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Seagrasses and Eutrophication: Interactions between seagrass photosynthesis, epiphytes, macroalgae and mussels
Stockholm University, Faculty of Science, Department of Botany.
2011 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Seagrass meadows are highly productive, ecologically and economically valuable ecosystems. However, increased human activities along the coastal areas leading to processes such as eutrophication have resulted in the rapid loss and deterioration of seagrass ecosystems worldwide. This thesis focuses on the responses of seagrasses to increases in nutrients, subsequent increases in ephemeral algae, and changes in the physical-chemical properties of seawater induced by interaction with other marine biota. Both in situ and laboratory experiments conducted on the tropical seagrasses Cymodocea serrulata and Thalassia hemprichii revealed that increased concentrations of water column nutrients negatively affected seagrass photosynthesis by stimulating the growth of the epiphytic biomass on the seagrass leaves. Interaction between seagrasses and other marine organisms induced different responses in seagrass photosynthesis. Ulva intestinalis negatively affected the photosynthetic performance of the temperate seagrass Zostera marina both by reducing the light and by increasing the pH of the surrounding water. On the other hand, the coexistence of mussels Pinna muricata and seagrass Thalassia hemprichii enhanced the photosynthetic activity of the seagrass, but no effect on the mussels' calcification was recorded. This study demonstrates that seagrass productivity is affected by a multitude of indirect effects induced by nutrient over-enrichment, which act singly or in concert with each other. Understanding the responsive mechanisms involved is imperative to safeguard the ecosystem by providing knowledge and proposing measures to halt nutrient loading and to predict the future performance of seagrasses in response to increasing natural and human perturbations. 

Place, publisher, year, edition, pages
Stockholm: Department of Botany, Stockholm University , 2011. , 44 p.
Keyword [en]
CO2, epiphytes, eutrophication, mussels, pH, Photosynthetic activities, seagrasses, Ulva
National Category
Botany
Research subject
Plant Physiology
Identifiers
URN: urn:nbn:se:su:diva-55808ISBN: 978-91-7447-250-9OAI: oai:DiVA.org:su-55808DiVA: diva2:407289
Public defence
2011-05-06, Föresläsningssalen, Botanicum, Lilla Frescativägen 5, Stockholm, 13:00 (English)
Opponent
Supervisors
Projects
Swedish Agency for Research Cooperation (Sida/SAREC) marine bilateral programme
Note
At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Papers 1, 3 and 4: Submitted. Paper 2: Manuscript.Available from: 2011-04-14 Created: 2011-03-29 Last updated: 2011-04-13Bibliographically approved
List of papers
1. When Zostera marina is intermixed with Ulva, its photosynthesis is reduced by increased pH and lower light, but not by changes in light quality
Open this publication in new window or tab >>When Zostera marina is intermixed with Ulva, its photosynthesis is reduced by increased pH and lower light, but not by changes in light quality
(English)In: Aquatic Botany, ISSN 0304-3770Article in journal (Refereed) Submitted
Abstract [en]

In eutrophic coastal waters, seagrasses often become intermixed with opportunistic algae, such as different species of Ulva that grow on top of, or between shoots in Zostera marina beds. When this occurs, the algae can both reduce the amount of light reaching the seagrasses and also alter the quality of that light so that it becomes dominated by the green part of the spectrum. Since Ulva has an efficient photosynthetic carbon uptake, its photosynthesis can drastically increase the pH of the surrounding seawater, and thus create conditions where Zostera marina is unable to acquire inorganic carbon (Ci). To evaluate the effects of Ulva on the photosynthetic capacities of the temperate seagrass Zostera marina, we compared it in the laboratory under normal light and light filtered through layers of Ulva intestinalis, and repeated the experiments with the addition of pH-induced changes in carbon speciation and availability. One thallus of Ulva reduced photosynthetically available irradiance to underlying seagrass by about 50% and shifted the quality of remaining light towards the green part of the spectrum. Interestingly, there was no significant difference in photosynthetic performance between Zostera marina under normal light and under Ulva-filtered green light when adjusted to the same irradiance as for the control plants, indicating that the green spectrum transmitted through Ulva layers may be efficient in driving photosynthesis in the seagrass bed. On the other hand, algae-generated pH shifts had drastic negative effects on the photosynthesis of the seagrass.

Keyword
Eutrophication, Inorganic carbon, Light quality, pH, Photosynthesis, Zostera marina
National Category
Biological Sciences
Research subject
Plant Physiology
Identifiers
urn:nbn:se:su:diva-55865 (URN)
Available from: 2011-04-07 Created: 2011-03-30 Last updated: 2011-04-14Bibliographically approved
2. Nutrient enrichment affects the seagrass Cymodocea serrulata and induces changes to its epiphytic cyanobacterial community
Open this publication in new window or tab >>Nutrient enrichment affects the seagrass Cymodocea serrulata and induces changes to its epiphytic cyanobacterial community
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

