AQUATIC VEGETATION ISSUES

The primary nuisance aquatic plant species in Lake Tarpon are Hydrilla verticillata and Typha latifolia, commonly referred to as hydrilla and cattail, respectively. Hydrilla is a rooted submersed exotic species whereas cattail is a native emergent species. Hydrilla grows and expands very quickly and becomes a problem when it clogs drainage ways and canals, prevents boating access for water-dependent recreation, and crowds out beneficial native plants. Cattail is an emergent native species which also grows and expands quickly, often dominating the littoral zone as dense monotypic stands which preclude shoreline recreational uses, obscure waterfront vistas, and crowd out other beneficial native plants.

Submerged Aquatic Vegetation

Hydrilla was probably introduced into Lake Tarpon sometime during the 1970s and since then it has become an ongoing management concern. Historically the Bureau of Aquatic Plant Management (formerly under FDNR, now under FDEP) has had sufficient funding to effectively maintain submergent and floating nuisance aquatics in the lake and manmade canals at insignificant levels. However, since 1986, budgetary constraints have limited the control of hydrilla to only within the natural lake, and within an arbitrarily determined management range of up to a 6 percent coverage, a level of coverage considered to be acceptable from the sport fishing and budgetary perspective. As a result, the control of hydrilla has been somewhat inconsistent in Lake Tarpon, characterized by fluctuations in the lake-wide coverage of this nuisance species. On the other hand, the spraying of floating nuisance aquatic vegetation, such as water hyacinth and water lettuce, has continued in both the lake and canals on a consistent basis, and has been generally very effective.

During the summer of 1992, hydrilla began spreading and establishing along the entire western shoreline covering more area than had previously been observed in Lake Tarpon. By the fall of 1992, it is estimated that approximately 500 acres of the lake surface area was covered by condensed hydrilla extending from the bottom to the surface, and that less dense hydrilla coverage extended across the majority of the lake bottom area (Personal Communication: John Rogers, FDEP). In recent years prior to this event, hydrilla coverage on Lake Tarpon had been effectively managed at a coverage of approximately 6-12 percent. Due to funding constraints during early 1992, the FDNR was unable to control the rapid growth of hydrilla in Lake Tarpon. As a result, no significant chemical treatments were made until October 1992.

To effectively treat the extensive hydrilla coverage it was necessary to implement several additional Sonar applications during March and May of 1993. These applications, which essentially resulted in a “whole lake” treatment, did not take effect until June and July 1993 when large floating mats of dead hydrilla were observed and persisted through the summer. No attempt was made to harvest these floating mats prior to their decomposition in the lake. These treatments were ultimately very successful in reducing hydrilla coverage back to a “maintenance” control level, as no follow up treatment was required in 1994, and only one treatment of 105 acres was required in 1995. The resulting massive die off of macrophytic plant tissue, however, appears to have adversely impacted water quality and increased the trophic state of the lake.

Although a clear cause and effect relationship cannot be established, it is very likely that the rapid release of nutrients organically bound up in this large mass of macrophytic plant tissue into the water column contributed significantly to the sharp increases in total nitrogen and chlorophyll-a concentrations observed in the lake during subsequent years. Based on hydrilla nutrient content data from Lake Okeechobee (Gremillion et al., 1988), the chemical treatment of approximately 500 acres of hydrilla potentially released as much as 2.4 tons of TP and 20 tons of TN back into the water column to be subsequently taken up by phytoplankton. Figure A-9 shows this relationship as a plot of the cumulative annual treatment acreage of hydrilla versus mean annual chlorophyll-a concentrations.

Trends in the coverage of desirable, native submerged aquatic vegetation, such as coontail (Ceratophyllum spp.) and eelgrass (Vallisneria spp.) have been difficult to assess due to the lack of a consistently applied quantitative monitoring program. An apparent reduction in eelgrass coverage occurred in the 1980s with the expansion of hydrilla in the lake (KEA, 1992); however, since the large scale eradication of hydrilla in 1993, coontail and eelgrass appear to be expanding their coverage in the lake (Personal Communication: John Rogers, FDEP).

