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Seagrasses in warming oceans: physiological and biogeochemical responses
Stockholm University, Faculty of Science, Department of Ecology, Environment and Plant Sciences. Tanzania Fisheries Research Institute (TAFIRI), P. O. Box 9750 Dar es Salaam, Tanzania. (Seagrass Ecology & Physiology Research Group, Department of Ecology, Environment and Plant Sciences, Stockholm University, SE-106 91 Stockholm, Sweden.)ORCID iD: 0000-0002-2793-2970
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

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.

Place, publisher, year, edition, pages
Stockholm: Department of Ecology, Environment and Plant Sciences, Stockholm University , 2019. , p. 98
Keywords [en]
Global warming, greenhouse gas, warming oceans, temperate, tropical, coastal waters, Western Indian Ocean (WIO), tidal variability, seagrass, photosynthesis, respiration, photorespiration, biogeochemical processes, sulphide, methane, nitrous oxide, carbon dioxide
National Category
Natural Sciences
Research subject
Plant Physiology
Identifiers
URN: urn:nbn:se:su:diva-167772ISBN: 978-91-7797-721-6 (print)ISBN: 978-91-7797-722-3 (electronic)OAI: oai:DiVA.org:su-167772DiVA, id: diva2:1302283
Public defence
2019-05-28, Vivi Täckholmssalen (Q-salen), NPQ-huset, Svante Arrhenius väg 20, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 2: Manuscript. Paper 3: Manuscript. Paper 4: Manuscript. Paper 5: Manuscript. Paper 6: Manuscript.

Available from: 2019-05-03 Created: 2019-04-04 Last updated: 2019-05-23Bibliographically approved
List of papers
1. High midday temperature stress has stronger effects on biomass than on photosynthesis: A mesocosm experiment on four tropical seagrass species
Open this publication in new window or tab >>High midday temperature stress has stronger effects on biomass than on photosynthesis: A mesocosm experiment on four tropical seagrass species
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2018 (English)In: Ecology and Evolution, ISSN 2045-7758, E-ISSN 2045-7758, Vol. 8, no 9, p. 4508-4517Article in journal (Refereed) Published
Abstract [en]

The effect of repeated midday temperature stress on the photosynthetic performance and biomass production of seagrass was studied in a mesocosm setup with four common tropical species, including Thalassia hemprichii, Cymodocea serrulata, Enhalus acoroides, and Thalassodendron ciliatum. To mimic natural conditions during low tides, the plants were exposed to temperature spikes of different maximal temperatures, that is, ambient (29-33 degrees C), 34, 36, 40, and 45 degrees C, during three midday hours for seven consecutive days. At temperatures of up to 36 degrees C, all species could maintain full photosynthetic rates (measured as the electron transport rate, ETR) throughout the experiment without displaying any obvious photosynthetic stress responses (measured as declining maximal quantum yield, Fv/Fm). All species except T.ciliatum could also withstand 40 degrees C, and only at 45 degrees C did all species display significantly lower photosynthetic rates and declining Fv/Fm. Biomass estimation, however, revealed a different pattern, where significant losses of both above- and belowground seagrass biomass occurred in all species at both 40 and 45 degrees C (except for C.serrulata in the 40 degrees C treatment). Biomass losses were clearly higher in the shoots than in the belowground root-rhizome complex. The findings indicate that, although tropical seagrasses presently can cope with high midday temperature stress, a few degrees increase in maximum daily temperature could cause significant losses in seagrass biomass and productivity.

Keywords
biomass loss, climate change, photosynthetic performance, tropical seagrass, Western Indian Ocean
National Category
Biological Sciences
Research subject
Plant Physiology
Identifiers
urn:nbn:se:su:diva-156604 (URN)10.1002/ece3.3952 (DOI)000431987300013 ()29760891 (PubMedID)
Available from: 2018-05-28 Created: 2018-05-28 Last updated: 2019-04-04Bibliographically approved
2. Effects of temperature and hypoxia on respiration, photorespiration and photosynthesis of seagrasses
Open this publication in new window or tab >>Effects of temperature and hypoxia on respiration, photorespiration and photosynthesis of seagrasses
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(English)Manuscript (preprint) (Other academic)
Keywords
Zostera marina, Thalassia hemprichii, Photorespiration, Respiration, Apparent photosynthesis, Fv/F0, PAM fluorometry
National Category
Biological Sciences
Research subject
Plant Physiology
Identifiers
urn:nbn:se:su:diva-167751 (URN)
Available from: 2019-04-03 Created: 2019-04-03 Last updated: 2019-04-04Bibliographically approved
3. Seagrass productivity during temperature variations: estimation of a whole plant Q10 for respiration and photosynthesis in Zostera marina
Open this publication in new window or tab >>Seagrass productivity during temperature variations: estimation of a whole plant Q10 for respiration and photosynthesis in Zostera marina
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(English)Manuscript (preprint) (Other academic)
Keywords
Seagrass meadows, Zostera marina, Primary productivity, Respiration, Photosynthesis, Q10
National Category
Biological Sciences
Research subject
Plant Physiology
Identifiers
urn:nbn:se:su:diva-167752 (URN)
Available from: 2019-04-03 Created: 2019-04-03 Last updated: 2019-04-04Bibliographically approved
4. Calcifying algae modify the air-sea flux of CO2 in tropical seagrass meadows
Open this publication in new window or tab >>Calcifying algae modify the air-sea flux of CO2 in tropical seagrass meadows
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(English)Manuscript (preprint) (Other academic)
National Category
Biological Sciences
Research subject
Plant Physiology
Identifiers
urn:nbn:se:su:diva-167754 (URN)
Available from: 2019-04-03 Created: 2019-04-03 Last updated: 2019-04-04Bibliographically approved
5. Methane emission and sulphide levels increase in tropical seagrass sediments during temperature stress: a mesocosm experiment
Open this publication in new window or tab >>Methane emission and sulphide levels increase in tropical seagrass sediments during temperature stress: a mesocosm experiment
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(English)Manuscript (preprint) (Other academic)
National Category
Biological Sciences
Research subject
Plant Physiology
Identifiers
urn:nbn:se:su:diva-167757 (URN)
Available from: 2019-04-03 Created: 2019-04-03 Last updated: 2019-04-04Bibliographically approved
6. Seagrass cover reduces emissions of methane, nitrous oxide and sulphide levels in organic rich tropical seagrass sediments during daytime
Open this publication in new window or tab >>Seagrass cover reduces emissions of methane, nitrous oxide and sulphide levels in organic rich tropical seagrass sediments during daytime
(English)Manuscript (preprint) (Other academic)
Keywords
tidal variability, intertidal seagrass meadows, biogeochemical processes, Western Indian Ocean, methane, nitrous oxide and sulphide
National Category
Biological Sciences
Research subject
Plant Physiology
Identifiers
urn:nbn:se:su:diva-167758 (URN)
Available from: 2019-04-03 Created: 2019-04-03 Last updated: 2019-04-04Bibliographically approved

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