CHANGES IN DISSOLVED NUTRIENT EXPORT WITH CONTINENTAL ICE SHEET RETREAT

Approximately 15% of the Earth’s land surface was exposed as kilometer thick ice sheets retreated across North and South America, Europe, and Asia following the Last Glacial Maximum (LGM). Simultaneously, watersheds formed that are disconnected from glacial meltwater by hydrologic divides. These “deglaciated” watersheds contain comminuted glacial sediments that are susceptible to chemical weathering, especially as ecosystems develop. Chemical weathering and biological activity should change nutrient concentrations of and exports from non-glacial streams compared to those of meltwater rivers. We observe this change in southwestern Greenland where deglaciated watersheds are juxtaposed with meltwater rivers. Although Greenlandic meltwater rivers have instantaneous discharge that are several orders of magnitude greater than non-glacial streams, their specific discharges (discharge normalized to watershed area) are similar. However, deglaciated watersheds near the coast with humid environments export more dissolved organic carbon (DOC) than meltwater streams or inland arid deglaciated watersheds. Non-glacial streams contain DOC with characteristics that suggest an origin primarily from terrestrial vegetation and degradation to recalcitrant forms, while meltwater stream DOC has an algal origin and is labile. Meltwater streams export more dissolved inorganic nitrogen (DIN) and phosphate (PO₄) than non-glacial streams. Average dissolved DIN/PO₄ ratios in meltwater rivers are similar demand of high latitude coastal phytoplankton, but in non-glacial streams are ~50 times higher than demand. Maximum discharge of non-glacial streams occurs before the solar equinox during spring freshet but after the equinox at peak melt season in meltwater rivers and suggests fertilization occurs during times of greater photosynthetic potential from non-glacial streams than meltwater rivers. These results suggest decreasing labile DOC export, increasing N compared to P export, and timing of elevated nutrient export relative to solar radiation may have changed primary productivity of coastal phytoplankton communities since the LGM. These effects may accelerate as polar amplification of global warming enhances future ice sheet retreat.