Research Physics controls on marine ecosystems

Satellite images all courtesy of the NERC Earth Observation Data Acquisition and Analysis Service (NEODAAS).

Distribution of fishing activity in the Celtic Sea. VMS vessel position data for 2008 were analysed to produce time series of vessel speeds. Fishing was assumed to be taking place when vessel speed <4 knots.

Main current projects: CarTRidge Enhanced carbon export driven by internal tides over the mid-Atlantic ridge starts from the findings of our Ridgemix research. We think that the Ridgemix results imply that our current understanding of how the ocean affects Earth’s climate is missing a significant process: we underestimate ocean carbon export because we do not include the effects of internal waves over mid-ocean ridges and seamounts. Internal waves are generated when tidal currents flow over rough or sloping seabed. Over the mid-Atlantic ridge we know that these waves cause turbulence and supply more nutrients upward to the plankton in the surface ocean. At the same time, the oscillations of the waves bring the phytoplankton closer to the sea surface, and so increase the sunlight received for photosynthesis. Increased supplies of light and nutrient resouces are thought to shift the size structure of the plankton community to larger species, and larger plankton are better at exporting particles of carbon to the deep ocean. Bacteria are constantly recycling dead plankton back to inorganic material, including changing organic carbon back to inoganic carbon (i.e. CO 2 ). For small particles the recycling to CO 2 can occur near the sea surface, and so the extra CO 2 can be released back to the atmosphere. Larger particles sink faster, so they get to the deep ocean before bacteria can break their organic carbon down to CO 2 . If the CO 2 is produced deeper in the ocean, then the density layers of the ocean prevent it from being mixed back upward to the sea surface and the atmosphere: the CO 2 is locked away from the atmosphere for the time it takes for the ocean’s deep-water to be recirculated back to the surface, which is several hundred to one thousand years. We estimate that, averaged over the whole Atlantic Ocean, internal waves over the mid-Atlantic ridge could be causing about 40% more carbon export that we currently think happens. Getting this right is important - we need to have an accurate budget of how carbon is cycled through the ocean to assess our progress to net-zero carbon emissions, and we need to understand the ocean processes that support the carbon export and how those processes might change as our climate warms. The CarTRidge project is led by the University of Liverpool, with collaborators at the University of Southampton, Heriott-Watt University and the National Oceanography Centre. We will be running a major expedition to the mid-Atlantic ridge in the South Atlantic in early 2025, making measurements linking from the fundamental physics of ocean turbulence, up through the growth of the plankton and the make-up of the plankton community, to the production of sinking particles and the export of carbon. We are funded by UKRI through the Pushing the Frontiers grant scheme. eSWEETS3 Enabling Sustained Wind Energy Expansion in Seasonally Stratified Shelf Seas is a project funded by UKRI aimed at determining how floating offshore wind turbines might affect phytoplankton growth in the thermally-stratifying seas around the UK. The seasonal thermocline, which separates the warm surface layer from cooler deeper waters during spring and summer, is a critical feature in the control of the growth of phytoplankton (see, for instance, our earlier work on turbulence and phytoplankton). As a result of controlling phytoplankton growth, the thermocline also plays a role in the productive shelf sea fisheries and in the ability of shelf seas to draw down atmospheric CO2. As offshore wind energy generation pushes out into deeper water, the turbines can no longer be fixed to the seabed and must instead be designed as large, floating structures (see image on the right). The base of these floating turbines reaches down into the thermocline, so as tidal currents flow past the structures we expect significant extra turbulence to be generated in the thermocline. This turbulence will mix nutrients from the deeper water up to the phytoplankton in the thermocline (and possibly further up into the surface water). Extra nutrients will drive extra phytoplankton growth - or, that’s our hypothesis at least. We will be working with several industry and regulatory partners towards understanding what the large-scale roll-out of floating offshore wind turbines will do to the productivity of shelf seas. The work begins in 2024 using computer simulations to get some initial pictures of the potential effects of the turbine