Assessing the impact of the Lake Kissimmee restoration on apple snails

Seasonal fluctuations in rainfall occur naturally in Florida, historically producing correspondingly large intra-annual and inter-annual fluctuations in lake water levels. As a result, "normal" lake levels are elusive; the norm is for wide fluctuations, a pattern which earned them the descriptive title, "astatic", or unstable (Brenner et. al. 1990). Lake Kissimmee, in Osceola County, FL, had a mean average water level fluctuation of 1.4 meters, but over several years varied as much as 3.7 meters (Grocki 1975).The substrate and vegetative communities, in part, reflect the pattern of hydrologic fluctuations, their timing, duration, and frequency. Topographic variability in lake systems, coupled with their hydrologic patterns, yield a mosaic of shrubby, emergent and submerged plant habitats in the littoral zone of the lake. Fish and wildlife have adapted accordingly, and successfully forage and reproduce in these highly fluctuating systems. Installation and operation of water control structures over the past 40 to 50 years have dampened the degree of hydrologic fluctuations in many Florida lakes, resulting in long term stabilization oflake levels [Holcomb and Wegener 1971, Wegener et. al. 1974, Fox et. al. 1977, Moyer and Williams 1982, Florida Game and Fresh Water Fish Commission (GFC) 1995]. The mean annual flux on Lake Kissimmee was reduced from 1.4 meters to 0.9 meters (Wegener et al 1973). In addition, increased nutrient inputs to Florida's watersheds from agricultural and urban development have resulted in lake eutrophication (Wegener et. al. 1974, Moyer and Williams 1982). As nutrients increased and water levels stabilized, rooted aquatic macrophytes expanded and organic debris accumulated to form undesirable lake substrates. Fish populations suffer from drops in accumulated to form undesirable lake substrates. Fish populations suffer from drops in dissolved oxygen associated with decaying algae and plant matter, the alteration of plant community composition, and poor habitat quality of the substrate (Wegener and Williams 1974).To remedy these problems in Florida lakes, the Florida Game and Fresh Water Fish Commission initiated a lake restoration program in 1971 which emphasizes the use of a lake draw down to expose sediments (GFC 95). The objective ofthe program is to reduce undesirable plant species, expand desirable plant communities, and consolidate flocculent organic sediments (Holcomb and Wegener 1971, Wegener et. al. 1974, Fox et.al. 1977, Cooke 1980, Tarver 1980, GFC 1995). In addition, muck removal operations following lake level draw downs are meant to resolve the problem of extensive build up of organic sediments (GFC 1995, Moyer et. al. 1995). The goal of the draw down and restoration activity is to improve aquatic habitat in support of fisheries and wildlife whileimproving lake quality for recreational usage (e.g. sport fishing) (Wegener and Williams 1974, Tarver 1980,. GFC 1995). The Florida Game and Fresh Water Fish Commission (GFC), in cooperation with the South Florida Water Management District (SFWMD), conducted a draw down ofLake Kissimmee from December 1995- June 1996, dropping lake levels by approximately 1.7 meters. The Lake Kissimmee draw down provided an excellent opportunity to investigate the impact of management techniques on an invertebrate which is a critical component of the central lakes food web, the Florida apple snail (Pomacea paludosa, SAY). Although data on the responses of some invertebrate species 2 to draw downs in central Florida are available (Wegener et. al. 1974, Fox et. al. 1977, Butler et. al. 1992), no published studies document the responses of P. paludosa. The Florida apple snail belongs to the tropical family of aquatic snails, Pilidae, which inhabitant wetland areas in Florida, Central and South America, Africa and India. Most Pilidae habitat typically (under natural hydrologic patterns) experiences a seasonal dry down, and the Florida apple snail is no exception. Pilidae snails utilize both a lung and gill for respiration (Andrews 1965). The dual respiratory system enables these animals to tolerate daily and seasonal dissolved oxygen fluctuations, permits egg-laying above water level (eggs thus avoid hypoxia and predation), and, in some Pilid species, enables adults to maintain aerobic respiration during the dry-season (McClary 1964, Burky et al. 1972, Aldridge 1983). Apple snails are critical components of lake food webs in Florida. They are prey to alligators (Alligator mississippiensis)( Delany and Abercrombie 1986), fish (Lepomismicrolophus)(Chable 1947), turtles (Trionyxferox) (Dalrymple 1977), and ibis (Eudocimus albus)(Kushlan 1974). Apple snails comprise over 75% ofthe diet of the limpkins (Aramus guaraunaXCottam 1942), and nearly 100% ofthe diet of endangered snail kite (Rostramus sociabilis)(Snyder and Snyder 1969). Although the majority of the snail kite population occupies wetland habitats in south Florida throughout the year, they have been found to emigrate from this area when the dry season escalates into a prolonged drought (Beissinger and Takekawa 1983, Bennetts and Kitchens 1997). Researchers have identified the central lakes system as a critical refuge for the snail kite during these droughts (Beissinger and Takekawa 1983, Bennetts and Kitchens 1997). In 3 addition, the kites have been consistently nesting (even in non-drought years) on LakeKissimmee and Lake Tohopekaliga since the early 1980's, providing further support (in the form of suitable foraging and nesting habitat) for the endangered kite population(Beissinger and Takekawa 1983, Bennetts and Kitchens 1997).The impetus for the Lake Kissimmee draw down is habitat improvement for fisheries and wildlife (GFC 1995). A conservation priority ranking system for Florida wildlife identifies the snail kite and limpkin, largely due to their specialized diet of apple snails, as deserving special attention for monitoring and management (Milsap et. a1.1990).Moyer et a1. 1991 specifically recommended an evaluation ofthe impact of lake draw downs on apple snails. In this report we present the findings of a three-year studyofthe immediate and long term impacts ofthe Lake Kissimmee draw down and restoration activity on apple snail populations. The objectives ofthis research are asfollows:• To study the impacts oflake restoration activity (e.g. draw down, muck removal)on snail populations in several sites throughout the lake by assessing pre-drawdown and post-draw down snail abundance~• To study the movement patterns of snails to find out what proportion ofthepopulation migrates to deeper water or become stranded in the dry down area aslake levels recede;• To study the capacity for snails to survive in dry down conditions (Document has 55 pages)