Bruce C. Cowell

A technique for optimizing sample size (replication)

Abstract Current methods for estimating sample size are not appropriate in situations where a minimum detectable difference cannot be specified a priori. For these cases, a method is proposed which incorporates resolving power as a primary factor and expended effort (feasibility) as a secondary factor. Trade-offs between resolving power and expended effort are evaluated over a range of sample sizes. The Se is used as a measure of resolving power and the greatest limiting factor on sample size is used as a measure of sample effort. Techniques for obtaining a range of sample sizes from a single large collection or from a preliminary experiment also are given.

The influence of whole lake aeration on the limnology of a hypereutrophic lake in central Florida

To determine the influence of a multiple inversion aeration system upon the limnology of a small sinkhole lake, we monitored physical-chemical and biological parameters for 15 months prior to starting aeration and for 24 months thereafter. Aeration eliminated thermal stratification and dissolved oxygen concentrations of bottom waters increased significantly. Secchi disk transparency increased during aeration while turbidity, pH, alkalinity, total nitrogen, hydrogen sulfide and iron concentrations decreased significantly. Primary production and mean chlorophyll a did not change significantly during aeration but total phytoplankton cell volume decreased 2-fold. This decrease was caused by a marked reduction in blue-green algae which appears to be attributable to rapid mixing of the lake and to decreases in the pH. Cell volumes of green algae remained constant but numbers of taxa increased 70%. Densities of crustacean zooplankton were reduced markedly by aeration while densities of rotifers increased significantly during the first year but then returned to preaeration levels during the second year. Large-bodied cladocerans were replaced by small-bodied forms during aeration, and copepod populations became dominated by nauplii (97%). Densities of benthic macroinvertebrates declined 2-fold during aeration due to to a marked reduction (10-fold) in the Chaoborus population which correlated strongly with decreases in crustacean zooplankton abundance. The total number of taxa collected on individual sample dates increased throughout the two year aeration period (from 12 to 25) and chironomids became the predominant group (70%).The multiple inversion aeration system successfully eliminated many of the undesirable features of eutrophication (e.g., oxygen depletion, blue-green algal blooms, low benthic diversity), but it did not change the trophic state. Aeration of hypereutrophic lakes for multiple years may be necessary to maintain desired conditions.

INTERACTION OF FACTORS GOVERNING THE DISTRIBUTION OF A PREDATORY AQUATIC INSECT

We investigated the interaction of factors regulating the larval distribution of the pred- atory chironomid Procladius culiciformis in Lake Thonotosassa, Florida. Hypotheses were tested experimentally to relate sediment grain size, organic content, prey density, diet, and feeding selectivity to larval growth, feeding habits, and distribution. The distribution of P. culiciformis was determined by monthly sampling (June 1978 to May 1979) of the benthic macroinvertebrate community along a littoral zone transect. Experimental and field observations were compared for final analysis. The distribution of P. culiciformis, within 0.2-1.5 m depth, correlated positively with density of total organisms, density of total chironomids, and median grain size of the sediment. However, the distribution was correlated negatively with density of oligochaetes, water depth. and sediment organic content. The annual mean density of P. culiciformis across all transect stations was 371 larvae/M2 and was highest (1024 larvae/M2) at the shoreline station (0.7 m depth). The influence of food on distribution was measured by larval preference for naturally occurring prey and by larval growth on four common diets. Feeding selectivity by P. culiciformis was positive for chironomids, ostracods, and cladocerans, and negative for rotifers. Survival and growth of larvae were highest on a diet of oligochaetes, significantly lower on diets of chironomids and zooplankton, and lowest on a diet of algae-detritus. Sediment grain size and organic content were tested for larval preference and their effect on predation rate. Larvae preferred sediment with high organic content, but no preference was found among natural sediments with different median grain sizes. Large-grain sand and high prey density promoted high predation rates in the laboratory. However, the presence of organic matter decreased the predation rate and nullified the effect of grain size. Multiple environmental factors, especially those influencing prey availability, interact to regulate the larval distribution of P. culiciformis.

