INTERACTION MECHANISMS BETWEEN GRACILARIA-CHILENSIS (RHODOPHYTA) AND EPIPHYTES

Epiphytes can have a considerable effect on Gracilaria production, and Ulva is one of the commonest algal species identified as an epiphyte, reaching loads of 60% (g of epiphytes per g of Gracilaria) in the int ertidal cultures of southern Chile. This study evaluates the relative importance of light reduction, addition of weight to the host thalli a nd nutrient depletion, as mechanisms determining the interaction effec ts of Ulva epiphytes on Gracilaria cultivation. Using field experiment s undertaken during the main Gracilaria growth season (spring), we eva luate the mechanisms of epiphyte-host algae interaction by manipulatin g artificial epiphytes. The results indicate that Ulva can significant ly depress Gracilaria biomass production and that the addition of weig ht to the host algae and the consequent dislodgement increase, appear to be the main mechanisms involved in the Ulva-Gracilaria interaction. However, the light reduction caused by the epiphytes can also partial ly explain the reduction in Gracilaria production. Nutrients depletion would not appear to fully account for the detrimental effects of Ulva over Gracilaria in intertidal farming areas of southern Chile.