Bulk Elastic Moduli and Solute Potentials in Leaves of Freshwater, Coastal, and Marine Hydrophytes. Are Marine Plants More Rigid?

Bulk modulus of elasticity (ɛ), depicting the flexibility of plant tissues, is recognized as an important component in maintaining internal water balance. Elevated ɛ and comparatively low osmotic potential (Ψπ) may work in concert to effectively maintain vital cellular water content. This concept, termed the ‘cell water conservation hypothesis’, may foster tolerance for lower soil-water potentials in plants while minimizing cell dehydration and shrinkage. Therefore, the accumulation of solutes in marine plants, causing decreases in Ψπ, play an important role in plant–water relations and likely works with higher ɛ to achieve favourable cell volumes. While it is generally held that plants residing in marine systems have higher leaf tissue ɛ, to our knowledge no study has specifically addressed this notion in aquatic and wetland plants residing in marine and freshwater systems. Therefore, we compared ɛ and Ψπ in leaf tissues of 38 freshwater, coastal and marine plant species using data collected in our laboratory, with additional values from the literature. Overall, 8 of the 10 highest ɛ values were observed in marine plants, and 20 of the lowest 25 ɛ values were recorded in freshwater plants. As expected, marine plants often had lower Ψπ, wherein the majority of marine plants were below −1.0 MPa and the majority of freshwater plants were above −1.0 MPa. While there were no differences among habitat type and symplastic water content (θsym), we did observe higher θsym in shrubs when compared with graminoids, and believe that the comparatively low θsym observed in aquatic grasses may be attributed to their tendency to develop aerenchyma that hold apoplastic water. These results, with few exceptions, support the premise that leaf tissues of plants acclimated to marine environments tend to have higher ɛ and lower Ψπ, and agree with the general tenets of the cell water conservation hypothesis

Sublethal Impacts of an Oil Spill on Rhizophora mangle, Avicennia germinans, and Laguncularia racemosa seedlings.

On the morning of June 19, 1991, approximately 3, 790 L (1,000 gallons) of heavy fuel oil were spilled into the waters of Port Everglades, Florida. The oil impacted one of several mitigation sites located in John U. Lloyd State Park (JUL). These sites were planted with Rhizophora mangle and Spartina alterniflora prior to the spill. Laguncularia racemosa and Avicennia germinans had also established themselves through natural recruitment. After the spill, these seedlings were coated with oil in varying amounts depending upon their location relative to the intervening oil slick and their elevation within the intertidal zone. To assess the impacts of the oil on the mitigation site, measurements were taken on the three species of mangroves. These measurements included: survival, growth rate, development of leaves, foliation, branches, and roots. Results indicated that L. Racemosa seedlings were less tolerant to oil contamination than A. germinans and R. mangle seedlings. In addition, there were some cases where growth stimulations were found in the exposed seedlings. The results from this investigation may aid in the selection of appropriate mangrove species that are to be utilized for wetland restoration sites located near ports, terminals, or refineries which are at high risk for oil pollution

Journal of Experimental Marine Biology and Ecology 350 (2007) 46–72 www.elsevier.com/locate/jembe Seagrasses and eutrophication

This review summarizes the historic, correlative field evidence and experimental research that implicate cultural eutrophication as a major cause of seagrass disappearance. We summarize the underlying physiological responses of seagrass species, the potential utility of various parameters as indicators of nutrient enrichment in seagrasses, the relatively sparse available information about environmental conditions that exacerbate eutrophication effects, and the better known array of indirect stressors imposed by nutrient over-enrichment that influence seagrass growth and survival. Seagrass recovery following nutrient reductions is examined, as well as the status of modeling efforts to predict seagrass response to changing nutrient regimes. The most common mechanism invoked or demonstrated for seagrass decline under nutrient over-enrichment is light reduction through stimulation of high-biomass algal overgrowth as epiphytes and macroalgae in shallow coastal areas, and as phytoplankton in deeper coastal waters. Direct physiological responses such as ammonium toxicity and water-column nitrate inhibition through internal carbon limitation may also contribute. Seagrass decline under nutrient enrichment appears to involve indirect and feedback mechanisms, and is manifested as sudden shifts in seagrass abundance rather than continuous, gradual changes in parallel with rates of increased nutrient additions. Depending on the species, interactions of high salinity, high temperature, and low light have been shown to exacerbate the adverse effects of nutrient over-enrichment. An array of indirect effects of nutrient enrichment can accelerate seagrass disappearance, including sediment re-suspension from seagrass loss, increased system respiration and resultin