It has been observed by numerous researchers that Florida lakes with severe algal bloom problems tend not to have rooted macrophyte problems (e.g., hydrilla), except perhaps for floating species like water hyacinth. Because increased algal abundance results in decreased water clarity, and thus reduces the euphotic zone, an inverse relationship should theoretically exist between macrophyte abundance (e.g., percent coverage) and algal abundance (e.g., chlorophyll-a concentration). This relationship underscores the importance of managing Lake Tarpon in such a manner that encourages the expansion of desirable, endemic submerged aquatic vegetation.

In summary, it is concluded that hydrilla can be, and has been, very effectively controlled in Lake Tarpon. Even though such control has been exercised inconsistently, resulting in ecological shocks to the system, it can be said that the FDEP Bureau of Aquatic Plant Management has essentially achieved its mandate of maintaining nuisance aquatics at their lowest feasible levels in Lake Tarpon. The larger question is whether or not this mandate always makes ecological sense from a holistic ecosystem management standpoint. Managing hydrilla coverage at some minimal maintenance level is probably a desirable goal for Lake Tarpon if: 1) chemical treatments are performed routinely on small areas such that the need for major whole lake treatments is avoided; and 2) the niche for rooted submerged aquatics is filled by other more desirable endemic species such as coontail (Ceratophyllum demersum) such that a minimum target lake-wide macrophyte coverage of 25 percent is achieved.

Emergent Aquatic Vegetation

As noted above, cattail may form what are essentially monocultures of densely growing plants along the lake shoreline. While cattail is a native species, problems occur when these plants proliferate unchecked. Cattail cause a litter buildup disproportionately high in comparison to most other aquatic plants, and reportedly become so dense that fish are restricted to the fringes rather than the interior of these stands. A more diverse assemblage of aquatic plant species is preferred since it provides a greater number of ecological niches. Increasing species diversity is equated with increasing environmental health.

Periodic and seasonal lake fluctuations, particularly on the high end of the scale, limit the expansion of cattail into deeper water. Lake level stabilization, therefore, tends to promote the expansion of cattail and allows dense, expansive stands to develop. While enhanced fluctuation would control cattail stand development, urban development in the watershed, and especially into the historic floodplain, has limited the vertical range over which Lake Tarpon can fluctuate. Historically, cattail was a relatively minor component of the emergent plant community in Lake Tarpon. Since the implementation of the water level fluctuation schedule in 1972, however, the coverage of cattail has expanded from less than 20-acres to approximately 120-acres (KEA, 1992). The cause of this expansion has been attributed primarily to the stabilized water levels in the lake which has allowed for the competitive dominance of cattail over other native species.

The effectiveness of increasing the upper range of water level fluctuation, even by a minor amount, as a means of controlling cattail stands has been recently observed in Lake Tarpon. During the past two years (1995 and 1996) cattail coverage has decreased by approximately 15 percent due to increased rainfall amounts and the associated slightly higher lake levels.

Currently, cattails are essentially managed on a piecemeal basis via the issuance of individual permits by FDEP for their removal along private waterfronts. Typically, applicants are required to replant their waterfronts with other desirable aquatic plants. No comprehensive program to improve the diversity of emergent aquatics in the littoral zone has yet been developed for Lake Tarpon. The removal and replacement of cattails with more desirable endemic species has occurred only on a limited piecemeal basis through the FDEP permitting program and other publicly funded habitat restoration. The largest such program was implemented by the District SWIM section where cattails were harvested from a total of 9.3 acres at five sites. The five sites were then revegetated with a more diverse mix of desirable native species. The success of these revegetation efforts, however, was generally poor due to the uprooting and erosion of the replanted areas by wave energy. Other smaller test revegetation projects have been successfully implemented by the FFWCC where bulrush was successfully established in areas previously dominated by cattail.

Cattail harvesting followed by revegetation with a more diverse assemblage of desirable emergent aquatics would likely provide significant ecological and aesthetic benefits. The benefits to the littoral plant community from this effort would, however, likely be supplemental to the greater lake-wide benefits derived from the increased lake level fluctuation range.

