Away from the tropics the average irradiance during daylight hours changes cyclically with the seasons during the year and we may reasonably suppose that it would be of advantage to the aquatic flora to adapt their photosynthetic systems to the seasonally varying light climate. This should especially be the case for the perennial benthic algae that survive throughout the year and therefore must photosynthesize as best they can at all seasons. However, the seasonal variation in temperature that accompanies the variation in irradiance does itself impose certain changes on the photosynthetic behaviour of aquatic plants, Light compensation and saturation points increase with temperature since the rates of respiration and carboxylation both increase with temperature whereas the rate of the light reactions is relatively unaffected. King and Schramm (1976) in their studies on a large number of benthic algal species from the Baltic found that there was a marked lowering of the compensation point in the winter compared to the summer or autumn. Since their measurements were carried out at the water temperature prevailing at the time, it was impossible to say whether there was any adaptation or whether the lower winter temperatures accounted entirely for the lower compensation points. In most of the species there was no marked seasonal change in the light saturation point but in two Laminaria (Phaeophyta) species the irradiance required to achieve saturation was much lower in winter than in autumn. In one of these species, L. saccharina, the light-saturated photosynthetic rate per g dry mass was about the same in both seasons suggesting that the reduction in light saturation point in the winter was not merely due to a fall in carboxylation rate caused by the lower temperature.
In the giant kelps Macrocystis integrifolia and Nereocystis luetkeana, growing off Vancouver Island (BC, Canada), Smith, Wheeler and Srivastava (1983) and Wheeler et al. (1984) found that the chlorophyll a and fucoxanthin levels (per unit area of blade) were high in winter, declined in spring-summer, and then rose in autumn once again to the winter level. The seasonal changes in pigment were substantial (two- to three-fold). Although a case might be made that the changes in pigment levels were an adaptive response to seasonally changing insolation, they appeared in fact to correlate best with the nitrate concentration in the water, i.e. perhaps these algae simply make more photosynthetic pigments when they have the nitrogen to do so.
In a number of red algal species it has been shown that the ratio of phycoerythrin to chlorophyll is higher in the winter than in the summer.194'932'1122 It seems likely in such cases that the irradiance required to achieve saturation would, quite apart from temperature effects, be lower in the winter, but experimental confirmation is required.
In the intertidal red alga Bostrychia binderi, growing in a subtropical location (Florida, USA), Davis and Dawes (1981) found little seasonal variation in chlorophyll content, but the light-saturated photosynthetic rate was at its highest in the late summer and autumn and at its lowest in the winter, the summer/autumn values being two- to four-fold greater than the winter values. Since all the measurements were carried out at 30°C, this probably represents a true seasonal variation in carboxylation capacity.
Was this article helpful?