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Carbon Flow Through Inshore Marine Environments of the Vestfold Hills, East Antarctica

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Carbon Flow Through Inshore Marine Environments of the Vestfold Hills, East Antarctica CARBON FLOW THROUGH INSHORE MARINE ENVIRONMENTS OF THE VESTFOLD HILLS, EAST ANTARCTICA by John Andrew Edwin Gibson B.Sc. (Hons), M.Sc. Submitted in fulfilment of the requirements for the degree of Doctor of Philosophy Antarctic CRC and Institute of Antarctic and Southern Ocean Studies University of Tasmania Hobart Australia March, 1997 For My parents and family and Kerrie in appreciation of their love and support DECLARATION • This is to certify that the material composing this thesis has never been accepted for any other degree or award in any tertiary institution and, to the best of my knowledge and belief, is solely the work of the author, and contains no material previously published or written by another person, except where due reference is made in the text. John Andrew Edwin Gibson AUTHORITY OF ACCESS This thesis may be made available for loan and limited copying in accordance with the Copyright Act 1968. John Andrew Edwin Gibson ABSTRACT The first integrated study in the Antarctic region of the marine carbon cycle over an entire annual period, including organic carbon formation, remineralisation, sedimentation and burial, was undertaken during two summers and the intervening winter from December 1993 to February 1995 at two sites in the Vestfold Hills region, East Antarctica: offshore at O'Gorman Rocks and in semi-enclosed Ellis Fjord. A single peak in uptake of dissolved inorganic carbon (DIC) occurred at the O'Gorman Rocks site in the first summer, while three peaks, resulting from different phytoplankton communities, were observed in the second. Net organic carbon (OC) production calculated from the uptake of DIC reached 2.1 mg kg'. The sum of particulate and dissolved OC was typically only 30 % of the calculated total, suggesting rapid sedimentation of OC or transfer to higher trophic levels. After the end of OC production in autumn the concentrations of DIC and nutrients increased slowly and steadily to late winter maxima, primarily as the result of remineralisation rather than nutrient supply by deep-water upwelling. Sediment trap flux of OC at O'Gorman Rocks totalled 10 g m -2 year', with peak sedimentation coinciding with maximum OC production, but with a significant proportion (30 %) occurring outside summer. The development of a bottom ice algal community in October contributed a pulse of sedimentation which was characterised by OC with high 813C (-13 % compared to the annual average -19 %0). Comparison of the fluxes with the OC content of a sediment core showed that 2 g nf2 year' OC was buried, with the majority (80 %) remineralisal by the benthic community. In Ellis Fjord DIC and nutrient uptake began earlier and ended later than at O'Gorman Rocks, though total DIC uptake and annual OC sedimentation were similar to those offshore. Early in summer the C:N uptake ratio calculated from concentrations in the II water column reached 40, considerably higher than the expected Redfield Ratio of 16:1. Sedimentation of bottom ice algae again resulted in the transfer of particulate matter with high 613C to the sediment. This study shows that OC production in the nearshore Antarctic environment does not necessarily occur in a single pulse after the loss of the ice cover, but rather can begin under the ice and continue after refreezing. Furthermore, OC production can be of similar magnitude in ice-covered systems to areas which are ice-free for part of the summer. Bottom ice algae play an important role in total OC production, and are a source of OC for sedimentation before and after the period of peak water column production. This community is also important in the formation of OC with elemental ratios and 613C different to material produced in open water. Interannual variability in OC production and the algal and phytoplankton communities, however, can be large. These factors must be taken into account when developing biological carbon uptake estimates from models of nutrient utilisation, in interpreting sedimentary isotope records, and considering the effects of climate change on Antarctic ecosystems. ifi ACKNOWLEDGMENTS My sincere thanks go to my supervisors Tom Trull, Tom McMeekin and Peter Franzmann for supporting and encouraging my work over the last four years. The winter expeditioners at Davis Base in 1994 helped immensely with the fieldwork. Special thanks are due to Kerrie Swadling, Graham Smith, Helen Cooley, Terry Newton, Dale Hughes, Trevor Bailey and Jenny Skerratt (for that first day) for their patience and support. Station Leaders Michael Carr and Diana Patterson made it possible to get out into the field to collect samples with the minimum of fuss. The assistance of the Australian