Canfield DE Jr, Bachmann RW, Hoyer MV. 2020. Restoration of Lake Okeechobee, Florida: mission impossible? Lake Reserv Manage. XX:XXX–XXX. Legally mandated eutrophication restoration goals for Lake Okeechobee (FL) are unachievable, therefore assigning managers a “mission impossible.” Since the 1970s, restoration efforts have focused on reducing pelagic total phosphorus (TP) to ∼40 µg/L. A total daily maximum load (TMDL) of 140 metric tons (t)/yr was adopted by the Florida Department of Environmental Protection in 1999 (effective date 2015) to restore the lake’s balance of flora and fauna. Phosphorus (P) loads (1975–2018) averaged 516 t/yr with no significant change over time, yet average TP significantly increased from 51 µg/L (1974–1977) to 146 µg/L (2015–2019). Greater TP values in 2019 were due to Hurricane Irma and an early June storm event. Annual P-loads and pelagic TP were not significantly correlated. Instead, TP was strongly correlated with turbidity (R ² = 0.85), which is generated by wave-driven resuspension of P-rich unconsolidated sediments. Since 1973, >13,000 t of TP has been added to Okeechobee’s sediments that have accumulated over the past century due to the lowering of water levels and the construction of the Herbert Hoover Dike. Prior to settlement, high water levels allowed turbid lake waters to flood large areas of adjacent wetlands, where suspended sediments were removed from the lake. With the minimization of this self-cleansing mechanism after construction of the Herbert Hoover Dike, P-rich fine sediments accumulated, and periodic hurricanes disrupted consolidated sediments. Unconsolidated sediments are easily resuspended into the water column, raising TP. Efforts to reduce Okeechobee’s pelagic TP through reductions of P-loads alone will not work due to sediment accumulation and resuspension.

Using data collected with 3 different methods, we found no decreases in the average water clarity of Maine (USA) lakes over different periods of time. Field measurements of Secchi disk depths in the summer months by volunteer samplers in several hundred lakes showed a small, statistically significant increase in water transparency during the period 1976 through 2013. A reanalysis of satellite-inferred Secchi depths between 1990 and 2010 showed no trend over time. In addition, diatom-inferred Secchi depths from short sediment cores in a randomly selected group of Maine lakes analyzed by the US Environmental Protection Agency showed no statistically significant difference between the average Secchi depths in a pre-1850 time period and the early 1990s. Lake maximum depth was the most important morphological variable associated with water clarity among Maine lakes. In individual lakes, both water color and chlorophyll were inversely correlated with Secchi disk depths. The statewide annual average Secc...

Canfield DE, Jr., Bachmann RW, Hoyer MV, Johansson LS, Søndergaard M, and Jeppesen E. 2018. To measure chlorophyl or phytoplankton biovolume: an aquatic conundrum with implications for the management of lakes. Lake Res Manage. 00:00–00. The log₁₀-transformed relationship between measured phytoplankton biovolumes and chlorophyll concentrations, surrogates for algal biomass, was examined using 13,000-plus paired samples collected from lakes in Denmark (250), the continental United States (1835), and Florida (159). A positive (R² = 0.57) relationship was found but predicted biovolumes had a 95% confidence interval of 11–912%. Regressing chlorophyll as opposed to phytoplankton biovolume against total phosphorus (TP) (R² = 0.43 vs. R² = 0.21), total nitrogen (TN) (R² = 0.24 vs. R² = 0.08), and Secchi disk (SD) (R² = 0.60 vs. R² = 0.39) yielded stronger relationships. Three algal groups (Cyanophyta, Chlorophyta, and Bacillariophyceae) contributed approximately 100% of the biovolume in some samples. For these groups and samples, biovolume increased significantly with chlorophyll (R² = 0.35, R² = 0.27, and R² = 0.31, respectively) and TP (R² = 0.18, R² = 0.13, and R² = 0.13) and decreased significantly with increases in SD (R² = 0.29, R² = 0.31, and R² = 0.17). All empirical relationships had substantial confidence intervals. Contingency tables for variance within the independent (horizontal variance) and dependent (vertical variance) variables are presented, providing managers information on how much change is required to insure noticeable effects. If resources are limited, chlorophyll is recommended for monitoring long-term trends because it provides an estimate of biomass magnitude and has better relationships with nutrients and SD. Managers may integrate occasional biovolume measurements if concerned with Cyanophyta abundance, taste and odor production, or changing algal population dynamics.

