The goal of this study was to look at changes in mean air temperatures, minimum air temperatures, maximum air temperatures, dew points, and precipitation over each of 1033 lakes in the coterminous United States over the summer months in the years 1981–2024. Near-surface water temperatures in the same lakes were calculated with equations using 8-day mean daily air temperatures, latitude, elevation, and the year of sampling. Over the past 43 years, there have been changes in air temperatures over many lakes of the United States with generally increasing trends for minimum air temperatures and mean air temperatures during the months of June through September. The greatest increases have been in daily minimum air temperatures followed by the mean daily air temperatures. Maximum daily air temperatures did not show a statistically significant increase for the summer season but did show a significant increase for the month of September. Along with the changes in the climate, the near-surface water temperatures of the lakes of the United States on average showed increases of 0.33 °C decade−1 for the four summer months and increases for each of the summer months.

Anthropogenic nonpoint source nutrient enrichment of lakes is a worldwide problem, but inputs from the watershed to Florida lakes may not be as severe as speculated. Long-term trends for total phosphorus (TP), total nitrogen (TN), chlorophyll (Chl), and Secchi disk transparency (SDT), enrichment surrogates, in 381 lakes monitored for 20-43 yr and having extensive management to none demonstrated improvements in trophic conditions following point source removals. Afterward, TP (R-2 = 0.85), TN (R-2 = 0.49), and Chl (R-2 = 0.50) continued to decline significantly (P < 0.05) and SDT (R-2 = 0.63) increased. For 99 state-designated "impaired" lakes, TP (R-2 = 0.73), TN (R-2 = 0.45), and Chl (R-2 = 0.44) also declined and SDT (R-2 = 0.69) increased. To consider natural background conditions, lakes were assigned to their ambient TP and TN zones. Geometric TP (93%) and TN (82%) zone averages for each lake after removal of point sources remained within each zone's 95% confidence interval. Individual lake trend analyses documented that similar to 80% showed no (most) or an improving trend for the trophic state variables. After correcting for "statistically meaningful" results or to remove false significances, <5% of the lakes had trophic state variable changes associated with eutrophication. If lakes are to be rehabilitated and/or protected to maintain designated uses, exclusive focus on watershed management of nonpoint source nutrients and associated regulatory actions should not be relied on to correct "impairments" in a timely manner. Management priorities should focus on actual causes of impairment and the most effective and efficient approaches for improvement.

ABSTRACT Rapid climate changes may potentially have strong impacts on the ecosystem structure and nutrient dynamics of lakes as well as implications for water quality. We used a space-for-time approach to elucidate such possible effects by comparing data from 1656 shallow lakes (mean depth <3 m) in north temperate Denmark (DK) and subtropical Florida (FL). The lakes were categorized into 7 total phosphorus (TP) classes within the range of 2 to 300 µg L−1. Physicochemical variables showed significant seasonal differences, which can be attributed to different sunlight regimes and temperatures. The FL lakes had overall higher fish biomasses (notably in the littoral zone) but a substantially lower zooplankton biomass and body mass of microcrustaceans, a much lower zooplankton:phytoplankton biomass ratio (lower grazing on phytoplankton), and a markedly lower biomass of benthic invertebrates, indicating much greater control of consumers by fish in the FL lakes. Accordingly, the summer phytoplankton biomass was higher in the FL lakes. Cyanobacteria in summer were proportionally more important in the FL lakes at all TP levels, whereas the proportion of dinophytes, chrysophytes, and cryptophytes was higher in the DK lakes at low TP. Submerged macrophytes occurred at higher TP (>100 µg L−1) in the FL lakes, but coverage was higher in the DK lakes at low TP. We also found lower oxygen saturation in the nutrient-rich FL lakes than in the DK lakes, suggesting lower net ecosystem production in the FL lakes. We discuss our results within the framework of climate warming.

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 similar to 40 mu g/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 mu g/L (1974-1977) to 146 mu g/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.

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.

Abstract 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 Reserv Manage. 35:181–192. 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 goals of the study were: (i) To describe the distribution of summer near-surface water temperatures in lakes of the coterminous United States and southern Canada (ii) to determine the geographic, meteorological and limnological factors related to summer water temperatures and (iii) to develop and test predictive models for summer near-surface water temperatures. We used data from the United States National Lakes Assessments of 2007 and 2012 as well as data collected from several different studies of Canadian lakes. Using multiple regressions, we quantified the general observations that summer water temperatures decreased when going from south to north, from east to west, and from lower elevations to higher elevations. Our empirical model using 8-day average air temperatures, latitude, longitude, elevations and month was able to predict water temperatures in individual lakes on individual summer days with a standard deviation of 1.7 °C for United States lakes and 2.3 °C for lakes in the southern regions of Canada.

