Impacts of internal waves on the twilight zone @ ICRS 2016, Hawaii (Dr Wyatt)

Ecological and biogeochemical impacts of internal waves on mesophotic coral ecosystems: testing eddy correlation and isotope approaches, Iriomote, Japan

Alex S.J. Wyatt1*, Toshihiro Miyajima1, James J. Leichter2, Tohru Naruse3, Tomohiro Kuwae4, Shoji Yamamoto5, Naomi Satoh1, Toshi Nagata1

1Department of Chemical Oceanography, Atmosphere and Ocean Research Institute, The University of Tokyo, Kashiwa, Chiba, JAPAN
2Scripps Institution of Oceanography, University of California at San Diego, La Jolla, California, USA
3Tropical Biosphere Research Center, University of the Ryukyus, Taketomi, Japan
4Coastal and Estuarine Environment Research Group, Port and Airport Research Institute (PARI), Nagase, Yokosuka, JAPAN
5Department of Earth and Planetary Science, The University of Tokyo, Bunkyo-ku, Tokyo, Japan

While mesophotic coral ecosystems (MCE) may be protected or damped from disturbances impacting shallower reefs insufficient information is available on the environmental conditions supporting these ‘deep water refugia’. Nutrient inputs and recycling have rarely been quantified over MCE but may differ fundamentally to that of shallow counterparts due to the reduction in light and increasing reliance on oceanic nutrients, leading to increased heterotrophy over autotrophy at species and ecosystem levels and stronger links to oceanic processes. For instance, due to the depth of MCE relative to typical water column density stratification, internal waves may be a highly significant process depending on community aspect and exposure. Preliminary observations of MCE along a continuum of oceanic exposure in Funauki Bay, Iriomote, Japan indicate that ocean-exposed MCE are subject to semi-diurnal temperature oscillations of up to 4 C during summer (range 23 – 29 deg C), while inner MCE occur shallower in more turbid but stable environments. Oceanic exposure along the bay may determine both the distribution and function of spatially extensive, but relatively homogenous, communities dominated by Leptoseris sp. or Acropora ?horrida. Combining bulk and compound-specific stable isotope analyses, depth-specific radioisotope markers such as radiocarbon, and eddy correlation experiments in these habitat promises a useful approach for elucidating the functional importance of internal waves in the development and persistence of MCE at local to regional scales.

Formation and maintenance of high-nitrate, low pH layers (Waite et al., Biogeosciences)

Formation and maintenance of high-nitrate, low pH layers in the eastern Indian Ocean and the role of nitrogen fixation

A. M. Waite, V. Rossi, M. Roughan, B. Tilbrook, P. A. Thompson, M. Feng, A. S. J. Wyatt, and E. J. Raes

We investigated the biogeochemistry of low dissolved oxygen high-nitrate (LDOHN) layers forming against the backdrop of several interleaving regional water masses in the eastern Indian Ocean, off northwest Australia adjacent to Ningaloo Reef. These water masses, including the forming Leeuwin Current, have been shown directly to impact the ecological function of Ningaloo Reef and other iconic coastal habitats downstream. Our results indicate that LDOHN layers are formed from multiple subduction events of the Eastern Gyral Current beneath the Leeuwin Current (LC); the LC originates from both the Indonesian Throughflow and tropical Indian Ocean. Density differences of up to 0.025 kg m−3 between the Eastern Gyral Current and the Leeuwin Current produce sharp gradients that can trap high concentrations of particles (measured as low transmission) along the density interfaces. The oxidation of the trapped particulate matter results in local depletion of dissolved oxygen and regeneration of dissolved nitrate (nitrification). We document an associated increase in total dissolved carbon dioxide, which lowers the seawater pH by 0.04 units. Based on isotopic measurements (δ15N and δ18O) of dissolved nitrate, we determine that ~ 40–100% of the nitrate found in LDOHN layers is likely to originate from nitrogen fixation, and that, regionally, the importance of N-fixation in contributing to LDOHN layers is likely to be highest at the surface and offshore.

