Document Detail
Alternate States in a Large Oligotrophic Lake: A Retrospective Analysis of Nutrient Loading and Food Web Change
Abstract/OtherAbstract :
Monthly or more frequent measures of internal and external nitrogen and phosphorus (NP) loading to Flathead Lake, Montana, in relation to biotic response variables and attendant interactive effects associated with invasion of a nonnative amphipod, <I>Mysis relicta</I> were made during 1977-2004. Trends and interactions were evaluated for statistical significance using Bayesian models against null hypotheses of no effects. <br> Aerosol deposition contributed 10.1% and 7.0% of the total annual NP load; nitrate and ammonium increased and soluble reactive phosphorus declined over the period of record. Deposition was highest during thermal inversions that entrained smoke and dust. Riverine nitrate increased and ammonium and SRP decreased. Increasing trends were coherent with increasing urbanization and forest disturbance. However, contribution of sewage treatment facilities to annual TP load decreased from 11% to 3% following improved nutrient removal technologies.<br> Increasing primary productivity per unit chlorophyll (PP/chl<I>a</I>) and decreasing hypolimnetic oxygen concentrations were coherent with the trends in NP loading and therefore indicative of human influences. The lake remains oligotrophic with productivity limited by availability of labile NP, although worrisome blooms of algae, including <i>Anabaena flos-aquae</i>, occurred several times during the period of record.<br> Catastrophic food web change was clearly associated with the establishment of <i>Mysis relicta.</i> Mysids exploded to 129 m2 in 1984-86. Their intense foraging on zooplankton caused an 83% reduction in the biomass of large species. Kokanee salmon, also a zooplankton feeder, were extirpated, whereas lake trout increased from 0.09 before to 1.7 cpue after <i>Mysis</i> in standardized gill net catches. Lake trout expansion and zooplankton changes corresponded with an 80% decline in native salmonid fishes. Mysids declined rapidly with increasing profundal fish predation during 1987-88 and then stabilized around 40 m2. A Bayesian analysis showed that during the <i>Mysis</i> upheaval there was a step increase in PP of 60 mgC m-2 day-1, but no trend in the period before or after <i>Mysis</i>. Herbivorous zooplankton increased post-<i>Mysis</i>, coherently with declining chlorophyll. Increased herbivory provided persuasive explanation for increasing PP/chl<i>a</i>.<br> The limnological legacy of Flathead Lake is a story of changing quasi-stable states mediated by a strong interaction between nutrient loading and mysid foraging.
Authors :
Ellis, Bonnie Kathleen
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Contributors :
Richard F. Hauer
Publication Detail :
Publisher :  The University of Montana     Type :  text     Format :  application/pdf    
Date Detail :
2007-03-29
Subject :
Division of Biological Sciences
Coverage :
-
Relation :
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Source :
http://etd.lib.umt.edu/theses/available/etd-03292007-094249/
Copyright Information :
unrestricted, I hereby certify that, if appropriate, I have obtained and attached hereto a written permission statement from the owner(s) of each third party copyrighted matter to be included in my thesis, dissertation, or project report, allowing distribution as specified below. I certify that the version I submitted is the same as that approved by my advisory committee. I hereby grant to University of Montana or its agents the non-exclusive license to archive and make accessible, under the conditions specified below, my thesis, dissertation, or project report in whole or in part in all forms of media, now or hereafter known. I retain all other ownership rights to the copyright of the thesis, dissertation or project report. I also retain the right to use in future works (such as articles or books) all or part of this thesis, dissertation, or project report.
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