John A. Osborne

Response of zooplankton to the reduction and elimination of submerged vegetation by grass carp and herbicide in four Florida lakes

Zooplankton populations were monitored monthly for a three year period in four Florida lakes. Each lake received an aquatic vegetation control program using grass carp (Ctenopharyngodon idella Val.) and herbicide, either alone or in combination. All aquatic vegetation was eliminated in the three lakes with grass carp; it was severely reduced and exhibited qualitative shifts in the lake treated with herbicide alone. Total number of recorded zooplankton species was reduced in each subsequent year as vegetation was eliminated, but mean density l−1 increased greatly. The overall trend in all lakes was toward increasing numbers of small filter-feeding herbivores, with a shift from copepod and copepod-cladoceran dominated to rotifer and small cladoceran dominated systems. It is suggested that these changes reflect primary plankton response to fluctuations in habitat, food base, and planktivore pressure resulting from gradual macrophyte alteration. Changes observed indicate apparent increases in eutrophy.

Phytoplankton responses to reduction and elimination of submerged vegetation by herbicides and grass carp in four Florida lakes

Abstract Four central Florida lakes were monitored for 3 years to determine the effects of macrophyte reduction and elimination by grass carp and herbicide on phytoplankton populations. Clear Lake, in which grass carp were stocked after a year of baseline studies, had all macrophytes eliminated within 14 months. The density of phytoplankton increased significantly from an initial 24-month mean of 165 000 cells 1 −1 to a mean level of 787 900 cells 1 −1 in the third year. In Little Lake Fairview, stocked with grass carp in both the first and third years of study, vegetation was not eliminated until the 34th month. Phytoplankton density increased significantly from a 24-month mean of 64 200 cells 1 −1 to a mean of 370 200 cells 1 −1 in the third year. Lake Orienta, stocked with grass carp in the first year, had all vegetation eliminated within 6 months. Phytoplankton abundance did not change significantly over the course of the study (mean: 2 700 000 cells 1 −1 ). Lake Mann was treated with herbicide alone after an initial year of baseline data collection. Submerged vegetation was reduced in the second year but increased to near initial biomass levels in Year 3, with Nitella sp. replacing Hydrilla verticillata (L.f.) Royle as the dominant macrophyte. No significant yearly changes were noted in density of phytoplankton in this lake. Lake Orienta was the only lake in which the number of phytoplankton species collected differed significantly between years. Cyanophytes (notably species of Anacystis, Microcystis and Anabaena ) and the diatom, Fragilaria sp., predominated in all lakes as macrophytes were removed. Most changes observed were in the direction of apparent increased trophic state. Major shifts in the phytoplankton assemblage (e.g., Shannon diversity indices and abundance) coincided with periods of maximal fluctuation in vegetation. Reversals of these tendencies were observed during prolonged stable periods of both high and low macrophute biomass.

Feeding and Growth Rates for Triploid Grass Carp as Influenced by Size and Water Temperature

ABSTRACT Thirty feeding trials were conducted using triploid grass carp (Ctenopharyngodon idella Val.) fed hydrilla (Hydrilla verticillata Royle) to establish feeding and growth rates as a function of fish size and water temperature. Size of fish ranged from 0.2 to 9.1 kg and water temperatures ranged from 17 to 27 C. Feeding trials consisted of fish and hydrilla housed in 4 m3 wire mesh cages suspended at weekly intervals in an experimental pond. Utilizing pre- and post-weights of fish and hydrilla, weight gain, relative and absolute feeding rates, and feeding efficiency were found to increase with increasing water temperatures. Relative and absolute feeding rates for 0.2—0.4 kg fish at 17 C were approximately 25% of that for similar sized fish at 27 C. Above 25 C, these small fish consumed their body weight per day of hydrilla. While relative feeding rates (gm hydrilla/gm fish/day) declined with increase in fish size (at constant temperature), this was not the pattern for absolute feeding rates (gm hydr...

Benthic Macroinvertebrates of a Hydrilla Infested Central Florida Lake

ABSTRACT Benthic macroinvertebrates and physicochemical parameters were monitored simnl taenously with hydrilla (Hydrilla verticillata Royle) biomass in a Central Florida lake. Twenty-seven of the 54 taxa of benthic macroinvertebrates collected were members of the family Quronomidae. Chironomids and oligochaetes numerically dominated the benthos, comprising 82% of the individuals collected. The greatest numbers of taxa and individuals were found during the winter when hydrilla biomass was low. Hydrilla biomass ranged from 0.382 kg/m2-FW in April, 1977 to 2.275 kg/m2-FW in October, 1977. Low numbers of macroinvertebrate taxa and individuals were collected from bottom sediments during sunnier and fall. Dissolved oxygen concentrations at the bottom were approximately 2.0 ppm during summer and fall and possibly limited benthic organisms. The annual means for the Shannon and Simpson indices for the benthos were 1.92 and 0.35, respectively.

