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Plant- and habitat productivity in a temperate seagrass system
Stockholm University, Faculty of Science, Department of Ecology, Environment and Plant Sciences.
Stockholm University, Faculty of Science, Department of Ecology, Environment and Plant Sciences.
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

Seagrass meadows are highly productive coastal habitats. Yet, little is known about the contribution of seagrass plants to the total seagrass habitat. To clarify the particular role of the seagrass plants for carbon capture in temperate environments, a one-year study was performed in seagrass meadows on the Swedish west coast. We aimed to assess the link between the net primary productivity of seagrass plants per se, the net production of the seagrass community and the net production of the entire system. To be able to predict effects of environmental changes on seagrass productivity, results were related to changes in water temperature, oxygen levels, light conditions and ice cover. Results showed large variations in net plant productivity across seasons, generally following light- and temperature variability, and ranging from very high (20.03 g C m-2 24h-1 ) in the summer to negative rates (-1.60 g C m-2 24h-1 ) in the least productive winter month. The patterns of variability in seagrass productivity were also influenced by depth- and site-specific dynamics in biomass. The high respiration of the benthic community did largely outbalance the productivity of the seagrass plants, probably as an effect of fast turnover rates. This resulted in an overall yearly low positive carbon balance of the entire seagrass system. Overall, the findings show that seagrass plants contribute substantially to the carbon capture in temperate seagrass habitats, but also that the rate of community respiration appears to be highly dependent on the degree of how much detritus material that is retained within the system. Thus, even though these seagrass systems are highly productive and may contain a large carbon stock, seagrass productivity per se seems not to be the most important determining factor for their carbon sink function.

Keywords [en]
Seagrass, Zostera marina, Productivity, Carbon capture, Inorganic carbon fluxes, Community respiration, PAM fluorometry, Seasonal patterns, Temperate environment
National Category
Biological Sciences
Research subject
Plant Physiology
Identifiers
URN: urn:nbn:se:su:diva-155870OAI: oai:DiVA.org:su-155870DiVA, id: diva2:1202696
Available from: 2018-04-30 Created: 2018-04-30 Last updated: 2020-02-06Bibliographically approved
In thesis
1. Seagrass productivity: from plant to system
Open this publication in new window or tab >>Seagrass productivity: from plant to system
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Seagrasses form one of the most productive habitats on earth and are recognized as very efficient carbon sinks. The levels and patterns of productivity within and across different seagrass systems vary widely due to natural or human-induced factors. Seagrass plants, being the foundation species of seagrass meadows, have a substitutional role as primary producers to the overall productivity of their habitat. Clarifying the variation in the carbon capture potential of these plants on physiological and ecological levels is essential to understand of the whole system’s carbon balance. In this thesis, the photosynthetic performance and productivity of seagrass plants were studied in relation to factors that have large impact on productivity, such as tissues age, season and water depth. Furthermore, the seagrass response, in terms of capacity to capture and sequester carbon, to human-induced stress factors such as shading and simulated grazing was evaluated in a tropical seagrass meadow. The research has included a multitude of seagrass productivity assessments from plant- to system level.

The results showed that age has a significant effect on the photosynthetic performance of the temperate seagrass Zostera marina L., both within a single shoot and between shoots. When comparing leaves among the same shoot, the photosynthetic capacity and efficiency were highest in mature tissues and significantly reduced in very young tissues as well as in tissues undergoing senescence. In response to high light stress, very young tissues seemed to cope better with dissipating excess light energy, which was demonstrated by the higher values of non-photochemical quenching (NPQ) observed compared to mature and senescent tissues. Such an effect was also observed when comparing the oldest and youngest shoots from the same genet; the youngest shoot showed higher ability to dissipate excess light energy compared to the oldest one, and might thus be able to better withstand light stress.

On a larger spatiotemporal scale, the areal productivity of seagrass plants was significantly affected by light availability and temperature, leading to a strong seasonal variation. In addition, depth had a strong site-specific effect on plant productivity in terms of biomass. On a yearly basis, productivity rates varied substantially, reaching up to 20 g C m-2 24h-1 in the summer months. This high carbon capture potential was, however, outbalanced by the high respiration rates of the benthic community. Overall, the whole system had a low but positive yearly carbon balance.

Both shading and simulated grazing negatively affected seagrass plants and the whole habitat after five months of experimental disturbance. On the plant level, photosynthesis, productivity and growth were all reduced. On the system level, a reduction in community productivity was recorded. The long-term refractory carbon was, however, not affected although erosion was observed in treatments subjected to simulated grazing.

In summary, this thesis has established that age, season, depth and exposure are factors highly responsible for natural variation in seagrass plant- and habitat productivity, and that seagrasses respond to human-induced stress by significantly reducing their productivity. Even though seagrass plants are generally capable of surviving stress periods, these results suggest that prolonged deteriorating stress conditions will lead to serious harm on the plants as well as the entire habitat, and thereby compromising the carbon burial capacity of the seagrass system.

Place, publisher, year, edition, pages
Stockholm: Department of Ecology, Environment and Plant Sciences, Stockholm University, 2018. p. 51
Keywords
Seagrass, Chlorophyll a fluorescence, Ageing, Productivity, Seasonality, Stress response
National Category
Botany Ecology
Research subject
Plant Physiology
Identifiers
urn:nbn:se:su:diva-155497 (URN)978-91-7797-116-0 (ISBN)978-91-7797-117-7 (ISBN)
Public defence
2018-06-08, 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 1: Manuscript. Paper 2: Manuscript.

Available from: 2018-05-16 Created: 2018-04-23 Last updated: 2018-05-09Bibliographically approved

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