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Investigates the composition of benthic microbial mats in permanently ice-covered Lake Hoare, Antarctica. Depth zones of mats; Precipitation of calcite within under-ice mats; Characteristics of moat mats.

1. The underwater light climate and benthic moss communities of Grane Langsø were investigated in May 1997 to determine the potential effects on benthic production of changing water column attenuation and spectral quality of light. 2. A reduction in water clarity in the lake since the 1960s was manifested as a marked increase in the attenuation of blue light, relative to red light, which can be attributed to increased dissolved organic carbon. 3. The biomass of the benthic moss community ranged from a maximum of 195 gDW m-2 at a depth of 4 m to 39 g DW m-2 at a depth of 10 m and Drepanocladus exannulatus contributed 70% of the biomass at all depths. 4. Absorption of PAR by D. exannulatus was maximal in the highly pigmented youngest parts of the plant and these correspondingly showed the highest rates of net photosynthesis. The absolute amount of chlorophyll-a per g dry weight was greater at 10 m than 2 m, but the ratio of accessory pigments to chlorophyll-a did not change. Deep growing plants did not show adaptation to changed light quality. 5. Increased attenuation of blue light and the resultant overall decrease in water clarity is likely to impact negatively on net annual production of benthic macrophytes of Grane Langsø. Any further increase in dissolved organic carbon concentration has the potential to markedly decrease the depth to which mosses grow by reducing the length of time in a year during which net photosynthesis occurs. [ABSTRACT FROM AUTHOR]