structures, and we will have a major ship-based programme of work in summer 2025. Useful background paper: Dorrell, R. M., C. J. Lloyd, B. J. Lincoln, T. P. Rippeth, J. R. Taylor, C. P. Caulfield, J. Sharples, J. A. Polton, B. D. Scannell, D. M. Greaves, R. A. Hall & J. H. Simpson, 2022. Anthropogenic mixing in seasonally stratified shelf seas by offshore wind farm infrastructure. Frontiers in Marine Science, 9, doi.org/10.3389/fmars.2022.830927. Other finished research: Ridgemix Shelf Sea Biogeochemistry Turbulence and phytoplankton in the thermocline Internal tides and the shelf edge ecosystem From physics to fish over banks in a shelf sea RidgeMix The RidgeMix project, funded by the UK Natural Environment Research Council, was based on the hypothesis that mixing generated by tidal flows over the mid-Atlantic ridge bring nutrients upward into the deep chlorophyll maximum (a layer of phytoplankton about 100 metres below the sea surface). In September 2015 we deployed a moored array of instruments on the crest of the ridge to measure mixing and nutrients in the upper 1 km of the ocean. In summer 2016 we revisited the area on a dedicated research cruise to make detailed measurements of nutrient supplies above and away from the ridge, and to recover the moored instruments. The first major result to be published was an analysis of the structure of the internal tidal waves, as seen by the mooring deployed on top of the mid-Atlantic ridge between September 2015 and July 2016. We found that about 50% of the internal tide energy went into mode 1 internal tidal wave, with the other 50% in higher modes (modes 2 - 10). The “mode number” of the wave decsribes how the water movement associated with the wave is layered through the ocean: mode 1 has 2 layers, mode 2 has 3 layers etc.. The importance of this modal structure is that it tells us how much energy is available locally for vertical mixing. The mode 1 wave propagates away from the ridge, adding low amounts of energy to the abyssal ocean mixing as it travels across the Atlantic Ocean. The higher modes all contribute to the local mixing over the ridge; this is what we were looking for, as this mixing can drive nutrients upward towards the sea surface. These results from the mooring data are published in Vic et al., 2017. The second major result was the analysis of our direct observations of the rate at which nutrients were being mixed upward over the mid- Atlantic ridge. Key papers: Vic, C., A. C. Naveira-Garabato, J. A. M. Green, C. Spingys, A. Forryan, Z. Zhao & J. Sharples, 2018. The lifecycle of semidiurnal internal tides over the northern mid-Atlantic ridge. Journal of Physical Oceanography, DOI:10.1175/JPO-D-17-0121.1. Tuerena, R. E., R. G. Williams, C. Mahaffey, C. Vic, J. A. M. Green, A. Naveira-Garabato, A. Forryan, & J. Sharples, 2019. Internal tides drive nutrient fluxes into the deep chlorophyll maximum over mid-ocean ridges. Global Biogeochemical Cycles, DOI:10.1029/2019GB006214. Shelf Sea Biogeochemistry We were the lead research institute in the pelagic component of the NERC research programme that aimed to address the role of the shelf seas in the global cycling of carbon and nutrients. The fieldwork work took place over 2014-15, with a major programme of 7 research cruises, working alongside other projects on shelf sea sediments and on the shelf seas as an iron supply to the global ocean. Our overall objectives were to: 1. Estimate the size of the shelf carbon pump over the whole NW European shelf, and its relation to the global carbon cycle. 2. Determine the relative importance of external nutrient sources and internal biogeochemical cycling in maintaining the shelf carbon pump. My cruise blog is at: http://jonathanatsea.wordpress.com/ A Special Issue of the journal Progress in Oceanography has been published, detailing some of the key results of this work (volume 177, October 2019). Turbulence and phytoplankton in the thermocline In summer in the temperate shelf seas around the UK large areas of the sea stratify, as solar heating warms up the upper 10 - 30 metres of water. This stratification plays a vital role in controlling phytoplankton growth. When the warm layer develops in spring the phytoplankton held within it grow rapidly (the “spring bloom”), removing nutrients from the surface waters. The thermocline, separating the warm upper layer from the colder bottom waters, severely limits the mixing of nutrients to the phytoplankton so that a lack of nutrients (particularly nitrate) limit phytoplankton growth in the surface layer during summer. The thermocline is not a perfect barrier to the mixing of deep water nutrients up towards the nutrient-starved phytoplankton. There is a weak turbulent flux of nutrients into the base of the thermocline, where it is intercepted by a community of