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Differences in top-down and bottom-up regulation of macroalgal communities between a reef crest and back reef habitat in Zanzibar
Stockholm University, Faculty of Science, Department of Systems Ecology.
Stockholm University, Faculty of Science, Department of Systems Ecology.
Stockholm University, Faculty of Science, Department of Systems Ecology.
Stockholm University, Faculty of Science, Department of Systems Ecology.
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.

Place, publisher, year, edition, pages
2011. Vol. 91, no 4, 511-518 p.
Keyword [en]
macroalgae, grazing, nutrients, coral reefs, water motion, East Africa
National Category
Biological Sciences
Research subject
Marine Ecotoxicology
Identifiers
URN: urn:nbn:se:su:diva-69450DOI: 10.1016/j.ecss.2010.12.003ISI: 000288310300006OAI: oai:DiVA.org:su-69450DiVA: diva2:477598
Note

authorCount :4

Available from: 2012-01-13 Created: 2012-01-12 Last updated: 2017-12-08Bibliographically 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.
Keyword
Macroalgae, Coral Reefs, Herbivory, Nutrients, Ecosystem change, Fish, Sea Urchins, East Africa
National Category
Biological Sciences
Research subject
Marine Ecotoxicology
Identifiers
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)
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
2. Macroalgae in tropical seascapes: regulating factors and functions in the coastal ecosystem
Open this publication in new window or tab >>Macroalgae in tropical seascapes: regulating factors and functions in the coastal ecosystem
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Although macroalgae usually are inconspicuous on pristine coral reefs, they often thrive on reefs that are subjected to various types of anthropogenic disturbance. This thesis consists of five papers and investigates how biomass and composition of macroalgal communities on coral reefs are affected by regulating factors, such as nutrient availability, herbivory, substrate availability and hydrodynamic forces. In addition, ecological functions and potential impacts of both wild and farmed macroalgal communities are evaluated. Paper I describes a method for using macroalgal tissue nutrient concentrations as bioindicator for nutrient availability, with the possibility to map nutrient loading from larger coastal cities. Papers II and III are manipulative studies comparing top-down and bottom-up regulation of macroalgal communities, where herbivore consumption seems to be the main regulator of biomass whereas nutrient availability mainly influences community composition. Exclosure of large-bodied herbivores had a positive influence on algal biomass in both studies, and during different climatic periods. Paper III also includes the influence of hydrodynamic forces on algal community biomass and structure by comparing a reef crest and a back reef-habitat. Alterations of top-down and bottom-up regulation generally had a stronger effect within the protected back reef-habitat, suggesting that such environments may be more sensitive to anthropogenic influence. Paper IV confirms the general conclusions from papers II and III by studying macroalgal biomass and composition on reef sites with different environmental prerequisites. This study also supports the notion that herbivorous fish can suppress accumulation of macroalgal biomass if substrate availability is low, but not where coral cover is reduced and plenty of substrate is open to macroalgal colonization. The study also found a large temporal variation of macroalgal standing stock and associated nutrients at sites with low top-down regulation. Paper V evaluates potential impacts of seaweed farming on coral reefs and nutrients in the seascape by experimentally studying growth, survival and nutrient binding capacity of Eucheuma denticulatum. This study showed that seaweed farms counteract eutrophication through nutrient extraction and that the risk of farmed algae colonizing local reefs seems to be small as they were rapidly consumed. In conclusion, the studies in this thesis contribute to the understanding of macroalgal regulation and function in tropical seascapes, thereby adding to the knowledge base for coastal management.

Place, publisher, year, edition, pages
Stockholm: Department of Systems Ecology, Stockholm University, 2010. 37 p.
Keyword
Macroalgae, Coral reefs, Nutrients, Herbivory, Ecological functions, East Africa, Regulating factors, Anthropogenic influence, Seaweed farming, Bioindicator
National Category
Biological Sciences
Research subject
Marine Ecotoxicology
Identifiers
urn:nbn:se:su:diva-45951 (URN)978-91-7447-188-5 (ISBN)
Public defence
2010-12-16, De Geersalen, Geovetenskapens hus, Svante Arrhenius väg 14, Frescati, Stockholm, 09:30 (English)
Opponent
Supervisors
Note

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 3: Manuscript. Paper 4: Manuscript. Paper 5: Manuscript.

Available from: 2010-11-24 Created: 2010-11-17 Last updated: 2013-12-09Bibliographically approved

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