Population dynamics of an invasive green alga, Codium fragile subsp. tomentosoides, in tidepools on a rocky shore in Nova Scotia, Canada

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. Survival of marked plants steadily decreased during summer and fall (to ≤ 20% by November 2001) and was greater in recruits transplanted to deeper and more wave-sheltered microhabitats within pools. Two pools in the low intertidal zone had lower temperatures, greater water movement, and fewer grazers than a third pool high on the shore. These environmental differences may account for variation in growth and survival of C. fragile among pools at different intertidal heights.