Distribution and seasonal abundance of benthic macroinvertebrates in a subtropical Florida lake

We studied the distribution and seasonal abundance of benthic macroinvertebrates from July 1975 through September 1976 in a hypereutrophic lake in subtropical Florida. The benthic community was comprised principally of oligochaetes (56.1%), chironomids (37.1%), and chaoborids (5.7%). Numbers of taxa and mean densities correlated negatively with depth and positively with mean grain size of the substratum and dissolved oxygen concentration at the mud-water interface. Seasonal abundances and life history information obtained for the predominant species of Chironomidae (Polypedilum halterale, Glyptotendipes paripes, Chironomus crassicaudatus, Cryptochironomus fulvus, C. blarina, Cladotanytarsus sp., Procladius culiciformis, and Coelotanypus concinnus) indicated that all of these species are multivoltine with rapid generation times. Larval lengths of life at summer temperatures, 27–31 °C, ranged from 14–22 days indicating that sampling in subtropical lakes should be at short intervals (approximately 3 days) if the life cycles, ecology, and function of the components of the benthic community is to be understood.

Factors influencing the distribution, abundance and growth of Lyngbya wollei in central Florida

Abstract Fifteen sites in the Kings Bay/Crystal River estuarine system, Florida, were sampled over 1 year to determine the spatial and temporal distribution and abundance of the filamentous blue-green alga, Lyngbya wollei (Farlow ex Gomont) Speziale and Dyck. Monthly biomass (dry weight) of benthic Lyngbya mats was constant at approximately 1 kg m −2 during most of the year, but it decreased from April to July when large mats were observed floating on the water surface. Multiple regression analyses showed that Lyngbya biomass was correlated negatively with conductivity and alkalinity; this is consistent with L. wollei being primarily a freshwater species. In regions of the bay with higher salinities, the vascular macrophyte, Hydrilla verticallata (L.f.) Royle, prevailed. Nutrient influences appeared to be small even though the northeast portion of Kings Bay/Crystal River has high concentrations of nitrogen, phosphorus and calcium. Controlled laboratory studies were conducted to determine the effects of pH, salinity, and nutrients on growth of L. wollei . Optimal growth occurred at a pH of 8.0 and a salinity of 0 ppt (g 1 −1 ); salinities in excess of 5.25 ppt killed more than 99% of the cells within 2 weeks. Growth of Lyngbya from five Kings Bay stations in six nutrients sources (spring water, NO 3 N, NH 4 N, PO 4 P, NO 3 N + PO 4 P, and NH 4 N, + PO 4 P) did not differ among stations, and nutrients additions produced no increases over spring water. Addition of Ca 2+ increased growth of L. wollei at low and intermediate concentrations of nitrogen and/or phosphorus; this partially explains why spring water, derived from the limestone aquifer, produces such good growth of Lyngbya .

A synoptic study of the limnology of Lake Thonotosassa, Florida

Limnological sampling of Lake Thonotosassa was initiated in 1970 to document current conditions following 15 years of artificial enrichment by organic wastes from primary treated sewage and citrus processing plants, and to provide base-level data for a long-term study of the rates of change following the installation of a secondary sewage treatment facility. Results from the first year of study indicated that the lake was in an advanced state of eutrophy. Inorganic nutrient levels were high; oxygen deficits occurred in the hypolimnion and at the mud-water interface; phytoplankton volumes were large and dominated by blue-green algae; primary productivity rates were comparable with those of grossly polluted lakes; small-bodied herbivores dominated the zooplankton; and benthic invertebrate populations were comprised of an overwhelming abundance of oligochaetes and chironomids. Construction of the new sewage treatment plant reduced the organic load and B. O. D. of the incoming waters and benthic diversity in the inlet subsequently increased. However, in the lake itself decreases in coliform bacteria concentrations were the only signs of immediate influence.

