Macroalgal Distribution Patterns and Ecological Performances in a Tidal Coastal Lagoon, with Emphasis on the Non-indigenous Codium Fragile ssp. Tomentosoides

Codium fragile ssp. tomentosoides is a North-West Pacific macroalgae that has invaded numerous lagoons. The success of C. fragile has been explained by its dispersal capacity, growth rate, nutrient uptake, salinity and temperature tolerances, and grazer resistance. I compared distribution, recruitment and growth of Codium to native algae in Hog Island Bay, a shallow water lagoon in Virginia. To determine the extent of invaded habitats algae were mapped from 1998-02. Codium was fourth most abundant, and hence considered successful compared to most species in terms of its biomass. Codium was found both unattached or attached to bivalve shells, while the majority of the dominant Gracilaria verrucosa and Ulva curvata were incorporated into tube caps of the polychaete Diopatra cuprea. Preference experiments showed that Diopatra incorporated Ulva and Gracilaria most, Agardhiella subulata intermediate, and Codium and Fucus vesiculosus least, and that the first 3 species were fragmented in the process. Thus, Diopatra facilitated Ulva and Gracilaria, by providing an abundant substrate, reducing flushing and maintaining a supply of fragments for regrowth. Tidal lagoons are characterized by sedimentation, desiccation, high turbidity, and high abundance of molluscan grazers. Short-term experiments showed that Codium was inferior under such conditions compared to Gracilaria, Ulva, Hypnea musciformis, and Agardhiella, decomposing faster when buried, being susceptible to desiccation, growing slower at high and low levels of nutrient and light, and being the only species grazed by Ilyanasa obsolata. To test if the success of Codium fragile could be related to its ability to colonize hard substrate, recruitment bricks were incubated in the shallow subtidal with i ii and without a cover of unattached algae or sediments. Codium recruited well onto control bricks, but not onto bricks covered by algae or sediments. After one year Codium, Gracilaria, Crassostrea virginica (oyster) and Agardhiella were space-dominants, having tolerated temperature regimes of 2-28ºC and desiccation at low spring tides. Thus Codium is only successful compared to native species by having moderate growth over a long season, and by being an effective colonizer of hard substrate in the shallow subtidal zone in the absence of high sedimentation or drift algae accumulations.

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