Effect of wave exposure on morphology, attachment strength and survival of the invasive green alga Codium fragile ssp. tomentosoides

Hydrodynamic forces are a key factor influencing morphology and survival of marine algae in intertidal and shallow subtidal habitats. Since 1989, the invasive green alga Codium fragile ssp. tomentosoides has spread rapidly in Atlantic Canada, forming dense stands in intertidal pools and shallow subtidal habitats. We measured the morphology and attachment strength of Codium over 4 seasons at 3 sites of differing wave exposure and water motion near Halifax, Nova Scotia. The bushiness index (circumference-to-length ratio) of Codium varied inversely with the degree of wave exposure and water motion. Attachment strength was weakly related to thallus morphology (circumference) and generally did not differ between sites and seasons. We examined the effect of morphology on survival of Codium by experimentally trimming the thallus (by cutting 33 or 66% of branches) in a field experiment over 1 year. Trimming, which simulated natural fragmentation, had a positive effect on survival relative to untrimmed controls, particularly at the most wave-exposed site and after a major storm event (Hurricane Juan), and the magnitude of this effect was related to the degree of trimming. We used a flume tank to measure drag forces at different flow velocities (0.3-0.5 m s^{- 1}) on experimentally trimmed plants (using the same levels of trimming as in the field experiment). Drag was positively related to plant circumference and water velocity, and negatively related to the degree of trimming. Attachment strength of these plants was 10 to 22 times greater than the drag force, suggesting that Codium is dislodged when flow velocities greatly exceed 0.5 m s^{- 1}. The ratio of attachment strength to drag force decreased with circumference, indicating a greater risk of dislodgment with increasing plant size.