Abstract
Many landscapes with large numbers of freshwater lakes experience ecological degradation linked to cultural eutrophication, pollutant runoff, changed runoff hydro-patterns, bank modifications, or other factors. Agencies tasked with maintaining or restoring these hydrosystems must evaluate and monitor lakes to identify those in need of corrective action and to determine when they are no longer impaired. That regulatory task could be greatly simplified if a mechanism could be found to give a preliminary assessment of a lake’s ecological health using widely available information such as visible conditions observed through aerial images. One objective of this study was to examine Canfield and Hoyer’s (1992) research finding that the concentration of nutrients in, and the water clarity of, inland shallow lentic freshwater hydrosystems are related to their proportion of cover with submerged aquatic vegetation (SAV) or emergent aquatic vegetation (EAV). The second objective was to determine whether the proportion of a lake’s cover with SAV plus EAV can be reliably determined using widely available aerial imagery. The research applied the proposed method to quantify SAV plus EAV cover in shallow subtropical lakes in Central Florida and attempted to identify a quantitative relationship between the cover determined in that way and the best available data about the lakes’ water quality, specifically the concentration of total phosphorus (TP) and total nitrogen (TN); and water clarity measured as concentration of chlorophyll-a (Chl-a), Secchi disk depth, and turbidity. The research assessed 19 lakes in Polk, Lake, and Hillsborough Counties, Florida, using (1) water quality data collected in single-day grab samples between May and September 2016 (as reported by county-level regulatory agencies); and (2) publicly-available aerial images of those lakes captured during the same months. Site visits to 10 of the 19 target lakes verified the method’s validity to estimate the presence of SAV and EAV. Results addressing the first objective showed that lakes with macrophyte cover (SAV plus EAV) of greater than about 10% to 20% tended to have lower concentrations of TN, TP, and Chl-a; and higher water clarity. The sample included few lakes with macrophyte cover in the range of 20% to 50%, because most central-Florida lakes tend to have cover either higher (for mostly natural lakes) or lower (for managed lakes) than that window, so the results are not as strongly supported as if it were drawn from a larger sample. The second objective was achieved by demonstrating that the method of quantifying proportion of vegetative cover of a given lake using publicly available aerial imagery is sufficiently precise to create those estimated relationships. The successful use of remote information gathering may greatly improve the capability of management agencies to identify and assess impairment. These remote methods can be used to enhance future regulatory efforts in two ways: (1) to assist in identifying lakes that may have degraded clarity and high nutrient concentrations using readily available remote information, which would allow limited field investigation resources to focus on lakes most likely to need analysis; and (2) to serve as an initial indicator of conditions under which a lake’s water quality has improved to no longer be impaired for nutrients or clarity.