Photosynthetic pathway diversity in a seasonal pool community

Aquatic plants have attracted attention in recent years because of the unexpected presence of CAM (e.g. Keeley 1981; Richardson et al. 1984) and C4 (Bowes & Salvucci 1984; Spencer, Wetzel & Teeri 1996; Reiskind et al. 1997; Keeley 1998a) photosynthetic pathways. Indeed, it now appears that the aquatic environment is home to as great a diversity of photosynthetic pathways as terrestrial environments. While the environmental factors responsible for selection of these pathways in terrestrial species are reasonably well understood (Teeri & Stowe 1976; Kluge & Ting 1978; Ehleringer & Monson 1993; Paruelo & Lauenroth 1996), far less is known about the conditions determining the distribution of aquatic C4 and CAM plants. Despite numerous studies of terrestrial landscape changes in abundance and diversity of C4 and CAM plants across various environmental gradients, relatively little attention has been given to the role of photosynthetic pathway diversity in controlling community composition. Factors determining the success of C4 and CAM are sufficiently different so that sites where one dominates the other is often poorly represented. While terrestrial communities comprising a combination C3, C4 and CAM species are known, the role of photosynthetic diversity in community coexistence has not been considered. In aquatic systems even less is known about community patterns of photosynthetic diversity. An aquatic community of particular interest is restricted to seasonal pools in California, USA, a habitat known as ‘vernal pools’. These are shallow rain-fed pools with large diel changes in physical and chemical characteristics (Fig. 1), which pose potential stresses for aquatic plant photosynthesis. On a typical spring day, water temperatures rise rapidly within the first few hours of the morning coupled with rapid photosynthetic production of O2 and depletion of CO2. Being rain-fed pools, they are poorly buffered and, Functional Ecology 1999 13, 106–118