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Regulation of macroalgal abundance on Western Indian Ocean coralreefs – Fishery closure induced phase shifts an indication of successfulmanagement?
Stockholm University, Faculty of Science, Department of Systems Ecology.
(Marine Programs, Wildlife Conservation Society, Bronx, NY)
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Grazing by herbivores is an important mechanism governing macroalgal distribution andbiomass on coral reefs. However, as feeding efficiency, preferences, mechanisms andeffects differs between grazers, the composition of the herbivore community determinesthe rates and impact of algal removal. Furthermore, environmental conditions such assubstrate availability can further affect the effects of herbivory, suggesting great spatialvariation in removal potential. Variation also occurs on a temporal scale as the herbivorecommunity and live coral cover are altered with recovering coral and fish populations.Thus, if the herbivore community changes across a recovery period, a phase shift mayoccur due to changing feedings rates and impacts. This study, therefore, investigated therelative importance of herbivorous fish and sea urchins, two groups with different feedingmethods and impacts, over different spatial and temporal scales in western Indian Oceanreefs that are mostly degraded through fishing. Overall, our results suggest that herbivoryby sea urchins is more important in controlling algal abundance than fish in the studiedregion. Sea urchins seem to be able to influence cover and community composition ofmacroalgae even though substrate available for macroalgal colonization is generally high.This same pattern was found at all investigated scales (the region as a whole, withinKenya and over time in Mombasa Marine Park). Furthermore, our results indicate thatincreasing macroalgal abundance may occur following the establishment of a fisheriesclosure, with declining sea urchin and recovering fish abundance. This may be a result ofweak recovery of herbivorous fishes in a seascape of heavy fishing but suggests thateliminating fishing of herbivores in small areas may not be able to recover intensegrazing and reduce macroalgal abundance.

Keyword [en]
Ecosystem change; Macroalgae; Herbivory; Fish; Sea Urchins; Coral Reefs; Scale
National Category
Biological Sciences
URN: urn:nbn:se:su:diva-48280OAI: diva2:374820
Available from: 2010-12-06 Created: 2010-12-06 Last updated: 2010-12-16Bibliographically approved
In thesis
1. Macroalgal community dynamics on coral reefs: Implications for management
Open this publication in new window or tab >>Macroalgal community dynamics on coral reefs: Implications for management
2011 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Although rather inconspicuous on healthy coral reefs, macroalgae form the basis of coral food webs. Today, macroalgae are generally increasing and many reefs undergo transitions from coral to macroalgal dominance resulting from e.g. enhanced nutrient loading or increased fishing.

This thesis aims to investigate the relative importance of top-down and bottom-up regulation, and different herbivore types, on macroalgal distribution, fecundity and community composition on coral reefs. Papers I and II indicate that macroalgal abundance in a coral reef system is largely governed by top-down regulation through grazing by herbivores, while bottom-up regulation through enhanced nutrient availability rather influence algal species composition. Paper II also shows that these regulating effects are not as evident in an area with relatively strong water motion, suggesting that impacts of anthropogenic disturbance may be site-specific. Paper III shows that herbivory is an important factor influencing macroalgal growth and subsequent reproduction. Furthermore, Paper IV and V conclude that efficiency in removing macroalgal biomass is dependent on the type of dominant herbivore, where sea urchins seem to be more effective than fish. Paper IV indicates a seasonal variation in macroalgal biomass and distribution in a small geographic scale but with relatively high temporal resolution. Paper V on the other hand shows these same effects, but with a focus on geographic variation, including a large part of the East African region, as well as between year temporal variations in Kenya. Together, results from the two latter studies indicate that herbivory by fish may not be able to prevent a macroalgal bloom in a degraded system where substrate availability for algal colonization is high, but that it may still facilitate coral recovery over time. Thus, a large algal biomass may not necessarily indicate a reef beyond the possibility of recovery.

Place, publisher, year, edition, pages
Stockholm: Department of Systems Ecology, Stockholm University, 2011. 46 p.
Macroalgae, Coral Reefs, Herbivory, Nutrients, Ecosystem change, Fish, Sea Urchins, East Africa
National Category
Biological Sciences
Research subject
Marine Ecotoxicology
urn:nbn:se:su:diva-48281 (URN)978-91-7447-200-4 (ISBN)
Public defence
2011-01-28, Nordenskiöldsalen, Geovetenskapens hus, Svante Arrhenius väg 12, Stockholm, 13:00 (English)

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

Available from: 2011-01-03 Created: 2010-12-06 Last updated: 2013-12-09Bibliographically approved

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