmer months, which coincides with the nearshore nuisance algae growth season. However, first order loss rates of SRP calculated during baseflow recession were also greatest during the summer, suggesting that increased river residence time during the summer could reduce export of bioavailable P. Observations of phosphorus partitioning combined with historic USGS monitoring data parallels trends seen in several Lake Erie watersheds with an increase in soluble reactive phosphorus (SRP) and decreases in particulate phosphorus (PP) export. Suspended sediment loads from the Milwaukee and Sheboygan watersheds are relatively P rich (mean 2.3 ± 0.66 ugP/mg) and relatively bioavailable at 39% and 33% respectively (using NaOH-extractable P as an index of bioavailability). Incubation experiments showed that over time river PP, and in particular NaOH-extractable P, releases P to the SRP pool. A seasonal increase in the PP:SRP ratio parallels increases in the percent bioavailability within the particulate P pool due to an increased contribution of NaOH-extractable P. This pattern coincides somewhat with prevalent midwest land use practices such as harvest of grain corn, but may also be driven by a combination of temperature and pH dependent coprecipitation and sorption phosphate buffering mechanisms on fine-grained particles that move SRP to the PP pool. However, this phosphate buffering mechanism may be offset by land use practices that increase soil P-Content, reducing the number of SRP sorption sites on particles. Moreover, P-rich fine-grained particles that do not settle in harbors but are discharged to the nearshore may desorb phosphate due to changes in equilibrium kinetics. Upon entering the lake, fine-grained P-rich particles can be intercepted by invasive mussel filtration, potentially releasing SRP to nearshore nuisance algae and thereby increasing the retention of P within the nearshore zone. Nearshore lake sediments were collected to test this hypothesis by examining lake sediments for fine-grained particles with high sorption capacity. Sorption experiments revealed that particles with high sorption capacity were present in the sediments. P-content saturation kinetics were modeled as a function of initial P-content, initial SRP concentration, and time. Results indicate that, though multiple size fractions of sediment exist in the lake sediment, it is the small (