Steven N. Murray

Impact of sewage on the distribution, abundance and community structure of rocky intertidal macro-organisms

The biological effects of a low-volume domestic sewage discharge were studied near Wilson Cove, San Clemente Island, California (USA), from February to June, 1972. There were fewer species and less cover near the outfall (7 macro-invertebrates, 17.6%; 13 macrophytes, 91.7%) than in nearby “unpolluted” control areas (9 macroinvertebrates, 9.2%; 30 macrophytes, 103.4%). The outfall biota was less diverse than that of the controls, as shown by 5 different diversity indices. A great reduction in community stratification (spatial heterogeneity) and, hence, community complexity occurred near the outfall; this reduction in stratification was primarilly due to the absence of Egregia laevigata, Halidrys dioica, Sargassum agardhianum and Phyllospadix torreyi. These were replaced in the mid-intertidal near the outfall by a low turf of blue-green algae, Ulva californica, Gelidium pusillum and small Pterocladia capillacea, and in the lower intertidal by Serpulorbis squamigerus covered with Corallina officinalis Var. chilensis. A statistically-based determination of assemblages or groups of organisms (i.e., cluster analysis) revealed 3 discrete outfall and 3 discrete control area groups; 3 assemblages contained samples from both areas. The distributional patterns of these groups indicate that near the outfall the degree of dilution of discharged sewage is more important in regulating zonation than is tidal height. The enhancement of the suspension feeder Serpulorbis squamigerus and the omnivores Ligia occidentalis, Pachygrapsus crassipes and Anthopleura elegantissima in the outer fringe of the outfall plume hypothetically is due to their ability to utilize sewage as a food source. A critical effect of the outfall may be to decrease environmental stability thereby favoring rapid-colonizers and more sewage-tolerant organisms. The outfall macrophytes were characterized by relatively higher net primary productivities, smaller growth forms, simpler and shorter life histories, and most were components of early successional stages.

Seasonal variations in net photosynthetic performance and cover of intertidal macrophytes

Abstract Standing stocks and net photosynthetic performances were determined concurrently for the 13 most abundant intertidal macrophytes from San Clemente Island, California over a four-season period. A summer cover maximum and winter minimum occurred for Colpomenia sinuosa (Roth) Derbes & Solier, Egregia menziesii (Turner) Areschoug, Gigartina canaliculata Harvey, Halidrys dioica Gardner, and Sargassum agardhianum J. Agardh, whereas Eisenia arborea Areschoug and Corallina officinalis var. chilensis (Decaisne) Kutzing had fall maxima and summer minima in cover. Ulva californica Wille and Gelidium pusillum (Stackhouse) Le Jolis had maximal cover during spring. Pterocladia capillacea (Gmelin) Bornet & Thuret was the only species with peak cover in winter while three macrophytes, Lithothrix aspergillum J.E. Gray, blue-green algae, and Phyllospadix torreyi Watson displayed little seasonality in standing stocks. Highest net production rates were measured during summer for nine of the 13 species and minimum rates were recorded in spring and to a lesser extent during winter. Correlations between seasonal fluctuations in standing stock and productivity were evident for only eight species. Total daily community production reached a peak in the fall (1.22 g C fixed m −2 of substratum day −1 ) and declined sharply through winter to a spring low (0.47 g C m −2 day −1 ), closely paralleling changes in ambient water temperature. Blue-green algae, C. officinalis var. chilensis, P. capillacea , and E. menziesii contributed 76% of the total community primary productivity for the year. Seasonal patterns of photosynthetic performances were highly variable with a tendency for most species to attain peak daily photosynthetic rates coincident with higher temperatures and longer daylength.