Detecting the presence of fish farm-derived organic matter at the seafloor using stable isotope analysis of phospholipid fatty acids

www.nature.com/scientificreports OPEN Received: 10 January 2017 Accepted: 25 May 2017 Published: xx xx xxxx Detecting the presence of fish farm-derived organic matter at the seafloor using stable isotope analysis of phospholipid fatty acids Daniel J. Mayor 1,2,3 , Nia B. Gray2, Giannina S. I. Hattich4 & Barry Thornton2 The expansion of global aquaculture activities is important for the wellbeing of future generations in terms of employment and food security. Rearing animals in open-exchange cages permits the release of organic wastes, some of which ultimately reaches the underlying sediments. The development of rapid, quantitative and objective monitoring techniques is therefore central to the environmentally sustainable growth of the aquaculture industry. Here, we demonstrate that fish farm-derived organic wastes can be readily detected at the seafloor by quantifying sediment phospholipid fatty acids (PLFAs) and their carbon stable isotope signatures. Observations across five farms reveal that farm size and/or distance away from it influence the spatial distribution of the generated organic wastes and their effect on benthic bacterial biomass. Comparison to the isotopic signatures of fish feed-derived PLFAs indicates that 16:0 and 18:1(n-9) are potential biomarkers for fish farm-derived organic wastes. Our results suggest that stable isotope analysis of sediment PLFAs has potential for monitoring the environmental performance of aquaculture activities, particularly given the increasing prevalence of terrigenous organic matter in aquaculture feed stocks because it is isotopically district to marine organic matter. Global aquaculture production must double by 2050 if current per-capita consumption levels of aquatic, animal-derived protein are to be sustained as the human population reaches 9 billion1, 2. Achieving the necessary level of growth and intensification of aquaculture activities within environmentally sustainable limits represents one of this global industry’s major bottlenecks2, 3. Marine finfish aquaculture, and in particular the production of Atlantic salmon, Salmo salar, has seen explosive growth over the past 3 decades and this species alone now has an estimated global value >US$14bn4. Rearing finfish in mesh cages allows the release of particulate organic wastes that can subsequently accumulate in the underlying and surrounding sediments. The extent to which the seafloor becomes organically enriched is determined by a variety of interacting environmental parameters, including distance away from the farm, current speed, water depth, and farm size5–9. Monitoring, understanding and mitigating the effects of aquaculture-derived organic wastes on the structure and biogeochemical functioning of the receiving communities are key to the expansion and intensification of this global industry2. The effects of fish farming activities on the underlying sediments can be quantified by examining changes in the structure of benthic macrofaunal communities, which exhibit predictable and readily discernible changes in response to organic enrichment10. This sensitive and reproducible approach has been widely-adopted6, 11, but is nevertheless time consuming and thus expensive12. Alternative approaches to quantify the fate of farm-derived organic wastes include the analysis of sediment bulk stable isotope signatures13–15, fatty acid compositions16–18, or combinations thereof19. Most recently, high-throughput sequencing metabarcoding of environmental samples has been proposed as a rapid and reproducible, albeit not quantitative, method for discerning the impacts of fish farming activities on benthic ecosystems20–22. Compound-specific stable isotope analysis (CSSIA) of sediment phospholipid fatty acids (PLFAs) potentially represents an additional, quantitative method for rapidly determining the benthic impacts of aquaculture. 1 National Oceanography Centre, Southampton, SO14 3ZH, United Kingdom. 2James Hutton Institute, Aberdeen, AB15 8QH, United Kingdom. 3Oceanlab, University of Aberdeen, AB41 6AA, Aberdeen, United Kingdom. 4GEOMAR, Helmholtz Centre for Ocean Research Kiel, Wischhofstr. 1-3, 24148, Kiel, Germany. Correspondence and requests for materials should be addressed to D.J.M. (email: ) Scientific Reports | 7: 5146 | DOI:10.1038/s41598-017-05252-w 1 www.nature.com/scientificreports/ PLFA Mol % δ13C (per mil) 14:0 0.92 ± 0.15 −18.93 ± 1.88 16:1(n-7) 1.51 ± 0.25 −21.37 ± 1.68 16:0 29.74 ± 0.49 −27.91 ± 0.09 18:2(n-6, 9) 17.04 ± 0.21 −29.49 ± 0.50 18:1(n-9) 23.37 ± 0.63 −31.33 ± 0.36 18:0 4.57 ± 0.09 −23.53 ± 0.96 20:5(n-3) 4.13 ± 0.23 −23.74 ± 1.78 22:6(n-3) 18.71 ± 0.66 −26.44 ± 0.16 Table 1. Composition and isotopic signatures of phospholipid fatty acids (PLFAs) (average ±SD) in salmon feed pellets (Skretting Label Rouge, 11 mm). Figure 1. Model-predicted estimates of bacterial biomass in the surficial sediments at 5 Scottish fish farms illustrating the effects of distance (m) from the cage edge. Common letters (a,b,c) denote that distances are not significantly different from each other (p > 0.05). Inset boxplot presents the analyzed data. Biological membranes are largely comprised of phospholipids and the constituent PLFAs have been widely used to provide insight into the biomass and structure of microbial communities for several decades23–26. The advent of CSSIA techniques has enabled the carbon isotopic (δ13C) signatures of individual PLFAs to be accurately determined, providing insight into the source of organic matter used by microorganisms27. This is particularly useful in the context of marine finfish aquaculture, where the feedstock may be isotopically distinct from benthic microbial communities, e.g. because it is derived from terrestrial sources28 and/or from fishmeal/oil harvested in other regions of the globe29. Here, we present data on the concentrations, relative abundances and isotopic signatures of sediment PLFAs in the vicinity of five Scottish fish farms alongside information on the composition of feed pellets. Our study explores the overarching hypothesis that aquaculture-derived organic matter at the seafloor, and the influences of fish farm size (maximum consented biomass, MCB), average current speed and distance from the cage edge, can be detected using CSSIA of sediment PLFAs. Results Composition of feed pellets. Feed pellets contained 48.17 ± 1.03% carbon (% dry weight ± SD, n = 5) and 6.11 ± 0.07% nitrogen, with bulk isotopic values of −22.56 ± 0.26 and 9.75 ± 0.17 for δ13C and δ15N respectively. The major phospholipid fatty acids and their isotopic signatures are presented in Table 1. Bacterial biomass. PLFA-derived estimates of bacterial biomass, which were analysed using a linear mixed-effects (LME) model that included Farm ID as a random effect (L. Ratio = 13.45, df = 1, p < 0.001), declined with increasing distance from the cage edge (Fig. 1; L. Ratio = 21.26, df = 4, p < 0.001). The effects of av (...truncated)