Photosynthetic strategies and productivity in aquatic systems

Abstract

Aquatic plants have physiological, morphological and growth-form adaptations that collectively determine a species' success in a given habitat. The primary productivity in various aquatic habitats is largely dependent on the constraints on both species composition and photosynthetic rates, thus, the characteristics of the assembled species should reflect such constraints. For the macrophytes, major adaptations for life in water are associated with constraints on photosynthesis: adaptations to increase carbon fixation, for example, include; (1) mechanisms to concentrate inorganic carbon for photosynthesis, such as C 4-like and CAM carbon fixation, and the ability to remove HCO 3 − from the water; (2) the use of alternate sources of carbon, such as sediment CO 2; (3) adaptation for net photosynthesis at low-light intensities; (4) morphological adaptation to increase inorganic carbon or mineral acquisition. We evaluate such characteristics at the species or community level, to provide insight into the ways in which adaptations for photosynthesis are integrated into a plant's strategy or suite of adaptations to constraints on growth or production. In nutrient-poor, softwater habitats, for example, the common rooted plants have adaptations that allow tolerance of low rates of nutrient delivery to the plant and the conservation of captured resources (e.g. small evergreen plants with inherently slow growth that use sediment CO 2 and CAM). These adaptations are compatible with the low rates of nutrient availability, although the slow growth results in low annual primary production. In habitats with generally higher levels of inorganic carbon and other nutrients, where competition among plants limits the availability of these resources, species have adaptations that allow rapid growth and biomass accumulation. Here, species are more likely to be able to remove HCO 3 − from the water for photosynthesis efficiently, and may have other carbon-concentrating mechanisms that allow rapid photosynthesis. These considerations serve as one possible framework for relating adaptations for photosynthesis to plant growth characteristics and the primary productivity of aquatic systems.