David Abel

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Abstract

A study of the spatial distribution and temporal occurrence and diversity of rotifers in relation to certain chemicophysical parameters was conducted in Barren Lake, Kentucky, a flood control lake, from January, 1970, through January, 1970. Paired samples were collected from six depths in the main pool area of the reservoir and from an automatic plankton sampler installed in the tailwater area where, from April through September, the effluent was derived from the upper 3 m of the pool . Diel studies were conducted in April, July, 1970, and January, 1971.

Barren Lake began to stratify in late April, was completely stratified by August and fall circulation was completed in late October; the metalimnion was at 6 m to 9 m from June to mid-October. The maximum temperature (30 C) was at the surface on 1 August, the lowest (3.1 C) was on 26 January. There was a trace amount of oxygen in the hyplimnion throughout the summer and the average pH was consistently slightly alkaline.

Average rotifer densities, in the main pool and tailwater, ranged from 2/liter to 565/liter. From June through September the large concentrations of rotifers at 6 m were statistically different from those at all other depths except 3 m (3 m was not significant from the remaining depths). The July diel study revealed that this vertical distribution persisted over a 24-hour period. During other months and during diel studies in April and January, rotifers were uniformly distributed at all depths. The vertical distribution was possibly affected by factors associated with stratification, such as dissolved oxygen concentrations and temperatures, and by the association of the rotifers with phytoplankton which was consistently abundant at 6-9 m.

The possible grazing effect upon phytoplankton was most prominent in May when both rotifer and crustacean densities were great. A decrease in total rotifers accompanied by an increase in phytoplankters in early June might have indicated a recovery from this grazing effect.

The number of species of rotifers per sampling date was greatest from June through mid-October (12 species). This period was characterized by a low reservoir discharge (50 cfs), decreased turbidity, increased Secchi disc transparencies (a mean of 3 m), and increased water temperatures accompanied by stratification. This increase in the number of species might have been indicative of the diversity found within a more mature, i.e., a more stable system.

A decrease in both species diversity and total rotifer density in December might have been attributed to the influences of increased turbidity, increased flushing rate, and decreased reproductive rates.

A total of 28 species referable to 18 genera of rotifers was identified during the study. Polyarthra spp. (3), Keratella spp. (5), and Conochilus unicornis were the dominant rotifers; they comprised 75-85% of the population when present. Keratella cochlearis, Polyarthra vulgaris, and Kellicottia bostoniensis were most persistent and characteristic of Barren Lake. Keratella Americana, Ploesoma sp., Hexarthra mira, Keratella crassa, and Barchionus angularis were warmwater forms. Keratella quadrata, Kellicottia bostoniensis, and Polyarthra minor were cold-water forms. Rotifers which were epilimnetic in distribution were: Conochilus unicornis, Synchaeta spp., Asplanchna spp., and Keratella crassa.

Keratella crassa occurred in samples after late May; low temperatures and low phytoplankton densities might have precluded its occurrence earlier.

Conochilus unicornis showed a major increase in May followed by two secondary occurrences, both at lower densities. It was uniformly distributed throughout the water column on all occasions.

The total rotifers observed in samples from the upper 3 m of the main pool were not statistically different from those in the tailwater from June through September. The exclusion of the bulk of the rotifer population might have occurred due to a significant major occurrence of rotifers at 6 m. This might have distorted a quantitative survey due to the exclusion of the bulk of the rotifers below the level of discharge.

Nine genera of microcrustacea (plus nauplii) were identified from March 1970 through January 1971. Their major increase corresponded with the spring rotifer increase. Major cladocerans were Bosmina sp. and Daphnia spp.; major copepods were Diaptomus spp. and Cyclops spp.

Disciplines

Biology | Environmental Microbiology and Microbial Ecology | Microbiology | Terrestrial and Aquatic Ecology

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