Hydrobiologia 172: 1-18, 1989 W. F. Vincent and J. C. Ellis-Evans(eds) High Latitude Limnology 0 1989 Kluwer Academic Publishers.Printed in Belgium Filamentous green algae in freshwater streams on Signy Island, Antarctica Ian Hawes British Antarctic Survey, Natural Environment Research Council, High Cross Site, Madingley Road, Cambridge CB3 OET, United Kingdom Key words: Antarctica, Chlorophyta,distribution, filamentousalgae, Klebsormidium, Mougeotia, periphy- ton, streams, Zygnema Abstract The streams of Signy Island are varied and extremely seasonal environments. Water flows from November/December to March/April; streamsare frozen for the rest of the year. Streams usually flow through small, barrencatchmentsand arenutrient poor, though they may be enriched by dense summer populations of seabirds and seals. Temperatures are consistentlylow. Stream depth is maximal during the spring melt period, declining over the course of the summer. Vegetation is exclusively algal, and filamentouschlorophytes from a particularly conspicuous component. Small numbersof vegetative cells survive the long frozen period in situ. A steadyincrease in standing crop results in a maximum 2 to 3 months after flow begins.Sloughing is the major loss mechanism and grazers areeffectively absent. Three taxa of tilamentous algaeare common in Signy streams,species of Zygnema, Mougeotia and Klebsor- midium. The distributions of thesealgaeare described and relatedto physical and chemical features of their environment. Introduction There is a wide variety of freshwater habitats on the Antarctic continent and its off-lying islands, ranging from small meltwater streams to large permanently ice-covered lakes. Recent reviewsof Antarctic freshwaterbiology have not presented the full extentof this rangeand insteadhavecon- centrated on the relatively well understood lake ecosystems with scant, if any, reference to streams(Heywood, 1977a, 1984; Priddle, 1985). This omission reflects more the lack of informa- tion on Antarctic streamsthan their abundance and probablesignificance as centres of biological activity (Heywood, 1977b; Broady, 1982; Howard-Williams et al., 1986).It is only in the last few years that these ephemeralecosystems have begunto be studied. Antarctica has been divided into 2 climatic zones,the continental and the maritime Antarc- tica (Holdgate, 1964).The latter comprises the western side of the Antarctic Peninsula and its off-lying islands and is characterisedby mean monthly winter temperatureswhich rarely fall below - 20 ’ C but whereat leastonemonth each summer has a mean temperature abovefreezing. The former has mean monthly temperatures which do not rise abovefreezingin summer and regularly fall below - 20 “C in winter. Antarctic stream studies have until now been confinedto continental Antarctica (Hirano, 1979; Broady, 1982; Howard-Williams et al., 1986).