DEEP-WATER AQUATIC PLANT-COMMUNITIES IN AN OLIGOTROPHIC LAKE - PHYSIOLOGICAL-RESPONSES TO VARIABLE LIGHT

1. Oligotrophic Lake Waikaremoana, New Zealand, is used for hydroelect ric power generation and the lake levels are manipulated within an ope rating range of 3 m. There was concern that rapidly changing water lev els adversely affected the littoral zone by decreasing light availabil ity in two ways: local turbidity caused by shoreline erosion at low wa ter levels; and decreased light penetration to the deep littoral zone caused by high water levels in summer. 2. The Littoral zone was domina ted by native aquatic plants with vascular species to 6 m and a charac ean meadow below this to 16 m. The biomass and heights of the communit ies in the depth zone 0-6 m were reduced at a site exposed to wave act ion relative to those at a sheltered site. However, the community stru cture below 6 m was similar at exposed and sheltered sites. The lower boundary of the littoral zone was sharply delimited at 16 m and this b ottom boundary remained constant throughout the year despite large sea sonal changes in solar radiation and the 3 m variation in lake level. 3. There was evidence that the deep-water community consisting of Char a corallina had adapted physiologically to low-light conditions. Net l ight saturated photosynthesis (CO2 exchange) per unit chlorophyll a (C hl a) was reduced to 1.7 mu g C (mu g Chl a)(-1) h(-1) at the lower bo undary, half of that recorded at 5 m. The concentration of Chl a per g ram of biomass (dry weight), was considerably greater at the lower bou ndary than higher in the profile [c. 7 mg Chl a (g dry wt)(-1) at 16 m vs. 4 mg Chl a (g dry wt)(-1) at 5 m]. Chl b also increased with dept h and there was no change in the ratio of Chl a and Chl b with increas ing depth. The saturation light intensity (I-k) of the community at th e lower boundary was only 78 mu mol photons m(-2) s(-1). photosyntheti c parameters (I-k and alpha) as well as the Chl a content remained rel atively constant throughout the seasonal and short-term changes in rad iation. 4. The photosynthetic characteristics of the littoral communit y were therefore not greatly affected by the lake level change caused by the present hydroelectric operations. However, the sharpness of the lower boundary and its extreme shade characteristics imply that the d eep-water community would be sensitive to any further changes in under water light availability.