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ThesisUniversity of Iceland>Verkfræði- og náttúruvísindasvið>Doktorsritgerðir>

Please use this identifier to cite or link to this item: http://hdl.handle.net/1946/13218

Title

Primary producers in sub-Arctic streams and the effectsof temperature and nutrient enrichment on succession

Published
November 2012
Abstracts
  • Streams in geothermal areas may offer good experimental opportunities for investigating the potential influences of climate changes upon freshwater ecosystems. Eight adjacent streams, varying in temperature, in the Hengill geothermal area, SW Iceland were selected to test hypotheses concerning the effects of nutrient addition and temperature on the community structure of primary producers. These were that: increasing temperature would have major effects on the structure of the primary producer communities, reflected in gross composition, species representation and growth form occurrence; that nitrogen addition would increase production whilst also changing community composition; and that there would be some interaction between temperature and nutrient effects that might give indications as to how sub–Arctic streams might be expected to change with progressive global warming and the associated secondary effects of changing land use.
    The density of the liverwort Jungermannia exsertifolia (Steph.) was low in the coldest streams but the biovolume of the biofilms on stones was high. In slightly warmer streams the density of the bryophyte Fontinalis antipyretica (Hedw.) was high along with high
    algal biovolume on stones. In the warmest streams the cover of F. antipyretica was very high. Green algal biovolume was high in all streams. Cyanobacteria and diatom biovolume was high in the cool and cold streams while it was lower in the warmest streams. Variables such as conductivity, pH and discharge along with the density of invertebrates and fish were examined in a relation to the temperature and nutrient influences on primary producers. Diatom communities included 64 taxa, 47 of them identified to species level. The coldest streams contained the most diverse diatom communities along with the highest growth–form diversity. The major grazer, Radix peregra (Mollusca) influenced growth forms by decreasing the diversity but it did not influence the density of diatoms. Small diatom
    populations were denser in warmer streams compared with colder streams.
    Experiments in which ammonium nitrate (NH4NO3) was dripped into each stream over several months showed that biomass of the bryophyte F. antipyretica increased significantly when nitrogen was added, along with increased biomass of filamentous algae
    (mainly Cladophora). However, the biomass of nitrogen fixing Cyanobacteria (e.g. Nostoc) and Oscillatoria decreased significantly with nutrient enrichment. The density of
    diatoms increased significantly when nutrients were added to the streams. The diversity of diatoms decreased but the TDI (Trophic Diatom Index) did not change, suggesting that this
    commonly used index may not be a reliable means of detecting nutrient pressures. The growth forms of diatoms were altered by nutrients and the diatom genus Nitzschia declined
    in numbers with nutrient enrichment.
    Consequently, the results suggest major changes in the future in subarctic streams as temperatures increase and more land becomes available for stock–keeping, and even cultivation, leading to increased eutrophication. Production will increase and microalgal communities will be displaced by bryophyte growth. There will be effects on invertebrate communities, and increased fish production may lead to additional effects, though these can only be hinted at from the present data.

  • is

    Lækir á jarðhitasvæðum geta verið hentugir til tilrauna í vistfræði, m.a. til að meta möguleg áhrif loftslagsbreytinga á vatnavistkerfi. Átta misheitir lækir á afmörkuðu svæði í Hengladölum á Hellisheiði voru notaðir til þess að prófa tilgátur um hugsanleg áhrif hlýnunar á samfélög frumframleiðenda. Einnig voru prófaðar tilgátur um áhrif næringarefna aukningar á sömu lífverusamfélög. Tilgáturnar voru; að aukinn hiti myndi
    hafa áhrif á uppbyggingu samfélaga frumframleiðenda, m.a. í breytingum á heildar samsetningu frumframleiðenda ásamt breytingum á samsetningu vaxtarforma kísilþörunga;
    að næringarefna aukning myndi auka frumframleiðni ásamt því að breyta samfélags uppbyggingu frumframleiðenda. Samverkandi áhrif yrðu af næringarefna aukningu og
    hlýnun sem gæti gefið mynd af því hvernig samfélög frumframleiðenda í lækjum á
    norðlægum breiddargráðum gætu breyst með hækkandi lofthita, breytingum á landnotkun og fólksfjölgun.
    Í ljós kom að í köldum lækjum var þéttleiki lækjarbleðlu Jungermannia exsertifolia (Steph.) lítill en lífmassi þörungaskánar á steinum var mikill. Í volgum lækjum var
    talsverður lífmassi ármosa (Fontinalis antipyretica Hedw.) ásamt miklum lífmassa þörungaskánar á steinum. Í heitustu lækjunum var þéttleiki ármosa mikill (F. antipyretica)
    en lítill lífmassi þörungaskánar var á steinum. Í öllum lækjum var lífmassi grænþörunga
    talsverður. Þættir eins og leiðni, pH og rennsli ásamt þéttleika hryggleysingja og fiska voru
    metnir með hliðsjón af hita og næringarefna aukningar áhrifum á frumframleiðendur.
    Greindir voru 64 kísilþörungahópar og þar af voru 47 greindir til tegunda. Flestar tegundir voru í kaldari lækjunum ásamt fjölbreyttustu vaxtarformunum. Vatnabobbi (Radix peregra
    (Müller)), sem er þörungaæta, virtist hafa mótandi áhrif á vaxtarform kísilþörunga í heitum
    lækjum en hafði ekki áhrif á þéttleika þeirra. Í heitu lækjunum voru smærri kísilþörungar
    meira áberandi en í þeim köldu.
    Tilraun þar sem magn ammoníumnítrats (NH4NO3) var aukið í lækjunum leiddi m.a. í ljós
    að þekja og lífmassi mosans F. antipyretica og þráðkenndra grænþörunga, einkum
    Cladophora spp. jukust marktækt. Aftur á móti minnkaði lífmagn blágrænna niturbindandi
    baktería (Cyanobacteria), eins og Nostoc spp. Lífmagn kísilþörunga jókst marktækt við
    aukningu næringarefna. Fjölbreytileiki kísilþörunga minnkaði samfara næringarefna
    aukningu, en TDI (Trophic Diatom Index) breyttist ekki. Vaxtarform kísilþörunga
    hliðruðust eftir að næringarefnum var bætt í lækina. Þéttleiki Nitzschia tegunda minnkaði
    marktækt við næringarefna aukningu í lækjunum.
    Samkvæmt niðurstöðum rannsóknarinnar mun hækkandi lofthiti, aukin áburðarefnanotkun
    á landi og fólksfjölgun (sem leiðir til aukningar á næringarefnum í vistkerfi) hafa mikil
    áhrif á samfélög frumframleiðenda í lækjum á háum breiddargráðum. Frumframleiðni
    eykst og samfélög smárra þörunga og blágrænna baktería mun verða skipt út fyrir samfélög mosa. Samfélög hryggleysingja munu að öllum líkindum breytast ásamt því að þéttleiki fiska mun aukast.

Issued Date
03/10/2012


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