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 oligotroph ic Lake Coleridge, New Zealand, were monitored over a 2 year period du ring which water clarity underwent significant fluctuation. Water clar ity, measured as vertical attenuation coefficient (K-d), varied betwee n 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 dept h of over 30 m at the start of the study in April 1993 but were reduce d to less than 20 m by July 1995. At depths of 5, 10 and 15 m, where c haracean meadows persisted through to July 1995, there was a change in species composition, with an upward shift in the proportions of domin ant 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 biom ass remained maximal at depths of 5 and 10 m at approximately 180 g (d ry weight) m(-2). At a depth of 5 m, biomass was not affected by chang ing water clarity and appeared to be limited by exposure to wave actio n and variable water level. At depths greater than 10 m, biomass decli ned significantly over the study period of 2 years. Reductions could b e related to the underwater light field which plants experienced over the period between each sampling. Total characean biomass declined bel ow maximal when irradiance fell below 1.0 mol m(-2) day(-1). Despite a n increase in water clarity in early 1995, recovery of biomass was not observed within the time scale of this study.