Water temperature and benthic light levels drive horizontal expansion of Caulerpa taxifolia in native and invasive locations

Caulerpa taxifolia is a marine alga native to tropical and subtropical regions, and invasive in temperate regions worldwide. The aim of this study was to quantify the impacts of water temperature, benthic light and nutrient enrichment on horizontal expansion of C. taxifolia, in the absence of competition from other benthic flora. Field experiments were undertaken in 1 native C. taxifolia population (Moreton Bay, Australia) and 2 invasive C. taxifolia populations (Pittwater and Port River Estuary, Australia). Manipulative experiments were conducted across a range of seasons and different shading and nutrient treatments to determine the effects of water temperature, benthic light dose and nutrient enrichment on horizontal expansion (stolon extension rate). Nitrogen and phosphorus enrichment had negligible effects on stolon extension rate at all locations, suggesting that nutrient conditions at the study sites were saturating for C. taxifolia. Shading significantly reduced stolon extension in Pittwater, and season/water temperature significantly affected stolon extension in Port River Estuary; only in Moreton Bay were both shading and season/water temperature significant. When all data were pooled in a general linear model, water temperature and average daily benthic light dose (log-transformed) significantly affected stolon extension, but nutrient enrichment did not. Site, season and the interaction between water temperature and benthic light dose also improved model performance, indicating that site-specific factors which varied between seasons also affected the measured stolon extension. Overall, our results indicate that C. taxifolia growth increased with light disproportionately at higher temperatures.