The Role of Methane-Derived Carbon As an Energy Subsidy to Benthic
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The role of methane-derived carbon as an energy subsidy to benthic invertebrates in streams by Aurora Sampson Submitted in partial fulfilment of the requirements of the Degree of Doctor of Philosophy School of Biological and Chemical Sciences Queen Mary, University of London 2015 1 Declaration I, Aurora Sampson confirm that the research included within this thesis is my own work or that where it has been carried out in collaboration with, or supported by others, that this is duly acknowledged below and my contribution indicated. Previously published material is also acknowledged below. I attest that I have exercised reasonable care to ensure that the work is original, and does not to the best of my knowledge break any UK law, infringe any third party’s copyright or other Intellectual Property Right, or contain any confidential material. I accept that the College has the right to use plagiarism detection software to check the electronic version of the thesis. I confirm that this thesis has not been previously submitted for the award of a degree by this or any other university. The copyright of this thesis rests with the author and no quotation from it or information derived from it may be published without the prior written consent of the author. Signature: A. Sampson Date: 31st December 2014 2 Abstract Recent evidence suggests that methane-derived carbon can provide an energy source to freshwater food webs. Cased caddis (Trichoptera) larvae, such as Agapetus fuscipes and Silo nigricornis, show consistently depleted carbon stable isotope ratios, suggesting a reliance on methane-derived carbon rather than the organic matter fixed by plants either in the stream or imported from the land. These two invertebrate species can be very abundant and thus there is considerable potential for trophic transfer of methane-derived carbon further up the food web. Hitherto, the evidence for this link between cased caddis and methane has been restricted to streams and rivers on permeable chalk geology across southern Britain. This thesis examines the geographical distribution of this phenomenon and specifically whether it occurs elsewhere in catchments on different geology. It also examines the potential magnitude and importance of this methane-derived carbon source to these cased caddis populations. Twenty-nine sites on varying geology across Britain were sampled from April to November 2011. The results suggest that the use of methane-derived carbon by cased caddis and other primary consumers is more widespread than first thought. To assess the proportion of methane-derived carbon contributing to cased caddis larvae, secondary production in the focal caddis taxa (Agapetus fuscipes and Silo nigricornis) was measured regularly, using the size-frequency method, at eight permanent sites selected from the various geologies. This, combined with stable carbon isotope measurements, suggests that methane-derived carbon may form a considerable subsidy in these freshwater systems and indeed may be widespread across the UK. 3 Table of Contents Abstract ......................................................................................... 3 Table of Contents ......................................................................... 4 List of Figures ............................................................................... 8 List of Tables............................................................................... 13 Acknowledgements .................................................................... 16 1 General Introduction ............................................................ 19 1.1.1 Methane as a greenhouse gas ................................................. 19 1.1.2 Methane in freshwater environments ....................................... 20 1.1.3 Methane as an energy source .................................................. 20 1.1.4 Biogenic and thermogenic methane ......................................... 22 1.1.5 Isotopic values of methane ....................................................... 23 1.1.6 Chemosynthesis in deep sea vent communities....................... 24 13 1.1.7 Freshwater organisms displaying depleted δ C values ........... 26 1.1.8 Evidence for methane-derived carbon uptake in streams ........ 27 1.1.9 Life history of ‘armoured’ grazing caddis .................................. 29 1.1.10 Uptake and assimilation of methane-oxidising bacteria by caddis larvae ......................................................................................... 33 1.1.11 Possible benefits of consumption of MOB ............................. 34 1.1.12 Secondary production ........................................................... 34 1.1.13 Thesis aims ........................................................................... 35 13 2 Geology and the distribution of δ C depletion in cased caddis .......................................................................................... 40 2.1 Introduction ................................................................................... 40 2.2 Methods ......................................................................................... 51 2.2.1 Site selection ............................................................................ 51 2.2.2 Collection of caddis larvae and resources for stable isotope analysis ................................................................................................. 55 2.2.3 Methane in river water .............................................................. 57 4 2.2.4 Data analysis ............................................................................ 59 2.3 Results ........................................................................................... 62 2.3.1 Glossosomatidae δ13C depletion and geology ......................... 62 2.3.2 Stream water methane concentration ....................................... 64 2.3.3 Cased caddis δ13C, geology and methane concentration ....... 67 2.4 Discussion ..................................................................................... 68 2.4.1 Widespread δ13C depletion in Glossosomatidae ...................... 68 2.4.2 Dissolved methane in stream water ......................................... 70 3 Life histories of the grazing caddis .................................... 76 3.1 Introduction ................................................................................... 76 3.1.1 Rationale of site choice ............................................................ 78 3.1.2 Caddis life histories .................................................................. 83 3.1.3 Generations per year ................................................................ 84 3.2 Methods ......................................................................................... 88 3.2.1 Measuring site characteristics .................................................. 88 3.2.2 Sampling caddis larvae ............................................................ 89 3.2.3 Identification and instar separation ........................................... 91 3.2.4 Calculating density of caddis larvae ......................................... 92 3.2.5 Weighing of caddis larvae ........................................................ 92 3.2.6 Analysis of measurements ....................................................... 93 3.3 Results ........................................................................................... 94 3.3.1 Site characteristics ................................................................... 94 3.3.2 Stream nutrients ....................................................................... 96 3.3.3 Caddis larvae collected ............................................................ 98 3.3.4 Agapetus fuscipes instar divisions ............................................ 99 3.3.5 Silo spp. instar divisions ......................................................... 107 3.3.6 Agapetus fuscipes instar distributions .................................... 110 3.3.7 Silo nigricornis instar distributions .......................................... 114 3.3.8 Silo pallipes instar distributions .............................................. 115 3.3.9 Pronotum length-mass regressions ........................................ 117 3.4 Discussion ................................................................................... 122 3.4.1 Separating instars .................................................................. 122 5 3.4.2 Generation time ...................................................................... 127 4 Secondary production in grazing caddis and its support by methane-derived carbon .................................................... 131 4.1 Introduction ................................................................................. 131 4.1.1 Secondary production ............................................................ 131 4.1.2 History of secondary production ............................................. 132 4.1.3 The size-frequency method .................................................... 133 4.1.4 Applications of secondary production ..................................... 134 4.1.5 Trophic basis of production .................................................... 135 4.1.6 Using stable isotopes to identify the trophic basis of production .. ............................................................................................... 136 4.1.7 Quantifying the contribution of the ‘third way’ ......................... 137 4.1.8 Lake food webs and assimilation