FISHERIES ISSUES

Lake Tarpon was formally designated as state Fish Management Areas by a special Resolution of the Pinellas County Board of County Commissioners in June, 1963. Section 39-20.005,Florida Statutes, sets forth the special regulations of state Fish Management Areas, and designates the Florida Fish and Wildlife Conservation Commission (FFWCC) as the state resource management agency with primary responsibility for sport fishery management.

The FFWCC has performed fisheries monitoring and management activities in Lake Tarpon on a periodic basis since the 1970s. Detailed fisheries investigations were conducted on Lake Tarpon from July 1987 to June 1991. During this time the FFWCC used a number of techniques to assess the fishery of the lake including aerial and boat surveys to estimate aquatic plant coverage, blocknetting and rotenone sampling in littoral sites, night electrofishing, and a creel survey. This combination of techniques allowed the FFWCC to assess fish population structure, the relationship between the fishery and macrophyte (aquatic plant) coverage, fishery utilization of differing habitats, and angler success and preferences.

For the most part, data presented by Champeau (1992) indicates an excellent sport fishery for Lake Tarpon. Being mesotrophic, Lake Tarpon has historically not supported the biomass of other more eutrophic (i.e., productive) Florida lakes, however, the population structure of the sport fishery has remained consistently within the preferred ranges. Sport fish are proportionately abundant with good percentages of harvestable and angler preferred sizes. Although most fishing effort in Lake Tarpon is focused on largemouth bass, the crappie population has expanded to the early 1980s levels to provide an excellent secondary sport fishery. Data further indicate good reproductive success, recruitment (survival) and rapid growth. Champeau (1992) did, however, suggest that the fishery could be enhanced through habitat management. Ideally it should not be necessary to manage habitat; however, two factors, lake level stabilization and the occurrence of exotic aquatic vegetation, necessitate an active management role with respect to aquatic vegetation.

The FFWCC again performed rotenone blocknetting and electrofishing in the summer of 1995 and spring of 1996 to assess densities and standing crops of all species present, and to determine the relative abundance and population structure of the largemouth bass population (Champeau, 1996). These results indicate that the electrofishing catch rates per unit effort, and the population age and size structure, for largemouth bass were comparable to data obtained during 1987 through 1992. In addition, the fish community balance is good with a forage biomass to carnivore biomass (f/c) ratio of 3.2. As shown in Figure A-10, however, the overall fish standing crop (biomass) in 1995 has increased by approximately 500 percent since last measured in 1990. This dramatic increase in fish biomass is due both to an increase in sport fish abundance as well as rough fish abundance.

Using a proprietary index of fish community balance (Champeau, 1996), the sport fishery in Lake Tarpon appears to be stable, or on a slight decline, since the 1970s, Figure A-11 shows this trend. The observed increase in the fish biomass in Lake Tarpon, however, is considered by the FFWCC to be an indicator of increasing eutrophication that threaten the future integrity of sport fishing in the lake if not addressed. The FFWCC has concluded that cultural eutrophication seems to have benefitted the fisheries of Lake Tarpon to date due to increased productivity, as measured in fish standing crops, however, increased fertility beyond current trophic levels may have future negative consequences. As a result, the FFWCC has recommended that strategies to abate significant nutrient sources to Lake Tarpon be implemented at this time (Champeau, 1996).

HYDROLOGIC AND HABITAT RESTORATION ISSUES

During development of the DBMP, PBS&J (1998) conducted a field survey and assessment of all natural habitats remaining in the Lake Tarpon watershed. This assessment did not extend into the Brooker Creek watershed within Pinellas or Hillsborough Counties. The results of this inventory are summarized below.

As shown in Figure A-12, the majority of the western portion of the study area has been developed and urbanized. The U.S. Highway 19 corridor is the most intensely developed area in the basin, with transportation and commercial land uses predominating. The northwest quadrant of the basin, which occurs mostly within the limits of the City of Tarpon Springs, is also intensely developed with commercial land uses. On the other hand, the southwest portion of the basin occurs entirely within unincorporated Pinellas County, and is intensely developed

with medium and high density residential land uses. The majority of the shoreline along the west side of the lake has been hardened or otherwise modified by residential and commercial development. Consequently, with the exception of Dolly Bay, Anderson Park, Salmons Bay, and Highland Park, there is relatively little littoral habitat on the western shoreline of the lake.

