Are seagrass growth and survival constrained by the reducing conditions ofthe sediment?

A literature review of the effects of the reducing conditions of the sedime nt on seagrass metabolism, growth and survival, and of the morphological an d physiological adaptations that seagrasses show to cope with sediment anox ia is presented and major gaps in knowledge are identified. The hypothesis that sediment anoxia controls the survival of seagrasses was tested experim entally by increasing the oxygen demand of the sediment with the addition o f sucrose. Experiments were performed in a tropical (Southeast Asia) multis pecific seagrass meadow, a Mediterranean Cymodocea nodosa meadow, and a tem perate Zostera marina meadow. Sulfide levels in pore water and vertical red ox profiles were used to characterise the effects of the sucrose additions on the sediment, while plant responses were quantified through the changes in shoot density and leaf growth. Sulfide levels in pore water:increased an d sediment redox potential decreased after the addition of sucrose to the s ediment of different seagrass meadows. The effect of the addition of sucros e to the sediment of seagrasses was species specific. Leaf growth was reduc ed and shoot mortality increased in some of the tropical species (e.g., Tha lassia hemprichii), but not in others. Neither mortality nor leaf growth of the Mediterranean species C. nodosa was affected by sucrose additions, and only leaf growth was reduced two months after the addition of sucrose in Z . marina. Our results suggest that increased sediment anoxia might be a fac tor promoting growth inhibition and mortality in seagrasses, although stron g differences have been found among different species and environments. (C) 1999 Elsevier Science B.V. All rights reserved.