Dynamics of a subtropical plankton community, with emphasis on the copepod Mesocyclops edax

The zooplankton community of a subtropical, eutrophic lake was studied over an annual cycle. Mesocyclops edax, the numerically dominant crustacean, exhibited a stationary age distribution for most of the year (July–March). Per capita growth rates (r) of M. edax approximated zero throughout most of the study, indicating a close correspondence between birth and death rates. High death rates in the spring and summer probably were due to intense predation, since physiological mortality was low and predators were abundant. Multiple regression analyses indicated that much of the variation in M. edax birth rates could be explained by abundance of their animal prey. Other plankton in this lake exhibited considerable seasonality, but not to the marked degree characteristic of temperate populations. The number of zooplankton species was low and cladocerans were scarce, as is characteristic of many eutrophic Florida Lakes.

Temporal patterns of abundance of epiphytic invertebrates on Typha shoots in a subtropical lake

We sampled epiphytic invertebrates and algae on Typha shoots in Lake Padgett, Florida, on a weekly basis from June 1988 through June 1989. The most abundant invertebrate taxonomic groups were oribatid mites, cyclopoid copepods, naidid oligochaetes, chydorid cladocerans, and chironomid larvae. The epiphytic invertebrates had seasonal patterns of abundance that were markedly different from those in temperate lakes where abundances tend to peak in the summer. In Lake Padgett, invertebrates showed strong peaks in abundance in September and October, with weaker, secondary peaks in December, March, and April. The only taxa that were most abundant in summer were oribatid mites and cyclopoid copepods. Invertebrate densities were higher on senescent shoots. In temperate lakes, abundances of epiphytic algae and invertebrate grazers tend to show strong temporal correlations, whereas, in Lake Padgett, they tended to be weakly correlated.

Benthic invertebrate recolonization of small-scale disturbances in the littoral zone of a subtropical Florida Lake

Construction of spawning beds by the exotic cichlid fish, Sarotherodon aurea, produces numerous small-scale disturbances in shallow Florida lakes. Two experimental methods were used to simulate these disturbances (containers filled with defaunated sediment and disturbed natural substratum) and to determine benthic invertebrate recolonization patterns and rates.Recolonization of experimental containers was rapid and densities of macroinvertebrates reached equilibrium with the control habitat by Day 6 following disturbance. However, Chironomidae and miscellaneous taxa usually were more abundant (p < 0.05) in the experimental containers, and Oligochaeta always were more abundant (p < 0.01) in the control. Species of the planktonically dispersed insects either attained equilibrium rapidly or showed significantly higher densities in the experimental containers (i.e. opportunistic species).In the second experimental approach, disturbance of natural substratum removed 91% of the benthic invertebrates. The common species of chironomids reached equilibrium by Day 6 but the two predominant oligochaetes, Limnodrilus and Pristina, reached equilibrium at 9 and 15 days respectively. For the entire community, similarity on Day 6 was 87.1% and over the remainder of the sample period averaged 86.8%. Chironomid instar analyses showed significantly higher proportions of early instars in the disturbed patch indicating that colonization is principally by younger organisms. Thus, small-scale disturbance in freshwater benthic communities may be more important in ordering the biomass or age-group structure than the numerical abundance.

Seasonal variation in benthic invertebrate recolonization of small-scale disturbances in a subtropical Florida lake

Approximately 11.5% of the littoral zone of a hypereutrophic Florida lake is disturbed by the construction of spawning beds by the cichlid fish, Sarotherodon aurea, during a single spawning season. Simulations of these beds were created during three seasons of the year to test for variation in recolonization rates and patterns in the benthic macroinvertebrate community.The seasonal variation in densities of benthic invertebrate populations suggests a direct relationship between reproductive activity and recolonization rate. Recolonization rates of the numerically dominant species, Polypedilum halterale (Diptera: Chironomidae), were much more rapid in the spring and summer than in the winter. In contrast, Hyalella azteca and Glyptotendipes paripes attained peak population densities during the winter season when Polypedilum was present in low densities. These organisms invaded the disturbed area in unusually high abundances and later declined to background levels. The final result was a winter population with densities comparable to the control (undisturbed) area, but the species composition was different.Similarity between disturbed and control communities during the winter season only reached 67% a month after disturbance, while communities during the warmer months attained nearly 90% similarity in less than 15 days. This lack of similarity during the winter indicates that disturbance, at the appropriate time, may play a role in community organization.