Mass and energy flow in seafloor ecosystems, testing and tuning deep-sea models with biochemical observations

Introduction
Deep-sea habitats at water depths over 2000 m cover ~60% of the Earth’s surface and are the largest and least explored environment on the planet (Smith et al. 2009). Deep-seafloor communities are sustained by the flux of particulate organic carbon, which originates from primary producers in the euphotic zone. A small fraction (<5%) of the POC fixed in the surface ocean reaches the seafloor. Recent work at two abyssal observatories has demonstrated that deep-seafloor communities are sensitive to changes in this flux (Smith et al. 2009). This food supply is likely to be impacted by climate change, which is predicted to impact surface ocean primary production leading to a reduction in the POC flux (Bopp et al. 2013). Modeling predicts that a reduction in POC flux will lead to a substantial reduction in biomass at the deep seafloor (Jones et al. 2013, Yool et al. 2013). Deep-seafloor communities play an important role in remineralizing POC to nutrients and dissolved inorganic carbon, over time periods of days to months. A small fraction of that POC is not remineralised by deep-seafloor communities and is sequestered by seafloor burial, an important process over geological timescales. Therefore, changes in deep-seafloor communities will have direct and indirect impacts on biologically controlled processes, ecosystem functioning, biogeochemical cycling and carbon burial. This project aims to bring together empirical measurements, theory and modeling to better understand energy flow and ecosystem function in deep-seafloor communities.
Project Summary
To better understand ecosystem functioning in deep-seafloor communities, and their role in biogeochemical cycling, this project will use a multifaceted approach to determine body size-structured energy flows within deep-seafloor communities. You will use your stable isotope data along with extensive data from the PAP-SO to quantitatively represent the transfer of energy through the seafloor ecosystem using existing numerical frameworks (e.g. the Metabolic Theory of Ecology [MTE], Brown et al. 2004; the Benthic Organisms Resolved In Size [BORIS] model, Yool et al. 2013).
Body size is thought to be of primary importance in controlling energy flow through biological communities (Peters, 1983). Across geometric body-size classes, animal abundance appears to decline as a -3/4 power function (Damuth, 1981). Given that metabolic rate scales as a +3/4 power function of body mass (Gillooly et al. 2001), then the total energy used by a size class will be approximately constant at steady state - this is known as Damuth’s Rule or the Energetic Equivalence Rule (EER). The theory provides a numerical framework to link organism physiology, activity, and abundance with energy flows through the ecosystem. Your project will aim to understand the fundamental mechanisms underlying the energetic equivalence rule and the distribution of resources amongst size classes in deep sea communities, which are currently not well known. You will examine pattern and process in energy transfer through seafloor communities by constructing food webs using stable isotopes and body-size spectra (Jennings et al. 2001).

The focus of this project will be the Porcupine Abyssal Plain sustained observatory (PAP-SO) site in the NE Atlantic (49°N 016°W, 4850 m water depth) and link directly to NERC’s Climate Linked Atlantic Sector Science programme (CLASS, http://projects.noc.ac.uk/class/). The abyssal seafloor community and water column biogeochemistry have been routinely studied at this site for the last three decades, providing a unique historical dataset on POC flux, benthic biomass and abundance (Hartman et al. 2012). Significant decadal-scale fluctuations in benthic ecology have been observed at the PAP-SO site (Smith et al. 2009), making it an ideal location for this work. The site will be accessed via CLASS-funded annual research cruises 2019-2023.

Work plan
You will be based at the University of Liverpool for the first 18 months. You will complete training in the Organic Biogeochemistry and LIFER (Liverpool Isotope Facility for Environmental Research) laboratories with Dr Jeffreys. Professor Hirst will provide guidance with theoretical ecology. You will participate in at least one research cruise. You will work closely with Dr Bett on cruise preparation and the numerical component of the project. During the second half of your second year you will spend 6 months at NOC Southampton with Dr Bett in the Ocean Biogeochemistry & Ecosystems Division and using MTE and BORIS models.