Algal Ecology of Southern Icelandic Hot Springs in Winter

Abstract

A survey of algal habitats of southern Icelandic hot springs (Hengill-Olfus district) was made between December 15, 1968, and January 20, 1969. Carbon-14 primary productivity was measured in experiments designed to compare the role of direct solar radia- tion on algal habitats of different aspect and inclination. Incident light (0.4-0.7 gLm) energy on December 20, was 23.1, 3.3, and 5.1 g-cal/cm2 day for south-facing 600 slopes, north- facing 600 slopes, and horizontal surfaces, respectively. Primary productivity for algal cores incubated (in vials) in these positions was 1110, 260, and 1130 mg C/M2 - day. Coincidence of productivity of south-facing and horizontally-incubated cores suggested that the latter re- ceived far more illumination than was indicated by light meter readings. Similar results were noted for incubations carried out on January 8, 1969. Submersed algal habitats exhibited a dramatic cutback from their summer appearance. Residual populations of Mastigocladus lanminosus Cohn were found up to 552C in quiet waters. Healthy mats of Phormidiuin corium (C.A. Ag.) Gomont, Lyngbya inartensiana Menegh. and Mastigocladus occurred locally in south flowing effluents (410-310C) in mod- erate to fast flow. The aerial blue-green algal community bordering the hot springs appeared unchanged from that of the summer. The cryptic persistence of Mastigocladus on submerged rock from habitats (550-40'C) having a swept bare appearance was demonstrated in the laboratory. This finding obviates the suggestion that nonthermal overwintering niches might serve as seeding sources for the large-scale resumption of growth in the spring. Eight of the 14 PVC substrates left for 15 days in thermal effluents showed signs of colonization, which indicated that the 24-h compensation point for this period had been exceeded.