EFFECTS OF CHIRONOMUS-TENTANS LARVAL GROWTH-RETARDATION ON ADULT EMERGENCE AND OVIPOSITING SUCCESS - IMPLICATIONS FOR INTERPRETING FRESH-WATER SEDIMENT BIOASSAYS

A laboratory study was conducted with Chironomus tentans to assess the significance of growth retardation of third-to fourth-instar larvae o ver a 10-d test period on long-term survival, adult emergence, and ovi positing success. Data were intended to provide interpretive guidance for the commonly used growth endpoint in 10-d sediment bioassays with C. tentans. Larval growth was controlled by using six feeding levels r anging from 0.2 to 5.9 mg dry weight Tetrafin(R) fish food per day. Me an 10-d survival was greater than or equal to 88% at all feeding level s, but larval growth decreased significantly (p<0.05) with each decrea se in feeding level. Cumulative successful emergence of adult C. tenta ns decreased significantly with decreasing larval growth. Mean times t o emergence always increased with decreasing growth rates and effects were generally more pronounced for females than males. At the lowest 1 0-d mean growth where successful emergence occurred (0.27 mg), the tim es to emergence doubled relative to the times observed at the highest 10-d mean growth (1.03 mg). Ten-day larval growth retardation was stro ngly correlated with reduction in adult emergence success (r(2) = 0.96 ). Growth retardations greater than or equal to> 64% resulted in 86-10 0% reductions in adult emergence. Growth retardation in the range of 3 5 to 50% equated with comparable percent reductions in adult emergence success. Although fewer females successfully emerged at 10-d growths of 0.37 and 0.74 mg relative to the highest 10-d growth (1.03 mg), ovi positing success of these females did not appear to be adversely affec ted by either their slower growth rates or their lower mean dry weight s (0.62 and 0.99 mg, respectively). Growth of second generation larvae did not appear to be affected by maternal growth rate, but rather app eared to be solely correlated with their own feeding level.