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Since its introduction to Nova Scotia in the late 1980s, the invasive green alga Codium fragile subsp. tomentosoides has spread from rocky subtidal habitats to tidepools on the Atlantic coast. We monitored recruitment, growth, and survival of C. fragile, and potential biotic and abiotic factors that regulate these processes in three tidepools at different tidal heights on a wave-exposed rocky shore over 4 y (2000–2003). Large seasonal and interannual fluctuations in population density (up to 520 plants · 0.25 m−2) were driven by recruitment of small thalli (≤ 2 cm length) in summer and subsequent mortality of larger plants in fall and winter. Variation in the timing and magnitude of recruitment among years may reflect differences in the mode of reproduction, with intensive recruitment via dispersing propagules establishing the dense populations that in subsequent years produce new thalli vegetatively. Growth rates of new recruits increased with water temperature between June and September. Surviv...