FACTORS AFFECTING THE WATER CLARITY OF PONDS ON THE MCMURDO ICE SHELF, ANTARCTICA

The clarity of 39 meltwater ponds on the McMurdo Ice Shelf was determi ned as the horizontal viewing range of a black disc. Visual ranges var ied widely from pond to pond from 0.14-5 m; so did the concentrations of optically-active constituents, including the suspended particulates , phytoplankton (10-fold variation) and inorganic suspensoids (> 100-f old), and dissolved yellow substance (10-fold). In six of the ponds th e ratio of beam attenuation coefficient to total suspended solids conc entration was low (< 0.6 m(2) g(-1)) compared to that in the others (0 .7-2.0 m(2) g(-1)), suggesting that generally larger particles were pr esent suspended in the water in these ponds. In both groups, relations hips between beam attenuation and constituent concentrations indicated that much of the attenuation was due to inorganic suspensoids. Organi c detritus also appeared to be important in many ponds, while phytopla nkton and dissolved yellow substance were generally less important. Ev en though the clarity of many of the ponds was poor, their relative sh allowness meant levels of underwater light were probably generally ade quate for benthic plant growth.