Seasonal changes in the diel vertical migration of Chaoborus punctipennis larval instars

We describe a novel seasonal shift in the vertical migration behavior of Chaoborus punctipennis second-instar larvae in Lake Opeongo, Ontario. An upward-looking 600 kHz acoustic Doppler current profiler (ADCP) moored at a fixed 22 m station in the lake recorded acoustic backscatter continuously during the study period. Zooplankton samples collected indicated that the abundance of C. punctipennis larval instars accounted for the greatest proportion of variance in the linear backscatter from the ADCP. The large fourth-instar larvae underwent normal diel vertical migration throughout the study. Smaller second-instar larvae underwent reverse migration during late June but switched to normal migration by late July. The acoustic record indicates that the switch occurred over a few days around late June, and at this time a double vertical migration appeared with the second instars leading and following the migration of fourth instars. We speculate that these changes in the migration of second instars are driven by seasonal shifts in predation risk from larval cisco (Coregonus artedi) and by the need to minimize spatial overlap with the larger fourth instars.

Developmental differences and a test for reciprocity in the tolerance of Chaoborus punctipennis larvae to ultraviolet radiation

We estimated the ultraviolet radiation (UVR) tolerance of Chaoborus punctipennis in five 2- to 4-day in situ experiments conducted with third and fourth instar larvae at 0.25, 1.25, 3.0, and 5.0 m in Ruth Roy Lake, a lake with low dissolved organic carbon levels (0.2 mg·L–1). UVR tolerance increased with larval age. Third and late fourth instar LD50 (median lethal dose) ranged from 52.3 to 62.2 J·cm–2 and from 82.4 to 119.6 J·cm–2, respectively, among the four depths. Reciprocity held for UVR exposure down to 5.0 m, i.e., toxicity was independent of dose rate. At any given depth, the LT50 (median lethal time) increased with larval stage, but even at 5.0 m, third and late fourth instars died in less than 10 and 20 h, respectively, under sunny skies. These results suggest that Chaoborus abundance and distribution might be affected by UVR, especially in clear lakes. If UVR levels continue to increase in some lakes because of stratospheric ozone loss and climate change, we predict that UVR damage to Chaoborus will increase in the future, thereby affecting trophic interactions in temperate freshwater food webs.

Influence of ingestion rate and food types on cadmium accumulation by the aquatic insect Chaoborus

We measured the influence of two food-related variables, prey type and ingestion rate, on Cd uptake by larvae of the aquatic insect Chaoborus punctipennis. Larvae collected from a low-Cd lake (0.1 nM total dissolved Cd) were held in the laboratory where they were exposed to water and to various quantities of either copepods (Diaptomus minutus) or cladocerans (Diaphanosoma birgei) from a high-Cd lake (1 nM total dissolved Cd). The Cd content of larvae exposed to the metal in water only did not increase, confirming the results of previous experiments in which food was demonstrated to be the major Cd source for C. punctipennis larvae. Prey type did not influence predator Cd content, likely because Cd concentrations in the two prey types were approximately the same (~10 µg Cd·g-1) and because Cd was assimilated with the same efficiency by the predator from the two prey types. Cadmium assimilation efficiencies were greater at low prey ingestion rates (nearly 100%) than at high prey ingestion rates (~30%), which is likely explained by a shorter residence time of food in the gut at high prey consumption rates.

Indirect effects of lake acidification on Chaoborus population dynamics: the role of food limitation and predation

A twofold increase in Chaoborus punctipennis abundance following experimental acidification of one basin of Little Rock Lake, Wisconsin, provided an opportunity to examine the hypotheses that C. punctipennis dynamics were regulated by (i) food resources for instars I and II, (ii) fish predation on instars III and IV, and (iii) invertebrate predation on instars I and II. Neither food limitation nor fish predation appeared to be important constraints on C. punctipennis abundance. Although rotifer biomass increased significantly during the acidification, C. punctipennis did not respond in a consistent manner to variation in food availability. Bioenergetics estimates of C. punctipennis consumption by rock bass (Ambloplites rupestris), the most acid-sensitive fish species, were small compared with C. punctipennis production. In contrast, invertebrate predation appeared to be an important constraint on C. punctipennis abundance. Chaoborus punctipennis abundance increased dramatically when abundance of the predator Mesocyclops edax declined to near zero. Comparison of M. edax predation rates and first instar production suggested that M. edax could exert strong predation pressure on C. punctipennis Thus, invertebrate predation may create a juvenile bottleneck for populations of small-bodied Chaoborus species in moderately productive acidified lakes.

Chaoborus populations: response to food web manipulation and potential effects on zooplankton communities

Chaoborus populations were studied in two lakes in which fish predation was experimentally reduced, and in a reference lake. In Tuesday Lake, major reduction of fish predation led to substantial increases in density of Chaoborus punctipennis. Analysis of crop contents and estimates of consumption rates suggested that C. punctipennis caused declines of rotifer and copepod populations following the manipulations. In Peter Lake, lesser changes in fish predation caused no major change in density of Chaoborus flavicans, perhaps because food limitation compensated for effects of reduced predation. In both Peter Lake and the reference lake, Paul Lake, C. flavicans preyed heavily and selectively on Daphnia less than about 1.4 mm in total length. Bioenergetic calculations indicated that up to 46% of the daphnids were consumed daily. At some times of the year, certain zooplankton populations (especially noncolonial rotifers, small copepods, and small cladocerans) were strongly influenced by Chaoborus predation.