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The microenvironmental and photosynthetic characteristics of Antarctic microbial mats were measured in a series of ponds near McMurdo Sound. As elsewhere in Antarctica, these cold‐water benthic communities were dominated by oscillatoriacean cyanobacteria. Despite large variations in mat thickness, surface morphology, and color, all of the communities had a similar pigment organization, with a surface carotenoid‐rich layer that overlaid a deep chlorophyll maximum (DCM) enriched in phycocyanin as well as chlorophyll a. Spectroradiometric analyses showed that the DCM population inhabited an orange‐red shade environment. In several of the mats, the deep‐living trichomes migrated up to the surface of the mat within 2 h in response to a 10‐fold decrease in surface irradiance. The euphotic layer of the mats was supersaturated in oxygen and contained ammonium and dissolved reactive phosphorus concentrations in excess of 100 mg N·m−3 or P·m−3. Integral photosynthesis by core samples was saturated at low irradiances and varied two‐ to threefold throughout the continuous 24‐h radiation cycle. Oxygen microelectrode analyses showed that the photosynthetic rates were slow to negligible near the surface and maximal in the DCM. These compressed, nutrient‐rich euphotic zones have some properties analogous to planktonic systems, but the integrated photosynthetic responses of the community reflect the strong self‐shading within the mat and physiological dominance by the motile, DCM populations.

A set of near-stream flowpaths in pasture, native forest and exotic pine plantations in New Zealand was sampled to describe differences in dissolved organic carbon (DOC). The quantity and bioavailability of DOC varied among flowpaths in different land uses, with higher concentrations of DOC in near-stream flow paths than the parent groundwater emerging from the hillslope. Tiles incubated in these waters did not consistently yield higher bacterial growth rates than tiles incubated in groundwaters. DOC composition, measured as fluorescence and absorbance properties and extracellular enzyme fingerprints, differed significantly among land uses and position along flowpath.Differences in riparian vegetation can indirectly affect DOC by altering exposure to ultraviolet radiation. A 2-h exposure of water from subsurface flowpaths to full sunlight caused marked changes in fluorescence characteristics of water from the pasture catchment but only small changes in water from the native forest catchment. There were up to fivefold differences in extracellular enzyme activities on tiles incubated in light-exposed water for the native forest site, but not for the pasture site. Bacterial growth and respiration were higher on tiles incubated in native forest water exposed to sunlight, but there was no light effect on growth for tiles incubated in water from the pasture flowpath. These results indicate that riparian flowpaths will affect the quantity and character of DOC delivered to streams and ultraviolet exposure may, at least in some cases, alter DOC bioavailability.

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Abstract:The extensive ablation zone on the McMurdo Ice Shelf (78"S, 165'30E) contains numerous ponds that are lined with benthic mats of cyanobacteria and associated micro-organisms. The photoautotrophic biomass content of these mats was examined in six contrasting ponds. Particulate carbon contributed only 3.2% of the mat dry weight, with C:N ratios generally less than 20:l. The chlorophyll a content was low relative to carbon (chlorophyll a : C < 0.01). Analysis of the mats by high performance liquid chromatography [HPLC] showed that the pigment fraction assayed spectrophotometrically as chlorophyll a contained large quantities (up to 70%) of the degradation product chlorophyllide u and the epimer chlorophyll a'. Photosynthetic rates per unit chlorophyll a [HPLC] were extremely slow: