Chironomids shed light on organic matter dynamics in macroinvertebrate communities in prairie pothole lakes in west-central Minnesota, USA

Abstract

The Prairie Pothole Region (PPR) in North America is characterized by a high density of wetlands that can range from temporary habitats to small lakes. Much of the PPR has been converted to agriculture resulting in outright habitat loss, eutrophication, sedimentation, introduction of agrochemicals, and the introduction of invasive plants and animals. We compared two prairie pothole lakes, Cottonwood Lake and Page Lake, to assess the functional roles and trophic structure of the macroinvertebrate communities in this highly altered landscape. We collected macroinvertebrate samples from littoral and profundal regions of each lake with an Ekman grab sampler in winters of 2007-2008. In fall 2012, 15 samples were collected from the near shore aquatic vegetation of Cottonwood Lake using D-frame nets. Macroinvertebrates, including Chironomidae, were identified to genus or morphospecies and assigned to functional feeding groups (FF-groups). Taxon richness of littoral and profundal samples were comparable in Cottonwood Lake but was significantly lower in the profundal zone of Page Lake. Macroinvertebrate abundance was significantly lower in Page Lake. Chironomidae made up 55% and 38% of the abundance in Cottonwood and Page Lakes, respectively and the similarity in chironomid composition was only 53%. Significantly more shredders, including the chironomid Glyptotendipes, occurred in Cottonwood Lake, including in the profundal zone. Macrophytes in Cottonwood Lake support additional taxa and shed light on the importance of near-shore sources of organic matter as a food resource. The FF-group distribution in Cottonwood Lake suggests that macroinvertebrates are taking advantage of a continuum of available organic matter resources that is dominated by coarse particulate organic matter (CPOM) near shore and by fine particulate organic matter (FPOM) in habitats farther from shore. In contrast, Page Lake had limited sources of organic matter; very few shredders were collected from littoral samples in Page Lake. In profundal samples most taxa were collector gatherers (Chironomus) or predators (Procladius and other Tanypodinae). Macroinvertebrates in Page Lake appear to be resource limited suggesting that the extensive algal blooms that characterize this eutrophic lake do not provide a comparable food source to that found in Cottonwood Lake. Analysis of the macroinvertebrate communities allowed us to discern the functional role of macroinvertebrates, including the Chironomidae, in processing organic matter in these highly impacted systems.