Ecosystem Studies & Management
Examining Human Impacts on Glaciers
Glaciers and ice sheets combined represent the second largest reservoir of water in the global hydrologic system and glacier ecosystems cover 10% of the Earth, yet the carbon dynamics underpinning these ecosystems remain poorly understood. Increased understanding of glacier biogeochemistry is a priority, as glacier environments are among the most sensitive to climate warming and industrial forcing.
Our recent work has found glacier derived dissolved organic matter (DOM) to be highly bioavailable to microbial communities compared to DOM in rivers that drain non-glacial terrestrial ecosystems (e.g. forests, wetlands, tundra). The biolability of riverine DOM exported from terrestrial ecosystems is often found to vary with age, with younger, relatively unaltered DOM being more easily metabolized by aquatic heterotrophs than older, heavily-modified material. The DOM derived from glacier ecosystems has been shown to be ancient in terms of its radiocarbon age, which typically exceeds ~3000 years before present. Surprisingly, the age and bioavailability of glacier DOM are positively correlated such that bioavailability increases with increasing DOM age. This is illustrated by a survey of DOM in rivers along the Gulf of Alaska that ranged in glacier coverage from 0-64%.
Age and biologically available dissolved organic carbon (BDOC) showed a strong linear correlation, with both DOM age and bioavailability increasing with watershed glacier coverage. Once exported from the glacier environment the respiration of this DOM represents a source of relic carbon to the contemporary atmosphere, analogous to carbon dioxide emissions from fossil fuel combustion. Our findings also have unrecognized impacts for coastal biogeochemistry. As bacterial production in coastal river plumes can largely be supported by terrestrial inputs of DOM, our results suggest that changes in the magnitude and timing of glacial runoff to the ocean could alter carbon availability and heterotrophic productivity in marine ecosystems. These changes could be particularly pronounced in regions that currently support commercially important fisheries such as the Gulf of Alaska and the North Atlantic, the latter influenced by a 41% increase in Greenland ice sheet discharge between 1961-1990 and 1998-2003.
The DOM exported from glaciers along the Gulf of Alaska appears to be predominantly anthropogenic in origin and enters glaciers in a pre-aged form. Carbon-containing aerosols, derived mainly from biomass and fossil fuel burning, are the original source of this aged-glacier DOM. Once deposited on glacier surfaces, aerosol DOM is exported downstream, where it provides an energy subsidy to receiving aquatic ecosystems. Our results from Alaska suggest that the flux of biolabile, ancient carbon from glaciers is in fact a modern phenomenon, attributable to the deposition of fossil fuel derived aerosols. If this depositional flux is ubiquitous and anthropogenic, then the surface biogeochemical cycles of today are universally post-industrial in a way we do not fully appreciate. Indeed, we hypothesize that the deposition of fossil fuel combustion derived organic matter is a universal phenomenon. However, we choose to study it against the near-pristine backdrop that glaciers provide. In warmer, more vibrant ecosystems, deposited OM is likely rapidly metabolized, becoming lost in the general milieu of life. In frigid glaciers any input stands out, making glaciers ideal sentinel ecosystems for the detection and study of anthropogenic perturbation of remote ecosystems through deposition.
Building on the ongoing work in Alaska we recently examined new glacier study sites in China on the Qinghai-Tibetan Plateau to see if they show similar characteristics – old yet biolabile DOM derived from fossil fuel combustion byproducts. China's glaciers comprise approximately 50 per cent of the total in Asia and 15 per cent of the World's ice fields and are thinning at an alarming rate. Further investigations are needed on glaciers and ice sheets around the globe to investigate whether just like modern humans, microbes at the base of aquatic foodwebs are supporting their energetic needs by utilizing previously sequestered and highly aged sources of organic carbon.
Eran Hood, University of Alaska Southeast; Pete Raymond, Yale University; Aron Stubbins, Skidaway Institute of Oceanography
Fellman, J., Spencer, R.G.M., Hernes, P.J., Edwards, R., D’Amore, D., Hood, E. 2010. The impact of glacier runoff on the biodegradability and chemical quality of terrigenous dissolved organic matter in near-shore marine ecosystems. Marine Chemistry, 121: 112-122.
Hood, E., Fellman, J., Spencer, R.G.M., Hernes, P.J., Edwards, R., D’Amore, D., Scott, D. 2009. Glaciers as a source of ancient, labile organic matter to the marine environment. Nature 462: 1044-1047, doi:10.1038/nature08580.