Status of freshwater resources and future management implications of sea level rise in the Lower Florida Keys Kristie Killam, Phillip Hughes, Anne Morkill Florida Keys National Wildlife Refuges Complex, Big Pine Key, FL Introduction
The following figure illustrates the sequence of cover types with increasing elevation (decreasing salinity) at the water holes sampled (all islands combined). Note that only freshwater wetland stands out from the strong inverse pattern.
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Freshwater Wetland
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For the wetlands assessed, there were no significant long-term trends in salinity (1987-2011). However, inequities in sampling effort (year X season X specific locations) and sampling conditions (e.g., precipitation levels), as well as real (high) interannual variation, preclude drawing definitive conclusions. For example, 1989 was a “severe” drought year (Folk 1992), and 2010 exhibited a relatively dry wet season and wet dry season.
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The survey effort was fully effective in terms of surveillance monitoring for whether water holes were protected from physical disturbance, exotic plants, and whether they remain generally suitable for wildlife (including general salinity level). Additionally, the study provided extensive data on variability in key factors, and sampling influences on those (e.g., season). Sampling of surface water without controlling for precipitation, season and other factors will not be sufficient to elucidate trends in salinity. To better determine potential trends and other relationships, a stratified design that allows for parsing out sources of variation will be required. Monitoring should be coupled with groundwater data in order to further isolate important factors, and ideally, be incorporated into a groundwater- surface water model.
Literature related to data sources
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proportion of sites gator present
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American alligator were observed in fresh to mildly brackish freshwater ponds (average 5.7 ppt) but generally avoided highly saline water holes. They were observed on 8 islands (see figure below), most frequently in buttonwood swamps in hardwood hammock cover. Alligators may serve as a useful indicator species for monitoring impacts of sea level.
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The frequency of occurrence of Key deer at water holes was negatively correlated with salinity (rs=0.8, 2-sided P=0.02), as illustrated (aggregated by island) in the following figure.
deer frequency of occurrence
Average of salin_ppt Average of lidar_rast_valu
Given the samples evaluated, frequency of occurrence (proportion of water holes with presence/sign) apparently increased in the case of Key deer and raccoon, and decreased in the case of the alligator (next figure).
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Not surprisingly, the elevation-salinity correlation among islands (below) is not as direct as that for cover types (above). This reflects differences in elevation profiles, size, and other factors among islands (the things that elevation alone does not explain). However, these differences will result in very different responses among islands to given amounts of sea level rise, and these responses will impact wildlife accordingly.
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proportion waterholes species present
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mean salinity
We explored whether there were trends in salinity, elevation, and wildlife relationships, with water holes aggregated by year, season, month, island, and cover type. Survey effort among years was imbalanced in terms of locations and dates; unlimited comparisons impose various biases. Additionally, the legacy data included many cases in which water holes were sampled more than once per year. We filtered the data to restrict it in various ways. We restricted comparisons to those water holes that were assessed in 2010-2011 (annual sample ≤277). Additionally, repeat samples for water holes that were sampled on multiple occasions within a given year were pooled (averaged) to yield a single estimate for the given variable, water hole, and year. Under these restrictions, sample size (all year-waterhole combinations) was reduced to about 450 depending on the variable. For one analysis, salinity by month, we included samples from repeat (e.g., monthly) visits to individual waterholes (overall sample size ≤ 2326), in order to consider the temporal information in the data.
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Materials and Methods We surveyed 277 freshwater wetlands on nine islands from January 2010 to January 2011. This targeted assemblage of water holes was relatively well represented in legacy data (sampled during one or more previous years) ranging back to 1987. We made single visits to those water holes. We sampled salinity, temperature, pH, presence of wildlife, and vegetation (cover of dominant species). Presence of invasive exotic flora and fauna were also documented. Water chemistry data was collected with a YSI 85 handheld oxygen, conductivity, salinity and temperature meter.
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Additionally, we explored the current and legacy data for trends and outliers in salinity and other variables, though recognizing inherent, substantial shortfalls of the data for this purpose. Such analyses may provide some insight for what would be needed for targeted monitoring (e.g., help inform strategies to monitor effects of sea level rise, restoration, or changes in wildlife distributions).
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The purpose of our study was to assess the status and general condition of waterholes, with emphasis on whether suitability for native wildlife has changed. We sampled at a subset of wetlands for which there were legacy data from previous surveys.
Most of the targeted wetland sites persist. Ten are now unavailable to wildlife because they are filled or fenced in (0.25% per year since 1991). However, only about 2% of the total were destroyed (filled in). This equates to 0.1% per year since 1991.
mean salinity (PPT)
The Miami oolite limestone formation found in the Lower Keys allows for the development of subterranean fresh water lenses as well as extensive surface fresh water wetlands. Rainwater collects in the shallow, impermeable limestone basins and solution holes that are distributed throughout the Lower Keys.
The more permanent freshwater wetlands are particularly critical to wildlife (e.g., Key deer) during the dry season (November-May). During the dry season, wildlife concentrate at fewer, more permanent locations. During the wet season, wildlife increase use of overflow and ephemeral wetlands, and are more dispersed. The next figure illustrates the proportion of wetlands with Key deer, raccoon, and alligator presence (including sign), by month.
frequency of occurrence (species present)
Results and Discussion
Freshwater wetlands are important habitat for many wildlife species, including those considered endangered, threatened or of special concern by federal and state resource agencies. A large portion of the Lower Florida Keys vertebrate and invertebrate diversity is dependent on these non-tidal wetlands in the National Key Deer Refuge and surrounding areas. For example, these wetlands are critical for persistence of the Endangered Key deer and Lower Keys marsh rabbit, amphibians, and, and native reptiles such as the American alligator and Key mud turtle, a Lower Keys endemic. The location, quality (salinity), and permanence of surface fresh water in the Florida Keys is critical to maintaining and managing this biodiversity.
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Folk, M.L. 1992. Habitat of the Key Deer. PhD. Dissertation, Department of Zoology, Southern Illinois University at Carbondale. 354 pp. Folk, M.J., W.D. Klimstra, C.R. Kruer, and M.L. Folk. 1990. Special report: Key deer accessibility to all of Big Pine Key. Report to U.S. Fish and Wildlife Service, Big Pine Key, Florida. Folk, M.L., W.D. Klimstra, and C.R. Kruer. 1991. Habitat evaluation: National Key deer range. Final report, Florida Game and Fresh Water Fish Commission. Nongame Program, Tallahassee, Florida. 366pp. USFWS 2009. Comprehensive Conservation Plan: Lower Florida Keys Refuges, Atlanta GA, 334 pp.
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Acknowledgments Funding for this project was provided by the American Recovery and Reinvestment Act of 2009. Special thanks to Monica Folk for extracting legacy data from archives. For further information please contact
[email protected]
