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Salp Swarms in the Tasman Sea: Insights Into Their Population Ecology

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Salp Swarms in the Tasman Sea: Insights Into Their Population Ecology Salp swarms in the Tasman Sea: Insights into their population ecology Natasha Henschke Evolution and Ecology Research Centre School of Biological, Earth and Environmental Sciences University of New South Wales A thesis in fulfillment of the requirements for the degree of Doctor of Philosophy August 2014 THE UNlVERSJTY OF NEW SOUTH WALES ThesisfDissertation Sheet Surname or Family name: Henschke First name: Natasha Other name/s: Abbreviation for degree as given in the University calendar: PhD School: Biological, Earth and Environmental Sciences Faculty: Science Title: Salp swarms in the Tasman Sea: Jnsights into their population ecology Salps are gelatinous zooplankton that regularly occur in intermittent swarms, quickly becoming the most dominant organism within an area. However, the factors affecting the occurrence and size of these swarms are stHJ widely unknown and little research attenti.on bas been paid to salps in the Tasman Sea. This thesis explores the trophic interactions of salps within the zooplankton community, the environmental drivers that influence the formation of sa1p S\''1-'arms, and impact of salp swarm demise on the benthic community. The trophic niche of an oceanic zooplankton community was studied using stable isotopes of carbon and nitrogen. T he trophic relationships among the zooplankton were dynamic and responded to different water types depending on food availability. The salp Thalia democratica likely competes with copepods for the same food source, however, kriJI were found to partition the niche to feed on other· prey items. The interannual abundance ofT. democratica was negatively related to the rates of asexual reproduction (buds per chain). T. democratica abundance was significantly positively related to food >2 Jlm in size and negatively related to the proportion of non-salp zooplankton. This suggests that salp swarm abundance may depend on the abundance of larger phytoplankton (prymnesiophytes and diatoms) and competition with other zooplankton. rn this thesis, a size-structured population model of the salp, T. democratica was developed to understand the relationship between environmental variables (temperature and chlorophyll a) and population dynamics. Temperature and chlorophyll a were sufficient as environmental variables to predict T. democratica population dynamics. The model simulated larger. faster growing salp swarms when availability of chlorophyll a (food) was high and temperature was near optimum (20.5°C). This thesis also identified that after the demise of a salp swarm, carcasses can rapidly si nk to the benthos as food fall events. Bentlii.c crustaceans and fish were observed feeding on carcasses, and the nutritional va.lue of the salp carcasses were high compared to other gelatinous organisms. The deposition of the mean yearly biomass (4.81 t km-2 WW) of salps in the Tasman Sea represents a 330% increase to the carbon input normally estimated for this region. Declaration relating to disposition of project thesis/dissertation I hereby grant to the University of New South Wales or its agents the right ro archive and to make available my thesis or dissertation in whole or in part in tbe University libraries in all forms of media, oow or here after known, subject to the provisions of the Copyright Act 1968. I retain all property rights, such as patent rights. I also retain the right to use in future works (such as articles or books) all or part of this thesis or dissertation. [also authorise University Microfilms to use the 350 word abstract of my thesis in Disseitation Abstracts Jntemational (this is applicable to doctoral theses only) . ..... ALJ .. ........................ ~.. .............................. .. ..1( o1/?9.1':/ ..... Signature w· ess Date The University recognises that there may be exceptional circumstances requiring restrictions on copying or conditions on use. Requests for restriction for a period of up to 2 years must be made in writing. Requests for a longer period ofrestriction may be considered in exceptional circumstances and require the approval of the Dean of Graduate Research. FOR OFFICE USE ONLY Date of completion of requirements for Award: Cover image from CL Griffiths ORIGINALITY STATEMENT "1 hereby declare that this submission is my own work and to the best of my knowledge it contains no materials previously published or written by another person, or substantial proportions of material which have been accepted for the institution, except where due acknowledgement is made in the thesis. Any contribution made to the research by others, with whom I have worked at UNSW or elsewhere, is explicitly acknowledged in the thesis. I also declare that the intellectual content of tllis thesis is the product of my own work> except to the ex'i.ent that the assistance from others in the project· s design and conception or· style, pr sentation and linguistic expression is acknowledged'. Signed Date . ...... L(.