Modelling the distribution and density of the invasive seaweed Sargassum muticum (Fucales, Sargassaceae) in shallow subtidal areas

Abstract

The present study describes the distribution and density of Sargassum muticum in shallow subtidal areas of the Marine Fishery Reserve (MFR) ‘Os Miñarzos’ (Galicia, north-western Spain) by using a data-collection strategy and a statistical approach fairly unused in literature. Our surveys showed a rapid spread of the invasive alga in the study area, where the number of patches increased more than 50% between 2008 and 2009. A model-selection approach was used to test the goodness of fit of Sargassum density data and the zero-inflated Poisson (ZIP) appeared to be the best model. The ZIP model quantified a probability of 22% of finding at least one Sargassum plant in a randomly placed quadrat within 11-m depth, demonstrating that a large part of the MFR has been invaded. In particular, the ZIP model showed that season, richness of macroalgal community, and abiotic factors, such as wave exposure and depth, are essential drivers for the establishment of S. muticum thalli in subtidal areas. Our results showed that the invasion of subtidal areas often follows patterns similar to the intertidal areas. The present study also demonstrated the usefulness of zero-inflated models to assess early and mid-stages of a seaweed invasion.