phytoplankton that have adapted their ability to photosynthesise to the low light levels. The plot on the right shows a water column typical vertical structure from the Celtic Sea in summer. Note the surface and bottom mixed layers (regions of homogeneous temperature) separated by a broad thermocline. The chlorophyll, indicating where the phytoplankton are, is highest in a broad peak within the thermocline, situated also within the region where the nutrient concentration (inorganic nitrogen in the form of nitrate) drops from the high levels in the bottom water to almost zero in the surface layer. This nitrate gradient, termed the “nitracline”, is where turbulent mixing can drive a flux of nitrate from the bottom waters into the phytoplankton layer. We measure the flux by combining the sample measurements in the vertical profile with observations of the turbulent diffusivity collected using a free-fall turbulent microstructure instrument. Typically we will try to collect profiles continuously over 25 hours, so collecting data over 2 full tidal cycles, and calculate the daily flux of nitrate to the phytoplankton. In regions of flat seabed on the shelf we find a vertical flux of about 1 - 2 mmol m -2 day -1 . This is at least a factor of about 10 higher than what is measured in the open ocean, mainly because of the action of the tidal currents, which are much stronger on the shelf, in generating turbulence at the base of the thermocline and also generating vertical oscillations of the thermocline (internal waves) which also cause turbulence. We have also shown that the very low levels of turbulent mixing in the thermocline allows different species of phytoplankton to grow in different parts of that broad chlorophyll layer. The vertical separation of species is largely determined by the vertical changes in the spectrum of light and the ability of different species to utilise different wavelengths of light in photosynthesis. The low turbulence provides sufficient time for the different species to grow in the regions where the light is optimal for them. Useful papers: Williams, C., J. Sharples, C. Mahaffey & T. Rippeth, 2013. Wind- driven nutrient pulses to the subsurface chlorophyll maximum in seasonally stratified shelf seas. Geophys. Res. Lett., 40, 5467-5472, doi:110.1002/2013GL058171 Hickman, A.E., P. M. Holligan, C. M. Moore, J. Sharples, V. Krivtsov, M. R. Palmer. 2009. Distribution and chromatic adaptation of phytoplankton within a shelf sea thermocline. Limnology and Oceanography, 54(2), 525-536. Sharples, J., C. M. Moore, T. P. Rippeth, P. M. Holligan, D. J. Hydes, N. R. Fisher, & J. H. Simpson. 2001. Phytoplankton distribution and survival in the thermocline. Limnology and Oceanography, 46(3), 486- 496. Internal tides and the shelf edge ecosystem The edges of the continental shelves, where the depth of the ocean begins to increase rapidly from 100 - 200 metres in the shelf seas to the abyssal depths of the open ocean, are often found to be sites of high biological activity. Physical processes associated with the relatively steeply-sloping seabed can supply nutrients from the deep ocean to the shelf, and we often find the shelf edge to be targeted by fishing vessels. A simple link is frequently made between the nutrient supply enhancing primary production and the environment being subsequently favourable to fish. The shelf edge of the Celtic Sea sees large levels of fishing, with vessels targeting spawning stocks of mackerel, horse mackerel, hake and whiting. This fishing activity is very tightly focused over the 200 metre depth contour, which marks the shelf edge. A key component of the physics here is a strong internal tide which, particularly at spring tides, breaks at the shelf edge and generates considerable turbulent mixing. This localised area of mixing is often seen in satellite images of summer sea surface as a band of cool water lying along the 200 metre isobath. An example is shown on the right, with a band of water at about 14.5 - 15 o C at the shelf edge compared to 16.5 - 18 o C away from the shelf edge. This cool water has been mixed upward and brings with it increased concentrations of nutrients (e.g. see Pingree & Mardell, Phil. Trans. Roy. Soc. Lond., A302, 663-682, 1981). We have found that this nutrient supply does indeed increase shelf edge primary production, by about a factor of 2 above that either on the shelf or in the adjacent NE Atlantic. The effect of this mixing and nutrient supply of the surface chlorophyll concentration is also clear in satellite images, with shelf edge chlorophyll being enhanced around much of the shelf edge to the southwest, west and northwest of the UK and Ireland (see the second satellite image on the right). A more intriguing observation that we have made over the shelf edge is that the species within the