The invasion of hydrilla into North American lakes has provoked concern over loss of native flora and fauna and resulted in costly suppression efforts. We used two data sets to determine if common measures of ecosystem health; abundance, species richness, diversity and evenness, were affected by the presence of hydrilla. Data Set 1 consisted of 27 Florida lakes, 11 of which had hydrilla present for approximately 4–8 years in varying abundances, and 16 did not have hydrilla. Given the number of lakes, each was sampled only once in the summers of 1986–1990 for community measures of aquatic plants, birds, and fish. Data Set 2 consisted of 12 lakes, six with abundant hydrilla for over 23 years and six without hydrilla. These lakes were sampled every year in the summer (with a few exceptions due to weather conditions) between 1999 and 2006 for community measures of aquatic plants and fish. The results for both data sets show that presence of hydrilla had no statistically significant effect (P > 0.05) on all community measures tested (i.e., richness, diversity, abundance). Our conclusions support the hypothesis that hydrilla in these Florida lakes has occupied a mostly vacant ecological niche and has not affected the occurrence or relative composition of native species of aquatic plants, birds, and fish. Because pond experiments have found negative effects of hydrilla, further focused research is needed to assist management decisions when considerable resources are to be spent each year on hydrilla suppression.

Because warming water temperatures have widespread consequences for freshwater communities, we were interested in estimating the patterns and rates of change of near-surface summer water temperatures in United States lakes. We developed multiple regression models to relate daily surface water temperatures in lakes of the conterminous United States to 8-day average air temperatures, latitude, elevation, and sampling month and year using data from 5723 lake samples in the months of June-September during the period 1981–2018. Our model explained 79% of the variation with a root-mean-square error of 1.69 °C. We predicted monthly average near-surface water temperatures for 1033 lakes for each year from 1981 through 2018. Lakes across the conterminous United States have been warming for the period 1981–2018 at an average heating rate of 0.32 °C per decade for the summer months (June–September). The average summer warming from 1981–2018 would be the equivalent of a lake decreasing 259 m in elevation or moving 233 km south. On the basis of national air temperatures starting in 1895, it was inferred that lake water temperatures are variable from year to year and have been steadily increasing since 1964, but that maximum temperatures in the 1930s were just as warm as those in 2008–2018.

Canfield DE Jr, Hoyer MV, Bachmann RW, Bigham Stevens D, Ruiz-Bernard I. 2016. Water quality changes at an Outstanding Florida Water: influence of stochastic events and climate variability. Lake Reserv Manage. 32:297–313. The Santa Fe Lake System (SFS) is an Outstanding Florida Water system in northern peninsular Florida and receives special protection from governmental agencies to prevent impairment of water quality from anthropogenic activities. Since 1986, periods of sudden nutrient increases and declines have occurred along with changes in water clarity documented within a 28-year monthly database. Changes were linked to stochastic events such as an influx of gulls in 1986, the adjacent 5100-ha Dairy Road forest fire in 2007, 3 Category 3 hurricanes that struck Florida in 2004, and droughts. However, increasing trends at SFS were also observed for the yearly measured minimum water chemistry values, as were synchronous changes in these baseline conditions at other nearby lakes, suggesting the lakes were being impacted by a regional environmental factor. These changes corresponded to a period of decreasing precipitation and were related to climate variability, perhaps reflecting phase changes in the Atlantic Multidecadal Oscillation. The possible mechanism for the observed changes most likely relates to alterations in regional precipitation/evaporation rates and resulting changes in groundwater chemistry and hydrology. Long-term trends in water quality at SFS may reverse if Florida enters a long-term period of increasing precipitation.