Scaling of photosynthetic production of aquatic macrophytes - a review

Most studies on photosynthetic production of aquatic macrophytes have been made on detached leaves and algal thalli. This may have given the false impression that production is often saturated by light and that inorganic carbon and nutrients are more important limiting factors. However, studies on the more relevant ecological scale of macrophyte communities lead to a completely different perception because community production is light limited due to intense self-shading. Relatively high irradiances are needed for photosynthesis to balance respiratory costs and not even maximum irradiances at noon during summer saturate photosynthetic production. The fundamental importance of light is confirmed by the close coupling to community light absorptance of both maximum production in high-light environments and efficiency of light use in low-light environments. The upper boundaries of light-limited and light-saturated production are distinctly and linearly related to community absorptance. Moreover, higher diversity in the community has a positive and stabilizing influence on light absorptance and production because different species supplement each other temporally and spatially. Close predictions of actual production rates in macroalgal communities throughout the year are possible solely from determinations of incident and absorbed irradiance. Along with the increasing regulating role of light from leaves or thalli to entire communities the importance of temperature, inorganic carbon and other resources decreases. Thus, ten-fold rise of CO₂ relative to atmospheric saturation does not enhance maximum production in dense communities of efficient HCO₃⁻-users and only doubles production of pure CO₂-users. A challenge therefore exists establishing the importance of light for photosynthetic production of macrophytes from individuals to communities and re-evaluating the postulated main importance of inorganic carbon and temperature in the scenarios of globally rising CO₂ and temperature.