Field measurements of water quality in Iowa lakes contradict paleolimnological studies that used 210Pb dating techniques in 33 lakes to infer accelerating eutrophication and sediment accumulation in recent decades. We tested this hypothesis by analyzing a series of water quality measurements taken in 24 of these lakes during the period 1972–2010. There was little change in the trophic state variables. Total phosphorus and algal chlorophylls did not increase, and Secchi depths did not decrease with no evidence that the lakes had become more eutrophic. Changes in daily sediment loads in the Raccoon River also did not match the paleolimnological inferred rates of soil erosion for the period 1905–2005, and an independent estimate of soil erosion rates showed a decline of 40% in the 1977 to 2012 period rather than an increase. We hypothesized that sediment mixing by benthivorous fish could be responsible for violating the basic assumption of 210Pb sediment dating that the sediments are not disturbed once they are laid down. We developed a mathematical model that demonstrated that sediment mixing could lead to false inferences about sediment dates and sediment burial rates. This study raises the possibility that sediment mixing in Iowa lakes and similar shallow, eutrophic lakes with benthivorous fish may cause significant sediment mixing that can compromise dating using 210Pb dating of sediment cores.

The State of Florida promulgated numeric nutrient criteria in 2013 because of a concern that nutrient enrichment had led to widespread increases in algal abundance and blooms. Chlorophyll was identified as a nutrient response variable and examination of historical chlorophyll trends was identified as one approach to help identify nutrient-impaired lakes. Examination of a 508-lake database with 10+ yr of data found that 371 (73%) lakes showed no statistically significant chlorophyll trends over time. Significant decreasing chlorophyll trends were identified at 67 (13%) lakes and 70 (14%) lakes had positive chlorophyll trends. For those lakes with significant trends, only 19 of the 67 lakes and 14 of the 70 lakes had R-2 values > 0.65. There were also 153 lakes with more than 20 yr of data, but only 53 had significant trends in chlorophyll concentration. Of those lakes, 32 had positive trends, but only 3 of those had R-2 values > 0.65. When the presence of an algal bloom was statistically defined as chlorophyll values exceeding two standard deviations of the individual lake's long-term average (geometric) chlorophyll or as a specific fixed chlorophyll value (> 20, > 40, > 80, or > 100 mu g/L), less than 5% of the lakes in the 153-lake database had increasing algal bloom trends. These lines of evidence suggest that there has not been widespread nutrient impairment of Florida lakes and that there is a frequent lack of nutrient limitation, suggesting why nonpoint nutrient control programs have yet to achieve management goals at some Florida lakes.

A probabilistic sample of lakes in the 48 coterminous US lakes was made by the United States Environmental Protection Agency in the 2007 National Lakes Assessment. Because of the statistical design, the results of our analyses of Secchi depths (SD) apply to a population of 45,265 lakes. We found statistically significant differences in mean Secchi depths between natural (1.57 m) and man-made lakes (1.18 m). The most important variable correlated with SD was turbidity, an optical measure related to suspended particles in the water column. For most lakes, chlorophyll a was highly correlated with both turbidity and SD, but several lakes had more turbidity and lower SD than expected based on chlorophyll a alone, indicating that non-algal suspended solids were an important factor. On an ecoregion basis, the non-algal suspended solids in the lake waters were related to the average levels of suspended solids in streams located in that ecoregion, and the non-algal suspended solids were more important in man-made than natural lakes. Phosphorus and nitrogen were directly correlated with chlorophyll a and turbidity and inversely correlated with SD. Based on diatom-inferred Secchi depths for the tops and bottoms of sediment cores from lakes in Ecoregions VIII and VII (excluding lakes in Minnesota) representing 40% of the natural lakes in the US, there has been no decrease in water transparency in that population of lakes in the past 70 or more years when the US population increased by 134%. We do not have information to determine if the other 60% of lakes have or have not changed.

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.

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 Secchi depths for the summer months were inversely correlated with water color and the amount of precipitation for the months of January through June. Drought years led to increased Secchi depths.

Individual Secchi disk measurements (975,760) were gathered from across the United States to understand regional distribution inwater transparency. Compiled Secchi diskmeasurements represented 14,421 US waterbodies. Average water transparency ranged from <0.1 to 31.6 m withwater transparencies <1.1 min 25% of the sampled waterbodies, <2.0 m in 50% of the waterbodies, and <3.3 m in 75% of the waterbodies. Of the population of examined waterbodies, 10% had mean Secchi measurements exceeding 4.8 m. US states with the lowest water transparency (<0.7 m) were Delaware, Louisiana, Mississippi, Nebraska, Oklahoma, and South Dakota, while the greatest water transparencies (>3.5 m) were in Alaska, Maine, Montana, New Hampshire, and Vermont. Regional differences in water transparency were identified across the US Environmental Protection Agency ecoregions (Level III), with lower water transparencies occurring in nutrient-rich regions and higher water transparencies occurring in nutrient-poor regions. Regional variability should be considered in the management of waterbodies at local to national levels.

