Seagrass Respiration : An assessment of oxygen consumption patterns of temperate marine macrophytes

Abstract: In coastal seas, the abundance of marine macrophytes has profound influence on the flows of oxygen and inorganic carbon through the water. Vast amounts of carbon dioxide are taken up by photosynthesis and part of this is respired back into the water column. The photosynthetic carbon uptake of the most common seagrasses of the northern hemisphere is nowadays extensively studied at both community- and individual levels, and its impact on coastal carbon- and oxygen fluxes is quite well defined. However, the coinciding release of carbon dioxide and consumption of oxygen by the processes of mitochondrial respiration and photorespiration in these organisms has as yet not been given much attention, especially concerning how these processes are affected by external factors. For estimations of the rates of mitochondrial respiration, the common approach has been to use values obtained during darkness and treat them as being constant over the day. This approach is questioned in this thesis where the effects of different abiotic and biotic factors on oxygen consumption were examined to elucidate possible variations of seagrass respiration rates (with primary focus on the species Zostera marina), explored mainly using gas exchange techniques. The initial aim was to investigate whether the rates of respiration are at all fluctuating. This was found to be the case. Secondly, impacts of various factors on the respiration process were examined on Z. marina, with additional studies on the seagrass Ruppia maritima and the common green alga Ulva intestinalis. It was found that respiration rates were lower in the light for all three species. Specific investigations on Z. marina showed that respiration rates also varied with time of the day. Moreover, the rates of both respiration and photosynthesis differed between Z. marina shoots of different age as well as among different parts of the leaves. These differences were observed at both ambient (19.1oC) and elevated (29.1oC) temperatures. Photorespiration, previously considered insignificant in seagrasses, was found to have a profound role, as high rates were observed in productive bays, i.e. in settings with low inorganic carbon availability and high oxygen. Overall, this thesis has identified important external and developmental factors influencing the patterns of oxygen consumption and associated carbon dioxide release of two common temperate seagrasses. Clearly, respiration in seagrasses is a dynamic process that responds to a variety of external and developmental factors, which should be carefully considered when assessing the carbon budget of coastal vegetated areas.