Nutrient exchange in an Antarctic macrolichen during summer snowfall snow melt events

Concentrations of NH4+, NO3-, PO43-, K+, Ca2+ and Mg2+ in snow meltwater resulting from summer snow showers were monitored before and after its passage through monospecific stands of the Antarctic macrolichen Usnea sphacelata R. Br. The sampling was conducted under field conditions near Casey Station in East Antarctica between January and March. Total snow deposition during the 61-d period was 44 +/- 1 mm (rainfall equivalent depth) delivering 362 +/- 10, 87 +/- 2 and 9 +/- 1 mu mol m(-2) of NH4+, NO3- and PO43-, respectively. Meltwater that had percolated through U. sphacelata was depleted in NH4+ and NO3- equating with a retention by the lichen of 87 and 92 %, respectively, of the total wet deposition of these ions. Lichen-modified meltwater was slightly enriched in PO43-, but because the volume of the lichen percolate was smaller than that of the original snow deposition, the lichen achieved a net gain of 9 % of the total P deposited. Lichen percolate was also enriched in metal cations. Potassium loss associated with the melting of the heaviest snowfall (18 mm) was equivalent to only 0.05 % of the total K in the lichen suggesting that ion loss did not signal significant cellular damage. There was also a progressive increase in NH4+ concentration in unmodified meltwater from 3 to 21 nmol ml(-1) over a 3-d period whereas levels in the lichen-modified meltwater remained unchanged at less than or equal to 4 nmol ml(-1). This enrichment in NH4+ might have resulted from dry deposition onto the melting snow pack of NH3 emitted from nearby penguin rookeries. During the study, tagged thalli of U. sphacelata made a 2 % loss in dry mass although they appeared healthy and, at the end of the study, showed an effective quantum yield (Delta F/Fm') comparable with field material. The results are discussed in relation to the time of year that is likely to be most suitable for lichen growth in this continental Antarctic environment and the potential growth-led demand for N in U. sphacelata.