Macro- and Microalgal Production Within a Nitella Opaca Bed in Lake Thingvallavatn, Iceland

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Abstract
(1) Nitella opaca forms a dense bed at a depth of 10-18 m in Lake Thingvallavatn, Iceland. In 1985 Nitella started growth in early May and achieved maximum height (70-80 cm) and biomass (up to 170 g dry wt m-2, or about 50 g C m-2) in October. During November and December the macroalgae collapsed and many of the thalli became detached. (2) During May-September the average Nitella biomass accumulation, calculated from successive harvesting, was 153 .+-. 10 (S.E) mg C m-2 at depth of 10-16 m. This figure was consistent with net production calculated from short-term (0.5-3 h) measurements of 14C-fixation under both laboratory and field conditions. However, measurements of O2 exchange yielded distinctly higher community production rates than did the 14C-method, possibly due to recycling of 14CO2. (3) Field measurements of 14C-fixation in 561 or 1121 acrylic plastic chambers showed that during July-September 40-78% of the total production of the Nitella community was attributable to epiphytic (+epipelic) and planktonic microalgae (mainly diatoms). Microalgal production would seem to be sufficient to sustain the rich benthic fauna (especially the larvae of Chironomus islandicus) living among the Nitella thalli.