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Biophysical drivers of coral trophic depth zonation

TitleBiophysical drivers of coral trophic depth zonation
Publication TypeJournal Article
Year of Publication2018
AuthorsWilliams GJ, Sandin SA, Zgliczynski B.J, Fox M.D, Gove JM, Rogers J.S, Furby KA, Hartmann A.C, Caldwell Z.R, Price N.N, Smith JE
JournalMarine Biology
Date Published2018/04
Type of ArticleArticle
ISBN Number0025-3162
Accession NumberWOS:000429377700002
Keywordsbleached corals; florida-keys; hawaiian corals; internal waves; macroalga enteromorpha; Marine & Freshwater Biology; nutrient-uptake; reef corals; scleractinian corals; spatial-patterns; stable carbon

Depth is used often as a proxy for gradients in energetic resources on coral reefs and for predicting patterns of community energy use. With increasing depth, loss of light can lead to a reduced reliance on autotrophy and an increased reliance on heterotrophy by mixotrophic corals. However, the generality of such trophic zonation varies across contexts. By combining high-resolution oceanographic measurements with isotopic analyses (delta C-13, delta N-15) of multiple producer and consumer levels across depths (10-30 m) at a central Pacific oceanic atoll, we show trophic zonation in mixotrophic corals can be both present and absent within the same reef system. Deep-water internal waves that deliver cool particulate-rich waters to shallow reefs occurred across all sites (2.5-5.6 events week(-1) at 30 m) but the majority of events remained depth-restricted (4.3-9.7% recorded at 30 m propagated to 10 m). In the absence of other particulate delivery, mixotrophs increased their relative degree of heterotrophy with increasing depth. However, where relatively long-lasting downwelling events (1.4-3.3 times the duration of any other site) occurred simultaneously, mixotrophs displayed elevated and consistent degrees of heterotrophy regardless of depth. Importantly, these long-lasting surface pulses were of a lagoonal origin, an area of rich heterotrophic resource supply. Under such circumstances, we hypothesize heterotrophic resource abundance loses its direct linkage with depth and, with resources readily available at all depths, trophic zonation is no longer present. Our results show that fine-scale intra-island hydrographic regimes and hydrodynamic connectivity between reef habitats contribute to explaining the context specific nature of coral trophic depth zonation in shallow reef ecosystems.

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