Nutrients and warming interact to force mountain lakes into unprecedented ecological states

Abstract

While deposition of reactive nitrogen (N) in the twentieth century has been strongly linked to changes in diatom assemblages in high-elevation lakes, pronounced and contemporaneous changes in other algal groups suggest additional drivers. We explored the origin and magnitude of changes in two mountain lakes from the end of the Little Ice Age at ca 1850, to ca 2010, using lake sediments. We found dramatic changes in algal community abundance and composition. While diatoms remain the most abundant photosynthetic organisms, concentrations of diatom pigments decreased while pigments representing chlorophytes increased 200-300% since ca 1950 and total algal biomass more than doubled. Some algal changes began ca 1900 but shifts in most sedimentary proxies accelerated ca 1950 commensurate with many human-caused changes to the Earth System. In addition to N deposition, aeolian dust deposition may have contributed phosphorus. Strong increases in summer air and surface water temperatures since 1983 have direct and indirect consequences for high-elevation ecosystems. Such warming could have directly enhanced nutrient use and primary production. Indirect consequences of warming include enhanced leaching of nutrients from geologic and cryosphere sources, particularly as glaciers ablate. While we infer causal mechanisms, changes in primary producer communities appear to be without historical precedent and are commensurate with the post-1950 acceleration of global change.

Keywords: chlorophyte; diatom; mountain lake; nitrogen deposition; palaeolimnology; warming.

Figure 1.

Summary of temporal trends in chemistry. Year versus δ¹³C, δ¹⁵N, C, N, C : N, are fitted with GAM-smoothing trends and 95% confidence intervals (light grey bands) for The Loch and Sky Pond. (a) δ¹³C of bulk sediment; (b) δ¹⁵N of bulk sediment; (c) carbon content as a percentage of dry mass; (d) nitrogen content as a percentage of dry mass; and (e) C : N ratio. Depths prior to ca 1850 are extrapolated from ²¹⁰Pb data and should be interpreted with caution. Electronic Supplemental Information for details. (Online version in colour.)

Figure 2.

Temporal trends in major algal functional groups inferred by pigment analyses. GAM-smoothing trends fitted are depicted with 95% confidence intervals for all major algal pigments in The Loch and Sky Pond time series. Pheophytin a (a) and β-carotene (b) are proxies for total algal biomass. Pheophytin b (c) is a proxy for total chlorophyte biomass. Lutein and zeaxanthin (d) are indicative of both chlorophytes and cyanobacteria. Diatoxanthin (e) is a proxy for total diatom biomass. Echinenone (f), canthaxanthin (g), and myxoxanthophyll (h) are proxies for various cyanobacteria (total, Nostocales, and filamentous and colonial, respectively). Alloxanthin (i) is proxy for total cryptophytes. (Online version in colour.)

Figure 3.

Ratio of Planktonic to Benthic (P : B) diatom valve counts in Sky Pond sediments since ca 1850. There was a significant positive trend (F_(1,20) = 22.69, R₂ = 0.53, p = 0.0001) in P:B with a shift towards more planktonic than benthic species, with an inflection point ca 1950.

Figure 4.

Trends in climate and nutrient drivers associated with lake algal production. (a) Mean summer (JJA) modelled air temperature (PRISM Climate Group, Oregon State University, http://prism.oregonstate.edu) for Loch Vale watershed. The 120-year PRISM record showed a significant breakpoint in which summer air temperatures increased more rapidly post-1985 (Davies test p < 0.0001). (b) Global N emission rates [82]. The shaded box highlights 1940–1960 when rates accelerated (GAM analyses). (c) P accumulation rates in sediments of alpine lakes 1900–present, extracted from [11]; the shaded box is the result of a GAM analysis that shows a significant increase in the rate of P accumulation.