Abstract

Between July 1993 and September 1994 Lake Coleridge, an oligotrophic New Zealand lake, underwent a period of abnormally low water clarity. Over this time, characean algal biomass in the littoral zone of the lake underwent a dramatic decline. Restoration of the lake’s normal high clarity followed and over the next 2 years there was a recovery of the community to close to pre-decline levels. Experimental and field studies investigated the mechanisms underlying the decline and recovery. The response of Chara corallina Willd. was investigated in detail. This plant responded to exposure to light below the instantaneous compensation point for photosynthesis by reducing rates of photosynthesis and respiration and depletion of storage products. This eventually led to the decay of the basal cells and detachment of plants from the sediment. In the lake, they were subsequently washed out of the littoral zone by wind-generated currents. When C. corallina, acclimated to below-compensating light levels, was returned to favourable light conditions in the laboratory, these trends were reversed. However, once biomass had been lost in the lake, recovery required growth of new plants. The most important methods of regeneration were by down-slope invasion and regrowth of buried shoots.