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The population biology of two intertidal seagrasses, Zostera Japonica and Ruppia Maritima, at Roberts Bank, British Columbia Bigley, Richard Ernest


Growing (vegetative) and seed components of co-occurring intertidal populations of Zostera japonica Aschers. and Graebn. and Ruppia maritima L. were studied in southwestern British Columbia to determine what factors affect their population maintenance. Results of repeated mapping and examination of shoots (ramets) in permanent plots on a tidal gradient showed that shoot flux, age structure, survivorship, leaf and rhizome growth, and seed production were all affected by the amount of exposure of plants to the air. Plots having the greatest exposure had fewer shoots, a greater percentage of shoots flowering early in the season, and substantially lower seed production than plots with less exposure to the air. The length of the R. maritima life cycle was the same regardless of differences in exposure to the air, while plants of Z. japonica with high exposure initiated and ended flowering and entered a quiescent overwintering state earlier than plants with less exposure. Seed of both species was shed innately dormant, and through enumeration of seed in the sediment it was found that most was exported from the site of production. Experimental seed burial showed that remaining seed suffered ~ 50% overwinter mortality. Simultaneous germination of Z. japonica and R. maritima seed was triggered by the warming of sediments in March when low tides occurred during the daytime. Germination was confined to aerobic sediments; seed buried in anaerobic sediment was maintained in an enforced dormancy year-round. Observations that low temperatures and anaerobic conditions retard seed germination were confirmed by laboratory studies. In addition, seed was found to be non-photoblastic and was prevented from germinating by local seawater salinities only when emerging from innate dormancy.

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