Mark V. Hoyer

Bird abundance and species richness on Florida lakes: influence of trophic status, lake morphology, and aquatic macrophytes

Data from 46 Florida lakes were used to examine relationships between bird abundance (numbers and biomass) and species richness, and lake trophic status, lake morphology and aquatic macrophyte abundance. Average annual bird numbers ranged from 7 to 800 birds km−2 and bird biomass ranged from 1 to 465 kg km−2. Total species richness ranged from 1 to 30 species per lake. Annual average bird numbers and biomass were positively correlated to lake trophic status as assessed by total phosphorus (r = 0.61), total nitrogen (r = 0.60) and chlorophyll a (r = 0.56) concentrations. Species richness was positively correlated to lake area (r = 0.86) and trophic status (r = 0.64 for total phosphorus concentrations). The percentage of the total annual phosphorus load contributed to 14 Florida lakes by bird populations was low averaging 2.4%. Bird populations using Florida lakes, therefore, do not significantly impact the trophic status of the lakes under natural situations, but lake trophic status is a major factor influencing bird abundance and species richness on lakes. Bird abundance and species richness were not significantly correlated to other lake morphology or aquatic macrophyte parameters after the effects of lake area and trophic status were accounted for using stepwise multiple regression. The lack of significant relations between annual average bird abundance and species richness and macrophyte abundance seems to be related to changes in bird species composition. Bird abundance and species richness remain relatively stable as macrophyte abundance increases, but birds that use open-water habitats (e.g., double-crested cormorant, Phalacrocorax auritus) are replaced by species that use macrophyte communities (e.g., ring-necked duck, Aythya collaris).

Relations between trophic state indicators and plant biomass in Florida lakes

We collected quantitative data on macrophyte abundance and water quality in 319 mostly shallow, polymictic, Florida lakes to look for relationships between trophic state indicators and the biomasses of plankton algae, periphyton, and macrophytes. The lakes ranged from oligotrophic to hypereutrophic with total algal chlorophylls ranging from 1 to 241 mg m−3. There were strong positive correlations between planktonic chlorophylls and total phosphorus and total nitrogen, but there were weak inverse relationships between the densities of periphyton and the trophic state indicators total phosphorus, total nitrogen and algal chlorophyll and a positive relationship with Secchi depth. There was no predictable relationship between the abundance of emergent, floating-leaved, and submersed aquatic vegetation and the trophic state indicators. It was only at the highest levels of nutrient concentrations that submersed macrophytes were predictably absent and the lakes were algal dominated. Below these levels, macrophyte abundance could be high or low. The phosphorus–chlorophyll and phosphorus–Secchi depth relationships were not influenced by the amounts of aquatic vegetation present indicating that the role of macrophytes in clearing lakes may be primarily to reduce nutrient concentrations for a given level of loading. Rather than nutrient concentrations controlling macrophyte abundance, it seems that macrophytes acted to modify nutrient concentrations.

The restoration of Lake Apopka in relation to alternative stable states

Lake Apopka (Florida, USA) changed in 1947 from being a clear, macrophyte-dominated lake, used primarily for fishing, into a turbid algal lake with a poor fishery. The lake has resisted various efforts to reverse the change and restore the previous state. The restoration approach emphasizes the reduction in phosphorus inputs to reduce algal blooms and clear the water. We examined the question of whether a deep-lake approach with nutrient reductions is going to work on this large (area 124 km2) and shallow (mean depth 1.7 m) lake, or if techniques such as drawdowns or wind barriers developed for shallow lakes using the theory of alternative stable states are more applicable.The assumptions upon which the current restoration is based are not supported. The poor transparency is due more to resuspended sediments than plankton algae, so the current Secchi disk depth of 0.23 m is predicted to increase to 0.34 m with any reasonable reduction in algal levels. The failure of the macrophytes to become re-established probably is due more to unstable sediments than lack of light reaching the lake bed, and the marsh flow-way developed by the St Johns River Water Management District will be ineffective in removing particles from the lake. It would take more than 300 years to remove the fluid mud and more than 800 years to remove the rest of the low density sediments. We conclude that the loss of macrophytes in Lake Apopka is an example of a forward switch in the theory of alternative stable states, and that it will take more than a nutrient reduction program to bring about the reverse switch to a macrophyte state. We suggest an alternative approach using wave barriers to create refuges for plants, macroinvertebrates, and fish to restore Lake Apopkas largemouth bass fishery.