Picoplankton and virus uptake by a coral reef (Patten et al., Coral Reefs)

Uptake of picophytoplankton, bacterioplankton and virioplankton by a fringing coral reef community (Ningaloo Reef, Australia)

Nicole L. Patten, Alex S.J. Wyatt, Ryan J. Lowe, Anya M. Waite

We examined the importance of picoplankton and virioplankton to reef trophodynamics at Ningaloo Reef, (north-western Australia), in May and November 2008. Picophytoplankton (Prochlorococcus,Synechococcus and picoeukaryotes), bacterioplankton (inclusive of bacteria and Archaea), virioplankton and chlorophyll a (Chl a) were measured at five stations following the consistent wave-driven unidirectional mean flow path of seawater across the reef and into the lagoon.Prochlorococcus, Synechococcus, picoeukaryotes and bacterioplankton were depleted to similar levels (~40% on average) over the fore reef, reef crest and reef flat (=‘active reef’), with negligible uptake occurring over the sandy bottom lagoon. Depletion of virioplankton also occurred but to more variable levels. Highest uptake rates, m, of picoplankton occurred over the reef crest, while uptake coefficients, S (independent of cell concentration), were similarly scaled over the reef zones, indicating no preferential uptake of any one group. Collectively, picophytoplankton, bacterioplankton and virioplankton accounted for the uptake of 29 mmol C m−2 day−1, with Synechococcuscontributing the highest proportion of the removed C. Picoplankton and virioplankton accounted for 1–5 mmol N m−2 day−1 of the removed N, with bacterioplankton estimated to be a highly rich source of N. Results indicate the importance of ocean–reef interactions and the dependence of certain reef organisms on picoplanktonic supply for reef-level biogeochemistry processes.

Keywords: Coral reef, Picoplankton, Virus, Uptake, Ningaloo Reef, Indian Ocean

Coral reef phytoplankton fluxes (Wyatt et al., MEPS)

Particulate nutrient fluxes over a fringing coral reef: relevant scales of phytoplankton production and mechanisms of supply

Alex S. J. Wyatt, Ryan J. Lowe, Stuart Humphries, Anya M. Waite

Seasonal observations of phytoplankton uptake at Ningaloo Reef, Western Australia, reinforce the importance of particulate organic nitrogen (PON) and carbon (POC) in reef nutrient budgets and identify wave action and the dynamics of regional currents (over a range of temporal and spatial scales) as important factors determining plankton supply to the reef. Phytoplankton uptake rates, calculated from declining chlorophyll a concentrations as water moved over the reef, appeared to be near the physical limits of mass transfer. Phytoplankton-derived PON flux of 2 to 5 mmol N m–2 d–1 was on the order of that typical for dissolved N uptake—confirming that particle feeding may supply the N missing in reef N budgets—while POC flux of 14 to 27 mmol C m–2 d–1 was on the order of net community metabolism. Phytoplankton supply was highly variable at daily-to-seasonal time scales in response to the dynamics of a regional current system dominated by the downwelling-favourable Leeuwin Current (LC). Acceleration of the LC in the austral autumn may supply as much phytoplankton to the reef as sporadic upwelling associated with the Ningaloo Current (NC) in summer. The ocean catchment concept is introduced as a basis for examining the spatial scale of pelagic processes influencing benthic systems: every day, Ningaloo may completely consume the phytoplankton over 87 km2 of LC water, compared to only 20 km2 of NC water. Production within this catchment appears insufficient to maintain offshore phytoplankton concentrations, and advection of remotely sourced production into the catchment is required to balance reef uptake. A functional dependence by reef organisms on externally sourced ocean productivity increases the potential scale at which human- or climatically induced changes may affect reef communities and suggests that processes such as changes in offshore currents and plankton communities require further consideration in reef-level biogeochemistry.

KEY WORDS: Ningaloo Reef · Nutrient budget · Oceanographic forcing · Particulate organic carbon · Particulate organic nitrogen · Leeuwin Current · Ocean catchment · Upwelling