↓↓ Check out Facebook for some images from our latest survey at Dongsha Atoll, South China Sea. Image above shows the very high soft coral cover on the atoll’s eastern reef terrace ↑↑
The Sumitomo Foundation has awarded an Environmenal Research Grant (環境研究助成) to Dr Wyatt and colleagues for their pioneering work on the environmental drivers of the structure and function of ‘twilight reefs’ (deep-water mesophotic coral ecosystems).
Title: A refuge for coral reef biodiversity: trophic function and reproduction in the twilight zone | 危機に瀕したサンゴ礁生物の避難場所：薄明帯の学際的解明による保全・再生の支援
Participants: Alex S.J. Wyatt, Toshihiro Miyajima, Toshi Nagata, James Leichter (Scripps), Satoshi Mitarai (OIST), Kazuhiko Sakai (U Ryukyu), Rob Toonen (U Hawaii).
Thank you to the Sumitomo Foundation for their support.
Functional understanding of ecosystem-scale inputs and recycling over coral reef communities from stable isotope analyses of organic matter
Alex S.J. Wyatt1*, James J. Leichter2, Benoit Thibodeau1, Toshihiro Miyajima1, Craig A. Carlson3, Craig E. Nelson4, Toshi Nagata1
1Marine Biogeochemistry Laboratory, Department of Chemical Oceanography, Atmosphere & Ocean Research Institute, The University of Tokyo, Kashiwa, Chiba, JAPAN
2Scripps Institution of Oceanography, University of California at San Diego, La Jolla, California, USA
3University of California at Santa Barbara, Santa Barbara, California, USA
4Center for Microbial Oceanography: Research and Education, University of Hawai’i, USA
Stable isotope analyses (SIA) are an increasingly useful tool for understanding functional links between water flow and nutrient cycling over coral reefs, including relative fluxes of oceanic and reef-derived material. SIA have suggested that oceanic particulate organic matter (POM) flowing over reefs can be rapidly metabolized, with the subsequent release of remineralized inorganic nutrients, as well reef-derived POM, representing a significant resource for downstream communities. High oceanic concentrations of dissolved organic matter (DOM) relative to POM suggests DOM may be an even more significant resource, especially around low-POM reefs such as mid-ocean islands and atolls. However, DOM fluxes have rarely been quantified, perhaps due to the refractory nature of oceanic DOM and difficulties linking small concentration changes with spatial changes in both hydrodynamics and macro- and microbial communities. Our Lagrangian studies of DOM around Moorea, French Polynesia and Ishigaki Island, Japan suggest that DOM changes occurring over short spatial scales reflect a balance between uptake and release. SIA further suggest that the release of reef-derived DOM (i.e. enriched in 13C), perhaps relatively labile and from nitrogen fixing organisms (i.e. depleted in 15N), may promote nutrient recycling and supply to downstream communities. Linking SIA and local hydrodynamics offers a promising path towards elucidating the relative functional importance of oceanic and reef-level processes for reef communities.
Key words: dissolved organic matter, fluxes, particulate organic matter, recycling, stable isotope analyses
Biological and Physical Interactions on a Tropical Island Coral Reef: Transport and Retention Processes on Moorea, French Polynesia
Leichter, J.J., Alldredge, A.L., Bernadi, G., Brooks, A.J., Carlson, C.A., Carpenter, R.C., Edmunds, P.J., Fewings, M.R., Hanson, K.M., Holbrook, S.J., Hench, J.L., Nelson, C.E., Schmitt, R.J., Toonen, R.J., Washburn, L. and Wyatt, A.S.J.
The Moorea Coral Reef Long Term Ecological Research project funded by the US National Science Foundation includes multidisciplinary studies of physical processes driving ecological dynamics across the fringing reef, back reef, and fore reef habitats of Moorea, French Polynesia. A network of oceanographic moorings and a variety of other approaches have been used to investigate the biological and biogeochemical aspects of water transport and retention processes in this system. There is evidence to support the hypothesis that a low-frequency counterclockwise flow around the island is superimposed on the relatively strong alongshore currents on each side of the island. Despite the rapid flow and flushing of the back reef, waters over the reef display chemical and biological characteristics distinct from those offshore. The patterns include higher nutrient and lower dissolved organic carbon concentrations, distinct microbial community compositions among habitats, and reef assemblages of zooplankton that exhibit migration behavior, suggesting multigenerational residence on the reef. Zooplankton consumption by planktivorous fish on the reef reflects both retention of reef-associated taxa and capture by the reef community of resources originating offshore. Coral recruitment and population genetics of reef fishes point to retention of larvae within the system and high recruitment levels from local adult populations. The combined results suggest that a broad suite of physical and biological processes contribute to high retention of externally derived and locally produced organic materials within this island coral reef system.
