The Role of Chironomids as Paleoecological Indicators of Eutrophication in Shallow Lakes Across a Broad Latitudinal Gradient

Abstract

The aquatic larvae of chironomids (Diptera, Chironomidae) were historically classified according to lake trophic status, and taxa classified as “eutrophic” were labeled as such because of adaptations for surviving hypoxic or anoxic conditions in the hypolimnion of stratified eutrophic lakes. As such, sedimentary chironomid assemblages have been used to reconstruct production-related variables (nutrients, chlorophyll-a), though this has been problematic, especially in shallow systems, because the response of chironomids to eutrophication is mediated through secondary environmental gradients including oxygen concentration, habitat, and food quality/quantity. In this thesis, eutrophic sewage ponds in the Canadian High Arctic were used to demonstrate that oxygen, not nutrients, is the primary control of chironomid species assemblages. The ability to explicitly test the influence of oxygen versus nutrients on chironomid distributions was made possible by the 24-hr daylight (continuous photosynthesis) and shallow, wind-mixed water column that resulted in oxygen concentrations that were decoupled from the effects of elevated nutrients and production. The subfossil chironomid assemblages were complacent during historical eutrophication, in contrast to marked changes in diatom assemblages, which have a direct physiological relationship with nutrients. Similarly, in shallow eutrophic ponds on islands in Lake Ontario, chironomid assemblages did not appear to be governed by the large gradient in total phosphorus due to the presence or absence of waterbird nesting colonies, but rather by habitat and possible bird-mediated heavy metal pollution. In a subarctic lake that was formerly used for sewage disposal, chironomid assemblages were relatively unresponsive to eutrophication in comparison with the larger turnover in diatom species. However, periods of low oxygen observed in the temperate and subarctic sites may explain the higher (but still low) relative abundances of hypoxia-tolerant Chironomus species at these sites compared to the High Arctic ponds. Together, this research demonstrates the problems associated with classifying chironomids based on nutrient levels. Since it is uncommon to examine chironomid responses to eutrophication across latitude, this thesis offers a relatively unstudied perspective of chironomid ecology, emphasizing that some of the assumptions of temperate chironomid ecology (with regards to eutrophication) may not necessarily hold true when applied at higher latitudes.