Between-Habitat Variation of Benthic Cover, Reef Fish Assemblage and Feeding Pressure on the Benthos at the Only Atoll in South Atlantic: Rocas Atoll, NE Brazil

<div><p>The Southwestern Atlantic harbors unique and relatively understudied reef systems, including the only atoll in South Atlantic: Rocas atoll. Located 230 km off the NE Brazilian coast, Rocas is formed by coralline red algae and vermetid mollusks, and is potentially one of the most “pristine” areas in Southwestern Atlantic. We provide the first comprehensive and integrative description of the fish and benthic communities inhabiting different shallow reef habitats of Rocas. We studied two contrasting tide pool habitats: open pools, which communicate with the open ocean even during low tides, thus more exposed to wave action; and closed pools, which remain isolated during low tide and are comparatively less exposed. Reef fish assemblages, benthic cover, algal turfs and fish feeding pressure on the benthos remarkably varied between open and closed pools. The planktivore <i>Thalassoma noronhanum</i> was the most abundant fish species in both habitats. In terms of biomass, the lemon shark <i>Negaprion brevirostris</i> and the omnivore <i>Melichtys niger</i> were dominant in open pools, while herbivorous fishes (mainly <i>Acanthurus</i> spp.) prevailed in closed pools. Overall benthic cover was dominated by algal turfs, composed of articulated calcareous algae in open pools and non-calcified algae in closed pools. Feeding pressure was dominated by acanthurids and was 10-fold lower in open pools than in closed pools. Besides different wave exposure conditions, such pattern could also be related to the presence of sharks in open pools, prompting herbivorous fish to feed more in closed pools. This might indirectly affect the structure of reef fish assemblages and benthic communities. The macroalgae <i>Digenea simplex</i>, which is uncommon in closed pools and abundant in the reef flat, was highly preferred in herbivory assays, indicating that herbivory by fishes might be shaping this distribution pattern. The variations in benthic and reef fish communities, and feeding pressure on the benthos between open and closed pools suggest that the dynamics in open pools is mostly driven by physical factors and the tolerance of organisms to harsh conditions, while in closed pools direct and indirect effects of species interactions also play an important role. Understanding the mechanisms shaping biological communities and how they scale-up to ecosystem functioning is particularly important on isolated near-pristine systems where natural processes can still be studied under limited human impact.</p></div

Density and biomass of reef fishes in open and closed pools at Rocas atoll.

<p>Displayed species were chosen based on a ranking combining density and biomass. Error bars represent standard errors.</p

Mean percent cover of the main benthic groups (i.e. those that pooled comprise between 80 and 100% of total cover).

<p>(*) Indicates significant differences between closed and open pools (t-test, p<0.05; <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0127176#pone.0127176.s009" target="_blank">S5 Table</a>). Error bars represent the standard errors.</p

Principal component analysis (PCA) on the composition of benthic cover of closed and open pools.

<p>CCA—Crustose coralline algae.</p

Multidimensional Scaling (MDS) of the composition of reef fish assemblages among different habitats based on a Bray-Curtis similarity matrix.

<p>Multidimensional Scaling (MDS) of the composition of reef fish assemblages among different habitats based on a Bray-Curtis similarity matrix.</p

Macroalgal removal and selectivity at the closed pool Tartarugas.

<p>(A) Mean proportion of consumed algae biomass; (B) and (C) Mean number of bites on each macroalgae by the herbivorous fishes <i>Acanthurus chirurgus</i> and <i>A</i>. <i>coeruleus</i>, respectively; (D) Mean proportion of bites taken by the two herbivorous species on each algae; (E) Strauss linear selectivity index for the two herbivores on each algae. (*) indicates significant differences at a 5% significance level. In (A), (B), (C) and (D) error bars represent standard errors, and in (E) it represents the 95% confidence interval.</p

Diet of the two main roving herbivores at Rocas atoll: <i>Acanthurus chirurgus</i> and <i>A</i>. <i>coeruleus</i>.

<p>Letters above bars indicate post-hoc comparisons of the Friedman test. Error bars represent standard errors.</p

Reef fish feeding pressure on the benthos in closed and open pools.

<p>Colored bars indicate species functional groups. Error bars represent standard errors.</p

Studied areas at Rocas atoll, NE Brazil, indicating the different habitats.

<p>The large scale map was plotted with the R software using the 'maptools' package [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0127176#pone.0127176.ref041" target="_blank">41</a>] based on data provided by the GEOphysical DAta System—Next Generation (GEODAS-NG) of the NOAA (<a href="https://www.ngdc.noaa.gov/mgg/geodas/geodas.html" target="_blank">https://www.ngdc.noaa.gov/mgg/geodas/geodas.html</a>). The illustration of the atoll was adapted from [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0127176#pone.0127176.ref039" target="_blank">39</a>] but is not identical.</p

Concentration of soluble sugars, starch, protein and lipid in the algae offered to herbivorous fishes.

<p>Letters indicate significant differences according to an analysis of variance followed by a Tukey HSD test. N.A.—not available. Error bars represent standard errors.</p