Seaweeds in Two Oceans: Beta-Diversity

Albertus J. Smit,John J. Bolton,Robert J. Anderson

doi:10.3389/fmars.2017.00404

Abstract

Several species assembly mechanisms have been proposed to structure ecological communities. We assess the biogeography of seaweeds along 2,700 km of South Africa's coastline in relation to a thermal gradient produced by the Agulhas Current, and contrast this with the environmental structure created by the Benguela Current. We subdivided the coastline into `bioregions' to examine the regional patterning. To investigate the assembly mechanisms, we decomposed Sorensen's Beta-diversity into `turnover' (Bsim) and `nestedness-resultant' (Bsne) dissimilarities, and used distance-based redundancy analysis (db-RDA) to relate them to the Euclidian thermal difference, dE, and geographical distance. Moran's eigenvector maps (MEM) were used as an additional set of spatial constraints. Variation partitioning was then used to find the relative strengths of thermal and spatially-structured thermal drivers. Spatial and environmental predictors explained 97.9% of the total variation in Bsim and the thermal gradient accounted for 84.2% of this combined pool. Bsim was the major component of overall Beta-diversity in the Agulhas Current region, suggesting niche influences (environmental sorting) as dominant assembly process there. The much weaker thermal gradient in the Benguela Current-influenced region resulted in a high amount of Bsne that could indicate neutral assembly processes. The intensification of upwelling during the mid-Pliocene 4.6--3.2 Ma (i.e. historical factors) were likely responsible for setting up the strong disjunction between the species-poor west coast and species-rich south and east coast floras, and this separation continues to maintain two systems of community structuring mechanisms in the Atlantic and Indian Ocean influenced sides of South Africa.

Highlights

The assembly processes that structure biodiversity across a range of scales form the theme of macroecology (Chave, 2013)
Using a detailed data set of seaweed presence and absence records coupled with coastal in situ seawater temperature climatologies that are able to resolve the coastal zone, we investigated the thermal properties and species composition of 58 coastal sections spaced around the South African coastline
Continuous sampling over three decades has resulted in a robust α-diversity data set, and it is still different from the animal data: fish and invertebrates become more abundant per section on the east coast but the seaweeds do not

Summary

Introduction

The assembly processes that structure biodiversity across a range of scales form the theme of macroecology (Chave, 2013). This paper deals with the composition of seaweed assemblages along the ~2,900 km South African coastline, including the identification, description and explanation for the spatially-structured patterns at scales from 100s to 1,000s of kilometers. Inshore conditions along this coastline range from cool through warm temperate to fringe tropical (Bolton and Anderson, 2004; Bolton et al, 2004), and are influenced by two major ocean currents in two oceans that set up a strong thermal gradient along the shore (Smit et al, 2013). Cognisance of spatial scaling should be deeply rooted in the study of biodiversity more generally, but such questions have only recently begun to be addressed (Barton et al, 2013)

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Discussion

Conclusion