The production ecology of benthic plants in some Antarctic lakes: I. In situ production studies
(1) The vegetation of three oligotrophic lakes on Signy Island (Antarctica) is dominated by benthic algal felts and aquatic mosses, and the phytoplankton is sparse. Five seasonal factors affect underwater irradiance--daylength, the sun's elevation, ice- and snow-cover and water clarity. Calculated irradiance at the bottom of the lake varies from c. 1 at midwinter to 2600 kJ m-2 day-1 in early summer. (2) Primary production of three representative benthic communities was measured in situ at monthly intervals over 11 months. Photosynthesis and respiration were estimated from changes in oxygen concentration in light and dark bottles using a semi-micro-Winkler method. There was measurable production for 9 months of the year, but the plants were below compensation point during early winter (June to early August). Compensation and light-saturation points were both at unusually low irradiance. (3) Maximum production rate (measured as oxygen evolution rate per unit ash-free dry weight, μg mg-1 day-1) was: Tolypothrix-Plectonema community (Sombre Lake) 10.8; Phormidium-dominated algal felt (Changing Lake) 4.2; aquatic mosses Calliergon and Drepanocladus (Moss Lake) 7.7. Calculated as equivalent carbon production, these rates are 4, 1.5 and 3 μg mg-1 day-1. Annual production as carbon per unit area of lake for the three communities was 9, 3.3 and 4 g m-2. (4) Comparison is made with similar lakes in the Arctic and Antarctic. The polar environment accentuates the differences between lakes dominated by either phytoplankton or by benthic vegetation.