Ecological niches of three abundant Caulerpa species in Port Phillip Bay, southeast Australia

• We model niches of three Caulerpa species in a temperate marine embayment.
• Boosted regression tree models explained deviance of 0.35–0.54 (ROC 0.86–0.97).
• Major niche predictors were sewage effluent exposure, sediment type and wave action.
• Distributions of Caulerpa species generally matched those observed last century.
• Two Caulerpa species form habitats that are likely to be ecologically important.

Caulerpa (J.V. Lamouroux) forms important habitat in temperate and tropical ecosystems. The temperate coastal waters of southern Australia have high Caulerpa diversity and abundance, yet little information is available on ecological differences between related species. The distribution of Caulerpa species in Port Phillip Bay was documented along 151 100 m long towed video transects. Data on physical variables and distributions of co-occuring species was compiled from historic and contemporary sources. The distributions three Caulerpa species were investigated using boosted regression tree models. The cross-validated boosted regression tree models achieved deviance explained of 0.35–0.54 and accurately predicted to held-out data (ROC scores 0.8–0.9). The models showed the distribution of each species correlated with distinct sets of environmental parameters with variables describing substrate often most important. The distribution of Caulerpa longifolia f. crispata (Harvey) Womersley near the Western Treatment Plant appeared to have contracted compared with previous studies. Caulerpa remotifolia (Sonder) was the most broadly distributed and its association with the solitary ascidian Pyura dalbyi (Rius & Teske) provides a substantial and unique benthic habitat. Caulerpa sedoides f. geminata (Harvey) Weber-van Bosse forms monospecific beds in parts of the study area, constituting an as-yet unstudied habitat with implications for nutrient cycling. The study provides information on the ecology of Caulerpa species in an area remote from regions where Caulerpa has typically been studied. It is hoped that the results contribute to a fuller understanding of this much maligned, but remarkable group of seaweeds.