Floristic and zonation studies of seaweeds from Mount Desert Island, Maine: an historical comparison

Based upon recent collections at 28 sites plus historical data from the last century, the macroalgal flora of Mount Desert Island consists of 41 Chlorophyceae, 50 Phaeophyceae, and 55 Rhodophyceae. Previously, 121 seaweeds were recorded from Mount Desert Island and 113 taxa were found during present sampling. A comparison of the two time periods shows 88 taxa in common or a 75% similarity. Varying percent similarity patterns are evident when historical and present collections at Otter Cliffs (68%), Seal Harbor (43%), and the Seawall-Southwest Harbor areas (54%) are compared. The reduced values for Seal Harbor may reflect anthropogenic effects, while the other values may represent varying levels of taxonomic characterization and/or temporal variability of floras. Pronounced habitat diversity on Mount Desert Island probably causes the relatively low intra-island similarity patterns (x̄ = 36.8 ± 7.6%), while interisland comparisons of other Northwest Atlantic islands are much higher (ca. 51.0—92.0%, x̄ = 72.3% ± 6.0%). In comparing species richness around Mount Desert Island, the largest numbers of taxa occur on the exposed coasts at Otter Cliffs and Seawall that experience intense wave activity, while the lowest numbers occur at several sheltered sites. Zonation patterns at three representative sites (exposed Otter Cliffs, protected Otter Cove, and sheltered Thompson Island) show pronounced localized differences. The biological zones at Otter Cliffs exceed mean tidal amplitude, and patterns of species richness there are also higher than at the other two Mount Desert sites. Green algae show the most conspicuous decrease in species richness with increasing shelter. Of the 32 intertidal species in common with a 1928 zonation study at Otter Cliffs, 13 showed a conspicuous reduction in their upper distributional limits (0.5 to 2.0 m), while none showed an upward expansion. Such reductional patterns may reflect either a general warming trend in the Gulf of Maine or the effects of air pollution during intertidal exposure.