Nitrogen Deposition Effects on Diatom Communities in Lakes from Three National Parks in Washington State.
Bottom Line: However, N deposition at the remaining nine lakes does not seem to exceed a critical load at this time.We used modeled precipitation for Hoh Lake and annual inorganic N concentrations from a nearby National Atmospheric Deposition Program station, to calculate elevation-corrected N deposition for 1980-2009 at Hoh Lake.An exponential fit to this data was hindcasted to the 1969-1975 time period, and we estimate a critical load of 1.0 to 1.2 kg N ha(-1) year(-1) for wet deposition for this lake.
Affiliation: US Geological Survey, Tacoma, WA 98402 USA.
The goal of this study was to document if lakes in National Parks in Washington have exceeded critical levels of nitrogen (N) deposition, as observed in other Western States. We measured atmospheric N deposition, lake water quality, and sediment diatoms at our study lakes. Water chemistry showed that our study lakes were ultra-oligotrophic with ammonia and nitrate concentrations often at or below detection limits with low specific conductance (<100 μS/cm), and acid neutralizing capacities (<400 μeq/L). Rates of summer bulk inorganic N deposition at all our sites ranged from 0.6 to 2.4 kg N ha(-1) year(-1) and were variable both within and across the parks. Diatom assemblages in a single sediment core from Hoh Lake (Olympic National Park) displayed a shift to increased relative abundances of Asterionella formosa and Fragilaria tenera beginning in the 1969-1975 timeframe, whereas these species were not found at the remaining (nine) sites. These diatom species are known to be indicative of N enrichment and were used to determine an empirical critical load of N deposition, or threshold level, where changes in diatom communities were observed at Hoh Lake. However, N deposition at the remaining nine lakes does not seem to exceed a critical load at this time. At Milk Lake, also in Olympic National Park, there was some evidence that climate change might be altering diatom communities, but more research is needed to confirm this. We used modeled precipitation for Hoh Lake and annual inorganic N concentrations from a nearby National Atmospheric Deposition Program station, to calculate elevation-corrected N deposition for 1980-2009 at Hoh Lake. An exponential fit to this data was hindcasted to the 1969-1975 time period, and we estimate a critical load of 1.0 to 1.2 kg N ha(-1) year(-1) for wet deposition for this lake.
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Mentions: Diatom communities showed some interesting similarities and differences across the four focus sites (Fig. 4a–d). In general, diatom assemblages were very diverse with 120–140 different species identified within each lake core. Historical reconstructions were focused on the upper 10 cm of each core, except for Hoh Lake where samples down to 20 cm were analyzed (see details below). Diatom communities in Snow Lake at MORA were represented mainly by taxa common in undisturbed oligotrophic alpine lakes (Wolfe et al. 2001; Saros et al. 2011) and included Aulacoseira alpigena, Psammothidium spp., and small fragilarioids (dominated by Pseudostaurosira brevistriata and Staurosirella pinnata) (Fig. 4a). The Psammothidium spp. were abundant (13–40 % relative abundance), dominated by Psammothidium curtissimum, and very diverse, represented by 12 species, including some that have not been formerly described (Enache et al. 2013). Together, the above species identified for Snow Lake (Fig. 4) represented from 69 to 81 % of the total assemblages from each sediment interval in the upper 8 cm of the lake core. DCA axis 1 captures a first shift above 5 cm (∼the turn of last century) with a 25 % drop in Psammothidium, a clean cold water genus preferring sandy substrates, and a slight increase in small chain-forming fragilarioids suggesting a change to a more organic substrate. A second gradient shift is captured above 3 cm (∼1990) when the small fragilarioids (Pseudostaurosia and Staurosirella spp.) become ten times more abundant and Psammothidium spp. decline two to three times, suggesting further changes in substrate type and composition.Fig. 4
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