Foundational marine macroalgae, Turbinaria ornata, faciliates foraging by herbivorous and invertivorous fishes in the South Pacific

Background/Question/Methods

An abundant brown macroalgal species in the South Pacific, Turbinaria ornata, may be an important facilitator species on the reef. T. ornata aggregations have been shown to increase macroalgal richness and invertebrate diversity by provision of a refuge and may also protect juvenile fish species as well. T. ornata also seems to be resistant to or even to benefit from anthropogenic change as increased nutrient levels cause a decrease in herbivory on the alga. Thus, anthropogenic changes such as increased terrestrial runoff with high nutrient levels may be in part responsible for the drastic increases in range and abundance exhibited by T. ornata since the 1980s. The goal of this study was to determine the impact increasing T. ornata persistence and abundance may have on important coral reef species such as herbivorous fish shown to serve an important role in maintaining coral dominance in these systems.

We conducted a density manipulation experiment with three replicates of T. ornata thalli densities: 0, 3, 7, 10, 15, 20, 25, 30 per 0.25 m². Aggregations of T. ornata with at least 30 thalli in 0.25 m²area were randomly selected at a site in Mo’orea, French Polynesia and thalli were removed to create the 24 plots. Foraging behavior was observed and recorded for different species of herbivorous fish; each plot was observed three times for 10 minutes. Density treatments were maintained for 18 days and at the conclusion of the experiment three thalli were collected and pictures were taken at the lab for digital analysis of epiphyte load.

Results/Conclusions

There was a positive increase in the percent cover by epiphytes on T. ornata thalli as a function of density until a density of 15 thalli per 0.25 m² area. Treatments with 3 thalli exhibited approximately 40% cover by epiphytes, which gradually increased to 65% cover in the 15 thalli treatments, and a slight decline at higher densities. This indicates that there is an optimal density of T. ornata at which it supports a maximum abundance of associated epiphytes. Foraging behavior by herbivorous fish was more variable and depends on the family of an individual fish. However, there was a positive relationship that increased sharply at 7 thalli (logistic model, r-squared=0.566, p<0.01) between number of bites taken by herbivorous fish and T. ornata density. This work suggests that T. ornata communities are complex and facilitate both primary producers and consumers on the reef.