We conducted a field survey to quantify the biomass (chlorophyll-a and ash-free dry mass), nutrient ratios (molar C:N:P), and isotopic signatures (δ13C and δ15N) of four microbial mat types (green, orange, black, and red) in the glacial meltwater streams of the McMurdo Dry Valleys, Antarctica. All samples were taken from late December to late January during the 2011-2012 and 2012-2013 austral summers, and included sites from Taylor, Miers, Garwood, and Wright valleys. Most collection sites were located at the lake outlet of streams, but for a subset (e.g. Delta, Von Guerard, Onyx, Miers, and Canada) more than one sample site is included per stream system.
Black, green, red, and orange mats were taken from established transects (descriptions in McKnight et al. 1998; Kohler et al. 2015) from 10 January 2012 to 23 January 2013. For all streams, sampling was executed by lifting mats from substrata with a spatula, and cores were taken with a brass cork borer (#13, 227 mm2). For each mat type, cores were taken for elemental and isotopic composition (C:N:P stoichiometry, and δ13C and δ15N isotopes) and biomass as ash-free dry mass (AFDM) and chlorophyll-a (chla) if sufficient material was available. Samples were dewatered onto Whatman® GF/F filters, wrapped in foil, and frozen at the field camp for later analysis. For samples from Taylor Valley, distances of sites to both its source glacier and the distance to the Ross Sea coast were further approximated with Google Earth©.
Biomass (as AFDM and chla) was determined as in Kohler et al. (2015). Briefly, chla was measured by extracting samples in 90% buffered acetone for 24 hours (Welschmeyer 1994) and analyzed on a Turner Designs 10-AU field fluorometer. AFDM subsamples were dried at 55 ºC for 24 h (or until a constant mass was achieved), weighed, burned at 450 ºC for 4 h and reweighed, then rewetted and dried to determine mass loss caused by hydration of sediments (Steinman et al. 1996). An autotrophic index (AI) was created by dividing chla values by their corresponding AFDM. All biomass analyses were performed in Crary Laboratory at McMurdo Station.
The C:N:P and isotope subsamples were dried at 50-55 ºC until a stable mass was achieved and ground to a powder. Prior to analysis, carbonates were removed from the C:N aliquot by fumigation (Hedges and Stern 1984) to prevent loss of acid-soluble carbon that may occur with rinsing. A second aliquot was analyzed without acidification to more accurately measure δ15N values, which may slightly increase following fumigation (Harris et al. 2001). Because separate aliquots were used to measure δ13C and δ15N, some samples do not have both isotope measurements. Furthermore, some samples did not have adequate C or N concentrations to make an accurate isotopic measurement, and these fields are designated by blank fields.
Percent C and N content was measured using a CE 1500 Elemental Analyzer (CE Instruments Ltd., Wigan, UK) and δ13C and δ15N isotope ratios were obtained with Finnigan-MAT Delta Plus XL mass spectrometer at the Center for Stable Isotope Biogeochemistry operated by the University of California, Berkeley. The %P aliquot was ashed in a muffle furnace at 500 ºC for 1 h, digested with 1N HCl, and analyzed as soluble reactive phosphorus (SRP) with a Lachat QuikChem 8500 Flow Injection Analyzer (Hach Company, Loveland, Colorado) by the Kiowa lab at the University of Colorado (Murphy and Riley 1962). A spinach standard (#1570a) was analyzed every ~10 samples to ensure method accuracy and digestion success. Resulting values were converted to molar C:N, C:P, and N:P ratios.
Streamwater chemistry samples were collected opportunistically for each stream throughout the flow season, ranging from 1 to 10 samples for each stream transect per summer, with an average of 4.6 (median = 4.0) samples per stream in 2011-12 and an average of 4.9 (median 5.5) in 2012-13. Raw water for nutrient analyses was collected in triple-rinsed 250 mL Nalgene® bottles, filtered through glass-fiber filters, and frozen at the field station. Stream water nutrient analyses were performed on either a Skalar San++ Continuous Flow Analyzer (2011-2012 summer) or a Lachat QuickChem Flow Injection Analyzer (2012-2013 summer) optimized for low concentrations. Dissolved inorganic nitrogen (DIN) was calculated as the sum of NH4+-N, NO2--N and NO3--N. In general, NH4+-N and NO2--N were mostly low or undetectable. Detection limits for 2011-2012 nutrient data were 1 μg L-1 for NO2--Nand NO3--N, and 3 μg L-1 for P-SRP. For 2012-2013 nutrient data, detection limits were 1 µg L-1 for NO3--N, 0.7 μg L-1 for NO2--N, 5 μg L-1 for NH4+-N, and 0.6 μg L-1 for P-SRP. In cases where the measured concentrations of stream water nutrients were below detection limits, a value of half the respective detection limit was designated. For each transect for each summer an average value for both DIN and SRP was calculated.
Funding: Provided by several grants from the National Science Foundation for Long Term Ecological Research including #OPP-1115245 and #OPP-1637708.
Provenance: Ash-free dry mass and chlorophyll-a measurements were largely derived from the MCM LTER algal microbial mat biomass core dataset (10.6073/pasta/519bfca4ea3cda94b09edb78ff8759db), and DIN and SRP values were derived from the MCM LTER stream nitrogen and phosphorus core dataset (10.6073/pasta/f6131f5ef67901bc98027e9df55ec364).