Bonnie K. Ellis

Long-term effects of a trophic cascade in a large lake ecosystem

Introductions or invasions of nonnative organisms can mediate major changes in the trophic structure of aquatic ecosystems. Here we document multitrophic level impacts in a spatially extensive system that played out over more than a century. Positive interactions among exotic vertebrate and invertebrate predators caused a substantial and abrupt shift in community composition resulting in a trophic cascade that extended to primary producers and to a nonaquatic species, the bald eagle. The opossum shrimp, Mysis diluviana, invaded Flathead Lake, Montana, the largest freshwater lake in the western United States. Lake trout had been introduced 80 y prior but remained at low densities until nonnative Mysis became established. The bottom-dwelling mysids eliminated a recruitment bottleneck for lake trout by providing a deep water source of food where little was available previously. Lake trout subsequently flourished on mysids and this voracious piscivore now dominates the lake fishery; formerly abundant kokanee were extirpated, and native bull and westslope cutthroat trout are imperiled. Predation by Mysis shifted zooplankton and phytoplankton community size structure. Bayesian change point analysis of primary productivity (27-y time series) showed a significant step increase of 55 mg C m−2 d−1 (i.e., 21% rise) concurrent with the mysid invasion, but little trend before or after despite increasing nutrient loading. Mysis facilitated predation by lake trout and indirectly caused the collapse of kokanee, redirecting energy flow through the ecosystem that would otherwise have been available to other top predators (bald eagles).

Microbial assemblages and production in alluvial aquifers of the Flathead River, Montana, USA

Microbial communities of alluvial floodplain aquifers of 5th- and 6th-order reaches of the Flathead River, Montana, were quantified by sampling wells drilled along transects (500-5100 m) perpendicular to the channels. Cell densities decreased progressively with distance from the river at both reaches. Only 2-3% of the total microbial densities measured in the river water column (3.1 × 105 cells/mL) occurred in wells most distant from the river (9.0 × 103 cells/mL). The rapid decline in bacterial densities could be a result of the filtering effect of the bed sediments. Multiple regression analysis indicated that site distance from the river channel was the only variable examined that explained a significant amount of the variance in bacterial densities in the transects in the down-welling region. Bioavailability of dissolved organic carbon (DOC) also may have been important in the distribution of bacteria, but no spatial correlation was observed using DOC. However, continuously low bacterial densities in wells far from the river suggested that DOC in phreatic waters was primarily refractory. Fungal biovolume exceeded bacterial biovolume in a few wells and hyphae were common in many well samples (up to 89/mL), suggesting strong connectivity with the surface environment. Low fungal densities in the river and elevated densities observed in wells located down-gradient in the flow path suggested that vertical immigration through infiltration and percolation of water from surface soils, which were typically high in fungi, may have been an important process influencing aquifer communities and productivity. Protozoa were observed at all sites with densities ranging from 0 to 64/mL in the river and 0 to 213/mL in wells. The discovery of 24 genera of algae and the presence of chlorophyll-containing cells 4 km from the river provided strong evidence that riverine microbiota were entrained great distances in these highly transmissive aquifers. Incubation of small rocks in wells revealed an extensive epilithic microbial community compared to sparse populations in interstitial waters; mean density of epilithic bacteria was 6.3× 106 cells/cm2, whereas fungi and protozoa averaged 134/cm2 and 350/cm2, respectively. Epilithic bacterial production, estimated by the rate of 3H thymidine incorporation into DNA, ranged from 0.4 to 6.9 μg C m-2 h-1. Lack of higher epilithic biovolume and metabolism in wells near the river suggested that the very pristine waters of the Flathead River did not stimulate production. However, sporadic increases in DOC during spring runoff, storm events, subsurface wetting of the unsaturated zone, and vertical percolation of rain and snow melt may be used very efficiently by the microbial community. Additional study linking hydrogeomorphic processes in these alluvial aquifers to temporal and spatial distribution of carbon and nutrients in a source-sink context is needed to fully elucidate controls on the microbial communities in these very oligotrophic groundwater systems.