~.~./! . ? . !.Y ............. .... .. 11 COPYRIGHT STATEMENT 'I hereby grant the University of New South Wales or its agents the right to archive and make available my thesis or dissertation in whole or part in the University libraries in all forms of media, now or here after known, subject to the provisions of the Copyright Act 1968. 1 retain all proprietary rights, such as patent rights. I also retain the right to use in future works (such as articles of books) all or part of this thesis or dissertation. I also authorise University Microftlms to use the 350 word abstTact of my thesis in Dissertation Abstract International (this is applicable to doctoral theses only). Thave either used no substantial portions of copyright material in my thesis or I have obtained permission to use copy1ight mate1ial; where permission has not been granted I have applied/will apply for a partial resttiction of digital copy of my thesis or distertation.' Signed r~~..u.. .............................. Date .. ,I. .(.?_ .~. (. ~ .1. Y-: .................... AUTHENTICITY STATEMENT 'I certify that the Library deposit digital copy is a direct equivalent of the fmal officially approved version of my thesis. No emendation of content has occurred and if there are any minor variations in formatting they are the result of the conversion to digital format.' Signed Date ....~ . /? . 6/ . ?-:.~. ~ .'f... ............. .... Ill Abstract Pelagic tunicates, including salps, are an important category of gelatinous zooplankton and yet are relatively understudied. Salps regularly occur in intermittent swarms and can quickly become the most dominant zooplankton within an area. My thesis explores the trophic interactions of salps within the zooplankton community using stable isotope analysis, the environmental drivers of salp population dynamics, and the occurrence of salp deposition on the sea floor. The trophic niche of salps within the oceanic zooplankton community was investigated using stable isotopes of carbon and nitrogen. Zooplankton and suspended particulate organic matter (POM) were sampled in three different water types: inner shelf (IS), a cold core eddy (CCE; cyclonic) and a warm core eddy (WCE; anti-cyclonic). Recent upwelling in the IS water type resulted in lower than expected trophic enrichment for all zooplankton species (0.53‰ compared to 3.4‰), and the salp Thalia democratica was depleted in 15N compared to POM. Trophic enrichment of zooplankton within the CCE (2.74‰) was higher than the IS, and more similar to expected results (3.4‰). Based on chlorophyll a and nitrate concentrations, the WCE was characterised as an oligotrophic environment and was associated with an increased trophic level for omnivorous zooplankton (copepods and euphausiids) to a similar level as carnivorous zooplankton (chaetognaths). This study shows that trophic relationships among the zooplankton are dynamic and can vary across water types. The demographic characteristics of three salp swarms were studied to examine factors influencing variations in salp swarm magnitude. The interannual abundance of Thalia democratica during spring was related to the rates of asexual reproduction (buds per chain). T. democratica abundance was significantly higher in October 2008 (1312 individuals m-3) than 2009 and 2010 (210 and 92 individuals m−3, respectively). Salp abundance was negatively related to buds per chain and relative growth rates, implying a faster release rate. As T. democratica abundance was significantly positively related to food >2 µm in size and negatively related to the proportion of non-salp zooplankton, salp swarm abundance may depend on the abundance of larger phytoplankton (prymnesiophytes and diatoms) and competition with other zooplankton. iv A discrete-time, size-structured Thalia democratica population model was developed to investigate the temporal resolution of a salp swarm. The model used size-dependent reproduction and mortality, where growth was dependent on food consumption (chlorophyll a biomass) and temperature. Average generation time (12 days) and mean abundances of each stage correspond to previously reported values. Salp ingestion rate and the doubling time of chlorophyll a were the most influential parameters, negatively influencing salp biomass and abundances for each individual stage. Forcing the model with a 10-year temperature and chlorophyll a time-series identified that salp abundances off the coast of Sydney (34ºS) were significantly greater during spring and summer compared to autumn and winter. This is consistent with observations of salp swarms which occur after
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