MARINE ECOLOGY PROGRESS SERIES Vol. 311: 15–28, 2006 Published April 13 Mar Ecol Prog Ser Spatial patterns and response to wave exposure of shallow water algal assemblages across the Canarian Archipelago: a multi-scaled approach Fernando Tuya1, 2,*, Ricardo J. Haroun1 1BIOGES, Department of Biology, Campus Tafira, University of Las Palmas de Gran Canaria, 35017 Las Palmas, Canary Islands, Spain 2Present address: Center for Ecosystem Management, Edith Cowan University, 100 Joondalup Drive, Joondalup, Western Australia 6027, Australia ABSTRACT: We conducted a mensurative survey to investigate spatial variability and the effect of wave exposure at a range of spatial scales including islands (100s of km apart), locations within islands (10s of km apart), and sites within locations (100s of m apart), on the composition, abundance and distribution of shallow water algal assemblages across subtidal hard bottoms of the Canarian Archipelago (eastern Atlantic). A multi-scaled hierarchical sampling design provided the framework for quantifying the variation among samples due to spatial scale and level of wave exposure. Ran- domly placed 50 × 50 cm squares were deployed in shallow rocky-reefs to assess community struc- ture and dominance. Non-parametric multivariate techniques, as well as univariate tests, provided evidence to collectively suggest that shallow water algal assemblages differed between protected (leeward) and exposed (windward) shores, with a consistency of effects across islands, while differ- ent spatial scales were also involved in the variability and patchiness of these assemblages. In this sense, differences were clearly taxon and/or group-specific. In general, the presence and abundance of frondose fucoid species was greater on exposed shores than on protected shores, whereas turf- algae dominated along the protected shores of each island. Dissimilarities between islands with regard to the overall algal assemblage generally increased with the distance between islands. In par- ticular, the presence and abundance of fucoid species was greater in the eastern islands, while turf and bush-like algae increased in the western islands. The large-scale gradient of the oceanographic conditions in an east-to-west direction across the Canarian Archipelago provided a partial explana- tion for this observation although some inconsistencies were observed in the overall regional pattern. KEY WORDS: Algae · Phytobenthic assemblages · Hierarchical design · Spatial variability · Wave exposure · PERMANOVA · Canary Islands Resale or republication not permitted without written consent of the publisher INTRODUCTION hitherto focused mainly on terrestrial systems (Boero 1999), with rare applications of these concepts to Differences in environmental conditions play an marine habitats (Fraschetti et al. 2001). Most studies important role in landscape heterogeneity at different have focused on a narrow range of spatial scales in a scales ranging from local patchiness to variation along limited number of habitats (Fraschetti et al. 2005). In biogeographic gradients (Levin 1992, Tilman & Ka- this sense, linkages between local geography and eco- reiva 1997, Fraschetti et al. 2001, 2005, Garrabou et al. logical features have seldom been considered with 2002). Consequently, linkages across multiple scales respect to the composition, distribution and structure are increasingly being considered by ecologists of subtidal assemblages on rocky reefs. (Brown 1995, Fraschetti et al. 2005). The use of macro- The main biological engineers of temperate rocky ecology to reconcile biogeography and ecology has reefs are macroalgae (Steneck et al. 2002, Graham *Email: [email protected] © Inter-Research 2006 · www.int-res.com 16 Mar Ecol Prog Ser 311: 15–28, 2006 2004). The existence of algae is influ- enced by pre-recruitment processes (Hoffmann & Ugarte 1985, Andrew & Veijo 1998, Coleman 2003), environ- mental conditions (e.g. wave expo- sure) (Santelices 1990, Coleman 2003, Taylor & Schiel 2003), post-recruit- ment biotic processes (Underwood & Jernakoff 1981, Jernakoff 1983, Bene- detti-Cecchi & Cinelli 1994), and phys- ical stress and disturbance (Kennelly 1987, Kendrick 1991). The role played by different processes operating at dif- Fig. 1. Study locations within islands. d: locations protected from the NE-swell. ferent scales in the composition, distri- j: locations exposed to the NE-swell bution and structure of algal assem- blages is a growing field of interest, and remains largely untested in the majority of coastal geneity on the spatial patterns of natural subtidal areas (Fraschetti et al. 2005). In this context, hierar- assemblages (Benedetti-Cecchi et al. 2003, Linde- chical spatially structured sampling programs provide garth & Gamfeldt 