Community composition, structure, and interrelationships in the marine intertidal Endocladia muricata – Balanus glandula association in Monterey Bay, California

Studies of the community composition, structure and species interrelationships of the Endocladia-Balanus association were carried out on the rocky shores at the Hopkins Marine Station, Pacific Grove, California, over the period 1959—1961. The organisms making up this biotic association form a horizontal band approximately two feet wide on intertidal rocks. The center of this belt averaged 4.6 ft in surveyed height above tidal datum, but field observations showed that the periods of exposure and submersion actually experienced under a variety of conditions at this level are those predicted for a level of about 3.8 ft above tidal datum. Selected aspects of the relatively mild marine and terrestrial climates were studied in relation to the high and low water periods. The composition of the association was determined from quadrat samples, qualitative collections and field observations. A total of at least 93 benthic and transient species was enumerated, belonging to 15 major groups. Samples taken near the center of the association in different areas and at different seasons demonstrated great similarity and stability in species composition. Thirty-five of the species present in the assemblage were of common occurrence, and where feasible the following aspects of the biology of each were studied: (a) habitat niche, (b) activity patterns, (c) organic matter content of the body (based on the dry weight and nitrogen content), (d) seasonal occurrence, (e) population structure, (f) reproductive activity, (g) food relationships, and (h) growth rate. The most characteristic species were: Rhodophyta-Endocladia muricata, Gigartina agardhii; Platyhelminthes-Notoplana acticola; Nemertea- Emplectonema gracile, Nemertopsis gracilis; Annelida-Syllis spenceri, S. vittata, S. armillaris, Nereis grubei, Perinereis monterea; Bryozoa- Filicrisia franciscana; Mollusca (8 gastropods, 1 chiton, and 3 bivalves)- Acmaea digitalis, A. scabra, A. pelta, Littorina scutulata, L. planaxis, Acanthina spirata, Thais emarginata, Tegula funebralis, Cyanoplax dentiens, Lasaea cistula, Mytilus californianus, Musculus sp.; Arthropoda (7 crustaceans, 2 insects, and 3 mites)- Balanus glandula, Chthamalus dalli, C. microtretus, Dynamenella glabra, Allorchestes ptilocerus, Hyale sp., Pachygrapsus crassipes, Diaulota densissima, Limonia marmorata, Agauopsis sp., a mesostigmatid mite, and Suidasia sp. A study of space relations shows that a multitude of species live among the holdfast branchlets and along the blades of E. muricata, and in the spaces formed by the tests of both living and dead B. glandula. Food studies of the 34 commonest animal species show that seven are filter feeders, ten are herbivorous browsers and scrapers feeding on encrusting forms, six are herbivores feeding on the large benthic algae, five are omnivores and scavengers, and six are carnivores which prey mostly on filter feeders and scraping herbivores. Pelagic materials present during high tide periods, in order of decreasing absolute volume, were: (a) large plant fragments, (b) phytoplankton, (c) other organic detritus, (d) zooplankton, and (e) inorganic detritus. Approximately 570 mg dry weight was available per m³ of sea water throughout the year. For the center of the Endocladia-Balanus zone the mean number of individuals above microscopic size of all species present at any time was 210,000/ m², the mean dry weight biomass was 2,640 g/m², and the mean nitrogen content was 25 g/m². In quantitative as well as qualitative terms, the Endocladia-Balanus association shows considerable similarity with the Gloiopeltis- Chthamalus association in the Sea of Japan and with the Chthamalus-Pygmaea zone on English rocky shores. Analysis of the association in terms of the protein content of the standing crop for the major trophic groups shows the following: filter feeding animals, 84 g/m²; larger red algae and their attached epiphytes, 58 g/m²; resident herbivores, 12 g/m²; a transient herbivore, 1.2 g/m²; resident carnivores, 0.9 g/m²; transient carnivores, 0.8 g/m²; omnivores and scavengers, 1.3 g/m². Although the scraping and grazing herbivores feed on algae produced in the zone, much of the food consumed in the association is derived by import of suspended detritus and plankton at high water.