Importance of canopy structure on photosynthesis in single‐ and multi‐species assemblages of marine macroalgae

Abstract

T. 2004. Importance of canopy structure on photosynthesis in single-and multi-species assemblages of marine macroalgae. Á/ Oikos 107: 422 Á/432. Plant communities utilize available irradiance with different efficiency depending not only on their photosynthetic characteristics but also on the canopy structure and density. The importance of canopy structure are well studied in terrestrial plant communities but poorly studied in aquatic plant communities. The objective of this study was to evaluate macroalgal community photosynthesis in artificial constructed communities of one to four species with different morphologies along a range of leaf (i.e.0/thallus) area densities. In a laboratory set-up we measured net photosynthesis and dark respiration in constructed assemblages of macroalgae, excluding effects other than photosynthesis of individual tissue and distribution of photons in the canopy from influencing metabolism. We hypothezised that 1) canopy structure determines the actual rates of photosynthesis relative to the optimal rates and 2) multi-species communities attain higher maximum photosynthetic rates than single species communities. We found that differences in canopy structure outweighed large differences in tissue photosynthesis resulting in relatively similar maximum community photosynthetic rates among the different single and multi-species assemblages (20.1 Á/40.5 mmol O 2 m (2 s (1 ). Canopy structure influenced community photosynthesis both at low and high leaf area densities because it determines the ability of macroalgae to use the photosynthetic potential of their individual tissues. Due to an averaging effect the photosynthetic rate at high leaf area density was more similar among multi-species community than among singlespecies communities. Multi-species communities had, on average, a slightly higher photosynthetic production than expected from photosynthesis of single species communities. Moreover multi-species communities were capable of exposing new tissue to irradiance up to high densities thereby avoiding a decrease in net photosynthesis. This finding suggests that multi-species communities may be able to maintain higher biomass per unit ground area than single-species communities.

Community photosynthesis of aquatic macrophytes

“…Field measurements of fluorescence in temperate field populations of most aquatic macrophytes confirm, however, that photoinhibition is indeed rare (Witt 2003). Also, when leaves or thalli of aquatic macrophytes are tested at high irradiances in the laboratory, they rarely show signs of photoinhibition (Binzer and Sand-Jensen 2002a;Middelboe and Binzer 2004), though a small (10-15%) depression of maximum photosynthesis may develop after prolonged exposure to high irradiances (Van der Bijl et al 1989;Witt 2003).…”

“…As macrophyte density changes so do photosynthetic parameters. The situation is obvious when succession introduces new species, but photosynthetic parameters will also change in single species communities because of variations in physiological variables, macrophyte density, and structure (Middelboe and Binzer 2004;Hirose 2005).…”

“…The present contribution presents mainly new data, but because the aim was to provide an integrated view, we also included data from published studies using the same experimental setup and methods (Binzer and Sand-Jensen 2002a;Middelboe and Binzer 2004). Freshwater plants, a marine seagrass, and natural and constructed marine macroalgal communities were used in the 190 experiments.…”

“…Understanding how photosynthesis responds to light at the proper ecological scale is therefore essential for our knowledge of ecosystem processes Englund and Cooper 2003). However, few measurements of aquatic community production are available in the literature (Van der Bijl et al 1989;Middelboe and Binzer 2004;Binzer and Middelboe 2005) and those that exist frequently lack key information on plant density and photosynthesis for the entire range of irradiances (photons, 0-2,000 mmol m 22 s 21 ) expressed in relevant units per square meter.…”

“…This situation is achieved when most available light is absorbed in the canopy and absorbed irradiance per biomass is low. Measuring photosynthetic characteristics, absorption properties of phytoelements, and community biomass are thus not sufficient to describe community production, since three-dimensional (3-D) structure and distribution of light in the canopy also contribute to determine photosynthetic production (e.g., Ceulemans and Saugier 1991;Binzer and Sand-Jensen 2002a;Middelboe and Binzer 2004). …”