PHOSPHORUS DYNAMICS OF THE LOCH VALE WATERSHED, ROCKY MOUNTAIN NATIONAL PARK, FROM THE LAST GLACIAL MAXIMUM TO PRESENT: GEOCHEMICAL MASS BALANCE, POTENTIAL ECOLOGICAL IMPACTS, AND PALEOCLIMATIC INFLUENCES

Phosphorus (P) is a plant macronutrient and in the absence of anthropogenic inputs is derived from the chemical dissolution of bedrock minerals. As with nitrogen (N), P is capable of contributing to algal blooms and eutrophication of surface waters. Despite being located in remote Rocky Mountain National Park (RMNP), the Loch Vale Watershed (LVW) has experienced algal blooms. The algal blooms are attributed, at least in part, to atmospheric deposition of anthropogenic N. However, much less is known about P behavior within the LVW.

Recent work in the LVW found that the dissolution of the bedrock mineral apatite (Ca₅(PO₄)₃(F,Cl,OH)) released P to surface waters at a rate of 13-47 mol/ha/year from 1984 to 2008. However, only 1.1 mol/ha/year of dissolved P was measured leaving the LVW. It was hypothesized that P was being sequestered in lake-bottom sediments of The Loch, a lake located at the watershed outlet. A core of sediment was collected from the The Loch with P-concentration and radiocarbon-age data of the sediment yielding a P flux into the sediment of 0.0070 mol/ha/year between 6.4-0.93 ka. Thus, P sequestration in lake-bottom sediments is unable to explain the apparent dissolved-P deficit at the outlet. However, the top 5 cm of the sediment contains approximately 640 kg of bioavailable P which may contribute to algal blooms during periods of high atmospheric N deposition.

Macrofossil ages in the The Loch sediment core reveal that sedimentation began 18.1 ka at the end of the last glacial maximum. Organic P increases systematically up-profile with an abrupt increase approximately coinciding with the advance of alpine glaciers at higher elevations 13.2-11.1 ka. This glacial advance was associated with the Younger Dryas Chronozone. The increase in organic P may reflect mechanical weathering associated with the glacial advance that facilitated apatite dissolution upstream from The Loch. Detrital apatite abundances in the core decrease up-profile during this period. The alpine glacial advance was followed by large fluxes of macrofossil-rich gyttja into The Loch. Such sedimentation patterns reflect climatic warming, higher lacustrine productivity, and increased vegetation within the LVW. Relatively high sedimentation rates ended approximately 7-6 ka with the onset of the greater climatic stability.