To better understand how elevated water column nutrient levels affect the performance of the seagrass Cymodocea serrulata as well as the composition and density of its associated epiphytes, two sets of experiments were carried out where nutrient concentration were manipulated in a flow though system containing seagrasses. The photosynthetic performance, growth characteristics and nutrient content (N:P) were followed for C. serrulata. Simultaneously the biomass, species composition for the epiphytic cyanobacteria, in particular diazotrophs was monitored. The photosynthetic capacity of seagrasses decreased with increase in nutrient concentrations and exposure time. Nutrient contents of seagrass leaves and epiphytes decreased after nutrient addition. A higher diversity of both heterocystous and non-heterocystous cyanobacteria was observed in the experimental seagrasses as compared with natural field samples. Many of the cyanobacterial sequences retrieved represented uncultured and potentially novel diazotrophic phylotypes. Diel nitrogenase activity measurements verified the presence of a distinct proportion of diazotrophs, which was negatively affected by moderate nutrient levels. These results demonstrate that seagrasses were physiologically stressed by the increased nutrient level as revealed by low maximum quantum yields, although the effect was not instant. In contrast the epiphytes whose response was apparent during the short term exposure to moderate nutrient concentration which also promoted rapid change in their composition.

Keyword
Cymodocea serrulata, diazotrophy, epiphyte, cyanobacteria, nutrient enrichment; photosynthesis, seagrasses
National Category
Botany
Research subject
Plant Physiology
Identifiers
urn:nbn:se:su:diva-55868 (URN)
Available from: 2011-04-06 Created: 2011-03-30 Last updated: 2011-04-13Bibliographically approved
3. Photosynthetic performance, epiphyte biomass and nutrient content of two seagrass species in two areas with different level of nutrients along the Dar es Salaam coast
Open this publication in new window or tab >>Photosynthetic performance, epiphyte biomass and nutrient content of two seagrass species in two areas with different level of nutrients along the Dar es Salaam coast
(English)In: Aquatic conservation, ISSN 1052-7613, E-ISSN 1099-0755Article in journal (Refereed) Submitted
Abstract [en]

Heavy nutrient loads in coastal waters often lead to excessive growth of micro- and macroalgal epiphytes on seagrass leaves, with varying effects on the underlying seagrasses. This study evaluates the photosynthetic performance, epiphytic biomass, and tissue nutrient content of two common tropical seagrasses, Cymodocea serrulata and Thalassia hemprichii, in two intertidal areas along the Dar es Salaam coast in the Indian ocean; Ocean Road (near the city centre, nutrient-rich) and Mjimwema (south of the centre, nutrient-poor). Epiphyte biomass was significantly higher at the nutrient-rich site, and epiphytes were associated with reduced photosynthetic performance in both seagrass species at both sites. Likewise, nitrogen and phosphorus tissue content was higher in both species at the nutrient-rich site than at the nutrient-poor site, further illustrating the documented difference in nutrients between the two areas. Epiphytic species composition on the seagrass leaves varied between seagrass species and between sites. Cymodocea serrulata had a higher number of epiphytic species at Mjimwema than at Ocean Road, while Thalassia hemprichii had more epiphytic species at Ocean Road than at Mjimwema. Our results indicate that seagrass photosynthetic performance, epiphytic biomass and nutrient content of the seagrass are influenced by nutrient gradients in the water.

National Category
Botany
Research subject
Plant Physiology
Identifiers
urn:nbn:se:su:diva-55839 (URN)
Available from: 2011-04-06 Created: 2011-03-29 Last updated: 2011-04-14Bibliographically approved
4. Interactions between seagrasses and mussels: CO2, pH and calcification
Open this publication in new window or tab >>Interactions between seagrasses and mussels: CO2, pH and calcification
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(English)In: Estuarine, Coastal and Shelf Science, ISSN 0272-7714, E-ISSN 1096-0015Article in journal (Refereed) Submitted
Abstract [en]

Mussels and other bivalves commonly found in tropical seagrass meadows are thought to increase seagrass productivity, and seagrass photosynthesis, through raising the pH of the surrounding water, has been shown to increase rates of calcification in calcareous algae. The effects of seagrass-driven increases in pH on mussel calcification and possible feedback effects of mussel metabolism on seagrass photosynthesis were studied in a seagrass bed on the south-western coast of Zanzibar, Tanzania. Seagrasses and mussels (Pinna muricata) were enclosed, separately or together, in transparent plastic cylinders. The pH and photosynthesis were measured and seawater samples were taken from the experimental cylinders to determine total alkalinity and total inorganic carbon concentration. Cylinders containing only sediments were exposed to light and dark and used as controls. The results showed no effects of increased pH on calcification rates in the mussels. However, photosynthetic rates of the seagrass Thalassia hemprichii rose by up to 15% in the presence of mussels, possibly as a result of water stirring caused by the mussels’ filter feeding and/or CO2 released by their respiration.

Keyword
Inorganic carbon, mussels, pH, photosynthesis, seagrasses
National Category
Botany
Research subject
Plant Physiology
Identifiers
urn:nbn:se:su:diva-55866 (URN)
Available from: 2011-04-06 Created: 2011-03-30 Last updated: 2011-04-14Bibliographically approved

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