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Macroalgal community dynamics on coral reefs: Implications for management
Stockholm University, Faculty of Science, Department of Systems Ecology. (Marin ekotoxikologi)
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.
Keyword [en]
Macroalgae, Coral Reefs, Herbivory, Nutrients, Ecosystem change, Fish, Sea Urchins, East Africa
National Category
Biological Sciences
Research subject
Marine Ecotoxicology
Identifiers
URN: urn:nbn:se:su:diva-48281ISBN: 978-91-7447-200-4 (print)OAI: oai:DiVA.org:su-48281DiVA: diva2:376404
Public defence
2011-01-28, Nordenskiöldsalen, Geovetenskapens hus, Svante Arrhenius väg 12, Stockholm, 13:00 (English)
Opponent
Supervisors
Note

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 2: Accepted.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

Available from: 2011-01-03 Created: 2010-12-06 Last updated: 2013-12-09Bibliographically approved
List of papers
1. Top-down and bottom-up regulation of macroalgal communities – implications for coral reef management in Eastern Africa.
Open this publication in new window or tab >>Top-down and bottom-up regulation of macroalgal communities – implications for coral reef management in Eastern Africa.
2008 (English)Licentiate thesis, monograph (Other academic)
National Category
Ecology
Identifiers
urn:nbn:se:su:diva-15636 (URN)
Available from: 2008-12-07 Created: 2008-12-07 Last updated: 2010-12-16Bibliographically approved
2. Differences in top-down and bottom-up regulation of macroalgal communities between a reef crest and back reef habitat in Zanzibar
Open this publication in new window or tab >>Differences in top-down and bottom-up regulation of macroalgal communities between a reef crest and back reef habitat in Zanzibar
2011 (English)In: Estuarine, Coastal and Shelf Science, ISSN 0272-7714, E-ISSN 1096-0015, Vol. 91, no 4, 511-518 p.Article in journal (Refereed) Published
Abstract [en]

Coral reef ecosystems are subjected to intense pressure from growing coastal populations and subsequently increased nutrient loading and extraction of marine organisms. This development has altered top-down and bottom-up regulation of macroalgae in the reef system. The relative importance of these regulating forces is also influenced by environmental prerequisites, such as exposure to wave action and water motion. Thus, the present study tested the importance of top-down and bottom-up regulation, by manipulation of nutrient availability and grazer abundance, at one reef crest- and one back reef-site in Chwaka bay (Zanzibar, Tanzania). Wave action and water motion may regulate macroalgal communities by affecting the mobility of herbivores and availability of nutrients. The present study was conducted at the onset of the monsoon period, with a general decline of macroalgal cover and biomass in the region; positive effects on biomass development were therefore manifested in reduced decline and not in an actual increase. The experimental study showed that both caging and fertilization had significant impacts on macroalgal community composition but only caging showed any significant effects on biomass development. However, the influences of both these structuring forces were lower at the more exposed crest-site. This period was chosen as most similar studies have been conducted during growth season, often overlooking the studied period. Such previous studies have shown that herbivore exclusion increases macroalgal biomass, while the present study shows that it can also reduce biomass decline during the seasonal die-off by approx 50%. Together, these results suggest an overall larger macroalgal presence on the reef when herbivory is reduced. In general, our results propose that exposure to wave action and water motion functions as an important regulating factor, affecting macroalgal communities by influencing both top-down and bottom-up regulation. In turn, these results suggest that anthropogenic disturbances may have a greater impact on more sheltered coral reef habitats.

Keyword
macroalgae, grazing, nutrients, coral reefs, water motion, East Africa
National Category
Biological Sciences
Research subject
Marine Ecotoxicology
Identifiers
urn:nbn:se:su:diva-69450 (URN)10.1016/j.ecss.2010.12.003 (DOI)000288310300006 ()
Note

authorCount :4

Available from: 2012-01-13 Created: 2012-01-12 Last updated: 2017-12-08Bibliographically approved
3. Effects of estimated herbivory on the reproductivepotential of four East African algal species – a mechanismbehind ecosystem shifts on coral reefs?
Open this publication in new window or tab >>Effects of estimated herbivory on the reproductivepotential of four East African algal species – a mechanismbehind ecosystem shifts on coral reefs?
2007 (English)In: Hydrobiologia, ISSN 0018-8158, E-ISSN 1573-5117, Vol. 575, no 1, 57-68 p.Article in journal (Refereed) Published
Abstract [en]

