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We tested the view that few generalizations are possible about the structure of subtidal algae assemblages and that the situation worsens or does not change as more sites are examined. We quantified the percentage cover of 4 morphological groups of benthic algae (encrusting and articulated coralline algae, and foliose and turfing algae) under canopies of kelp Ecklonia radiata and in areas without kelp (macroalgae > 4 cm high). This was repeated over 4 spatial scales ranging from quadrats (separated by 10s of m), sites (separated by km), locations (separated by 100s of km) to regions (separated by 1000s of km) across the southern coastline of Australia: Western Australia (WA), Southern Australia (SA) and Eastern Australia (EA). The key result was that while comparison among sites revealed substantial and often inconsistent variation in abundance of benthic algae between habitats (kelp vs open), clear patterns emerged when locations and regions were compared. In EA, where grazers are effective in maintaining extensive areas of encrusting corallines, patterns of algal cover between habitats (kelp vs open) were generally reversed to WA and SA, where grazers are substantially less effective. These results indicate a large distinction in the ecology of these regions (WA = SA ≠ EA) and how lack of understanding of this pattern at the regional scale tends to suggest overwhelming variation when single studies are compared among regions. These differences also highlight that comparisons of studies done at small scales, even if done at several sites in a locality, provide a difficult basis to understand the generality of pattern in algal assemblage structure due to large variation at this scale. While we acknowledge that these broad patterns were not possible to validate at the scale of sites, it was possible to increase the scale of observation to encompass broader patterns that might be organized around a relatively simple set of ecological predictions.

The ecology of Australia's most extensive canopy-forming alga, Ecklonia radiata , is often studied with little regard as to whether it occurs in monospecific stands or as part of a mixed assemblage of canopy-forming algae. We tested the hypothesis that E. radiata does not primarily occur as monospecific stands, rather it occurs more often in stands of mixed algae. At a 1-m 2 scale we recognized three main configurations within forests of algae (hereafter called stands): E. radiata that occurs as (i) monospecific stands; (ii) clumps (four or more individuals together) surrounded by species of Fucales; or (iii) individual plants (or clusters of fewer than three plants) interspersed among species of Fucales. All three types of stand occurred in similar proportions (percentage cover) across two regions of Australia's southern coastline (Western and South Australia). We also tested the hypothesis that these three types of stands (identified at 1 m 2 ) contain different assemblages of invertebrates associated with the holdfast of E. radiata . Assemblages of invertebrates varied between monospecific and interspersed stands, but not between monospecific and clumped stands. These results suggest that variation in the configuration of subtidal algae (stands measured at a 1-m 2 scale) has the potential to influence the composition and abundance of associated biota. We suggest that although studies in stands of monospecific E. radiata may provide useful information for the majority of forests containing E. radiata (monospecific and clumped stands made up 65% of forests sampled), caution must be used when extrapolating to stands of mixed, interspersed algae (>31% of forests sampled).

Intraspecific variation in morphology is common among marine algae and may allow plants to exist across a wide geography and range of environmental conditions. Morphological variation of Ecklonia radiata has been described over thousands of kilometres of the temperate Australian coastline; however, the degree to which this morphological variability is related to geographic and environmental variation is unknown. We tested the hypotheses that: (1) variation in the morphology of Ecklonia radiata, growing in both monospecific and mixed stands, is related to variation in latitude, longitude, wave exposure, temperature, depth and plant density (collectively referred to as ‘physical variables’); and (2) measures of morphological dissimilarity in E. radiata are greatest among locations that are separated by the largest geographic distances. The combined effect of the physical variables accounted for 74% of the variation in both monospecific and mixed stands. The majority of this variation was related to longitude and the remainder to wave exposure, water temperature and plant density. In monospecific stands, measures of morphological dissimilarity were consistently large between locations that were separated by the greatest geographical distances (>2500 km). The existence of such relationships may not indicate causality, but do contribute to a broad based understanding of major ecological patterns across temperate Australia’s coastline.

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