Particulate nutrient fluxes over a fringing coral reef: Source-sink dynamics inferred from carbon to nitrogen ratios and stable isotopes
Alex S. J. Wyatt, Ryan J. Lowe, Stuart Humphries and Anya M. Waite
We examined spatial and temporal variations in particulate organic matter (POM) dynamics over a fringing coral reef (Ningaloo Reef) in Western Australia during the austral autumn and spring. Total POM concentrations generally did not differ between seasons or reef zones, but the composition of POM, in terms of carbon isotope ratios (δ13C-POM), carbon to nitrogen ratios (C : N), and fatty acids, changed consistently in water flowing across the reef. Both δ13C-POM and C : N increased from the fore reef to the reef flat and lagoon, −23.0‰ to −20.1‰ and 7.31 to 8.34, respectively. Average rates of net POM uptake by the reef community were highest over the reef crest (4 to 30 mmol N m−2 d−1 and 6 to 130 mmol C m−2 d−1), with a Bayesian isotope model confirming independent measurements of high uptake rates of allochthonous POM (oceanic phyto- and zooplankton). In contrast, over the reef flat, net release of POM was observed (−4 to −5 mmol N m−2 d−1 and −50 mmol C m−2 d−1), with gross release rates (estimated as −6 to −8 mmol N m−2 d−1 and −30 to −90 mmol C m−2 d−1) indicating that the release of autochthonous POM may be of similar magnitude to allochthonous uptake. Examining POM dynamics in terms of gross fluxes reinforces the dependence of coral reef systems on oceanographic processes for allochthonous POM supply, as well as highlighting the potential for autochthonous POM production to supply nutrients to benthic and pelagic communities downstream.
Stable isotope analysis reveals community-level variation in fish trophodynamics across a fringing coral reef
A. S. J. Wyatt, A. M. Waite, S. Humphries
In contrast to trophodynamic variations, the marked zonation in physical and biological processes across coral reefs and the concomitant changes in habitat and community structure are well documented. In this study, we demonstrate consistent spatial changes in the community-level trophodynamics of 46 species of fish across the fringing Ningaloo Reef, Western Australia, using tissue stable isotope and fatty acid analyses. Increasing nitrogen (δ15N) and decreasing carbon (δ13C) isotope ratios in the tissues of herbivores, planktivores and carnivores with increasing proximity to the ocean were indicative of increased reliance on oceanic productivity. In contrast, detritivores and corallivores displayed no spatial change in δ15N or δ13C, indicative of the dependence on reef-derived material across the reef. Higher δ13C, as well as increased benthic- and bacterial-specific fatty acids, suggested reliance on reef-derived production increased in back-reef habitats. Genus-level analyses supported community- and trophic group-level trends, with isotope modelling of species from five genera (Abudefduf sexfasciatus, Chromis viridis, Dascyllus spp.,Pomacentrus spp. and Stegastes spp.), demonstrating declining access to oceanic zooplankton and, in the case of Pomacentrus spp. and Stegastes spp., a switch to herbivory in the back-reef. The spatial changes in fish trophodynamics suggest that the relative roles of oceanic and reef-derived nutrients warrant more detailed consideration in reef-level community ecology.
Alex S. J. Wyatt, James L. Falter, Ryan J. Lowe, Stuart Humphries and Anya M. Waite
Nitrate and nitrite (NOx) and phosphate (PO4) dynamics over Ningaloo Reef, Western Australia, are shown to depend on oceanographic forcing of coupled mass transfer limited (MTL) gross uptake and gross release from remineralized oceanic particulate organic matter (POM). Estimates of gross release rates increased significantly with increasing POM uptake and were of the same order as gross uptake rates. Gross uptake rates increased significantly with increasing oceanic concentrations and wave energy dissipation, were 35–80% higher over the reef crest (7–9 mmol NOx m−2 d−1 and 4–5 mmol PO4 m−2 d−1), and were significantly correlated with independent estimates of POM-mediated gross NOx uptake, supporting both MTL uptake and the strong role of oceanic POM supply. The relative supply of NOx and POM was linked to the seasonal dynamics of a regional current system. In late spring, upwelling associated with seasonally strong equator-ward winds led to increased NOx concentrations (0.71 ± 0.2 µmol L−1), POM < NOx and the reef was a net nutrient sink (5390 mmol NOxm−1 d−1 and 270 mmol PO4 m−1 d−1). In contrast, during the autumn, NOx was low (0.16 ± 0.06 µmol L−1), but POM > NOx and the reef was a net nutrient source (−7060 mmol NOx m−1 d−1 and −730 mmol PO4 m−1 d−1). The autumn enhancement of oceanic POM supply to the reef can be attributed to a regional phytoplankton bloom associated with acceleration of the oligotrophic Leeuwin Current, which may result in a significant supply of dissolved nutrients to downstream communities.