Southern Madagascar rocky shores : a primary description and comparison with northern KwaZulu-Natal shores.

Rudman, Cleo (2017-03)

Thesis (MSc)--Stellenbosch University, 2017.

Thesis

ENGLISH ABSTRACT: The rocky shores of southern Madagascar are poorly studied with no publications describing intertidal biodiversity and community structure. This dissertation aimed to provide a first description of the rocky shore communities on the south-western shores of Madagascar and secondly to compare rocky shore communities of this region with northern KwaZulu- Natal. This study was part of a broader collaboration with the Suitcase Project which aimed to test the hypothesis that westward moving eddies may connect marine ecosystems in southern Madagascar and northern KwaZulu-Natal through the transport of larvae and propagules. The intertidal shores of exposed and sheltered shores in Madagascar had an unexpected small tidal range. Despite this narrow tidal range, the distribution patterns across the shore were similar to global patterns with distinct vertical zonation patterns, high species diversity and an abundance of algae within the low-shore. Across all the wave exposure levels, a significant effect of zonation was detected between mid-and low-shore especially among communities on the low shore. Two-way ANOVA detected that (a) in sheltered conditions, fewer grazers were recorded and (b) identified differences in algal cover at each exposure level which contributed significantly to the differences seen between the shore levels (mid and low-shore). Algae species were affected by wave exposure with a trend of decreasing abundance with increasing wave exposure. The possible contributing factors for the high algae abundance include wave exposure, human impacts, combined which limited tidal ranges. Understanding community composition and the factors which influence intertidal communities are important in managing these systems sustainably. The description of undocumented shores similar to these in southern Madagascar is a vital contribution to baseline studies in a developing country. This chapter aimed to address this need and has provided an important first description for the region, one which can form the foundation for future studies. Using the data collected in Chapter 2, Chapter 3 analysed community structure data from eight intertidal sampling sites in Madagascar and four sites in northern KwaZulu-Natal, across three exposure levels. The Madagascan shores were on average 86.47% dissimilar to shores in northern KwaZulu-Natal, with 29 taxa accounting for 90% of this dissimilarity. The overall numbers of species recorded on the low-shores for both Madagascar and KwaZulu-Natal were 56 species of which 26 of those species were present in both countries. The articulate coralline seaweed Jania sagittata and brown algae Sargassum elegans were the predominant species, characteristic of rocky shores in both countries. The Madagascan shores supported 21 distinct species in the low shore and only nine taxa were confined to northern KwaZulu-Natal. The key species defining the assemblages present on Madagascan shores were algae (particularly Arthrocardia spp., Spyridia hypnoides), whereas distinguishing species of northern KwaZulu-Natal included the polychaete Idanthyrsus pennatus and the zoanthid Palythoa nelliae. In addition, the KwaZulu-Natal shores featured three ascidian species that were absent from the Madagascan shores. The brown mussel Perna perna was the only filter feeding species that the two countries shared and this occurred at low abundances in both countries (less than 2% mean percentage cover). The low abundances of this key food species and the field observations of high subsistence fishing effort suggests that both southern Madagascar and northern KwaZulu-Natal shores are exposed to extensive harvesting pressures. Harvesting pressures on both shores could be masking the fact that P.perna have higher species abundance at high wave exposure. The overlap of communities between the countries, were within exposed and semi-exposed sites, these exposures had higher P.perna abundance. The results from this dissertation represent the first attempt to quantitatively examine rocky shore assemblages in contribution to a study of potential linkages between southern Madagascar and northern KwaZulu-Natal rocky shores. I quantitatively compared the rocky shore community structure between the shores of these countries and found the biocoenosis was similar but that there were significant differences between countries and within each country, shores differenced with wave exposure. This aligns with intertidal ecology elsewhere as shown by previous literature. In a global context, southern Madagascar falls within the Western Indo-Pacific and northern KwaZulu-Natal fall within the Indo-Pacific bioregion based on geographic proximity. The biogeographic province of southern Madagascar may not be part of the Western Indo-Pacific realm nor part of the tropical Western Indian Ocean Province but rather a region of overlap, in which both tropical and temperate species flourish. According to the species present on the Madagascan shores, there seems to be an overlap between Western Indo-Pacific and tropical Western Indian Ocean Province, which could be a major contributing factor to the differences in algal species. This work has implications for the biographic affinities and classification of regions and the species associated within those regions. The baseline data collected will contribute to further studies for KwaZulu-Natal and the first documented biodiversity and community structure study for the southern Madagascan region. In terms of connectivity between the countries the shores were less similar that expected with an overlap in species composition between semi-exposed and exposed shores. The low levels of similarity could be attributed to (1) the transport rate and frequency of eddies across the Mozambique Channel (2) the length of the larval and propagules phase of particular species and their ability to regenerate once on the other side and (3) both countries falling within realms of biogeographic overlap.

AFRIKAANSE OPSOMMING: Geen opsomming beskikbaar.

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