The role of light in determining the maximum depth limits of characean algae in New Zealand lakes

Abstract

The role of light in determining the maximum depth limits of characean algae in New Zealand lakes was investigated in this thesis. A data-set comprising the maximum depth of colonisation (Zc) of aquatic plants and water clarity was compiled from 63 New Zealand lakes. Characeans formed the deepest aquatic macrophyte communities (excluding bryophytes) in 71% of the 63 lakes, and a close relationship was found between water clarity (defined as the reciprocal of the attenuation coefficient for downwelling irradiance (l/A'd)) and the maximum depth to which aquatic plants penetrated. Chara corallina was the most common species at Zc in 40% of the lakes, occurring to depths of up to 35 m. In South Island lakes Zc was linearly related to water clarity by the relationship Zc = -2.0 + 4.5/ATd. While a similar relationship held in most North Island lakes, in the clearest of these (Kj < 0.2) this relationship began to over predict Zc. The relationship was better described by an asymptotic equation which predicted Zc to be maximal at a depth of 21 m. Field studies of lake optical properties and the characteristics of plants growing at Zc were conducted in detail in 17 South Island lakes. In these lakes a Zc comprised of characean algae was able to be closely predicted by a compensation depth for photosynthesis. Despite a wide range of water colours in the study lakes, accounting for spectral composition did not markedly improve these predictions. Lake Coleridge in the South Island was the main study lake and here the relationship between the attenuation of photosynthetically available radiation (PAR) in the water column and characean biomass was investigated over a two year period. Community biomass at depths approaching Zc were found to be closely related to the light history of the plants. A balance between light mediated loss processes and net photosynthesis appeared to dictate biomass decline and ultimately the depth of Zc during a period of low water clarity in this lake. Given the relationships that were established between PAR, biomass and Zc, the most common, and likely reason for discrepancies in the relationship between Zc and PAR established in the thesis, and Zc measured in South Island lakes, was considered to be a variable amount of light at depth. Experiments were conducted to investigate the ability of C. corallina to acclimate during periods when PAR fluctuated above and below the light compensation point for photosynthesis. C. corallina was able to withstand unfavourable periods of PAR for up to four months depending on the light history of the material. Acclimation of photosynthetic parameters under low light conditions was to a lower threshold limit by reducing maximum rates of net photosynthesis to no less than approximately 1.0 mg O2 (g DW)'1 h'1. The saturation onset parameter (£0 and respiration rates also reduced in the early stages of acclimation. This level of net photosynthesis was not sufficient to maintain biomass indefinitely and plants in these conditions eventually died. There was evidence that uptake of dissolved organic compounds (as determined using acetate concentrations between 15 and 30 pg L'1) may help delay loss processes, but with continued time in light conditions unfavourable for photosynthesis, starch reserves were depleted, basal cell decay occurred and, in the field, plants were gradually lost from the littoral zone. Remaining plants retained a variable ability to resume higher rates of net photosynthesis within two weeks of a return to a favourable PAR environment. Given that natural and anthropogenic disturbances may cause episodic perturbations to water clarity in New Zealand lakes, the timing of such events has important implications for lake management if a healthy littoral zone community is to be maintained. The resilience of deepwater characean communities to unfavourable light conditions and thus the lag time before plants die and biomass is lost, will depend on the light history of the community. Results from this thesis suggest that if unfavourable periods are to occur it is desirable to maximise the intervening period over which plants are exposed to a favourable light climate.