Temporal boundaries of invertebrate adaptation to abiotic anthropogenic stressors in freshwater streams

We investigated invertebrate diversity in field studies of stream communities impacted by ultraviolet (UV) radiation and mixtures of transition metals in the Colorado Rockies. Some stream communities have adapted to adverse conditions for over a century. In other streams, naive species emigrate from low-metals tributaries, diluting local effects. Finally, metals remediation has provided approximately a decade for adaptation in other streams. An important mediator of metals bioavailability and biologically-damaging UV radiation is dissolved organic matter (DOM). However, photooxidation by ambient sunlight photobleaches (loss of UV absorbance) and photomineralizes DOM (conversion from organic to inorganic carbon) and differs in degree depending on DOM source. Such source-dependent transformations alter DOM composition, its metal complexation, and subsequently, metal bioavailability. From stream geochemistry and spectral qualities of DOM, paired with laboratory studies of photochemical processes, we developed a dynamic model linking abiotic mechanism to communities challenged by multiple stressors and unstable natural protection (e.g., differing degrees of DOM influx and photooxidation). Broadly ranging conditions in our streams, which encompassed those of many streams in the western United States, defined upper and lower limits of tolerance to UV radiation and metal stress. Synoptically, model results indicate that community abundance and diversity were determined by synergistic effects of metals and UV. However, temporal diversity was mainly a function of invertebrate adaptation and gene flow from undisturbed upstream communities. Depending on the extent of in situ and upstream disturbance, our findings indicate the temporal limits required for adaptation within communities confronting multiple, anthropogenic forces.