Bio-physical interactions

Hubbs (1959) put forward the theory that seamounts and banks may act as stepping stones for the transgression of bathyal and benthic fauna and/or their larvae across otherwise abyssal depths. Many studies into this theory have focussed on indirect genetic methods for tracking population connectivity over an evolutionary timescale, but on a shorter timescale other methods are needed. Within this project being undertaken by Rebecca Ross and is supervised by Dr Kerry Howell, Dr Alex Nimmo Smith and Dr Vasyl Vlasenko, we are coupling the outputs from an established oceanographic numerical model with a Lagrangian particle tracer to map the possible movements of larvae, treated as passive tracers with limited behaviours, from seamounts and banks in the NE Atlantic (Figure 1). This method will be combined with biological habitat suitability modelling to track the potential for recruitment at other seamounts and banks and assess Hubbs's theory around UK and Irish waters. Results from this study will also have applications in conservation, where dynamics of source and sink populations are important in the establishment of an ecologically coherent network of marine protected areas. Although in its infancy the project is based on a pilot study being prepped for publication where larval dispersal to and from Anton Dohrn Seamount was simulated at regular compass directions and at various depths. The results of this study show that only species with a planktonic larval duration of greater than 50 days would be able to disperse to and from Anton Dohrn Seamount summit when restricted to travelling at the same depth they were released from. Deeper release locations were found to require even longer larval durations which would make it difficult to connect populations. Hard substrate dwelling organisms at all depths must be heavily reliant upon the fragmented habitat provided by glacial drop stones and cobbles along the continental shelf in order to maintain connectivity between the larger known areas of suitable habitat.

This project, funded by NERC and in collaboration with Plymouth Marine Laboratory (PML), aims to identify the physical factors in the marine environment that are important to top predator foraging. We are considering, firstly, how is predator foraging influenced by temporal changes in the structure of the water column and, secondly, is predator foraging influenced spatially by physical oceanography (i.e. frontal zones, thermocline depth).

To address these separate questions, we conducted tidal station and transect surveys at the frontal region off the north Cornwall coast during 2012 and 2013 (Figure 2). During both surveys two observers continuously recorded the abundance and behaviour of marine mammals and seabirds whilst physical characteristics of the water column were measured using, during the tidal stations, an MSS 90 microstructure/turbulence profiler and, during the transect surveys, a towed undulating conductivity-temperature-depth sensor equipped with a chlorophyll-tuned fluorometer. Currents were measured in both cases using a hull-mounted 300kHz ADCP. The analysis phase of this project is still in its early stages but initial results demonstrate a clear correspondence between gannet foraging and front location.

In addition to the in-situ observations made off the north Cornwall coast we have, in collaboration with the Marine Biology and Ecology Research Centre (MBERC) at the University, also tagged a long ranging foraging predator, the northern gannet (Morus bassunus), from Grassholm island to determine how the physical structure of the water column can influence the fine-scale behaviour of the gannets (Figure 3). Birds were equipped with a GPS logger and TDR (temperature-depth) recorder during the summer field seasons of 2010, 2011, 2012 and 2013. This allowed for bird location, dive location and depth of dive to be measured. The first question to be addressed within this project will be if the thermal structure of the water column influences the depth to which gannets dive (Figure 4). This will be established using the GPS and TDR data alongside outputs from the coastal ocean circulation model FVCOM.

A recently completed project used the combination of a free-falling microstructure profiler (MSS) and holographic imaging (the holocam) to better understand the dynamics of suspended particles in coastal seas. Surveys were conducted across a single tidal cycle, whereby both instruments were deployed near-simultaneously. The MSS is able to resolve important parameters such as temperature, salinity and optical backscatter (OBS), in addition to its principal function which is to generate an estimate for the rate of turbulent dissipation, ε. The holocam delivers in situ images of suspended particles, allowing identification of both biological and flocculated suspended particulate matter (SPM) (Figure 5).

During a survey in September 2010, the combination of enhanced vertical mixing brought about by an increase in wind stress (Figure 5b), and advection of a water mass from the south of the L4 site in the Western English Channel, resulted in marked changes to the phytoplankton population. The data from the holocam allows the user to count individual phytoplankton particles, thus determining how events brought about by the changing conditions may impact the phytoplankton population. The L4 site is dominated by large, chain-forming diatoms. On this basis, phytoplankton > 200μm in size were identified and counted. The counts revealed that far greater numbers of phytoplankton were observed during the period of relative calm.

Supporting data from the holocam indicate that a combination of factors contribute to this change. A reduction of particle size, likely a result of the enhanced mixing breaking the diatom chains, and a change in composition as diatoms begin to aggregate with other SPM forming flocs can be directly attributed to the increased turbulence. Further, advection of water from the south is likely to transport water away from the region being sampled, advecting the phytoplankton population with it.

Cross, J., W. A. M. Nimmo Smith, R. J. Torres, P. J. Hosegood, 2013. Biological controls on resuspension and the relationship between particle size and the Kolmogorov length scale in a shallow coastal sea. Marine Geology, 343, 29-38, doi:10.1016/j.margeo.2013.06.014.

Cross, J., W. A. M. Nimmo Smith, P. J. Hosegood, R. J. Torres, 2013. The dispersal of phytoplankton by enhanced turbulent mixing in a shallow coastal sea. Journal of Marine Systems, submitted.

Did you think the ocean is just plain blue? Look again. Ocean colour sensors have been measuring reflectances since the late 1970s, documenting the variations in how much light is backscattered out of the water in several visible wavebands. Figure 6a shows the sea surface from the satellite's perspective more or less as we might see it with the human eye. Figure 6b shows the detail that can be found using a simple colour enhancement.

Current satellite sensors only measure a few wavebands but as technology progresses, spectral resolution is increasing. If we could measure at very fine spectral resolution, we would see something like the in situ reflectance data in (Figure 7). Instead of a spiky, blue-biased spectrum, there are now smaller features that correspond to absorption by phytoplankton pigments.

Several approaches have been developed around the world to interpret ocean colour information in terms of particular phytoplankton species or groups, especially those that are important for nutrient cycling or carbon sequestration. However, we don't know the typical absorption characteristics of all phytoplankton species (there are thousands!), or even the variation in absorption characteristics within a particular phytoplankton group. That is the gap we are trying to address in this project, beginning with measurement protocols and slowly working through species and groups. Preliminary work is being funded by the School of Marine Science & Engineering here at the University.