Chironomus

Saether, Ole A., 2012, The Chironomus group (Diptera: Chironomidae) in Lake Winnipeg, Canada, Zootaxa 3275, pp. 1-19 : 17

The Chironomus group in Lake Winnipeg

At least 16 species of the Chironomus group occur in Lake Winnipeg. Chironomus (C.) bifurcatus and C. (C.) entis both are very common and distributed all over the lake ( Fig. 1 ). The larvae of C. bifurcatus are most common in the South Basin, while C. entis is more common in the Narrows and in the North Basin. The remaining species of Chironomus are represented by only a few specimens in the bottom samples ( Fig. 1 ).

The fluviatilis type probably is one of two species described by Wülker (2007) from the near-shore sediments of Lake Michigan, C. mozleyi Wûlker and C. winnelli Wülker. The significance of the different Chironomus species as indicators of trophic levels is nicely demonstrated by their distribution in Lake Winnipeg. The oligotrophic fluviatilis type is limited to the extreme northern part of the lake which is isolated from the rest of the lake by a strong current from the outlet of the Saskatchewan River to the outlet. The mesotrophic to moderately eutrophic C. entis is most common in the narrows and in the north basin, while the more eutrophic C. bifurcatus is among the dominant chironomids of the south basin ( Figs. 1 , 5 , 7 ) present in the same locality and especially the early instars difficult to separate. The inverse of Dyar's Rule or 0.71 is more useful in order to identify earlier instars. This rule is applicable for all examined chironomid larvae from Lake Winnipeg and is further discussed in Saether (2009; 2010a, b; 2011). The head capsule lengths of the instars of are given in Fig. 9 . For both species the variations in head capsule lengths were the median ±11–25%, and the median of one instar was 50–56% higher than the preceding instar. The variation thus is somewhat larger than in the other species examined from Lake Winnipeg and the difference between instars is slightly smaller. (57%–63% in other examined species). The larger than normal range probably is caused by the spring generation being slightly larger than the fall generation.

Figures

FIGURE 1. Distribution of Chironomus spp. in the benthic samples of Lake Winnipeg.

FIGURE 5. Chironomus (Chironomus) bifurcatus Wülker, Martin, Kiknadze, Sublette et Michiels. Numbers of adults caught in light traps and emergence traps in 1969 and 1971, and numbers of immatures in benthic samples from June to November 1969 and in March 1970. (LT = light traps, ET = emergence traps, short vertical lines = 1 – 25 specimens, second shortest lines = 26 – 50 specimens, medium long lines = 50 – 200 specimens; second longest vertical lines = 201 – 400 specimens, longest vertical lines = more than 400 specimens, 1 = first instar larvae, 2 = second instar larvae, 3 = third instar larvae, 4 = fourth instar larvae, P = pupae.)

FIGURE 7. Chironomus (Chironomus) entis Shobanov. Numbers of adults caught in light traps and emergence traps in 1969 and 1971, and numbers of immatures in benthic samples from June to November 1969 and in March 1970. (LT = light traps, ET = emergence traps, short vertical lines = 1 – 25 specimens, second shortest lines = 26 – 50 specimens, medium long lines = 50 – 200 specimens; second longest vertical lines = 201 – 400 specimens, longest vertical lines = more than 400 specimens, 1 = first instar larvae, 2 = second instar larvae, 3 = third instar larvae, 4 = fourth instar larvae, P = pupae.)

FIGURE 9. Head capsule lengths of different instars of Chironomus (Chironomus) entis Shobanov and Chironomus bifurcatus Wülker, Martin, Kiknadze, Sublette et Michiels in Lake Winnipeg.