Antarctic Division with logistics was appreciated. The help of the following people was also greatly appreciated: Mark Pretty and Bronte Tilbrook (CSIRO Division of Oceanography, Hobart), who provided assistance with the determination of total DIC and also provided standards; Brian Popp and Terri Rust (University of Hawaii), who measured 6 13C in some samples; Mike Power and Chris Cooke (Central Science Laboratory, University of Tasmania), who assisted with 6 13C measurements in Hobart, Graham Rowbottom (Central Science Laboratory, University of Tasmania) for microanalytical work; Roger Francey, Colin Allison and Emily Welch (CSIRO Division of Atmospheric Research), who provided flasks and a pump unit for the collection of air samples and for analysis of the samples; Andrew McMinn (IASOS, University of Tasmania) provided identifications for some diatoms; Professor Frank Millero (University of Miami) provided a computer program for calculation of carbonate parameters; Professor Bob Byrne (University of South Florida) for clarification of pH measurement techniques; and John Cox (Australian Antarctic Division), who drafted some of the figures. I also wish to thank Kerrie Swadling, Harry Burton, Andrew Davidson, Harvey Marchant, Bronte Tilbrook and Dennis Mackey for discussions on various aspects of the work. The card players in the Chemistry Department, University of Tasmania, also deserve a special mention. I iv would like to acknowledge the financial assistance of the Antarctic CRC, University of Tasmania, and the Australian Postgraduate Award scheme. Finally I would like to thank Kerrie Swadling and all members of my family for their continued support throughout the past four years. ABBREVIATIONS Standard SI unit and prefix abbreviations are used, and are not listed except for some less common units. yarn microatmosphere pE microeinstein pequiv microequivalent Allcr total alkalinity ANSTO Australian Nuclear Science and Technology Organisation BOD biological oxygen demand chl a chlorophyll a chl b chlorophyll b DIC dissolved inorganic carbon DO dissolved oxygen DOC dissolved organic carbon • fCO2 fugacity of CO2 mS rnillisiemen (unit of electrical conductance) Gt gigatonne (10" g) OC organic carbon PFIf pH measured on the free hydrogen scale POC particulate organic carbon POM particulate organic matter psu practical salinity unit salinity SDL submersible data logger WW winter water vi TABLE OF CONTENTS ABSTRACT i ACKNOWLEDGMENTS i i i ABBREVIATIONS v TABLE OF CONTENTS vi FIGURES xii TABLES xviii 1 INTRODUCTION 1 1.1 Introduction 1 1.2 The global biogeochemical carbon cycle 2 1.3 The role of the Antarctic region in the global carbon cycle 7 1.4 Components of the Antarctic carbon cycle 8 1.4.1 Measurements of/CO2 8 1.4.2 Primary production: conversion of CO2 to organic material 10 1.4.3 Carbon export: sediment trap studies 13 1.4.4 Carbon export: advection 15 1.5 The themes of this thesis 16 2 O'GORMAN ROCKS SITE: LOCATION, PREVIOUS STUDIES AND PHYSICAL OCEANOGRAPHY 18 2.1 Introduction 18 2.2 The Vestfold Hills 23 2.3 The inshore marine environment of the Vestfold Hills 23 2.4 The climate of the Vestfold Hills 26 2.5 Physical oceanography at the O'Gorman Rocks site, December 1993 - February 1995 28 2.5.1 Ice cover 28 2.5.3 Water temperature 29 2.5.4 Water salinity 31 2.6 Conclusions 36 3 PHYTOPLANKTON AT THE O'GORMAN ROCKS SITE: SEASONAL CYCLE AND INTERANNUAL VARIABILITY 37 3.1 Introduction 37 3.2 Results 38 3.2.1 Seasonal phytoplankton cycle 38 3.2.1.1 Cryptomonad A 40 3.2.1.2 Diatoms 42 3.2.1.3 Phaeocystis cf. antarctica 44 3.2.1.4 Dinoflagellates 45 vii 3.2.1.5 Pyramimonas gelidicola 46 3.2.2 Chlorophyll a 46 3.3 Discussion 47 3.3.1 Comparison of the seasonal cycle to previous studies 47 3.3.2 Factors contributing to variation in the phytoplankton community 50 3.3.3 What effect could phytoplankton variability have on carbon export? 54 3.4 Conclusions 57 4 ORGANIC MATTER PRODUCTION AND ATMOSPHERIC CO2 UPTAKE AT O'GORMAN ROCKS 58 4.1 Introduction 58 4.2 Results 58 4.2.1 Dissolved inorganic carbon 58 4.2.2 Dissolved organic carbon 59 4.2.3 Particulate organic carbon 60 4.2.4 Carbon isotope ratios 62 4.2.5 pH 62 4.2.6 Chlorophyll a 63 4.2.7 Dissolved oxygen 64 4.2.8 Nutrients 66 4.2.9 Calculated carbon parameters 68 4.2.9.1 Total alkalinity 69 4.2.9.2 The fugacity of CO2 70 4.2.10 Atmospheric carbon dioxide: concentration and isotopic ratio 70 4.3 Discussion 71 4.3.1 Seasonal cycles of primary production and related parameters 71 4.3.1.1 The general seasonal cycle of primary productivity 71 4.3.1.2 DIC and nutrients 73 4.3.1.3 Dissolved oxygen 76 4.3.1.4 pH 77 4.3.1.5 Total alkalinity 78 4.3.1.6 fCO2 79 4.3.2 Correlations: DIC, nutrients and dissolved oxygen 80 4.3.2.1 Correlation of DIC and the nutrients 80 4.3.2.2 Correlations of dissolved oxygen with DIC and the nutrients 84 4.3.3 Carbon budget calculations and correlation
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