Photosynthesis and Canopy Structure of a Submerged Plant, Potamogeton Pectinatus, in a Danish Lowland Stream

Abstract

SUMMARY (1) The depth distributions of photosynthesis and respiration were examined in a Potamogeton pectinatus population in the River SusA. This examination was based on calculated vertical profiles of photon flux density (PFD) and biomass in the field and photosynthesis-light curves for different depth strata measured in the laboratory. (2) Potamogeton showed no sign of shade acclimatization with increasing depth in the canopy. On the contrary, basal leaves had lower chlorophyll content and lower photosynthetic efficiency than apical leaves in both low and high light. Potamogeton has an apical growth form leading to increasing age of the tissue with depth in the canopy. The reduced activity of basal leaves can be explained by light limitation and the beginning of senescence. (3) Potamogeton stands concentrated their biomass at the water surface during summer, and thereby compensated for high water attenuation. The calculated photosynthetic rate of the stand was strongly reduced if the biomass was assumed constant with depth. Simultaneously, the depth strata with highest photosynthetic and respiratory activity and the light compensation point in the canopy moved towards the surface. (4) Integrated photosynthesis of the stand showed a hyperbolic dependency on surface PFD. Photosaturation was usually not reached at maximum PFD, due to extensive selfshading. Gross 02 production was primarily regulated by PFD and biomass and showed a unimodal seasonal pattern with maximum in mid-August (7 5 g02 m-2 day- 1). Gross 02 production corresponded to total stream photosynthesis at maximum biomass during mid-summer. Dark respiration was a high proportion of gross photosynthesis, and this proportion increased with biomass and age of tissue during the growing season. Net 02 production increased linearly from early May to early August and was positive until late August. Changes in biomass followed the same seasonal pattern.