Has Nitrogen Supply to Coral Reefs in the South Pacific Ocean Changed Over the Past 50 Thousand Years?

Erler, Dirk V.; Shepherd, Benjamin; Linsley, Braddock K.; Nothdurft, Luke D.; Hua, Quan; Lough, Janice

doi:10.1029/2019pa003587

Cited by 9 publications

(10 citation statements)

References 84 publications

(117 reference statements)

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“…Despite having overall lower δ 15 N, some types of cyanobacteria have still been shown to incorporate nitrogen from seabirds, although they have high standard deviations around these δ 15 N values 68 , which may make detection of seabird effects more difficult in cyanobacteria-feeding organisms. A more likely explanation is that seabird-derived nutrients facilitate nitrogen fixation by cyanobacteria by supplying phosphorous, as has been shown for terrestrial run-off and groundwater 69 . Such a pathway is consistent with the observed elevated nitrogen, but not δ 15 N, in C. sordidus feeding on cyanobacteria.…”

Section: Discussionmentioning

confidence: 95%

Natural nutrient subsidies alter demographic rates in a functionally important coral-reef fish

Benkwitt

Taylor

Meekan

et al. 2021

Sci Rep

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By improving resource quality, cross-ecosystem nutrient subsidies may boost demographic rates of consumers in recipient ecosystems, which in turn can affect population and community dynamics. However, empirical studies on how nutrient subsidies simultaneously affect multiple demographic rates are lacking, in part because humans have disrupted the majority of these natural flows. Here, we compare the demographics of a sex-changing parrotfish (Chlorurus sordidus) between reefs where cross-ecosystem nutrients provided by seabirds are available versus nearby reefs where invasive, predatory rats have removed seabird populations. For this functionally important species, we found evidence for a trade-off between investing in growth and fecundity, with parrotfish around rat-free islands with many seabirds exhibiting 35% faster growth, but 21% lower size-based fecundity, than those around rat-infested islands with few seabirds. Although there were no concurrent differences in population-level density or biomass, overall mean body size was 16% larger around rat-free islands. Because the functional significance of parrotfish as grazers and bioeroders increases non-linearly with size, the increased growth rates and body sizes around rat-free islands likely contributes to higher ecosystem function on coral reefs that receive natural nutrient subsidies. More broadly, these results demonstrate additional benefits, and potential trade-offs, of restoring natural nutrient pathways for recipient ecosystems.

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Section: Discussionmentioning

confidence: 95%

Natural nutrient subsidies alter demographic rates in a functionally important coral-reef fish

Benkwitt

Taylor

Meekan

et al. 2021

Sci Rep

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“…In addition, the ratio of DIN to DIP, which has rarely been reported in coral-diazotrophy studies (Table S1 ), may control N 2 fixation more than the DIN concentration alone [ 34 ]. As coral reefs form within a range of nutrient conditions, it is critical to constrain rates of N 2 fixation for a variety of reef settings, including reefs impacted by human activities and urbanization, as well as in understudied regions, such as Southeast Asia [ 6 , 35 , 36 ]. Moreover, all previous measurements of coral-associated N 2 fixation have been performed in aquaria (Table S1 ).…”

Section: Introductionmentioning

confidence: 99%

Coral-associated nitrogen fixation rates and diazotrophic diversity on a nutrient-replete equatorial reef

et al. 2021

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The role of diazotrophs in coral physiology and reef biogeochemistry remains poorly understood, in part because N2 fixation rates and diazotrophic community composition have only been jointly analyzed in the tissue of one tropical coral species. We performed field-based 15N2 tracer incubations during nutrient-replete conditions to measure diazotroph-derived nitrogen (DDN) assimilation into three species of scleractinian coral (Pocillopora acuta, Goniopora columna, Platygyra sinensis). Using multi-marker metabarcoding (16S rRNA, nifH, 18S rRNA), we analyzed DNA- and RNA-based communities in coral tissue and skeleton. Despite low N2 fixation rates, DDN assimilation supplied up to 6% of the holobiont’s N demand. Active coral-associated diazotrophs were chiefly Cluster I (aerobes or facultative anaerobes), suggesting that oxygen may control coral-associated diazotrophy. Highest N2 fixation rates were observed in the endolithic community (0.20 µg N cm−2 per day). While the diazotrophic community was similar between the tissue and skeleton, RNA:DNA ratios indicate potential differences in relative diazotrophic activity between these compartments. In Pocillopora, DDN was found in endolithic, host, and symbiont compartments, while diazotrophic nifH sequences were only observed in the endolithic layer, suggesting a possible DDN exchange between the endolithic community and the overlying coral tissue. Our findings demonstrate that coral-associated diazotrophy is significant, even in nutrient-rich waters, and suggest that endolithic microbes are major contributors to coral nitrogen cycling on reefs.

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“…For example, at the same location, hard corals vary in their isotope values temporally as available nutrients fluctuate (Erler et al, 2019;Radice et al, 2019). Consequently, as plankton communities are replenished, these fish may exhibit trophic plasticity (reflected in the large range in their isotope values) by adapting to prevailing environmental conditions (Berg & Ellers, 2010).…”

Section: Two-dimensional Isotopic Niches Help Explain Resource Use Bu...mentioning

confidence: 99%

Improving stable isotope assessments of inter‐ and intra‐species variation in coral reef fish trophic strategies

Cybulski

Skinner

Wan

et al. 2022

Ecology and Evolution

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Fish have one of the highest occurrences of individual specialization in trophic strategies among Eukaryotes. Yet, few studies characterize this variation during trophic niche analysis, limiting our understanding of aquatic food web dynamics. Stable isotope analysis (SIA) with advanced Bayesian statistics is one way to incorporate this individual trophic variation when quantifying niche size. However, studies using SIA to investigate trophodynamics have mostly focused on species‐ or guild‐level (i.e., assumed similar trophic strategy) analyses in settings where source isotopes are well‐resolved. These parameters are uncommon in an ecological context. Here, we use Stable Isotope Bayesian Ellipses in R (SIBER) to investigate cross‐guild trophodynamics of 11 reef fish species within an oceanic atoll. We compared two‐ ( δ 15 N and δ 13 C) versus three‐dimensional ( δ 15 N, δ 13 C, and δ 34 S) reconstructions of isotopic niche space for interpreting guild‐, species‐, and individual‐level trophic strategies. Reef fish isotope compositions varied significantly among, but also within, guilds. Individuals of the same species did not cluster together based on their isotope values, suggesting within‐species specializations. Furthermore, while two‐dimensional isotopic niches helped differentiate reef fish resource use, niche overlap among species was exceptionally high. The addition of δ 34 S and the generation of three‐dimensional isotopic niches were needed to further characterize their isotopic niches and better evaluate potential trophic strategies. These data suggest that δ 34 S may reveal fluctuations in resource availability, which are not detectable using only δ 15 N and δ 13 C. We recommend that researchers include δ 34 S in future aquatic food web studies.

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Has Nitrogen Supply to Coral Reefs in the South Pacific Ocean Changed Over the Past 50 Thousand Years?

Cited by 9 publications

References 84 publications

Natural nutrient subsidies alter demographic rates in a functionally important coral-reef fish

Natural nutrient subsidies alter demographic rates in a functionally important coral-reef fish

Coral-associated nitrogen fixation rates and diazotrophic diversity on a nutrient-replete equatorial reef

Improving stable isotope assessments of inter‐ and intra‐species variation in coral reef fish trophic strategies

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