Carbon and nitrogen dynamics in a maritime Antarctic stream

1 The carbon and nitrogen dynamics in a maritime Antarctic lake outflow stream were investigated. The stream and the algal communities could be split into two zones: a semi-aquatic margin consisting of a perennial cyanobacteria/diatom mat and a flowing channel with a similar perennial mat that was overgrown by annual filamentous chlorophytes during the course of the summer. 2 Neither algal community was limited by nutrient availability. Major nutrients were always available in the stream water. There were slight differences in the atomic ratios of the mats, the N:P ratios in the channel mat being lower than those in the marginal mat. However, both these and the total dissolved N:P ratio in the stream water were all close to those that indicate a balanced supply. 3 There was no net carbon or nitrogen accumulation by the marginal mat suggesting that uptake processes were balanced by loss processes. 4 Maximum rates of carbon fixation (0.1–0.5mgCg−1 dry weight h−1) were similar to those of other perennial Antarctic algal mats. Productivity appeared to be limited by physical factors, but the effects of irradiance and temperature could not be separated. 5 There were no heterocystous cyanobacteria in the mat communities and rates of atmospheric nitrogen fixation were very low (0–10ngNmg−1 mat Nh−1). Fixation accounted for only 0.3% of the nitrogen accumulation of the channel mats, but was higher in the marginal mat where uptake of other sources of nitrogen was also low. 6 Nitrogen accumulation by the channel mat averaged 0.34gNm−2 day−1. Only 0.05gNm−2 day−1 was accounted for by the uptake of dissolved inorganic nitrogen (nitrate plus ammonium). The major (80%) source of nitrogen appeared to be dissolved organic nitrogen. Recycling of nitrogen within the stream ecosystem may also be important.