A comparison of spatial and temporal patterns in epiphytic macroalgal assemblages of the seagrasses Amphibolis griffithii and Posidonia coriacea

Abstract

Studies of patterns in assemblages of epiphytic macroalgae growing on seagrasses have largely focussed at the scale of leaves or shoots. Few have examined patterns at larger spatial scales or the influence of host seagrass type on these patterns, despite interest in monitoring changes in epiphyte composition that can lead to the death of seagrasses. We studied spatial pat- terns in assemblages of epiphytic macroalgae on 2 seagrass species with different morphologies and longevity, Amphibolis griffithii and Posidonia coriacea, which co-occur on Success Bank, West- ern Australia. Epiphyte assemblages were described for each species at a hierarchy of spatial scales (sites separated by 10s, 100s and 1000s m) in summer and winter. At all locations and at both times, A. griffithii supported a higher species richness. A total of 91 taxa were recorded on A. griffithii and 70 on P. coriacea. Both seagrass species showed strong temporal patterns in their epiphyte macro- algal assemblages, but this was stronger in P. coriacea. For A. griffithii spatial variation was the dominant pattern. Epiphyte assemblages of the 2 seagrasses displayed striking differences in pat- terns of dissimilarity at different spatial scales. For A. griffithii, Bray-Curtis dissimilarity (D) among pairs of samples progressively increased with increasing spatial scale (D = 0.31, 0.37 and 0.47 at scales of 10s, 100s and 1000s m, respectively). For P. coriacea, the differences among samples sepa- rated by 10s m or 100s m were relatively small (0.29 and 0.32, respectively), but the differences among samples 1000s m apart were much greater (0.52). We suggest that the differences in spatial and temporal patterns between species of seagrass may reflect an interaction of timescales of seagrass longevity with timescales of algal reproductive biology and the dispersal ranges of algal propagules. Due to the high turnover of P. coriacea leaves, the assemblages present at any given time are strongly influenced by recent recruitment events. The epiphytic assemblage of long-lived A. griffithii integrates a much longer period of recruitment, so that recent events may produce comparatively smaller shifts in composition. The longevity of A. griffithii may also permit local reinforcement of existing assemblages, so that differences in assemblage structure increase with spatial scale. The high turnover of P. coriacea leaves results in less opportunity for an assemblage to reinforce its presence, so patterns at the smaller scales are comparatively weaker. Strong patterns across km are more likely to be the result of environmental gradients. Patchiness within these gradients leads to complex, many layered spatial patterns.