Lophelia pertusa at VK826

The Lophelia pertusa community at Viosca Knoll (VK826) is the most extensive found to date in the Gulf of Mexico. As part of a multi-disciplinary study, the physical setting of this area was described using benthic landers, CTD transects and remotely operated vehicle observations. The site was broadly characterised into three main habitats: (1) dense coral cover that resembles biogenic reef complexes, (2) areas of sediment, and (3) authigenic carbonate blocks with sparse coral and chemosynthetic communities. The coral communities were dominated by L. pertusa but also contained numerous solitary coral species. Over areas that contained L. pertusa, the environmental conditions recorded were similar to those associated with communities in the north-eastern Atlantic, with temperature (8.5–10.6 °C) and salinity (35) falling within the known species niche for L. pertusa. However, dissolved oxygen concentrations (2.7–2.8 ml l⁻¹) and density (σ_Θ, 27.1–27.2 kg m⁻³) were lower and mass fluxes from sediment trap data appeared much higher (4002–4192 mg m⁻² d⁻¹). Yet, this species still appears to thrive in this region, suggesting that L. pertusa may not be as limited by lower dissolved oxygen concentrations as previously thought. The VK826 site experienced sustained eastward water flow of 10–30 cm s⁻¹ over the 5-day measurement period but was also subjected to significant short-term variability in current velocity and direction. In addition, two processes were observed that caused variability in salinity and temperature; the first was consistent with internal waves that caused temperature variations of 0.8 °C over 5–11 h periods. The second was high-frequency variability (20–30 min periods) in temperature recorded only at the ALBEX site. A further pattern observed over the coral habitat was the presence of a 24 h diel vertical migration of zooplankton that may form part of a food chain that eventually reaches the corals. The majority of detailed studies concerning local environmental conditions in L. pertusa habitats have been conducted within the north-eastern Atlantic, limiting most knowledge of the niche of this species to a single part of an ocean basin. Data presented here show that the corals at VK826 are subjected to similar conditions in temperature, salinity, and flow velocity as their counterparts in the north-east Atlantic, although values for dissolved oxygen and density (sigma-theta: σ_Θ) are different. Our data also highlight novel observations of short-term environmental variability in cold-water coral habitat.

In press in Deep Sea Research vol 1.

Full Citation

Davies, A.J., Duineveld, G.C.A., van Weering, T.C.E., Mienis, F., Quattrini, A.M., Seim, H.E., Bane, J.M. & Ross, S.W. (in press) “Short-term environmental variability in cold-water coral habitat at Viosca Knoll, Gulf of Mexico.” Deep-sea Research Vol 1. DOI: 10.1016/j.dsr.2009.10.012.

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SAMS/Bangor Mini-lander

With my long standing collaborator Kim Last from SAMS and MSc student Harri Condie, we recently conducted a field trip to Sabellaria spinulosa habitats in the Wash. Sabellaria spinulosa is a polychaete worm that builds dwelling tubes out of suspended particles. As part of a project with Kim, funded by the Aggregate Levy, we are working to better understand these amazing reef-building creatures.

Armed with the best gear we could find at short-notice, we constructed a mini-lander system to be deployed near a S. spinulosa reef. We equipped the system with sensors that included a laser particle sizer set to provide information on the particle supply in the region, a current meter, temperature loggers and a CTD system logging not only temperature, salinity and depth but also light attenuation and dissolved oxygen.

Kim with the VideoRay ROV

We left the lander on the seafloor for 24 hours, providing a valuable insight into the particles that may be supplied to the S. spinulosa reef. To supplement this, we also placed out sediment traps on the amazing intertidal reef at Scotsman’s Sled. These traps are basically tubes that allow sediment to settle to the bottom, we then take the sediment back to the lab and look at the type and quantity of particles, including any organic food particles that land into the trap. These particles allow us to verify the readings from the particle sizer but also provide a vital clue to why S. spinulosa is found in that area.

We were out with the Eastern Sea Fisheries Joint Comittee on board their boat the Three Counties. They even let us have a go with their amazing little ROV, a VideoRay. Seriously, we’re thinking that we could do with one! Many thanks to ESFJC and the crew for supporting our research.

We’ve only been back a few days, so our data is still being downloaded off our instruments. But these observations will hopefully be the first of many, as we attempt to determine the underlying environmental controls for S. spinulsa. This work was related to the MEPF/ALSF project (MEPF 08/p76), where we are using our novel VoRT tanks to better understand the impact of aggregate dredging on a range of different species.

In 2006 and 2007, multiple deployments of current meters and optical sensors on landers and moorings were made in the first detailed in situ study of the particle supply to the coral community in the Mingulay Reef complex in the Sea of Hebrides at 140 m water depth. Two distinct and predictable supply mechanisms were resolved. One mechanism consisted of the rapid downwelling of surface water caused by hydraulic control of tidal flow that transports particles from the surface to the corals in less than an hour. The rapid downwelling was recorded on the reef top as a pulse of warm, fluorescent, and relatively clear water at the onset of the flood and ebb tides. The pulse was strongest after flood tide and lasted for up to 3 hours. The second mechanism consisted of advection onto the reef of deep bottom water with a high suspended matter load. This advection occurred during peak tides and was combined with topographical current acceleration on the reef top enhancing delivery of particles to the corals.

Published as feature article in Limnology and Oceanography

http://www.aslo.org/lo/pdf/vol_54/issue_2/0620.pdf

Full citation

Davies, A.J., Duineveld, G.C.A., Lavaleye, M.S.S., Bergman, M.J.N., van Haren, H. & Roberts, J.M. (2009) “Downwelling and deep-water bottom currents as food supply mechanisms to the cold-water coral Lophelia pertusa (Scleractinia) at the Mingulay Reef Complex.” Limnology and Oceanography 54(2): 620-629.

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