Production in aquatic macrophyte communities: A theoretical and empirical study of the influence of spatial light distribution

Abstract

Many terrestrial plant canopies regulate spatial patterns in leaf density and leaf inclination to distribute light evenly between the photosynthetic tissue and to optimize light utilization efficiency. Sessile aquatic macrophytes, however, cannot maintain the same well-defined three-dimensional structure because of the strong drag and shear forces of moving water. This difference in canopy structure has been suggested to account for the three-to fivefold higher gross production rates in terrestrial than aquatic communities. To evaluate the effect of community structure in aquatic habitats, we combined a simple mechanistic model and empirical measurements on artificially structured macroalgal communities (Ulva lactuca) with varying thallus absorptance and community density. Predicted and measured values corresponded closely and revealed that gross production in high-light environments was markedly enhanced by a vertical orientation of thalli when absorptance and community density were both high. This result implies that aquatic macrophytes of high thallus absorptance and community density exposed to high light are limited in attaining high gross production rates because of their inability to distribute photons evenly between the photosynthetic tissues. As scattering and attenuation in the water column increase, the effect of thallus structure on production declines and thin transparent macrophytes are more efficient at utilizing light than thick opaque macrophytes. The results confirm that inefficient distribution of light can account for the low community production rates in aquatic habitats and the depth distribution of form-functional groups of macroalgae with different canopy structure.

Community photosynthesis of aquatic macrophytes

“…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). …”

“…Photosynthetic parameters are thus dependent on the structure of the community. Theoretical models (e.g., Kuroiwa 1970;Sands 1995a,b;Binzer and Sand-Jensen 2002b) and experimental tests (Monsi et al 1973;Ishii 1998;Binzer and Sand-Jensen 2002b) show that if light is distributed evenly among phytoelements, irradiance on each phytoelement is low and can be used efficiently for photosynthesis. This situation is achieved when most available light is absorbed in the canopy and absorbed irradiance per biomass is low.…”

Impact of riparian trees shade on aquatic plant abundance in conservation islands

“…Aquatic macrophyte control methods (MURPHY 1988a, b;PIETERSE and MURPHY 1990;JADHAV et al 2008;PALMER et al 2010) provide studies of aquatic macrophyte abundance and growth (SPENCE 1981, SMART and BARKO 1986, SAND-JENSEN and MADSEN 1991, SCHWARZ et al 1996, MIDDELBOE and MARKAGER 1997, BINZER and SAND-JENSEN 2002, RAEDER et al 2010. These authors identified light as a primary factor limiting growth.…”