The relationship between total phosphorus concentration (TP) and chlorophyll-a concentration (Chla; as an indicator of algae) has been well-established for temperate freshwater lakes. Understanding the annual variations in the Chla response to TP (Chla-TP relationship) by lake is important for uncovering the additional drivers of harmful algal blooms. A recent two-stage study was conducted using EPA Great Lakes National Program Office water quality data collected from open lake monitoring stations in the Great Lakes from 1999-2016. This study primarily utilized statistical analyses and multilevel modeling. The first stage of the study focused on Lake Erie and the changes in the Chla-TP relationship over time. The second stage of the study incorporated all five Great Lakes along with additional data on broader spatio-temporal climate factors in the region to better understand the annual variations in the Chla-TP relationship. The data were subdivided by three basins for each of the lakes as well as differentiated by depth of measurement in the water column.
Results/Conclusions
The results of both stages of the study showed that the relationship between the levels of TP and Chla concentrations changed erratically from year to year—in addition to the expected seasonal variation—with no overall temporal trend during that time period. Referencing Lake Michigan as an example, the summer time percent change in Chla varied over that time period from -0.076+/-0.048 to 0.528+/-0.049 with each percent increase in TP. The four lakes typically characterized as oligotrophic exhibited a somewhat more moderated amount of annual variation in comparison to Lake Erie—particularly the western basin—which is considered eutrophic. The amount of variability indicates that there are additional factors, likely on a regional scale, that are influencing the year-to-year variation in the Chla-TP relationship. Additionally, the analysis showed that changes in the Chla-TP relationship are linked to regional climate factors, especially precipitation. With the Great Lakes region expected to experience marked climate change related impacts, the task of managing eutrophication in Lake Erie will become increasingly complex as factors related to climate change will play more prominent roles—beyond just managing for nutrient reduction.