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

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

Lock, stock and two different barrels: comparing the genetic composition of morphotypes of the Indo-Pacific sponge Xestospongia testudinaria

The giant barrel sponge Xestospongiatestudinaria is an ecologically important species that is widely distributed across the Indo-Pacific. Little is known, however, about the precise biogeographic distribution and the amount of morphological and genetic variation in this species. Here we provide the first detailed, fine-scaled (<200 km(2)) study of the morphological and genetic composition of X. testudinaria around Lembeh Island, Indonesia. Two mitochondrial (CO1 and ATP6 genes) and one nuclear (ATP synthase β intron) DNA markers were used to assess genetic variation. We identified four distinct morphotypes of X. testudinaria around Lembeh Island. These morphotypes were genetically differentiated with both mitochondrial and nuclear markers. Our results indicate that giant barrel sponges around Lembeh Island, which were all morphologically identified as X. testudinaria, consist of at least two different lineages that appear to be reproductively isolated. The first lineage is represented by individuals with a digitate surface area, CO1 haplotype C5, and is most abundant around the harbor area of Bitung city. The second lineage is represented by individuals with a predominantly smooth surface area, CO1 haplotype C1 and can be found all around Lembeh Island, though to a lesser extent around the harbor of Bitung city. Our findings of two additional unique genetic lineages suggests the presence of an even broader species complex possibly containing more than two reproductively isolated species. The existence of X. testudinaria as a species complex is a surprising result given the size, abundance and conspicuousness of the sponge.The project was supported by the Percy Sladen Memorial Fund of the Linnean Society of London and the Treub-Maatschappij Fund. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. We thank the Research Centre for Oceanography of the Indonesian Institute of Sciences (RCO-LIPI) for allowing us to use their research facilities and the University of Sam Ratulangi (UNSRAT) for logistic support. We thank Lin Dong, Peter Kuperus and Betsie Voetdijk for advice and help with molecular lab techniques. Hans Breeuwer provided valuable comments on the original manuscript.publishe

Factorial ANOVA results of the effects of neighbour type (CCA or turfalgae) and nutrient enrichment on turfalgal overgrowth.

<p>ns  =  not significant.</p

The abundance of turf algae relative to all other benthic groups along Curaçao's south-west shore.

<p>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 <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0014312#pone-0014312-g001" target="_blank">Figure 1</a>).</p

Overview of the surveyed sites along Curaçao's south-west shore.

<p>Buoy 0 is the location where the herbivore exclusion experiment was conducted but the benthic community was not surveyed at this site.</p

Repeated-measures ANOVA analysis of the effects of herbivore exclusion on the photosynthetic efficiency of corals.

<p>ns  =  not significant.</p

Repeated-measures ANOVA results of the effects of neighbour type (CCA or turfalgae) and nutrient enrichment on the photosynthetic efficiency of corals.

<p>ns  =  not significant.</p

Food preference of five common herbivorous fishes: <i>Scarus vetula</i> (queen parrotfish), <i>Scarus taeniopterus</i> (princess parrotfish), <i>Sparisoma viride</i> (stoplight parrotfish), <i>Acanthurus bahianus</i> (ocean surgeonfish), <i>Acanthurus coeruleus</i> (blue tang).

<p>Food preference of five common herbivorous fishes: <i>Scarus vetula</i> (queen parrotfish), <i>Scarus taeniopterus</i> (princess parrotfish), <i>Sparisoma viride</i> (stoplight parrotfish), <i>Acanthurus bahianus</i> (ocean surgeonfish), <i>Acanthurus coeruleus</i> (blue tang).</p

Overgrowth rates when turf algae are present (Turf) or absent, i.e., corals border crustose coralline algae (CCA).

<p>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.</p