phytoplankton community change quite dramatically in the region influenced by the internal tide mixing. Phytoplankton cells in the band of chlorophyll along the shelf edge tend to be much larger than those either on the shelf or off in the NE Atlantic, with the shelf edge being particularly important for diatoms. About 60% of shelf edge chlorophyll was made up of phytoplankton >5 microns in diameter, compared to 25 - 30% elsewhere. Rather than the increased primary production, it is likely that the shift in cell size of the phytoplankton is key to the existence of the spawning fish stocks. First- feeding fish larvae are known to ingest phytoplankton, generally requiring phytoplankton > 5 microns in size. Useful papers: Sharples, J., C. M. Moore, A. E. Hickman, P. M. Holligan, J. F. Tweddle, M. R. Palmer & J. H. Simpson. 2009. Internal tidal mixing as a control on continental margin ecosystems. Geophys. Res. Lett., 36, L23603, doi:10.1029/2009GL040683. Sharples , J., Tweddle, J. F., Green, J. A. M., Palmer, M. R., Kim, Y- N, Hickman, A. E., Holligan, P. M., Moore, C. M., Rippeth, T. P., Simpson, J. H., and Krivtsov, V. 2007. Spring-neap modulation of internal tide mixing and vertical nitrate fluxes at a shelf edge in summer. Limnology & Oceanography, 52(5), 1735-1747. From physics to fish over banks in a shelf sea The distribution of fishing activity in a shelf sea is not uniform.The figure on the right illustrates which areas of the Celtic Sea are visited by commercial fishing vessels, based on an analysis of data from the Vessel Monitoring System (VMS). The most heavily fished region is over the shelf edge and slope (see Internal Tides and the Shelf Edge Ecosystem above), followed by a small region (the Celtic Deep) in the north of the Celtic Sea. In the central Celtic Sea there is a broad region of moderate fishing activity. During research cruises in 2005 and 2008 we attempted to observe the physics, biogeochemistry and fish distributions in this region in an attempt to gain some understanding of why it is attractive for fishing. The main contrast between this central Celtic Sea region and elsewhere is that the seabed is marked by several large banks. These banks are typically 5 - 30 km long, 1 - 10 km wide, and have broad tops about 50 metres above the surrounding shelf. The surrounding water depth is 100 - 120 metres, so the banks are not high enough to reach the seasonal thermocline. During the cruise in 2008 we carried out work ranging from the physics of lee waves, mixing and dispersion over the banks, through the effects on nutrient supplies to the phytplankton, the basic sediment biogeochemistry over and adjacent to the banks, to the distribution and behaviour of fish and seabirds. The results are all detailed in a special issue of Progress in Oceanography. Briefly, our results show that the action of breaking internal waves caused by the tidal flow of stratified water over the banks drives very large fluxes of nitrate into the seasonal thermocline. This nitrate fuels primary production, but the effects of the nitrate mixing and increased production are smeared over a broader area because of mean flows and dispersion away from the banks. We think it is this increase in biological productivity, driven by mixing at the banks, that results in the region being favourable for fish and fishing. We can use bottom mixed layer chlorophyll concentration as a tracer of water that has recently experienced increased mixing at the base of the thermocline. The region of increased fishing activity is bounded by the region of elevated bottom layer chlorophyll concentrations, indicating the correspondence between water influenced by mixing and the fishing vessels. For the nitrate-fuelled increased primary production to be useful to fish, we would expect to see some correlated pattern in the biomass of the zooplankton. Our observations of zooplankton over and immediately adjacent to a bank showed the biomass and species of zooplankton to be similar; this is what we would expect as a result of the dispersion around the area of banks. However, overall zooplankton biomass in the region of the banks was higher than in the non-fished area between the banks and the shelf edge; this provides the link between the enhanced nitrate- fuelled primary production and the fish. Useful papers: Sharples, J., B. E. Scott & M. E. Inall, 2013. From physics to fishing over a shelf sea bank. Progress in Oceanography, 117, 1-8. Sharples, J., J. R. Ellis, G. Nolan & B. E. Scott, 2013. Fishing and the oceanography of a stratified shelf sea. Progress in Oceanography, 117, 130-139. Tweddle, J. F., J. Sharples, M. R. Palmer, K. Davidson & S. McNeill, 2013. Enhanced nutrient fluxes at the shelf sea seasonal thermocline caused by stratified flow over a bank. Progress in Oceanography, 117, 37-47.