The eastern portion of the study area, although substantially developed, still retains many large contiguous remnants of the natural plant communities originally found within the basin. The most conspicuous habitat type in the east lake area is the large contiguous cypress and mixed hardwood forested wetland communities which run from southwest to northeast and form the Brooker Creek corridor. Although these wetland systems have been filled, ditched and impounded in many areas, some of the natural wildlife dispersal corridors and hydrologic flow ways still exist. Another conspicuous feature in the east lake area is the contiguous cypress and mixed hardwood forested wetland communities which occur along virtually the entire eastern shoreline of the lake. Waterward of this linear forested wetland system is a littoral fringe of herbaceous marsh vegetation. These marshes consist predominantly of cattails; however, some small areas of planted bulrush occur sporadically along this shoreline.

The majority of the relatively pristine habitat units remaining within the basin occur on the east side of the lake. Furthermore, the larger contiguous forested wetland systems, both those along the eastern shoreline of the lake, and those oriented southwest to northeast in the eastern basin, are clearly the most undisturbed natural systems remaining within the study area.

The majority of major wetland systems in the western portion of the basin have been hydrologically modified as a result of adjacent development. Two cypress systems, however, still show natural characteristics of both mature canopy and representative hydrophytic understory. The most notable example is the large mature cypress dome located west of U.S. 19 near the center of Lake Tarpon. This ±30-acre cypress swamp has, in the past, been a Southern bald eagle nesting site. Although no eagles have nested in this wetland for several years, it should continue to be preserved.

The remaining upland and wetland communities in the eastern portion of the basin are more representative of the historical habitat distribution and natural ecological characteristics of the watershed. Less intensive agricultural uses such as silviculture and cattle ranching preceded the current urbanization patterns in this area. This, in combination with more environmentally conscious development planning and regulation associated with the more recent development, has resulted in most of the historical wetland systems remaining intact. Unlike the west side of Lake Tarpon, over two-thirds of the eastern shoreline's natural wetland systems remain relatively undisturbed. Extensive cypress and mixed hydric forested systems also are distributed linearly throughout the adjacent upland areas of the basin to the east and represent historical and existing drainage patterns.

The habitat evaluation performed for the DBMP (PBS&J, 1998) did not take into account the Brooker Creek watershed. Many of the relatively pristine wetlands identified on the east side of the lake coincide with channels of Brooker Creek (Figure A-13). The Brooker Creek Preserve is located in Pinellas County, east of the Lake Tarpon watershed and extends to the Hillsborough/Pinellas county line. The Preserve encompasses approximately 8,500 acres of land acquired by the County and the District. Additionally, Hillsborough County has acquired approximately 1,440 acres for conservation adjacent to the Preserve. Wetland impacts within the Brooker Creek Preserve and in the Brooker Creek watershed in Hillsborough County are not as extensive as on the western side of the lake, but they do exist. These impacts include transportation and utility corridors, drainage improvements, development and wellfields.

There appear to be more opportunities to enhance and restore wetlands east of the lake and in the Brooker Creek watershed due to the less impacted nature of the area and the land in public ownership. Therefore, the potential for hydrologic restoration may be greater on the eastern side of the lake within Pinellas and Hillsborough counties. However, hydrologic and habitat restoration projects should be pursued where opportunities exist to improve or enhance water quality, water quantity or wetland and aquatic habitat. Hydrologic restoration of impacted wetlands to restore historic surface water flow patterns (i.e., ditch blocks and rehydration) would provide multiple benefits. Initially, the restored wetlands would provide habitat for wetland dependant animals and plants. Hydrologic restoration could result in increased flows through the historic channels of Brooker Creek and other unnamed tributaries to Lake Tarpon. Ultimately, increased surface inflows to Lake Tarpon could lead to increased flushing and dilution of the lake, which could lead to improved water quality.