INTRAPOPULATION FEEDING DIVERSITY IN BLUE TILAPIA: EVIDENCE FROM STABLE-ISOTOPE ANALYSES

Stable carbon and nitrogen isotopes (δ13C and δ15N) in animal tissues are largely determined by isotope composition of assimilated food. Variations in δ13C and δ15N values within a fish population are often small as a result of feeding on the same food source. We report here, however, strikingly broad ranges of δ13C (−25.9 to −9.5‰) and δ15N (5.8 to 14.4‰) in an exotic fish from a Florida lake. Together with isotope data for food sources and gut contents, we demonstrate significant diet variation within a population of blue tilapia.

Volunteer Lake Monitoring: Testing the Reliability of Data Collected by the Florida LAKEWATCH Program

ABSTRACT Because the use of volunteer samplers is a very cost-effective way to collect large amounts of information on lakes over space and time, we studied the reliability of the protocols used by the Florida LAKEWATCH program. We found that chlorophyll extractions with hot ethanol gave values that were no different from those obtained with the standard method of grinding with acetone. In a comparative study of 125 lakes we found the data collected by volunteers were comparable to those collected by professionals. Mean Secchi disk depth, TP, TN, and chlorophyll values obtained by the citizens were strongly correlated (r > 0.99) to the mean values obtained by the professionals. To determine if freezing was a valid means of preserving water samples prior to analysis, we compared estimates of chlorophyll, TP, TN, pH, total alkalinity, and specific conductance obtained from fresh samples with estimates obtained from samples frozen up to 150 days. For most lakes there was little difference in chemical measurements made in samples preserved by freezing for different lengths of time, and various statistical tests indicated that freezing was a valid means of preserving lake water samples prior to analysis. Water quality data produced by volunteer samplers following the LAKEWATCH protocols were just as good as those from samples collected by professional biologists and handled using standard methods of sample preservation. The fact that volunteers can collect credible data means that lake management agencies could amplify their limited budgets by using volunteer monitoring, to sample more lakes and to sample them more frequently.

The Potential For Wave Disturbance in Shallow Florida Lakes

ABSTRACT We applied wave theory to calculate the extent and frequency that we would expect wave-driven surface water movements to disturb the sediments in 36 Florida lakes covering a broad range of surface areas and mean depths. The calculated per cent of the lakebed subject to wave disturbance at one time or another ranged from 6 to 100% and the per cent of the time 50% of the lakebed was disturbed ranged from 0 to 65%. The large Florida lakes, Apopka, Okeechobee, and Istokpoga showed high levels of calculated wave disturbance, which was consistent with the conclusions of previous investigations. Historic water level fluctuations in Lake Apopka were calculated to have major effects on wave disturbance in that lake. The dynamic ratio (the square root of lake surface area in square kilometers divided by the mean depth in meters) was significantly related to various measures of wave disturbance in our sample lakes. For lakes with ratio values above about 0.8 the entire lakebed was subject to wave disturbance at least some of the time. The dynamic ratio was also related to lake water quality. We found that increases in the dynamic ratio were significantly related to decreases in water quality as measured by total phosphorus, total nitrogen, chlorophyll, and Secchi disk depth. Calculations of wind disturbance by waves need to be modified in lakes with extensive beds of macrophytes, where water levels change and in periods where climatic fluctuations result in changes in wind regimes.

