Sublittoral, Benthic Marine Algae of Southern Cape Cod and Adjacent Island: Seasonal Periodicity, Associations, Diversity, and Floristic Composition

Abstract

The periodicity, associations, species diversity, and composition of the sublittoral, attached algal vegetation were studied at four ecologically distinct stations in southern Cape Cod and adjacent islands. In situ collections and observations of 142 species of attached sublittoral algae (Chlorophyta, Phaeophyta, and Rhodophyta) were made with the aid of diving equipment. These were categorized into four groups with respect to seasonal periodicity (seasonal annuals, aseasonal annuals, pseudoperennials, and perennials) based on thallus longevity, plant form during their adverse season, and the period in which each species population was present in the community. Using these criteria we recognize the influence of environmental change on algal development, and the plants' temporal influence on the community. Based on qualitative and quantitative sampling, 10 sublittoral alga associations have been recognized from 2 bottom substrata types. The distribution of sublittoral attached algae at our four stations was more complex than can be accounted for by the three sublittoral algal associations as described in studies of other geographic areas. This may be due to the different levels at which associations have been recognized, to the type and degree of substratum stability, and to whether consideration is given to algal seasonal periodicity. The concept of a universal tripartite zonation of sublittoral algae is unacceptable when these factors are considered in vegetational analyses. An in—habitat, seasonal comparison of species diversity and composition was based on adjacent quantitative collections from a uniform shell bottom at 6 and 12 m. Species diversity remained seasonally stable at these depths; species composition, however, changed dramatically throughout the year. At 6 m, brown algae dominated in winter and were replaced by red algae during summer. At 12 m, the seasonal floristic changes were less pronounced, and red algae dominated year—round. A between—habitat comparison of species numbers and composition was based on year—round qualitative collections and observations made at four regularly visited stations (5—22 m depth). Species numbers decreased with increased station depth from 5 to 22 m except for one heavily silted station. The seasonal change of floristic composition was most striking at West Chop (5—8 m) where short— and long—lived species were about equally represented. Seasonal differences in species composition between summer and winter were striking at a 5—8 m station but were less pronounced with increased station depth where long—lived species made up an increasingly larger percentage of the total vegetation. Floristic composition also differed among stations. About equal numbers of red and brown species occurred at 5—8 m in contrast to the occurrence of about 4 times more red than brown algal species at 22 m. The crust vegetation also made up an increasingly larger percentage of the deep vegetation when compared with the vegetation of shallower areas sampled. Species diversity at 20—22 m was much higher in southern Cape Cod than reported for similar depths to the north (Cape Ann, Massachusetts, New Hampshire; Halifax, Nova Scotia); this may be due in part to the intensive collecting of crustose forms during the present study.