Phosphorus bioavailability of fluvial sediments determined by algal assays

Eroding bank soils and riverine suspended sediments from the Flathead River-Lake ecosystem, Montana, USA, were cultured with the alga Selenastrum capricornutum Printz in PAAP medium with the sediments as the sole source of phosphorus. Extraction of phosphorus by NaOH and nitrilotriacetic acid (NTA) solutions were compared to results from algal bioassays. The fine sediment particles transported into Flathead Lake during spring runoff had the highest availability (i.e. 6% of total phosphorus). Bank soils which contained the greatest percentage of fine clays exhibited similar (i.e. 4% of total phosphorus) availability. Bank soils containing predominantly organic phosphorus had the lowest availability. Spearmans rank correlation indicated significance at the 5% test level between algal assay estimates of available phosphorus and both chemical extraction techniques.

Long-term atmospheric deposition of nitrogen, phosphorus and sulfate in a large oligotrophic lake

We documented significantly increasing trends in atmospheric loading of ammonium (NH4) and nitrate/nitrite (NO2/3) and decreasing trends in total phosphorus (P) and sulfate (SO4) to Flathead Lake, Montana, from 1985 to 2004. Atmospheric loading of NO2/3 and NH4 increased by 48 and 198% and total P and SO4 decreased by 135 and 39%. The molar ratio of TN:TP also increased significantly. Severe air inversions occurred periodically year-round and increased the potential for substantial nutrient loading from even small local sources. Correlations between our loading data and various measures of air quality in the basin (e.g., particulate matter <10 µm in size, aerosol fine soil mass, aerosol nutrient species, aerosol index, hectares burned) suggest that dust and smoke are important sources. Ammonium was the primary form of N in atmospheric deposition, whereas NO3 was the primary N form in tributary inputs. Atmospheric loading of NH4 to Flathead Lake averaged 44% of the total load and on some years exceeded tributary loading. Primary productivity in the lake is colimited by both N and P most of the year; and in years of high atmospheric loading of inorganic N, deposition may account for up to 6.9% of carbon converted to biomass.

Zooplankton induced decrease in inorganic phosphorus uptake by plankton in an oligotrophic lake

Experiments conducted on samples collected from a large oligotrophic lake revealed the following: (1) excretion rates of POinf4sup3−by single Daphnia thorata were below detection (5 pmol animal−1 min−1) in 20 ml of oligotrophic lake water over a period of 10 min, (2) experimental addition of D. thorata to 20 ml aliquots of lake water decreased community-wide microbial uptake of POinf4sup3−on two occasions (as measured by 32POinf4sup3−incorporation), and (3) the presence of D. thorata increased uptake by organisms smaller than 1µm, and decreased uptake by large phytoplankton. The specific mechanism for these responses remains unclear, but the results imply that when phytoplankton larger than 1µm encounter cm scale patches of water recently occupied by Daphnia they may experience decreased POinf4sup3−availability rather than elevated concentrations of POinf4sup3−caused by excretion. We show that 32P uptake experiments using natural plankton assemblages can be influenced by the presence or absence of large zooplankton, and that neither grazing, turbulence, nor POinf4sup3−excretion can account for this influence.

Spatial and Temporal Dynamics of Invasive Freshwater Shrimp (Mysis diluviana): Long-Term Effects on Ecosystem Properties in a Large Oligotrophic Lake

Invasion of Mysis diluviana from upstream stockings drastically altered the food web of 480 km2 Flathead Lake, Montana (USA). Mysis increased exponentially after establishment in 1982, preying upon large zooplankters, thereby substantially altering zooplankton community composition, favoring small-sized species. In consequence, primary production increased by 21% owing to changes in zooplankton feeding efficiency. Moreover, the abundant Mysis provided forage for the nonnative lake trout that also rapidly expanded, causing concomitant extirpation of kokanee salmon and near loss of native fishes. This has become a case history of how introduced species can mediate trophic cascades. Here we examine the long-term (1982–2014) dynamics of Mysis in Flathead Lake and how distribution and abundance of this invasive species is related to chemical, physical, and biological factors. We show that Mysis is a strong interactor, regulating zooplankton and phytoplankton biomass interactively with nutrient (N and P) dynamics. Moreover, changes in life history and changing spatial dynamics are strengthening the regulatory role of the Mysis, despite seemingly strong top-down pressure via predation of the Mysis by lake trout. The Mysis are structuring nearly all interactions within and between the biota of Flathead Lake.