SPATIAL VARIATION IN AN ALGAL TURF COMMUNITY WITH RESPECT TO SUBSTRATUM SLOPE AND WAVE HEIGHT

A mid-shore colonial algal turf community is described. Samples were a nalysed to determine if species composition, invertebrate epiphyte den sity, algal mat biomass, and the amount of sediment trapped among alga l thalli differed with respect to substratum slope and wave height. Co lonial algal turf was approximately 1-3 cm in height and predominantly composed of a mixture of four species of eurythermal red algae, inclu ding Gelidium crinale (Turner) Lamouroux (Rhodophyceae: Gelidiales), C entroceras clavulatum (C. Agardh) Montagne, Polysiphonia denudata (Dil lwyn) Kuetzing, and Bryocladia cuspidata (J. Agardh) De Toni (Rhodophy ceae: Ceramiales). More delicate algae and algal epiphytes were found at stations with lower mean wave height, while non-epiphytic coralline algae were found only at stations with higher mean wave height. Algal mat mass was significantly greater on horizontal slopes than on verti cal slopes, but did not differ with respect to wave-height levels. Alg al mats trapped significantly more sediment on horizontal slopes with lower mean wave height. Densities of the three most abundant invertebr ate epiphytes appeared to be related to sediment deposition. Like sedi mentation patterns, disparities in population densities on different s ubstratum slopes increased with lower mean wave height. The highest de nsities of the amphipods Elasmopus rapax Costa and Hyale frequens Hasw ell (Crustacea: Peracarida) occurred on horizontal slopes at stations with significantly lower mean wave height. The highest densities of th e tree oyster Isognomon bicolor C.B. Adams (Mollusca: Pterioida) were on vertical slopes at stations with significantly lower mean wave heig ht. Barnacle epiphytes occurred in higher densities at stations receiv ing greater mean wave height, while dominant species of other crustace ans and molluscs had higher densities at stations with significantly l ower mean wave height. Species of polychaetes and peracarideans living among the bases of algal thalli had significantly higher densities on horizontal slopes; but densities with respect to wave-height levels w ere not significantly different.