2005, Micheli et al. 2005). We took a means of partitioning and quantifying the magni- advantage of the natural conditions across the Ca- tude of variation at different spatial scales (Under- narian Archipelago to assess the role played by envi- wood & Chapman 1996, Underwood 1997, Menconi et ronmental factors in determining the composition, al. 1999, Benedetti-Cecchi 2001, Benedetti-Cecchi et structure and organization of shallow water algal al. 2003, Anderson & Millar 2004, Dethier & Schoch assemblages on rocky reefs. In this sense, we con- 2005, Fraschetti et al. 2005). ducted a mensurative, multi-scaled, observational ex- The Canary Islands lie between 100 and 600 km periment (sensu Underwood 1997, Anderson & Millar off the north-west coast of Africa (~28° N) and com- 2004, Fraschetti et al. 2005) to study the effects of: the prise 7 major islands, as well as a group of small islets degree of wave exposure and spatial variability asso- (Chinijo Archipelago) (Fig. 1). Nearshore waters of ciated with a hierarchy of spatial scales — ranging north-western Africa are characterized by almost from (1) islands (100s of km apart), to (2) locations year-round wind-driven upwelling that brings cold, within islands (10s of km apart), and (3) sites within nutrient-rich sub-surface waters to the surface, locations (100s of m apart) — on the composition, extending as a 50 to 70 km band along the shore abundance and distribution of shallow water algal (Davenport et al. 2002). Consequently, the Canarian assemblages in a regional context (<1000 km). More Archipelago lies in the transition between the oligo- specifically, we tested the hypothesis that the role of trophic open ocean and the northwest African wave exposure is significant in determining the struc- upwelling (so-called Northwest African Coastal Tran- ture and organization of shallow water algal assem- sition Zone [NACTZ]). Large spatial variation in sea blages, and assessed the consistency of this pattern surface temperature (SST) occurs across an east–west across the islands constituting the Canarian Archipel- gradient perpendicular to the African coast (Daven- ago. Since frondose fucoid algae may be considered port et al. 2002), with an average difference of 2°C as temperate-water elements of the shallow subtidal between the eastern and western islands (Barton et zone (Lüning 1990, Steneck et al. 2002), whereas turf al. 1998, Davenport et al. 2002). As a result, marine and bush-like algae are more common in tropical assemblages for islands 100s of km apart are sub- waters (Lüning 1990), we additionally hypothesized jected to different oceanographic conditions and re- that the presence and abundance of fucoid algae gimes of ‘bottom-up’ effects (sensu Menge 2000) that should be greater in the eastern islands, while turf produce qualitative and quantitative differences and bush-like algae should increase in the western between the eastern and western islands, as has been islands. Algae can be expected to be more susceptible observed for demersal fish (Tuya et al. 2004a). At the to disturbance by wave action and/or be less able to same time, persistent trade winds induce strong tur- recover after disturbance, when other factors make bulence (swell and wind) on exposed north and the environment stressful. As a result, we predicted northeast facing shores, while south and southwest that the effects of wave exposure would interact with facing shores are more sheltered. variability among islands, and that the different algal Islands have provided valuable systems to test taxa and/or algal groups would show different pat- hypotheses about the effect of environmental hetero- terns in this regard. Tuya & Haroun: Organization of shallow water algae 17 MATERIALS AND METHODS in general forming low diversity algal stands. Under- storey algae were excluded from the surveys as their Area of study and sampling design. The study was coverage is hard to determinate, and a meticulous carried out on basaltic rocky bottoms between 2 and investigation of the whole substratum is too time- 8 m deep on the Canarian Archipelago (28° N, eastern consuming. However, crustose coralline algae (e.g. Atlantic Ocean), during March 2005. In this region, the genera Lithothamnion, Lithophyllum, Neogo- the long-spined black sea urchin Diadema antillarum niolithon, Titanoderma, etc.) were counted when not (Philippi) plays a key role in the structure of subtidal overgrown by other algae. rocky reefs (Tuya et al. 2004a), transforming areas Our sampling design tested the effect of the degree previously covered by erect algae to unvegetated of wave exposure to the dominant, trade wind induced substrates. In general, water turbulence inhibits con- NE swells (categorized as high versus low exposure, siderably