Nitrate-rich inland waters of the Ross Ice Shelf region, Antarctica

Abstract

Nutrient and major ion concentrations were measured in surface water samples from lakes, ponds and streams at sites 30–320 km south of McMurdo Sound: the Darwin Glacier region (79.7–80.0°S), Pyramid Trough in the southern Dry Valleys (78.2°S), and the McMurdo Ice Shelf ablation zone (77.8–78.4°S). These aquatic environments ranged from dilute meltwaters (conductivity <0.05 mS cm⁻¹) to concentrated brines (>50 mS cm⁻¹). The lowest nitrate concentrations were recorded at the sites closest to the seasonally open waters of the Ross Sea. Much higher values (100–142000mg NO₃–Nm⁻³) were recorded at sites further south. These observations support the hypothesis that NO₃ precipitation over Antarctica is of stratospheric rather than coastal marine origin. The nitrogen-rich waters contained chloride and nitrate in the ratio 5.45g Cl:1g N (C.V.=8.4%) which is within the range for Antarctic snow, and indicative of nitrate enrichment by freeze concentration processes. Cyanobacterial mats were conspicuous elements of the biota across the full range of salinities, and were usually dominated by oscilatoriacean species. Nitrogen-fixing cyanobacteria and diatoms were also represented in these benthic microbial communities at the more northern sites, but were absent from all samples from the Darwin Glacier region.