Effects of changing water clarity on characean biomass and species composition in a large oligotrophic lake

The biomass and species composition of characean meadows in oligotrophic Lake Coleridge, New Zealand, were monitored over a 2 year period during which water clarity underwent significant fluctuation. Water clarity, measured as vertical attenuation coefficient (K-d), varied between 0.1 and 0.4 m(-1) and there was a period of prolonged low clarity in the middle of the study. Characean meadows extended to a maximum depth of over 30 m at the start of the study in April 1993 but were reduced to less than 20 m by July 1995. At depths of 5, 10 and 15 m, where characean meadows persisted through to July 1995, there was a change in species composition, with an upward shift in the proportions of dominant taxa. This change was manifested as an increase in the percentage contribution to biomass of Chara globularis and Chara corallina at the expense of Chara fibrosa. Over the entire study period characean biomass remained maximal at depths of 5 and 10 m at approximately 180 g (dry weight) m(-2). At a depth of 5 m, biomass was not affected by changing water clarity and appeared to be limited by exposure to wave action and variable water level. At depths greater than 10 m, biomass declined significantly over the study period of 2 years. Reductions could be related to the underwater light field which plants experienced over the period between each sampling. Total characean biomass declined below maximal when irradiance fell below 1.0 mol m(-2) day(-1). Despite an increase in water clarity in early 1995, recovery of biomass was not observed within the time scale of this study.