Florida LAKEWATCH is a successful example of a long-term volunteer water qualitymonitoring program that started in 1986. Working with thousands of volunteers, these dedicated citizen scientists have collected reliable long-term water quality data for over 1100 lakes, 175 coastal sites, 120 rivers, and 5 springs. These data encompass water resources in 57 Florida counties. This manuscript describes the start and evolution of LAKEWATCH, including discussions of the following two major (of the many) hurdles to the continued success of the program: 1) demonstrating to professional groups that trained volunteers are capable of collecting credible (research and regulatory quality) data, and 2) maintaining consistent long-term funding. Funding is especially critical because trained and committed core staff is needed to work along with volunteers. Quality staff members are also important to provide direction, ensuring consistent data are collected and enough sites are monitored to answer statewide questions such as how geology impacts water chemistry in Florida. Examples are also provided on how LAKEWATCH data have been used to address lake management issues (i.e., ‘‘fixing’’ the problem) in the State of Florida. We hope the Florida LAKEWATCH experience assists other groups who have a vast army of citizen scientists waiting to get involved and then to best develop a successful monitoring program.

Limnology and OceanographyVolume 59, Issue 6 p. 2231-2239 CommentFree Access Response to comments: Quantification of the extent of cultural eutrophication of natural lakes in the United States Roger W. Bachmann, Roger W. Bachmann Fisheries and Aquatic Sciences, School of Forest Resources and Conservation, University of Florida, Gainesville, FloridaSearch for more papers by this authorMark V. Hoyer, Mark V. Hoyer Fisheries and Aquatic Sciences, School of Forest Resources and Conservation, University of Florida, Gainesville, FloridaSearch for more papers by this authorDaniel E. Canfield Jr., Daniel E. Canfield Jr. Fisheries and Aquatic Sciences, School of Forest Resources and Conservation, University of Florida, Gainesville, FloridaSearch for more papers by this author Roger W. Bachmann, Roger W. Bachmann Fisheries and Aquatic Sciences, School of Forest Resources and Conservation, University of Florida, Gainesville, FloridaSearch for more papers by this authorMark V. Hoyer, Mark V. Hoyer Fisheries and Aquatic Sciences, School of Forest Resources and Conservation, University of Florida, Gainesville, FloridaSearch for more papers by this authorDaniel E. Canfield Jr., Daniel E. Canfield Jr. Fisheries and Aquatic Sciences, School of Forest Resources and Conservation, University of Florida, Gainesville, FloridaSearch for more papers by this author First published: 12 October 2014 https://doi.org/10.4319/lo.2014.59.6.2231Citations: 5 Corresponding author: [email protected] AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Citing Literature Volume59, Issue6November 2014Pages 2231-2239 RelatedInformation

We used paleolimnological data for 240 lakes from the U.S. Environmental Protection Agency's (USEPA) 2007 National Lakes Assessment to estimate the extent that natural lakes in the coterminous United States have been changed by anthropogenic activities. In order to detect cultural eutrophication, we analyzed data on diatom‐inferred concentrations of total nitrogen (TN), total phosphorus (TP), specific conductance (SC), and pH as determined from lake sediments from the tops and bottoms of sediment cores from 240 natural lakes where the bottom of the core was judged to represent conditions prior to European settlement. We found no statistically significant increases in the average concentrations of TN, while TP decreased by 14% in this population of lakes since the time of European settlement. We also analyzed data from 48 reference lakes used by the USEPA to determine the relative condition of the sample lakes. Paired t‐tests showed the TN concentrations were not significantly different between the two time periods (p > 5%), while the average TP concentrations had significantly decreased by 26% since presettlement times (p < 5%). There were statistically significant increases in SC (17%) and pH (0.05 pH units). There were no statistically significant differences between the changes in TP, TN, SC, and pH in the 240 sample lakes and the changes in the 48 reference lakes. The proportions of lakes categorized as oligotrophic, mesotrophic, eutrophic, and hypereutrophic for the presettlement time period were not significantly different from the proportions found in 2007.