Epiphytic Macroinvertebrates on Dominant Macrophytes in Two Central Florida Lakes

ABSTRACT The density and biomass of epiphytic macroinvertebrates on Eichhornia crassipes, Nuphar luteum, Panicum hemitomon, Paspalidium geminatum, Ceratophyllum demersum, Utricularia spp. and Hydrilla vertlcillata were determined In Orange and Henderson lakes, Florida, during December 1982 and February, April, June and August 1983. Density and biomass of epiphytic macroinvertebrates ranged from 6 to 9714 individuals kg−1 wet weight of plant and 2 to 3118 mg dry weight kg−1 wet weight of plant, respectively, among macrophyte species. The quantities of epiphytic macroinvertebrates were greater on floating and submergent plants than on emergent plants. Abundant invertebrate taxa included Hirudinea, Gastropoda, Amphipoda, Odonata, Trichoptera, and Diptera. There were significant (P <0.05) temporal variations in the abundance of invertebrates; however, consistent temporal trends were not evident.

The Eutrophication of Lake Okeechobee

ABSTRACT Average annual total phosphorus concentrations in Lake Okeechobee increased from 0.049 mg/L in 1973 to 0.098 mg/L in 1984. The increases in total phosphorus concentrations were not correlated to external phosphorus inputs, but to changes in lake water levels. It is proposed that the flooding of marshes and exposed lake bottom primarily caused increased total phosphorus concentrations, but that the resuspension of bottom sediments by wind action may also be a major factor influencing in-lake total phosphorus concentrations. Reducing phosphorus inputs by 40 percent as proposed by the Lake Okeechobee Technical Advisory Committee (LOTAC) will not significantly improve water quality in Lake Okeechobee. Average total phosphorus concentrations will not decline significantly. Chlorophyll a concentrations and Secchi transparencies are not correlated to total phosphorus concentrations so massive expenditures of public funds to reduce nutrient inputs will not protect long-term water quality at Lake Okeechobee.

Use of Otoliths to Determine Age and Growth of Largemouth Bass in Florida

Abstract The annual formation of otolith annuli was validated through age 5 for Florida largemouth bass Micropterus salmoides floridanus. Sectioned otoliths (sagittae) gave more reliable ages than whole otoliths. Beginning at age 2, at least one annulus was obscure in some whole-otolith mounts, a problem that worsened with fish age. By age 5, over 20% of the whole otoliths gave under-estimates of true age; otoliths producing these errors came from fish with slower than average growth rates. Sectioned otoliths gave poorer regressions (lower coefficients of determination) of otolith radius on body length than intact structures because the sectioning plane varied slightly among otoliths. Nevertheless, back calculations of fish size and growth from sectioned and correctly aged whole otoliths were equally accurate. Otolith sectioning is the procedure of choice for age-and-growth analysis of Florida largemouth bass. Whole otoliths, however, require much less time for analysis than sectioned ones and may be used...

An Index of Biotic Integrity: A Test with Limnological and Fish Data from Sixty Florida Lakes

Abstract An index of biotic integrity (IBI) that used eight fish assemblage metrics was examined for effectiveness in estimating anthropogenic impacts to 60 Florida lakes ranging in size from 2 ha to more than 12,400 ha. The lakes ranged in trophic status from oligotrophic to hypereutrophic and had aquatic macrophyte abundances (percent lake volume infested) ranging from less than 1% to 100%. Fish species were classified by trophic feeding guild and tolerance to increases in turbidity or warming and decreases in dissolved oxygen concentration. Fish assemblage metrics tested were as follows: Number of fish species, number of native fish species, number of Lepomis species, number of piscivorous species, number of generalist species, number of invertivore species, number of species intolerant of increased turbidity or warming and decreased dissolved oxygen concentration, and number of species tolerant of increased turbidity or warming and decreased dissolved oxygen concentration. The total IBI scores and the...