The effects of nutrient enrichment and herbivore abundance on the ability of turf algae to overgrow coral in the Caribbean

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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 Curac ̧ao (and likely elsewhere in the Caribbean), new conservation strategies are required to mitigate their negative impact on coral communities. © 2010 Vermeij et al.

Figures

  • Figure 1. Overview of the surveyed sites along Curaçao’s south-west shore. Buoy 0 is the location where the herbivore exclusion experiment was conducted but the benthic community was not surveyed at this site. doi:10.1371/journal.pone.0014312.g001
  • Figure 2. The abundance of turf algae relative to all other benthic groups along Curaçao’s south-west shore. The abundance of five major functional groups was quantified between depths of 7 and 10 m (for an overview of the locations of surveyed sites, see Figure 1). doi:10.1371/journal.pone.0014312.g002
  • Figure 3. Overgrowth rates when turf algae are present (Turf) or absent, i.e., corals border crustose coralline algae (CCA). The treatment in which corals bordered CCA’s served as ‘‘controls’’ for the turf algal treatment. The presence and absence of added nutrients is indicated by +Nut and -Nut respectively and the presence (+Hbv) or absence (-Hbv) of herbivores is indicated using the same methodology. Letters above the markers indicate significant groupings based on post-hoc analyses. doi:10.1371/journal.pone.0014312.g003
  • Table 1. Factorial ANOVA results of the effects of neighbour type (CCA or turfalgae) and nutrient enrichment on turfalgal overgrowth.
  • Table 3. One way ANOVA results of the effects of herbivore exclusion on turfalgal overgrowth.
  • Table 2. Repeated-measures ANOVA results of the effects of neighbour type (CCA or turfalgae) and nutrient enrichment on the photosynthetic efficiency of corals.
  • Table 4. Repeated-measures ANOVA analysis of the effects of herbivore exclusion on the photosynthetic efficiency of corals.
  • Figure 4. Food preference of five common herbivorous fishes: Scarus vetula (queen parrotfish), Scarus taeniopterus (princess parrotfish), Sparisoma viride (stoplight parrotfish), Acanthurus bahianus (ocean surgeonfish), Acanthurus coeruleus (blue tang). doi:10.1371/journal.pone.0014312.g004

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CITATION STYLE

APA

Vermeij, M. J. A., van Moorselaar, I., Engelhard, S., Hörnlein, C., Vonk, S. M., & Visser, P. M. (2010). The effects of nutrient enrichment and herbivore abundance on the ability of turf algae to overgrow coral in the Caribbean. PLoS ONE, 5(12). https://doi.org/10.1371/journal.pone.0014312

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