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Macroalgae in tropical seascapes: regulating factors and functions in the coastal ecosystem
Stockholm University, Faculty of Science, Department of Systems Ecology. (Marine Ecotoxicology)
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 [en]
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: urn:nbn:se:su:diva-45951ISBN: 978-91-7447-188-5 (print)OAI: oai:DiVA.org:su-45951DiVA: diva2:370545
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
List of papers
1. Tissue Nutrient Content in Ulva spp. (Chlorophyceae) as Bioindicator for Nutrient Loading Along the Coast of East Africa
Open this publication in new window or tab >>Tissue Nutrient Content in Ulva spp. (Chlorophyceae) as Bioindicator for Nutrient Loading Along the Coast of East Africa
2009 (English)In: The Open Environmental & Biological Monitoring Journal, ISSN 1875-0400, Vol. 2, 11-17 p.Article in journal (Refereed) Published
Abstract [en]

Eutrophication is an increasingly occurring problem that causes degradation of coastal ecosystems worldwide.Traditional monitoring of dissolved nutrients in the water column can be complicated by short term fluctuations, especiallywhen levels of nutrients are low and turnover rapid. A proposed alternative method is the use of macroalgal tissuenutrient concentrations for indication of ambient nutrient availability, as they integrate nutrients over time. This studyevaluates the use of macroalgae within the genus Ulva (Chlorophyceae), regarding their ability to reflect nutrient gradientsoff the coast of East Africa (i.e. Kenya and Tanzania). Ulva was able to reflect nitrogen (N) gradients from all three outletsources, with up to 90% higher tissue N levels in vicinity of major cities compared to adjacent areas. Together with resultsfrom nutrient- uptake and fluctuation experiments, this study suggests that macroalgal tissue nutrient content has potentialas a complement to regular water nutrient sampling.

Keyword
Macroalgae, Eutrophication, Ulva, Nutrients, Monitoring, East Africa
Identifiers
urn:nbn:se:su:diva-45782 (URN)
Available from: 2010-11-11 Created: 2010-11-11 Last updated: 2010-11-17Bibliographically approved
2. Top–down and bottom–up regulation of macroalgal community structure on a Kenyan reef
Open this publication in new window or tab >>Top–down and bottom–up regulation of macroalgal community structure on a Kenyan reef
2009 (English)In: Estuarine, Coastal and Shelf Science, ISSN 0272-7714, E-ISSN 1096-0015, Vol. 84, no 3, 331-336 p.Article in journal (Refereed) Published
Abstract [en]

Top–down and bottom–up regulation in the form of grazing by herbivores and nutrient availability areimportant factors governing macroalgal communities in the coral reef ecosystem. Today, anthropogenicactivities, such as over-harvesting of herbivorous fish and sea urchins and increased nutrient loading, arealtering the interaction of these two structuring forces. The present study was conducted in Kenya andinvestigates the relative importance of herbivory and nutrient loading on macroalgal communitydynamics, by looking at alterations in macroalgal functional groups, species diversity (H0) and biomasswithin experimental quadrats. The experiment was conducted in situ for 42 days during the dry season.Cages excluding large herbivorous fish and sea urchins were used in the study and nutrient addition wasconducted using coated, slow-release fertilizer (nitrogen and phosphorous) at a site where herbivory isgenerally low and nutrient levels are relatively high for the region. Nutrient addition increased tissuenutrient content in the algae, and fertilized quadrats had 24% higher species diversity. Herbivoreexclusion resulted in a 77% increase in algal biomass, mainly attributable to a >1000% increase in corticatedforms. These results are in accordance with similar studies in other regions, but are unique in thatthey indicate that, even when prevailing nutrient levels are relatively high and herbivore pressure isrelatively low, continued anthropogenic disturbance results in further ecological responses and increasedreef degradation.

Keyword
macroalgae, grazing, nutrients, coral reefs, community composition, reef degradation
National Category
Biological Sciences
Identifiers
urn:nbn:se:su:diva-45795 (URN)10.1016/j.ecss.2009.03.033 (DOI)000270122400006 ()
Available from: 2010-11-11 Created: 2010-11-11 Last updated: 2017-12-12Bibliographically approved
3. 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
4. Regulation of macroalgal biomass accumulation and community structure by herbivory and nutrients on Kenyan coral reefs
Open this publication in new window or tab >>Regulation of macroalgal biomass accumulation and community structure by herbivory and nutrients on Kenyan coral reefs
(English)Manuscript (preprint) (Other academic)
Keyword
Macroalgae, Nutrients, Herbivory, Coral reefs, East Africa
Identifiers
urn:nbn:se:su:diva-45806 (URN)
Available from: 2010-11-11 Created: 2010-11-11 Last updated: 2010-11-17Bibliographically approved
5. Seaweed farming in the tropical seascape – implications for coral reefs and nutrients in Chwaka Bay, Tanzania
Open this publication in new window or tab >>Seaweed farming in the tropical seascape – implications for coral reefs and nutrients in Chwaka Bay, Tanzania
(English)Manuscript (preprint) (Other academic)
Keyword
Aquaculture, Seaweed farming, Macroalgae, Eucheuma denticulatum, Growth, Nutrient extraction, Coral reef, Herbivory, Top-down control
Identifiers
urn:nbn:se:su:diva-45807 (URN)
Available from: 2010-11-11 Created: 2010-11-11 Last updated: 2010-11-17Bibliographically approved

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