The aim of this field study was toinvestigate effects of estimated fish- and seaurchin herbivory on the reproductive potential offour species of macroalgae; Halimeda macroloba(Decasine), H. renschii (Hauck), Turbinaria ornata(Turner) and Padina boergesenii (Allender etKraft). Fish and sea urchin herbivory were calculatedbased on reported consumption rates fortheir biomass estimates. We hypothesized thatreduced herbivory would increase algal size andthe reproductive potential, which may promotealgal recruitment and be one of the drivingmechanisms behind algal shifts and persistent algae-dominated reefs. Algae were investigated infield sites where the estimated fish- and or seaurchin herbivory differed. Our results suggest thatalgal fecundity of T. ornata and P. boergesenii arepositively correlated to their size. Fecundity ofT. ornata was higher and individuals grew largerin areas where estimated fish herbivory was lower.The two species of Halimeda grew larger andhad higher fecundity in areas where estimatedsea urchin herbivory was lower. P. boergeseniiresponded ambiguously to patterns in herbivory.Due to species-specific responses to differentherbivores, it is difficult to generalize abouteffects of overfishing on algal fecundity.

Keyword
Macroalgae, Herbivory, Reproduction, Coral Reefs, Ecosystem change
National Category
Biological Sciences
Identifiers
urn:nbn:se:su:diva-48271 (URN)10.1007/s10750-006-0282-1 (DOI)000242505000005 ()
Available from: 2010-12-06 Created: 2010-12-06 Last updated: 2017-12-11Bibliographically approved
4. Differences between fish and sea urchin herbivory – effects onmacroalgal abundance and influence on status of an EastAfrican coral reef
Open this publication in new window or tab >>Differences between fish and sea urchin herbivory – effects onmacroalgal abundance and influence on status of an EastAfrican coral reef
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Macroalgal removal by herbivores is a key mechanism influencing recovery potential ofcorals after a disturbance event in a reef ecosystem. Both fish and sea urchins are usuallyconsidered important herbivores maintaining a healthy reef, but grazing mechanisms andefficiency differ between them. This study was conducted in a Kenyan coral reef lagoon,using two sites with different herbivore community compositions as a model system. Theherbivore community at the first site is dominated by fish, whereas sea urchins dominatein the second site. Temporal variation of macroalgal distribution, biomass and communitycomposition was investigated at five occasions during a nine months period. The studyillustrates the influence that different herbivore community compositions may have oncoral reef status. Despite an almost 4 times higher estimated total herbivore consumptionwithin the fish-dominated park, macroalgal biomass was found to be up to 10 timeshigher than in the sea urchin-dominated unprotected site. Thus, sea urchins may notconsume as much algal biomass per individual, but seem to have a higher algal clearancecapacity in comparison to herbivorous fish. Such disparity between consumption andactual algal biomass removal could be the result of inter- and intraspecific differences ingrazing mechanisms as well as grazing behavior between grazer types in the two sites.Despite the higher macroalgal biomass, the fish dominated site has over time experienceda higher coral recovery, possibly resulting from a higher structural complexity,heterogeneous macroalgal distribution and the composition of the herbivore community.The results suggest that the fish dominated community does not ensure low algalbiomass, but may still increase the potential for coral recovery. The urchin dominatedcommunity on the other hand resulted in low macroalgal biomass, but may have a longerroute to recovery after disturbance

Keyword
Ecosystem change; Macroalgae; Herbivory; Fish; Sea Urchins; Coral Reefs
National Category
Biological Sciences
Identifiers
urn:nbn:se:su:diva-48279 (URN)
Available from: 2010-12-06 Created: 2010-12-06 Last updated: 2010-12-16Bibliographically approved
5. Regulation of macroalgal abundance on Western Indian Ocean coralreefs – Fishery closure induced phase shifts an indication of successfulmanagement?
Open this publication in new window or tab >>Regulation of macroalgal abundance on Western Indian Ocean coralreefs – Fishery closure induced phase shifts an indication of successfulmanagement?
(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
Ecosystem change; Macroalgae; Herbivory; Fish; Sea Urchins; Coral Reefs; Scale
National Category
Biological Sciences
Identifiers
urn:nbn:se:su:diva-48280 (URN)
Available from: 2010-12-06 Created: 2010-12-06 Last updated: 2010-12-16Bibliographically approved

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