We performed a field survey to assess changes in biomass, nutrient ratios, and isotopic signatures of three different microbial mat types in two glacial meltwater streams in the McMurdo Dry Valleys, Antarctica. Samples were taken in January 2013 from pre-existing transects over Von Guerard Stream and the Relict Channel, and stretched from near the Von Guerard Glacier terminus to the stream outlets at Lake Fryxell. In addition to the three microbial mat types, which included Nostoc (black), Chlorophyte (green), and Oscillatorean (orange) mats, particulate organic matter (POM) was collected from Von Guerard Stream in January 2008 over a full diel cycle in order to determine the origin of material in transport by comparing its elemental and isotopic characteristics with stream mats.
To characterize and compare POM in transport with stream mats, POM samples were retained from an experiment described previously (“drift 1”; Stanish 2011). Briefly, a temporary ‘drift transect’ was established in Von Guerard Stream (-77.613883°, 163.259550°) between two MCM LTER algal monitoring transects (i.e. Von Guerard Middle and Gage; Fig. 1). A plankton net (40 μm mesh) with a 10 μm mesh collection bucket was anchored to the streambed, and POM samples were collected for 2-8 hours at a time over the course of one diel flow cycle over 22-23 January 2008. Trapped material was rinsed into the collection bucket with distilled water, filtered onto pre-combusted GF/F filters, and frozen until analysis. Samples for isotopic and elemental composition were retained from six times periods (times: 21:30, 1:26, 5:17, 9:35, 13:12, and 13:35). Adjacent mats for comparison were sampled at the drift transect on 19 December 2007 (black n=3, orange n=3) and were collected and processed as described below.
To test how the distance from the source glacier influences mat elemental and isotopic composition, at least three replicates of black, green, and orange mats were taken from the three established Von Guerard transects (descriptions in McKnight et al. 1998; GPS coordinates in Kohler et al. 2015). Samples were collected from Von Guerard Gage on 14 January 2013, and from the Middle and Upper transects on 15 January 2013. To achieve a higher resolution in longitudinal patterns, duplicate (and occasionally triplicate) samples of black and orange mats were also taken along 7 of the established Relict Channel transects (McKnight et al. 2007) and the Harnish Stream gage on 20 January 2013. For both 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. Transect distances from Von Guerard Glacier were 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 55ºC for 24 h (or until a constant mass was achieved), weighed, burned at 450ºC 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 and ground to a powder. 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. Percent C and N content was measured using a CE 1500 Elemental Analyzer, and δ13C and δ15N isotope ratios were obtained with Finnigan-MAT Delta Plus XL mass spectrometer at the University of California Berkeley Center for Stable Isotope Biogeochemistry. The %P aliquot was combusted in a muffle furnace at 500 ºC for 1 h, digested with 1N HCl, and analyzed as orthophosphate (Murphy and Riley 1962) with a Lachat QuikChem 8500 Flow Injection Analyzer at the Kiowa Lab, University of Colorado. A spinach standard (#1570a) was analyzed every ~10 samples to ensure method accuracy and digestion success. Resulting values were converted to molar C:N:P ratios.
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