Title

Flathead Lake - Long Term Trends and Rising Threats

Abstract

Flathead Lake, a jewel in the Crown of the Continent, has experienced dramatic changes from human actions over the past few decades. To examine these impacts, monthly measurements of primary production, nutrients, and other physical and biological variables in Flathead Lake, Montana, were made in the period 1977-2004. This time spanned increasing development in the watershed, a prolonged drought and warming episode, and introduction and population explosion of the nonnative opossum shrimp, Mysis relicta. Trends and interactions were evaluated for statistical significance using frequentist and Bayesian analyses. Aerosol deposition of nitrate and ammonium increased while soluble reactive phosphorus (SRP) declined over the period of record. Riverine loading of nitrate increased and ammonium and SRP decreased. Blooms of the blue-green alga Anabaena flos-aquae (an indicator of over-enrichment) occurred several times during the period of record, and hypolimnetic oxygen concentrations have decreased. Catastrophic foodweb change was clearly associated with the establishment of Mysis relicta. Mysids exploded to 129 m2 in 1984-86 then declined rapidly with increasing profundal lake trout predation. Intense foraging by mysids caused an 83% reduction in the biomass of large zooplankton. Lake trout expansion and zooplankton changes corresponded with extirpation of established nonnative Kokanee salmon and a decline in native salmonid fishes. Coincident with the Mysis upheaval there was a step increase in primary production. 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. Continued population growth and the potential for more exotic species introductions are ongoing concerns for Flathead Lake.

Start Date

5-3-2010 10:00 AM

End Date

5-3-2010 10:45 AM

Document Type

Presentation

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Mar 5th, 10:00 AM Mar 5th, 10:45 AM

Flathead Lake - Long Term Trends and Rising Threats

Flathead Lake, a jewel in the Crown of the Continent, has experienced dramatic changes from human actions over the past few decades. To examine these impacts, monthly measurements of primary production, nutrients, and other physical and biological variables in Flathead Lake, Montana, were made in the period 1977-2004. This time spanned increasing development in the watershed, a prolonged drought and warming episode, and introduction and population explosion of the nonnative opossum shrimp, Mysis relicta. Trends and interactions were evaluated for statistical significance using frequentist and Bayesian analyses. Aerosol deposition of nitrate and ammonium increased while soluble reactive phosphorus (SRP) declined over the period of record. Riverine loading of nitrate increased and ammonium and SRP decreased. Blooms of the blue-green alga Anabaena flos-aquae (an indicator of over-enrichment) occurred several times during the period of record, and hypolimnetic oxygen concentrations have decreased. Catastrophic foodweb change was clearly associated with the establishment of Mysis relicta. Mysids exploded to 129 m2 in 1984-86 then declined rapidly with increasing profundal lake trout predation. Intense foraging by mysids caused an 83% reduction in the biomass of large zooplankton. Lake trout expansion and zooplankton changes corresponded with extirpation of established nonnative Kokanee salmon and a decline in native salmonid fishes. Coincident with the Mysis upheaval there was a step increase in primary production. 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. Continued population growth and the potential for more exotic species introductions are ongoing concerns for Flathead Lake.