Biophysical drivers of coral trophic depth zonation
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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 (δ13C, δ15N) 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.
Permission to work within Palmyra Atoll National Wildlife Refuge was granted by a Special Use Permit (SUP 12533-13025) issued by the U.S. Fish and Wildlife Service (USFWS). We thank the USFWS and The Nature Conservancy for providing logistical support, Stephen Monismith, Robert Dunbar and David Koweek for providing additional temperature data, and Mattias Green for constructing Fig. 1b. Scripps Institution of Oceanography and Stanford University are members of the Palmyra Atoll Research Consortium (PARC). This is PARC publication number PARC-TBD
GJW and SAS conceived the project; GJW led the writing with SAS; GJW, SAS, BJZ, KAF, JMG, JSR, ZRC, NNP and JES collected data; GJW, MDF, KAF and ACH performed the lab work; GJW, JMG, and JSR analysed the data.
Compliance with ethical standards
Research involving animals
All applicable international, national, and/or institutional guidelines for the care and use of animals were followed.
Conflict of interest
The authors declare that they have no conflict of interest.
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