Seagrasses in warming oceans : physiological and biogeochemical responses

Abstract: The exponential increase of atmospheric greenhouse gas concentrations over the past 50 years has caused a rise in the global average temperature by more than 1ºC above pre-industrial levels. Ninety-three percent of this heat energy has been absorbed and stored by the oceans, increasing their temperatures, particularly in surface waters. This can produce both negative and positive impacts on the health and function of vital coastal shallow-water communities, hosting seagrasses and macroalgae, which are key primary producers and ecosystem engineers in the coastal zone. The physiological processes of these plants and the biogeochemical processes in associated sediments operate over a wide range of temperatures and their response can serve as early indicators of changes in their ecosystem function. This thesis employed a combination of laboratory, mesocosm and field based experiments to understand: 1) the responses of key physiological processes to elevated temperatures occurring frequently (and likely to occur in a future warming scenario) in seagrass meadows, and how these will affect biogeochemical processes in associated sediments, 2) the exchange of carbon dioxide between seagrass, water and atmosphere, and 3) effects of the tidal variability on biogeochemical processes of tropical seagrass sediments.The results showed that elevated water temperatures cause increased rates of photosynthesis in seagrasses up to a threshold temperature above which rates declines rapidly. The negative effects of temperatures reaching beyond threshold levels increased with repeated days of exposure. The rates of mitochondrial respiration in seagrasses increased with elevated temperatures until a collapse of their respiratory machinery occurred. Photorespiration did not increase linearly with elevated temperatures. The responses of the different components of the seagrass plant (i.e. leaves, shoots, rhizomes and roots) to temperature increase clearly differed, and varied within different parts of each component. Spikes of very high water temperatures, up to 40-44ºC, occur frequently during daytime at low spring tides during the northeast monsoon in the tropical intertidal areas of the western Indian Ocean, and if they occur repeatedly over several days, lead to large biomass loss in seagrasses. Such temperatures also increased methane emission and sulphide levels in seagrass-associated sediments. Submerged macrophytes in shallow coastal waters had pronounced effects on air-water fluxes of carbon dioxide, with an upward flux occurring when partial pressure of carbon dioxide is higher in the seawater than in the air and carbon dioxide escapes the water phase, and a downward flux when carbon dioxide enters the water phase. Plant cover, time of day and tidal level had pronounced consequences on emissions of methane and nitrous oxide as well as sulphide levels in tropical seagrass sediments. Emissions of methane and nitrous oxide positively correlated to sediment organic matter content and the relationship became stronger during high tide.The findings of this thesis indicate that intertidal seagrasses of the tropical WIO region are at special risk of declining under future warming, as they are currently living in an environment where ambient water temperatures frequently reach at, or beyond, threshold levels of key physiological processes during midday hours of low spring tides of the northeast monsoon. The negative effects of high temperature spikes may be further intensified by other anthropogenic stressors (e.g. eutrophication by land-based pollution sources). Taken together, these will reduce seagrass cover and promote the release and emission of historically deposited carbon back to the atmosphere, and this would possibly change these ecosystems from being carbon sinks to being sources and further exacerbate the negative impacts of greenhouse gases.