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Anthropogenic Disturbances and Shifts in Tropical Seagrass Ecosystems
Stockholm University, Faculty of Science, Department of Systems Ecology.
2008 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Seagrasses constitute the basis for diverse and productive ecosystems worldwide. In East Africa, they provide important ecosystem services (e.g. fisheries) but are potentially threatened by increasing resource use and lack of enforced management regulations. The major aim of this PhD thesis was to investigate effects of anthropogenic distur-bances, primarily seaweed farming and coastal fishery, in East African seagrass beds. Seaweed farming, often depicted as a sustainable form of aquaculture, had short- and long-term effects on seagrass growth and abundance that cascaded up through the food web to the level of fishery catches. The coastal fishery, a major subsistence activity in the region, can by removing urchin predators indirectly increase densities of the sea urchin Tripneustes gratilla, which has overgrazed seagrasses in several areas. A study using simulated grazing showed that high magnitude leaf removal – typical of grazing urchins – affected seagrasses more than low magnitude removal, typical of fish grazing. Different responses in two co-occurring seagrass species furthermore indicate that high seagrass diversity in tropical seagrass beds could buffer overgrazing effects in the long run. Finally, a literature synthesis suggests that anthropogenic disturbances could drive shifts in seagrass ecosystems to an array of alternative regimes dominated by other or-ganisms (macroalgae, bivalves, burrowing shrimp, polychaetes, etc.). The formation of novel feedback mechanisms makes these regimes resilient to disturbances like seagrass recovery and transplantation projects. Overall, this suggests that resource use activities linked to seagrasses can have large-scale implications if the scale exceeds critical levels. This emphasizes the need for holistic and adaptive management at the seascape level, specifically involving improved techniques for seaweed farming and fisheries, protection of keystone species, and ecosystem-based management approaches.

Place, publisher, year, edition, pages
Stockholm: Systemekologiska institutionen , 2008. , 53 p.
Keyword [en]
Aquaculture, feedback mechanisms, Kenya, management, overgrazing, regime shift, resilience, seagrass, seaweed farming, sea urchins, Tanzania, trophic cascades, Zanzibar
National Category
Natural Sciences
Research subject
Marine Ecotoxicology
Identifiers
URN: urn:nbn:se:su:diva-7285ISBN: 978-91-7155-552-6 (print)OAI: oai:DiVA.org:su-7285DiVA: diva2:197989
Public defence
2008-02-08, Nordenskiöldsalen, Geovetenskapens hus, Svante Arrhenius väg 8 C, Stockholm, 09:00
Opponent
Supervisors
Available from: 2008-01-17 Created: 2008-01-08Bibliographically approved
List of papers
1. Differences in macrofaunal and seagrass assemblages in seagrass beds with and without seaweed farms
Open this publication in new window or tab >>Differences in macrofaunal and seagrass assemblages in seagrass beds with and without seaweed farms
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2005 In: Estuarine, Coastal and Shelf Science, ISSN 0272-7714, Vol. 63, no 3, 385-396 p.Article in journal (Refereed) Published
Identifiers
urn:nbn:se:su:diva-24639 (URN)
Note
Part of urn:nbn:se:su:diva-7285Available from: 2008-01-17 Created: 2008-01-08Bibliographically approved
2. Effects of tropical open-water seaweed farming on seagrass ecosystem structure and function
Open this publication in new window or tab >>Effects of tropical open-water seaweed farming on seagrass ecosystem structure and function
2006 In: Marine Ecology Progress Series, ISSN 0171-8630, Vol. 325, 73-84 p.Article in journal (Refereed) Published
Identifiers
urn:nbn:se:su:diva-24640 (URN)
Note
Part of urn:nbn:se:su:diva-7285Available from: 2008-01-17 Created: 2008-01-08Bibliographically approved
3. How do seaweed farms influence fishery catches in a seagrass-dominated setting in Chwaka Bay, Zanzibar?
Open this publication in new window or tab >>How do seaweed farms influence fishery catches in a seagrass-dominated setting in Chwaka Bay, Zanzibar?
2006 In: Aquatic Living Resources, ISSN 0990-7440, Vol. 19, no 2, 137-147 p.Article in journal (Refereed) Published
Identifiers
urn:nbn:se:su:diva-24641 (URN)
Note
Part of urn:nbn:se:su:diva-7285Available from: 2008-01-17 Created: 2008-01-08Bibliographically approved
4. Fishing, trophic cascades, and overgrazing of Kenyan seagrass beds
Open this publication in new window or tab >>Fishing, trophic cascades, and overgrazing of Kenyan seagrass beds
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(English)Manuscript (Other academic)
National Category
Environmental Sciences
Identifiers
urn:nbn:se:su:diva-24642 (URN)
Note

Part of urn:nbn:se:su:diva-7285

Available from: 2008-01-17 Created: 2008-01-08 Last updated: 2013-02-19
5. Physical responses of two co-occurring seagrasses to different grazing regimes
Open this publication in new window or tab >>Physical responses of two co-occurring seagrasses to different grazing regimes
Show others...
Manuscript (Other academic)
Identifiers
urn:nbn:se:su:diva-24643 (URN)
Note
Part of urn:nbn:se:su:diva-7285Available from: 2008-01-17 Created: 2008-01-08 Last updated: 2010-01-13Bibliographically approved
6. Seagrass loss and feedback mechanisms: multiple regimes in seagrass ecosystems
Open this publication in new window or tab >>Seagrass loss and feedback mechanisms: multiple regimes in seagrass ecosystems
Manuscript (Other academic)
Identifiers
urn:nbn:se:su:diva-24644 (URN)
Note
Part of urn:nbn:se:su:diva-7285Available from: 2008-01-17 Created: 2008-01-08 Last updated: 2010-01-13Bibliographically approved

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