Alternate States in a Large Oligotrophic Lake: A Retrospective Analysis of Nutrient Loading and Food Web Change

Bonnie Kathleen Ellis, The University of Montana

Abstract

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, Mysis relicta were made during 1977-2004. Trends and interactions were evaluated for statistical significance using Bayesian models against null hypotheses of no effects.
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.
Increasing primary productivity per unit chlorophyll (PP/chla) 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 Anabaena flos-aquae, occurred several times during the period of record.
Catastrophic food web change was clearly associated with the establishment of Mysis relicta. 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 Mysis 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 Mysis upheaval there was a step increase in PP of 60 mgC m-2 day-1, but no trend in the period before or after Mysis. Herbivorous zooplankton increased post-Mysis, coherently with declining chlorophyll. Increased herbivory provided persuasive explanation for increasing PP/chla.
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.