Who can replace the mayflies?: Insect biomass and emergence in salinized headwaters of Central Appalachia

Secondary salinization is a concern as salts enter freshwaters through anthropogenic activities. In central Appalachian headwaters, benthic macroinvertebrate diversity declines in response to mining-induced salinization. Elevated ion concentrations can be energetically taxing for organismal osmo- and ion-regulatory processes. However, effects of salinization on macroinvertebrate-driven processes remain largely unknown. Moreover, macroinvertebrates may have varied responses to salinization dependent upon identity and life stage. We predicted benthic macroinvertebrate biomass would either decrease with taxa lost or be maintained or increase via compensation by salt-tolerant taxa from a salt subsidy effect, whereas emergent biomass would decrease disproportionately relative to benthic biomass due to late instar and pupae succumbing to salt stress and oxygen deprivation during metamorphosis. We sampled six streams in central Appalachia that represent a salinization gradient (annual mean specific conductance 25-1422 µS/cm). We collected emerging insects for 1-3 weeks. Our results suggest that macroinvertebrate biomass was maintained along a salinization gradient despite lost salt-sensitive mayflies. Salt-tolerant taxa, including flies, showed greater biomass in salinized streams. Emergent biomass showed variability among streams with peak emergence in spring, though a strong, negative response to salinity was unapparent. Comparisons of macroinvertebrate biomass and emergence show differences between insect life stages and indicate the movement of organic material and energy in response to salinization. Although richness declined following salinization, biomass was sustained but by different taxa that may shift the allocation of energy in food webs. We infer subsidies to riparian consumers are altered by quantity, quality, or phenological timing driven by the macroinvertebrate community.