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Effects of Marine Protected Areas on Tropical Seagrass Ecosystems
Stockholm University, Faculty of Science, Department of Ecology, Environment and Plant Sciences.ORCID iD: 0000-0002-3973-1703
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Seagrass beds are highly productive coastal ecosystems that sustain a rich and diverse associated fauna and flora. Increasing anthropogenic pressures threaten seagrass ecosystems and have already led to major seagrass losses across the world. Marine Protected Areas (MPAs) have become one of the key strategies to manage coastal ecosystems and associated resources worldwide and have been often shown to successfully protect marine ecosystems. However, relatively few studies have assessed the effects of MPAs on seagrass ecosystems, and there are indications that MPAs may not be able to fully protect seagrasses, especially from disturbances originating outside their boundaries. Within this context, this thesis aimed to investigate the direct and indirect effects (those mediated by biotic interactions) of MPAs on tropical seagrasses, associated fish communities, and ecosystem processes.

The thesis consists of three parts. First, we used 10-years of seagrass monitoring data within a MPA to evaluate the temporal variability in seagrass cover and species composition in relation to changes in environmental conditions (Paper I). Second, we investigated the potential of MPAs to enhance the temporal stability of seagrass ecosystems using a 10-month field study. We surveyed seagrass-associated fish communities (Paper II) and estimated seagrass growth and herbivory rates (Paper III) during three different seasons within MPAs and unprotected sites. Finally, to evaluate the effects of MPAs and land-use on seagrass ecosystems we surveyed seagrass species and trait composition within government-managed MPAs, community-managed MPAs, and unprotected sites (Paper IV).

The seagrass bed monitored in Paper I showed a high temporal and spatial variability, with a temporal decline in cover and change in species composition, followed by a period of recovery. This pattern could not be associated with any of the climate and tidal variables considered, suggesting that potential drivers of decline may have originated outside MPA boundaries. The results from the seasonal field study showed that MPAs increased the temporal stability of seagrass-associated fish communities, particularly juvenile fish (Paper II), and strengthened a positive link between herbivorous fish, herbivory rates, and seagrass growth (Paper III), suggesting the presence of a positive feedback that promotes stability. Finally, MPAs affected seagrass species and trait composition (by selecting for more stress-sensitive species) but did not seem to be able to protect seagrasses from land-use effects, with seagrasses showing similar changes in species and trait composition within and outside MPAs (Paper IV). Considering these results, this thesis builds to a body of literature indicating that MPAs alone may not be sufficient to protect seagrass ecosystems and that improved management strategies may be necessary to preserve these important coastal habitats.

Place, publisher, year, edition, pages
Stockholm: Department of Ecology, Environment and Plant Sciences, Stockholm University , 2018. , p. 54
Keywords [en]
coastal ecosystems, seagrass, marine protected areas, management, conservation, fish, herbivory, Western Indian Ocean, East Africa, tropical
National Category
Biological Sciences Ecology
Research subject
Marine Ecology
Identifiers
URN: urn:nbn:se:su:diva-154966ISBN: 978-91-7797-268-6 (print)ISBN: 978-91-7797-269-3 (electronic)OAI: oai:DiVA.org:su-154966DiVA, id: diva2:1196100
Public defence
2018-06-05, Vivi Täckholmssalen (Q-salen), NPQ-huset, Svante Arrhenius väg 20, Stockholm, 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 1: Manuscript. Paper 3: Manuscript. Paper 4: Manuscript.

Available from: 2018-05-09 Created: 2018-04-09 Last updated: 2018-05-04Bibliographically approved
List of papers
1. Monitoring of a protected multi-specific tropical seagrass meadow reveals a pattern of decline and recovery
Open this publication in new window or tab >>Monitoring of a protected multi-specific tropical seagrass meadow reveals a pattern of decline and recovery
Show others...
(English)Manuscript (preprint) (Other academic)
Abstract [en]

In a changing environment, there is an increasing interest to monitor ecosystems to understand their responses to environmental change. Seagrass meadows are highly important ecosystems, but at the same time they are under a constant threat from human activities, as well as climate impacts, and marked declines have been observed worldwide. Despite increasing efforts, monitoring of multi-specific tropical seagrass meadows is scarce, particularly in developing regions. Here we analysed data from the first 10 years of a monitoring programme in a marine protected area in Zanzibar (Tanzania) to assess temporal changes in seagrass cover and species composition and to detect potential drivers of change. The seagrass meadow experienced a strong gradual decline in seagrass cover and changes in species composition, followed by a period of recovery. However, the timing and length of these temporal patterns varied in space (between transects). Of the climate variables considered, cloud cover, temperature, storm occurrence, sunspot activity and the height of the diurnal low tide seemed to influence seagrass cover, although only to a small extent, suggesting that the monitored seagrass meadow may be influenced by other unmeasured factors. Considering our results, seagrass meadows seem to be highly dynamic at small spatial scales even in the absence of major local anthropogenic impacts. Further monitoring programmes should be developed in the region to gain a better understanding of seagrass temporal variability and causes of change.

