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Summer and winter distribution of foraminifera are similar in Tomales Bay, a long narrow embayment, 64 km north of San Francisco, California. Using three different numerical techniques (cluster analysis, multiple discriminant analysis, and multidimensional scaling), three groups of foraminifera were distinguished. One group including Glabratella ornatissima, Rotorbinella campanulata, and Cribrononion lene characterizes the nearshore turbulent zone near the mouth of the bay, an area of direct marine influence in which wind-driven waves impinge upon the shoreline. Two estuarine groups occupy the bay proper: a middle bay group is composed of Bulimina denudata, Fursenkoina pontoni, Hopkinsina pacifica, Nonionella basispinatus, N. stella, Quinqueloculina ackneriana, and several species of Brizalina; and a group at each end of the bay is composed of Buliminella elegantissima, Buccella tenerrima, Elphidiella hannai, and other less common species. Nearshore turbulent conditions at the northern mouth of the bay grade to estuarine conditions towards the south. Several of the species present in the middle bay group are more characteristic of warmer waters found in southern California and Baja California; these may represent a relict fauna from a time of warmer coastal waters or incursions of species brought by El Nino conditions. In contrast, most species found in the rest of the bay are typical of the colder-water Oregonian Province that reaches from Point Conception in the south to Vancouver Island in the north. The only apparent environmental factor that distinguishes the middle bay from the bay ends is the fine-grained sediment size of the middle bay region.

Summer and winter samples showed similar foraminiferal composition and abundance, except in some stations where certain species dominate at one season or the other. The winter middle bay is dominated by Hopkinsina pacifica and Bulimina denudata. Glabratella ornatissima dominated the summer samples in the northernmost turbulent zone at the mouth of the bay.

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