Thomas Binzer

Oxygen Movement in Seagrasses

Seagrasses are, like all vascular plants, obligate aerobes, which require a continuous supply of oxygen to sustain aerobic metabolism of both aboveand below-ground tissues. Compared to their leaves, seagrass roots and rhizomesmay experience oxygen deprivation for shorter periods, but these below-ground tissues exhibit physiological adaptations which allow them to rely temporarily on anaerobic fermentative metabolism (Pregnall et al., 1984; Smith et al., 1988). Aerobic respiration is energetically about 10 times more efficient than fermentative processes, which tend to accumulate ethanol, acetate, and other potentially toxic metabolites representing a threat to tissue survival (Smith et al., 1988; Crawford and Braendle, 1996). The meristematic tissues, located in the transitionbetweenwater columnand sediment, are especially vulnerable to low oxygen supply and exposure to anaerobic metabolites due to their high metabolic activity and the continuous oxygen supply required for mitotic growth. In addition to the importance of oxygen inside seagrass tissues, maintenance of oxic conditions around roots may provide efficient protection against invasion of reduced toxic compounds and metal ions from the surrounding sediment (Armstrong et al., 1992; Crawford and Braendle, 1996; see also Marba et al., Chapter 6). Accordingly, there are several benefits to plant performance in maintaining a rich oxygen supply to all tissues including roots and rhizomes.

Highly predictable photosynthetic production in natural macroalgal communities from incoming and absorbed light

Photosynthesis–irradiance relationships of macroalgal communities and thalli of dominant species in shallow coastal Danish waters were measured over a full year to test how well community production can be predicted from environmental (incident irradiance and temperature) and community variables (canopy absorptance, species number and thallus metabolism). Detached thalli of dominant species performed optimally at different times of the year, but showed no general seasonal changes in photosynthetic features. Production capacity of communities at high light varied only 1.8-fold over the year and was unrelated to incident irradiance, temperature and mean thallus photosynthesis, while community absorptance was a highly significant predictor. Actual rates of community photosynthesis were closely related to incident and absorbed irradiance alone. Community absorptance in turn was correlated to canopy height and species richness. The close relationship of community photosynthesis to irradiance is due to the fact that (1) large differences in thallus photosynthesis of individual species are averaged out in communities composed of several species, (2) seasonal replacement of species keeps communities metabolically active, and (3) maximum possible absorptance at 100% constrains the total photosynthesis of all species. Our results imply that the photosynthetic production of macroalgal communities is more predictable than their complex and dynamic nature suggest and that predictions are possible over wide spatial scales in coastal waters by measurements of vegetation cover, incoming irradiance and canopy absorptance.