Seasonal Migration of Water Boatmen (Hemiptera: Corixidae) as a Wetland-River Ecosystem Linkage

Organisms that undertake seasonal migrations can act as important ecosystem linkages by subsidizing food webs. Such transfers of material mean that even food webs which seem isolated may be closely connected. One such linkage that has largely gone unstudied is the seasonal migration of a family of aquatic insects, water boatmen, or corixids (Hemiptera: Corixidae) that fly from geographically isolated wetlands into large rivers in the Prairie Pothole Region (PPR) of North America every fall, to overwinter. This thesis provides further documentation of the phenomenon of corixid migration in the North American prairies, while also investigating the ecological importance and drivers of this movement across the landscape. \n\nFirst, I quantified and recorded the shifts in abundance and species composition of corixids in wetland and river ecosystems in different seasons. I found that these migrations can lead to drastically increased riverine corixid densities as high as ~3,000 individuals/m2 within areas of standing or slow-moving water, with ~500 g of corixid material entering every meter of water immediately adjacent to the banks of rivers, where landings are concentrated. This movement shifts the corixid species assemblage in rivers to one dominated by wetland-breeding species, namely Callicorixa audeni, Sigara bicoloripennis, and Sigara decoratella. Stomach content analyses of riverine fishes revealed that goldeye (Hiodon alosoides), mooneye (Hiodon tergisus), longnose sucker (Catostomus catostomus), and white sucker (Catostomus commersoni) make heavy use of this forage subsidy, with corixids occurring in 97% to 100% of these fishes and accounting for 38% to 97% of stomach contents by weight during the corixid migration period in fall. This could have implications for the productivity and overwintering survival of corixid feeding fish, with the potential for cascading effects in riverine food webs. Across the landscape, I estimated that seasonal migrations could result in ~1500 metric tons of corixids entering the North and South Saskatchewan rivers (~12,000 river km) within Saskatchewan, and ~12,000 tons of biomass moving between wetlands and rivers across the entire PPR. \n\nNext, by studying changing patterns in abundance and evidence of flight into rivers, I designated different corixid species from my study area as being predominantly migratory (62% of encountered species), acting as cyclic colonizers between wetlands and rivers, or non-migratory residents of either habitat type (27% of encountered species). This information allows for the identification of the corixid assemblage that is driving the seasonal flux between the two habitat types, and helps to fill a knowledge gap which exists on the migratory abilities of corixids at the species level.\n\nThird, I examined the use of the stable isotope ratio of sulfur, δ34S, as a tracer of corixid movement and the incorporation of these insects as a dietary subsidy by riverine fish. I found that both corixids and other invertebrate taxa originating from wetland ecosystems exhibited lower δ34S values, with wetland taxa averaging -10.5 ± 5.8‰ overall, as opposed to riverine taxa at -4.1 ± 4.1‰, allowing the use of δ34S as a tracer of insects out of wetlands. Specifically, δ34S values of invertebrates from the South Saskatchewan River (-5. 1 ± 4.1‰) were more 34S depleted than those from the North Saskatchewan River (-1.4 ± 2.8‰). In the fall season, the corixid-feeding fish species goldeye, mooneye, and longnose sucker exhibited lower δ34S values in fast-turnover liver tissue than non-corixid feeding species, shorthead redhorse, northern pike, and walleye, with mixing models indicating that ~17 to 94% of liver tissue may be derived from wetland sources during this season. However, goldeye was the only species to exhibit a significant seasonal reduction in liver δ34S values in fall compared to summer. These findings indicate that δ34S has utility in tracing flows of energy between wetland and riverine food webs.\n\nFinally, I examined the overwintering strategy of corixids that do not migrate to rivers in the fall, documenting the little understood ability of these insects to survive in wetlands that freeze solid. I found that while multiple corixid species were present in wetlands at ice-over, those embedded within the ice were almost entirely composed of two non-migratory species, Cymatia americana and Dasycorixa hybrida, of which only the former revived after thawing. These findings indicate that migratory species are likely incapable of survival within the ice, driving the need to leave shallow waters in fall. The percent of C. americana that revived after being experimentally thawed out from the ice ranged from 4% to 10% in both winters of this study. The majority of corixids were grouped together within air pockets, which could enable them to limit direct contact with the surrounding ice. Other invertebrate taxa were also found overwintering within the ice, including adults and larvae of crawling water beetles (Coleoptera: Haliplidae) and adults of predaceous diving beetles (Dytiscidae) within air pockets alongside the corixids or on their own, as well as damselfly nymphs (Odonata: Coenagrionidae), caddisfly larvae (Trichoptera: Phryganeidae, Leptoceridae), midge larvae (Diptera: Chironomidae), and snails (Gastropoda: Physidae, Planorbidae) that appeared to be encased in solid ice.\n\nTaken together, this thesis has demonstrated an extensive cross-boundary flux that occurs between spatially separated wetland and river ecosystems, highlighting a need for conservation to ensure that this connection is maintained. By examining migratory patterns, I have identified which species drive this flux, which may allow for increased protection of habitats that these corixids require. δ34S was shown to have the potential to trace insect movement and consumer use between isotopically distinct freshwater systems in the prairies. The study of corixids overwintering in ice represents a little understood survival mechanism of aquatic invertebrates in shallow wetlands, knowledge of which could help predict how the abundance of these organisms might change in the face of altered overwintering conditions due to global warming. The seasonal flights of corixids between wetlands and rivers may represent one of the world’s great insect migrations, which has largely gone unnoticed, but could have important implications for ecosystem functioning and conservation in the North American prairies.

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