Growth and survival of the invasive green alga Codium fragile ssp. tomentosoides in tide pools on a rocky shore in Nova Scotia

We monitored growth and survival of the invasive green alga Codium fragile ssp. tomentosoides, and physical environmental factors (temperature, water flow, wave force), in eight tide pools in the low/mid intertidal zone on a rocky shore near Halifax, Nova Scotia, Canada, from May to October 1999. Growth in thallus length of individually tagged plants generally increased from May to a peak in August, and did not differ between pools. Growth in circumference also increased during this period but differed significantly between pools in each month, as did growth in fresh weight (estimated from length and circumference). An index of thallus bushiness (circumference-to-length ratio) increased gradually between May and August and then decreased in September. Fragmentation of Codium occurred throughout the summer, as evidenced by tagged plants that decreased in length, and the percentage of fragmenting plants nearly doubled between August and October as wave forces increased. Growth rate based only on plants that increased in length (i.e., did not undergo substantial fragmentation) was positively correlated with pool temperature and peaked in August-September (5.6 cm/month) when temperatures were warmest (16°C). Survival of tagged Codium did not differ between pools each month and decreased exponentially over the study to 30% by October. Survival of Codium in tide pools was negatively related to bushiness index, suggesting that large bushy plants are more likely to be dislodged by wave forces. Mortality of Codium was offset by recruitment between May and August, such that plant density remained relatively stable in four of five pools, and increased markedly in one other. Our study demonstrates that Codium has become successfully established in wave-exposed intertidal habitats at the northern limit of its current geographic range in the northwest Atlantic.