Comparative Ecology of Submersed Grass Beds in Freshwater, Estuarine, and Marine Environments

• Published in: Limnology and oceanography Vol. 33; no. 4; pp. 867 - 893
• Main Author: Stevenson, J. Court
• Format: Journal Article Conference Proceeding
• Language: English
• Published: Waco, TX American Society of Limnology and Oceanography 01-01-1988
• Subjects: Angiosperms; Animal and plant ecology; Animal, plant and microbial ecology; Biological and medical sciences; Brackish; Fresh water; Freshwater; Fundamental and applied biological sciences. Psychology; General aspects; Grasses; Lakes; Macrophytes; Marinas; Marine; Marine ecology; Plants; Productivity; Sea water; Synecology; Freshwater environment; Marine environment; Brackish water environment; Angiospermae; Estuaries; Spermatophyta; Ecology
• ISSN: 0024-3590; 1939-5590; 0024-3590
• DOI: 10.4319/lo.1988.33.4_part_2.0867

Worldwide, there are 500-700 species of submersed angiosperms adapted to freshwater and estuarine environments compared with 50 species adapted to marine waters. In their evolution from freshwater ancestors, seagrasses have undergone extensive anatomical changes (e.g. reduction in floral and leaf structures, reduction of xylem tissue with a lacunal gas transport system), as well as physiological adaptations (bicarbonate utilization in photosynthesis). Seagrasses appear to have more annual production than do their freshwater counterparts because they develop greater standing crops and have the capacity to store photosynthetic products in extensive rhizome systems in the sediments. For example, maximum productivity of $>10 g C m^-2 d^-1$ has been reported for tropical seagrass species (Cymodocea nodosa and Thalassia testudinum), but the maximum productivity of temperate freshwater species such as Myriophyllum or tropical freshwater species such as Hydrilla is usually $<5 g C m^-2 d^-1$. In addition, the marine environment provides ample supplies of inorganic carbon (C) and increased mixing energies, making $CO_2$ limitation less likely. One calculation suggests that marine macrophytes impact the global C budget by sequestering as much as $10^9 t$ of C per year. Secondary productivities of seagrass communities can also be high. For example, stable isotopic ratios suggest that macrophytic C is important in sustaining several species of commercial fish species in Australia, accounting for $>50%$ of their diets. Also, sea urchins (Diadema antilarum) consume plant material, creating bare halos around tropical patch reefs in the Caribbean Sea. It is difficult to generalize regarding brackish submersed aquatics in estuarines because their coverage is variable due to light limitation and algal overgrowth from eutrophication. Freshwater macrophytes seem rarely grazed by fish (except via exotic introductions of Tilapia or carp), but waterfowl use is often significant at the end of the growing season. Thus, trophic relations in freshwater macrophyte beds may be qualitatively different and much more pulsed than in seagrass systems, with more r-selection in lakes and more K-selection in marine environments.