Limitation of carbon assimilation of intertidal Zostera noltii and Z. marina by desiccation at low tide

Carbon dioxide assimilation of emersed Zostera noltii leaves was measured by infrared gas analysis to determine its dependence on leaf water content and the duration of air exposure in the tidal flats of the German Wadden Sea (south east North Sea). After 5 h of exposure to air during low tide, leaves of Z. noltii had lost up to 50% of their water content. The degree of leaf water loss rapidly increased with the distance from the water line and, thus, was a function of the duration of exposure. A decrease in leaf water content by about 30% (from 340–390 to 190–250% DW) resulted in a reversible reduction in light-saturated net photosynthesis rate ( A max) of Z. noltii by more than 50% (24–31 nmol CO 2 g DW −1 s −1 vs. 6–14 nmol g DW −1 s −1). Similarly, laboratory experiments with artificially dried leaves showed a decrease of A max by about 40% for a water content reduction from 400 to 270% DW. Drying experiments with leaves of the co-occurring species Z. marina showed a rapid reduction of A max from 30 nmol CO 2 g DW −1 s −1 (at 500% leaf water content) to zero or even negative CO 2 fluxes (at 180% water content). These results indicate that photosynthesis is more sensitive to desiccation in Z. marina than in Z. noltii under a given leaf water content. The two intertidal Zostera species use low tide periods for rapid assimilation of atmospheric CO 2 as long as their leaf water status is favourable, but desiccation can significantly reduce carbon gain at low tides. We conclude that the presence of seagrasses in the tidal flats of the Wadden Sea is primarily restricted by desiccation at the upper limit.