Damselfish Stegastes nigricans increase algal growth within their territories on shallow coral reefs via enhanced nutrient supplies

Abstract

Coral reefs are highly productive ecosystems that typically exist in nutrient-poor waters, stimulating considerable research on the sources of nutrients that may support this productivity, including nutrient recycling. We examined the potential for resident herbivorous damselfish (Stegastes nigricans) to recycle nutrients within their territories by conducting an exclusion experiment on S. nigricans territories and measuring algal turf growth on shallow fringing reefs in Mo'orea, French Polynesia. A nutrient bioassay was used to evaluate whether the enhanced growth was due to increased nutrients. We chose 15 isolated damselfish territories for a caging experiment, with a large cage obstructing all fish access on one half of the territory (−Damselfish, −Herbivory) and a small low-profile cage allowing access but not herbivory to a portion on the other half (+Damselfish −Herbivory). Additionally, we measured turf in a randomly-chosen uncaged area within each territory where both damselfish and herbivory were present (+Damselfish +Herbivory). Behavioral observations confirmed damselfish rarely swam over the large-caged half of the territory, effectively eliminating their physical presence, but continued to interact with the other half. Over the 16-day experiment, time and treatment had a significant effect on turf growth. By day 16, the turf receiving the +Damselfish −Herbivory treatment experienced average net turf growth of 5.34 mm, which was significantly greater than turf growth within the territories receiving the −Damselfish −Herbivory (2.13 mm) and the +Damselfish +Herbivory (1.50 mm) treatments. The macroalgae in the nutrient bioassay grew 21% more on the side of the territory where damselfish were present compared to the side where they were excluded, supporting our conclusion that the mechanism that supported faster turf growth was increased nutrients. Thus, our study was the first to establish experimentally that the presence of damselfish increased algal growth by enhancing nutrient supplies via recycled waste products, a mechanism that has been advanced but never tested. Surveys established that damselfish territories covered 28–64% of the fringing reef flat along the north shore of Mo'orea, suggesting this increased productivity within territories could be a significant contribution to total reef productivity. These findings highlight the importance of continuing research into the long-term and spatial-scale effects of damselfish as nutrient recyclers on coral reefs.