The size of grass carp as a factor in the control of hydrilla

Abstract Grass carp (Ctenopharyngodon idella Val.) were restocked into a 5.42 ha central Florida lake in November, 1978, to study the effect of fish size and number on the control of hydrilla (Hydrilla verticillata (L.f.) Royle). The results were compared with those from a previous two-year study on the lake which indicated a loss of hydrilla control during the third year. The hydrilla biomass in October 1979 approached that of October 1976, when the grass carp were initially stocked. The mean weight of the fish at the end of the previous study was 5.42 kg, while at the end of this study they had reached a mean weight of 11.37 kg. Since no significant loss in numbers of grass carp occurred between the second and third years (8%), the reduction in hydrilla control during this study was suspected to be due to the increased size of the grass carp. This supposition was tested utilizing three size-classes of grass carp in matched experimental ponds. A significant linear relationship (P = 0.05) was determined between the size of the grass carp and the consumption of Egeria densa Planchon by the fish (kg Egeria fish−1 day−1 = 2.42−0.17 kg grass carp; r2 = 0.9997), while at the same time the growth rate of small grass carp was found to be much higher than that for larger grass carp. It was concluded that only small grass carp should be stocked in hydrilla-infested waters, and that one can expect the fish to become ineffective as a means of weed control as they approach a body weight of 14 kg, due to their reduction in the absolute, as well as the relative, amount of vegetation consumed with increasing size.

Zooplankton abundance and diversity in Central Florida grass carp ponds

The effect of the Asian grass carp (Ctenopharyngodon idella Val.) upon the zooplankton in three adjacent experimental ponds (0.139 ha each) was studied for one year. The ponds contained nine species of aquatic macrophytes. Grass carp were stocked into Pond 1 (65 per ha) and Pond 2 (611 per ha) three months after the study was started. At the time of stocking, physichochemical and biological parameters were similar among the ponds.Grass carp did not affect water quality and had little direct or indirect effect upon the zooplankton in the ponds. The abundance and species diversity of zooplankton (number of species, Shannon Index, and Simpson Index) were not significantly different (P < 0.05) between ponds prior to grass carp stocking. After stocking with grass carp, the number of species and species diversity were found to be significantly different (P < 0.05) between Pond 1 (low grass carp stocking rate) and Pond 2 (high grass carp stocking rate). Additionally, significant differences (P < 0.05) in populations between ponds were found for Lecane luna, Monostyla bulla, Lepadella ehrenbergii, Polyarthra sp., and Synchaeta sp. Temporal variation rather than grass carp was probably responsible for those differences. The number of zooplankton per group (Rotifera, Cladocera, and Copepoda) and species diversity were not significantly different (P<0.05) between ponds containing different stocking rates of grass carp.During the study, zooplankton were collected with a shallow-water sampler. No significant differences (P < 0.05) between collections were found for the sampler and a No. 20 mesh nylon zooplankton net.

Benthic fauna species diversity in six central Florida lakes in summer

The summer macroinvertebrate fauna was studied in six central Florida monomictic lakes during July, 1973. No direct relationships were found between physiochemical conditions and species diversity. Of the 22 species of benthic organisms recorded, only four species were found in all the lakes. Mean species diversity (d) values for the lakes ranged from 0.735 to 1.731.

The Potential of the Hybrid Grass Carp as a Weed Control Agent

ABSTRACT Hybrid grass carp, spawned at the Malone Fish Hatchery in 1979, were stocked into two Central Florida lakes containing hydrilla (Blue Lake) and coontail (Crealde Lake) at the rate of 183 fish ha−1 after nearly two years vegetation control was not achieved. A 24% reduction in the number of hybrid grass carp occurred in Blue Lake while an 89% reduction occurred in Crealde Lake. Hybrid grass carp in the coontail lake had a growth rate of 1.3 gm mo−1 while the growth rate in the hydrilla lake was 28 gm mo−1. A total of 49% of the fish in the hydrilla lake was less than 450 gn after 20 mo. Feeding trials in 0.14 ha experimental ponds using triploid hybrid grass carp from Blue Lake revealed that 0.8 kg hybrid grass carp had a feeding rate of approximately one-third less than that of similar sized grass carp and had a growth rate of approximately seven times less. The feeding efficiency of two year old hybrid grass carp was nearly 50% less than for one and one-half year old grass carp. Due to the slower...

Zooplankton densities in a Hydrilla infested lake

The number of individuals and species of zooplankton were sampled concurrently with Hydrilla biomass and water quality for one year in a small, eutrophic central Florida lake. Throughout the study, rotifer species and individuals dominated the zooplankton. The abundance of the zooplankton tended to remain high when Hydrilla biomass was at its seasonal low during late winter and early spring. When hydrilla growth increased in the late spring and summer months causing a decrease in total alkalinity, specific conductivity, water color, turbidity, orthophosphate and chlorophyll a concentrations; the abundance of the zooplankton declined. During this time, there was a shift from limnetic to littoral species, principally rotifers. Hydrilla growth did not affect the mean number of cladoceran or copepod species, but may have led to an increase in rotifer species.

The Zooplankton Community in an Acidic Central Florida Lake

ABSTRACT Since Floridas sandy soils have low buffering capacities, many of Floridas lakes are susceptible to acid rain effects. The objective of this study was to provide a portrait of the zooplankton community within Lake Ruby, an acidic, central Florida lake. Water chemistry and zooplankton were monitored during January—December, 1989. Mean pH ranged from 3.72 to 5.08 (annual mean—4.35). Low nutrient concentrations (phosphorous, nitrogen, carbon) and high water clarity were characteristically oligotrophic. The zooplankton community contained an average of three species of rotifers, three species of cladocerans and four species of copepods. Annual mean zooplankton abundance was 60 individuals/L. Species diversity of the zooplankton community was low; its annual mean was 1.81; monthly mean values ranged from 1.08 (February, 1989) to 2.24 (November, 1989).