Use of hypolimnetic enclosures for in-situ experiments on profundal Chironomus: results of pilot experiments
In order to test ecological factors limiting production of profunda1 benthos, techniques for in-situ, mesoscale experimentation on undisturbed profunda1 sediments are being developed. Pilot experiments involving enhancement of seston deposition (increased food supply) and enhanced density of Chironomus cucini larvae have been conducted in two mesotrophic lakes rthern Wisconsin. The technique involves deployment of polyethylene cylinders (3m diameter X6m high) in the hypolimnion, buoyed 1m above the sediment surface. In a feeding experiment, artificial seston (commercial fish food) was periodically re1ease into the cylinder and allowed to’ settle over the enclosed 7m area. Larval density was enhanced by similarly releasing concentrations of larvae collected with an epibenthic sled. In Crystal Lake, where growth of the dense C. cucini population appeared to be food-limited, the natural fTux–or- organic carbon to the profunda1 was supplemented 120-240\% over the period July 1986 - April 1987. When the population was sampled in April, three out of five larval age/sex categories from inside the treated enclosure were significantly heavier than unenclosed larvae. A preliminary test of larval density-enhancement showed that 38\% of the larvae added to an enclosure could be detected one month after stocking. The enclosures do not significantly alter temperature or dissolved oxygen in water overlying the experimental sediments.