We found that the Florida Lake Vegetation Index (LVI) did not identify Florida lakes that were impaired due to excess loading of phosphorus or nitrogen from anthropogenic sources. The index is based on 4 plant metrics: the Coefficient of Conservatism of the dominant or co-dominant taxa, the percent of sensitive taxa, the percent of native taxa, and the percent of invasive exotic taxa. Our analysis of the data used by the Florida Department of Environmental Protection to establish and calibrate this biotic index found no link between nutrient concentrations and the LVI. The LVI was primarily associated with the pH and specific conductance of the lake waters, with the best scores found in acidic lakes and the poorest scores in more alkaline lakes. These variables are the result of natural gradients, not pollution, and are not included in the calculation of the LVI. Our study illustrates the importance of considering natural factors that determine the value of any index of biological integrity before it is used to indicate anthropogenic pollution.

Bachmann RW, Bigham DL, Hoyer MV, Canfield DE Jr. 2012. Factors determining the distributions of total phosphorus, total nitrogen, and chlorophyll a in Florida lakes. Lake Reserv Manage. 28:10-26.Using data from 1387 lakes collected over 3 decades, we found a wide range in the concentrations of total phosphorus (TP), total nitrogen (TN) and chlorophyll (Chl-a) in Florida lakes, and that edaphic factors as outlined by the United States Environmental Protection Agency's Florida Lake Regions were dominant in determining the concentrations of plant nutrients in the state's lakes. The hypothesis that the majority of the eutrophic lakes in Florida without known point source pollution are the result of nonpoint source nutrient pollution was tested in several ways and rejected. There was no correlation between the Landscape Development Intensity index and the concentrations of TP, TN, and Chl-a examined in Florida lakes. Several of Florida's 30 benchmark lakes (lakes with minimal human impact and meeting designated uses) were eutrophic, and there was no significant difference between the mean concentrations of TP and TN in these lakes versus all remaining Florida lakes. Paleolimnological studies also showed that several lakes were eutrophic to hypereutrophic prior to 1900, a time before significant population growth in the State of Florida. To help develop numeric nutrient criteria for Florida lakes that take regional differences into account, we grouped similar lakes into 6 TP zones and 5 TN zones.

Bachmann RW, Bigham DL, Hoyer MV, Canfield DE Jr. 2012. A strategy for establishing numeric nutrient criteria for Florida lakes. Lake Reserv Manage. 28:84-91.We used our knowledge of the factors that determine the nutrient concentrations in Florida lakes to develop a strategy for establishing numeric nutrient criteria for lakes in the State of Florida. Based on previous findings that natural factors were most important in determining the current nutrient concentrations in Florida lakes, we used current distributions of nutrients as a basis for setting criteria. We started with the US Environmental Protection Agency's Florida Lake Regions and grouped similar regions into 6 total phosphorus (TP) zones and 5 total nitrogen (TN) zones that were used to set numeric nutrient criteria for each zone. We propose criteria that will identify the lakes in each zone with the highest concentrations of TP and TN for subsequent investigation to determine the degree anthropogenic sources or natural backgrounds are responsible for their trophic status. To provide special protection to Florida's oligotrophic lakes, site-specific criteria are proposed, with oligotrophic lakes defined as those lakes that have (1) a long-term average of not more than 6 mu g/L of chlorophyll and (2) less than 30% coverage by surface area of submersed aquatic macrophytes. The site-specific criteria for TP and TN will be the long-term (7 year) average concentrations. These proposed criteria are protective of the designated uses of Florida lakes and prevent the misclassification of many naturally eutrophic lakes as impaired when they are not undergoing cultural eutrophication. The proposed numeric standards can be implemented immediately in the State of Florida.

Bachmann RW, Bigham DL, Hoyer MV, Canfield DE Jr. 2012. Phosphorus, nitrogen, and the designated uses of Florida lakes. Lake Reserv Manage. 28:46-58.We reviewed published information on the biology of Florida lakes to determine what concentrations of total phosphorus (TP) and total nitrogen (TN) might impair their designated uses. For the designated use of swimming, lake users preferred oligotrophic to mesotrophic lakes. Eutrophic lakes in Florida generally support their designated use of the propagation and maintenance of a healthy, well-balanced population of fish and wildlife. Fish standing crops in Florida lakes increased as the concentrations of TP increased from 1 to 1000 mu g/L. Florida lakes did not show the kind of changes in fish species with trophic state as might be found in northern lakes. Populations of aquatic birds and alligators also increased with increases in trophic state. Benthic macroinvertebrate indices of lake condition were not related to anthropogenic nutrient pollution when estimated by the Landscape Development Intensity index. We found no evidence that the concentrations of TP and TN in the water were responsible for excessive populations of aquatic macrophytes. A study of open-water concentrations of the cyanobacterial toxin microcystin in 187 Florida lakes found only 3 individual water samples collected from 2 lakes exceeded the World Health Organization guidance level of 20 mu g/L for swimming, although high levels of microcystin can sometimes be found in some lakes in surface accumulations of cyanobacteria.