The nearshore waters of the Laurentian Great Lakes have historically suffered from beach fouling and clogged water intakes due to proliferation of the native, filamentous green alga Cladophora. A resurgence in nuisance growth of the alga has led to a demand for an improved model platform to better guide management. The Great Lakes Cladophora model (GLCM v3) predicts algal biomass (g dry matter m−2) and stored phosphorus content (P as % of dry matter) based on simulations forced by time series of incident light (I), water temperature (T) and water column soluble reactive phosphorus concentration (SRP, μgP L-1). A particular strength of the GLCM v3 is its foundation in ecologically sound biokinetic mechanisms, supported by field and laboratory measurements. These measurements, advancing the credibility and reliability of the biokinetic framework, include improved characterization of the growth and respiration responses to light and temperature, addition of a self-shading algorithm replacing an overly deterministic carrying capacity term, a new treatment of phosphorus uptake based on radioisotope experiments, additional observational support for Droop-based simulation of growth as a function of stored P, and implementation of a new physiologically and physically driven sloughing function. Uncertainty associated with processes collectively termed “environmental friction” (the I, T, and P growth forcing functions) is reduced, leaving the model sensitive to the maximum specific growth rate and the coefficient for extinction of photosynthetically active radiation through the algal mat. The model was performance tested by multi-lake (Erie, Huron, Ontario, and Michigan) calibration employing a common set of biophysical coefficients. This common set of calibration coefficients provides enhanced corroboration that GLCM v3 is suitable for examining the phosphorus–Cladophora dynamic across the Great Lakes. In particular, it greatly strengthens the model's efficacy for establishing a phosphorus standard to maintain levels of algal biomass below those constituting a nuisance condition, as per the Great Lakes Water Quality Agreement of 2012. In addition, the model structure can be applied to other lakes experiencing problems with attached filamentous algae.

Dijkstra ML, Corcoran MJ, Sloan JJ, and Lutz BL. 2020. Assessing phosphorus distribution and bioavailability in Lake Decatur, IL. Lake Reserv Manage. XX:XXX-XXX. In freshwater systems phosphorus is generally considered to be the limiting nutrient for algal growth, but not all phosphorus species are equally bioavailable to algae. Furthermore, the ratio of bioavailable to nonavailable phosphorus, impacted by in-lake processes and tributary loadings, is known to vary. Limited information, however, is available regarding this variation. Here we present the results from our investigation into spatial patterns in phosphorus concentration and bioavailability for 12 sites on Lake Decatur, a eutrophic 6-basin reservoir in central Illinois. We found that soluble phosphorus was distributed fairly homogeneously among basins, but the particulate fraction varied significantly (representing similar to 11% and similar to 89%, of phosphorus present in the system, respectively). Bioavailable phosphorus was also not evenly distributed throughout the reservoir. The highest concentration of bioavailable phosphorus (0.063 mg/L) was found in a basin that receives return flow of supernatant water from the dredged sediments dewatering pond. The reservoir's outflow basin, which has more tributaries and coastline than the other 5 basins, had a similar concentration (0.061 mg/L). Bioavailable phosphorus concentrations in the other basins had a mean of 0.024 mg/L, making these less susceptible to algal blooms and subsequent water quality degradation. Current monitoring and reduction efforts regarding nitrogen (to remediate blooms in the Gulf of Mexico) need to (more fully) include phosphorus. Site-specific kinetics, presented in this work, will support model (re)development to improve the selection and prioritization of effective strategies to remove/control the limiting nutrient in Lake Decatur.

The total phosphorus analyte (TP) has a long history of use in monitoring and regulatory applications relating to management of cultural eutrophication in freshwaters. It has become apparent, however, that the fraction of the TP analyte ultimately available to support algal growth varies significantly spatially (within a system), seasonally, and among systems. The algal bioassay methods described here provide an approach for determining the bioavailable fraction of the three operationally defined components of TP: soluble reactive phosphorus (SRP), dissolved organic phosphorus (DOP), and particulate phosphorus (PP) in effluents and tributaries discharging to lakes and reservoirs. Application of the technique facilitates a quantitative ranking and targeting of bioavailable phosphorus sources for management.•One congruent method to fractionate particulate and soluble phosphorus (found in aquatic samples) into bioavailable and unavailable fractions was developed based on compilation, adaptation and expansion of two methods from the late 1970s and early 1980s.•Detailed descriptions for culturing phosphorus-starved algae, sub-sampling schedules, kinetics determination, and data presentation are provided•Reproducibility is demonstrated by replication and closure of a mass balance on phosphorus.