The Effects of Nutrient Enrichment and Herbivore Abundance on the Ability of Turf Algae to Overgrow Coral in the Caribbean

Abstract

Turf algae are multispecies communities of small marine macrophytes that are becoming a dominant component of coral reef communities around the world. To assess the impact of turf algae on corals, we investigated the effects of increased nutrients (eutrophication) on the interaction between the Caribbean coral Montastraea annularis and turf algae at their growth boundary. We also assessed whether herbivores are capable of reducing the abundance of turf algae at coral-algae boundaries. We found that turf algae cause visible (overgrowth) and invisible negative effects (reduced fitness) on neighbouring corals. Corals can overgrow neighbouring turf algae very slowly (at a rate of 0.12 mm 3 wk−1) at ambient nutrient concentrations, but turf algae overgrew corals (at a rate of 0.34 mm 3 wk−1) when nutrients were experimentally increased. Exclusion of herbivores had no measurable effect on the rate turf algae overgrew corals. We also used PAM fluorometry (a common approach for measuring of a colony's “fitness”) to detect the effects of turf algae on the photophysiology of neighboring corals. Turf algae always reduced the effective photochemical efficiency of neighbouring corals, regardless of nutrient and/or herbivore conditions. The findings that herbivores are not capable of controlling the abundance of turf algae and that nutrient enrichment gives turf algae an overall competitive advantage over corals together have serious implications for the health of Caribbean coral reef systems. At ambient nutrient levels, traditional conservation measures aimed at reversing coral-to-algae phase shifts by reducing algal abundance (i.e., increasing herbivore populations by establishing Marine Protected Areas or tightening fishing regulations) will not necessarily reduce the negative impact of turf algae on local coral communities. Because turf algae have become the most abundant benthic group on Curaçao (and likely elsewhere in the Caribbean), new conservation strategies are required to mitigate their negative impact on coral communities.

40 Years of benthic community change on the Caribbean reefs of Curaçao and Bonaire: the rise of slimy cyanobacterial mats

“…Besides macroalgae, algal turfs appear to benefit from a changing reef environment, in particular from eutrophication (Gorgula and Connell 2004;Vermeij et al 2010). Due to their opportunistic life-history characteristics, turfs are able to rapidly occupy newly available substratum and algal turfs have become the dominant cover on many coral reefs worldwide (e.g.…”

“…Curaçao (Vermeij et al 2010), Australia (Gorgula and Connell 2004), the Gulf of Arabia (Haas et al 2010) and the remote Line Islands in the Pacific (Sandin et al 2008). The general consensus is that algal turfs benefit from elevated nutrient loads (Gorgula and Connell 2004;Vermeij et al 2010).…”

Diazotrophs: Overlooked Key Players within the Coral Symbiosis and Tropical Reef Ecosystems?

“…Therefore, turfs represent one of the most dominant benthic components on many coral reefs in the Caribbean (Vermeij et al, 2010), central Pacific (Haas et al, 2010;Barott et al, 2012), Red Sea (Haas et al, 2010), and Indonesia (Wangpraseurt et al, 2012), and their high contribution to total reef N 2 fixation has received increasing attention in the past years. Overall, it has been estimated that ca.…”

Energetic differences between bacterioplankton trophic groups and coral reef resistance

“…The DDAM model predicts that overfishing and/or nutrient-loading promote the growth of benthic turf and fleshy macroalgae [57][58][59], leading to increased amounts of algalderived DOC thereby fuelling heterotrophic microbial growth and coral disease. Fluxing a larger proportion of energy and materials through fast, non-pathogenic energy channels (i.e.…”