The effects of global climate change on seagrasses

The increasing rate of global climate change seen in this century, and pred icted to accelerate into the next, will significantly impact the Earth's oc eans. In this review, we examine previously published seagrass research thr ough a lens of global climate change in order to consider the potential eff ects on the world's seagrasses. A primary effect of increased global temper ature on seagrasses will be the alteration of growth rates and other physio logical functions of the plants themselves. The distribution of seagrasses will shift as a result of increased temperature stress and changes in the p atterns of sexual reproduction. Indirect temperature effects may include pl ant community changes as a result of increased eutrophication and changes i n the frequency and intensity of extreme weather events. The direct effects of sea level rise on the coastal oceans will be to increase water depths, change tidal variation (both mean tide level and tidal prism), alter water movement, and increase seawater intrusion into estuaries and rivers. A majo r impact of all these changes on seagrasses and tidal freshwater plants wil l be a redistribution of existing habitats. The intrusion of ocean water in to formerly fresh or brackish water areas will directly affect estuarine pl ans distribution by changing conditions at specific locations, causing some plants to relocate in order to stay within their tolerance zones and allow ing others to expand their distribution inland. Distribution changes will r esult from the effects of salinity change on seed germination, propagule fo rmation, photosynthesis, growth and biomass, Also, some plant communities m ay decline or be eliminated as a result of increased disease activity under more highly saline conditions. Increased water depth, which reduces the am ount of light reaching existing seagrass beds, will directly reduce plant p roductivity where plants are light limited. Likewise, increases in water mo tion and tidal circulation will decrease the amount of light reaching the p lants by increasing turbidity or by stimulating the growth of epiphytes. In creasing atmospheric carbon dioxide will directly elevate the amount of CO2 in coastal waters. In areas where seagrasses are carbon limited, this may increase primary production, although whether this increase will be sustain ed with long-term CO2 enrichment is uncertain. The impact of increases in C O2 will vary with species and environmental circumstances, but will likely include species distribution by altering the competition between seagrass s pecies as well as between seagrass and algal populations. The reaction of s eagrasses to UV-B radiation may range from inhibition of photosynthetic act ivity, as seen fur terrestrial plants and marine algae. to the increased me tabolic cost of producing UV-B blocking compounds within plant tissue. The effects of UV-B radiation will likely be greatest in the tropics and in sou thern oceans. Then is every reason to believe that, as with the predicted t errestrial effects of global climate change, impacts to seagrasses will be great. The changes that will occur in seagrass communities an difficult to predict; our assessment clearly points out the need for research directed t oward the impact of global climate change on seagrasses. (C) 1999 Elsevier Science B.V. All rights reserved.