National Category
Biological Sciences
Research subject
Marine Ecology
Identifiers
urn:nbn:se:su:diva-154964 (URN)
Available from: 2018-04-09 Created: 2018-04-09 Last updated: 2018-04-10Bibliographically approved
2. Marine protected areas increase temporal stability of community structure, but not density or diversity, of tropical seagrass fish communities
Open this publication in new window or tab >>Marine protected areas increase temporal stability of community structure, but not density or diversity, of tropical seagrass fish communities
2017 (English)In: PLoS ONE, ISSN 1932-6203, E-ISSN 1932-6203, Vol. 12, no 8, article id e0183999Article in journal (Refereed) Published
Abstract [en]

Marine protected areas (MPAs) have been shown to increase long-term temporal stability of fish communities and enhance ecosystem resilience to anthropogenic disturbance. Yet, the potential ability of MPAs to buffer effects of environmental variability at shorter time scales remains widely unknown. In the tropics, the yearly monsoon cycle is a major natural force affecting marine organisms in tropical regions, and its timing and severity are predicted to change over the coming century, with potentially severe effects on marine organisms, ecosystems and ecosystem services. Here, we assessed the ability of MPAs to buffer effects of monsoon seasonality on seagrass-associated fish communities, using a field survey in two MPAs (no-take zones) and two unprotected (open-access) sites around Zanzibar (Tanzania). We assessed the temporal stability of fish density and community structure within and outside MPAs during three monsoon seasons in 2014-2015, and investigated several possible mechanisms that could regulate temporal stability. Our results show that MPAs did not affect fish density and diversity, but that juvenile fish densities were temporally more stable within MPAs. Second, fish community structure was more stable within MPAs for juvenile and adult fish, but not for subadult fish or the total fish community. Third, the observed effects may be due to a combination of direct and indirect (seagrass-mediated) effects of seasonality and, potentially, fluctuating fishing pressure outside MPAs. In summary, these MPAs may not have the ability to enhance fish density and diversity and to buffer effects of monsoon seasonality on the whole fish community. However, they may increase the temporal stability of certain groups, such as juvenile fish. Consequently, our results question whether MPAs play a general role in the maintenance of biodiversity and ecosystem functioning under changing environmental conditions in tropical seagrass fish communities.

National Category
Biological Sciences
Research subject
Marine Ecology
Identifiers
urn:nbn:se:su:diva-147901 (URN)10.1371/journal.pone.0183999 (DOI)000408693600076 ()28854231 (PubMedID)
Available from: 2017-10-19 Created: 2017-10-19 Last updated: 2018-04-12Bibliographically approved
3. Fishing weakens a positive link between herbivore abundance and plant growth in tropical seagrass beds
Open this publication in new window or tab >>Fishing weakens a positive link between herbivore abundance and plant growth in tropical seagrass beds
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Herbivory is a key process influencing the structure and function of both terrestrial and aquatic systems. In seagrass ecosystems, moderate levels of herbivory may stimulate plant growth, compensating for the loss of eaten tissue. However, the relationship between herbivory and seagrass growth can be influenced by an array of factors, such as seasonality, herbivore abundance, and presence of epiphytes, many of which can be directly or indirectly affected by human activities like fishing. Here, we used data from a multi-season field survey in fished and protected seagrass beds to assess how fishing and seasonality affect the link between herbivores, herbivory, and plant growth in seagrasses. Path analyses revealed an interactive effect of seasonality and protection. In protected seagrass beds, seasonally high herbivore abundance positively affected herbivory rates, which in turn enhanced seagrass growth. This link was however not apparent in seagrass beds subjected to fishing activities. At the same time, seasonality effects seemed stronger in fished areas, suggesting that in addition to weakening a positive herbivory-plant growth interaction, fishing increases temporal instability of ecosystems. Our results highlight the need for evaluating not only the direct effects of fisheries exploitation on fish populations, but also the potential indirect effects on ecosystems, to improve fisheries management.

National Category
Biological Sciences
Research subject
Marine Ecology
Identifiers
urn:nbn:se:su:diva-154965 (URN)
Available from: 2018-04-09 Created: 2018-04-09 Last updated: 2018-04-10Bibliographically approved
4. Additive effects of marine protected areas and land-use on inter- and intraspecific trait variability in tropical seagrass assemblages
Open this publication in new window or tab >>Additive effects of marine protected areas and land-use on inter- and intraspecific trait variability in tropical seagrass assemblages
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Marine ecosystems are under increasing human pressure and therefore in need of effective management. Marine protected areas (MPAs) can reduce effects of local disturbances (e.g. fishing), but their ability to buffer stressors originating outside MPA borders (e.g. runoff) is more unclear. Moreover, while MPAs are well-known to affect species composition, we know much less about their effects on organisms’ traits (physiological, morphological, and/or behavioural characteristics), which in turn dictate how organisms respond to stressors and influence ecosystem processes and services. Here, we conducted a field survey in southern Kenya to assess the single and joint (interactive) effects of MPAs and land-use on species and trait composition of seagrass assemblages; a key group of habitat-forming plants in shallow coastal areas. We measured five morphological traits on multispecies seagrass assemblages (leaf length and width, number of leaves per shoot, and above- and below-ground biomass) within three types of sites: government-managed MPAs, community-managed MPAs, and unprotected areas. Using single- and multi-trait statistical analyses, we found that both MPAs and land-use influence seagrass species and trait composition. Changes in community-level traits were mostly explained by species turnover. However, management and land-use also had a direct influence on trait composition, with MPAs promoting wider leaves and higher above-ground biomass, while land-use mostly influenced seagrass leaf length and below-ground biomass. Moreover, even though there was an interactive effect of management and land-use, the largest and oldest MPAs did not seem to buffer effects of intense land-use. In conclusion, it appears that MPAs influence seagrass assemblages by reducing local disturbances, but do not buffer spatially distant land-use effects. Consequently, to sustain seagrass ecosystems and the important services they support, there is a need for a more integrated coastal zone management that regulates resource use and human impacts in both the marine and the terrestrial parts of tropical coastal zones.

National Category
Biological Sciences
Research subject
Marine Ecology
Identifiers
urn:nbn:se:su:diva-154962 (URN)
Available from: 2018-04-09 Created: 2018-04-09 Last updated: 2018-04-10Bibliographically approved

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