Ecology of the diatom Aulacoseira pusilla in oligotrophic mountain lakes, with implications for paleoclimate reconstructions

Figures & data

Figure 1. Location of the study lakes.

Table 1. Select parameters of the study lakes

	Beauty	Beartooth	Kersey
Coordinates	44.97° N, 109.57° W	44.95° N, 109.6° W	45.95° N, 109.54° W
Altitude (m.a.s.l.)	2,874	2,713	2,460
Lake area (km^2)	0.36	0.45	0.47
ZMax (m)	35	26	21
Mixing depth (m)	4	5	5
Z1percentPAR (m)	>17	12.8	10
Nitrate (µM)	< 0.07	<0.07	< 0.07
TP (µM)	0.1	0.1	0.06
SRP (µM)	< 0.02	<0.02	<0.02
DSi (µM)	28.7	27.2	NA

Notes. Maximum depth (Z_Max), depth at which 1 percent of surface photosynthetically active radiation remains (Z_1percentPAR), total phosphorus (TP), soluble reactive phosphorus (SRP), and dissolved silica (DSi). NA = not available. Data from Saros et al. (2003, 2012) and Kessler et al. (2008).

Figure 2. Temperature profile (colored contours) and Z_1percentPAR depth (solid pink line) on left-hand panels and vertical profile of the distribution of Aulacoseira pusilla on right-hand panels between 11 to 28 July 2017 in (A), (B) Kersey Lake, (C), (D) Beartooth Lake, and (E), (F) Beauty Lake.

Figure 3. Nutrient concentrations and ratios in Kersey, Beauty, and Beartooth lakes during the study. DIN:TP ratio calculated using concentrations in micrograms per liter. Geometric shapes indicate the type of nutrient limitation in each lake strata. Colors of geometric shapes identify the lake strata. Circles indicate N limitation, squares indicate P limitation, and triangles indicate N + P limitation.

Figure 4. Phytoplankton composition in Kersey, Beauty, and Beartooth lakes. (A) Percentage of the total cell density. Vertical distributions over time of the main phytoplankton taxa found in (B) Kersey Lake, (C) Beartooth Lake, and (D) Beauty Lake. Empty spaces in (D) correspond to the depths where the algae were not observed.

Table 2. Average values of physical and chemical parameters in Kersey and Beauty Lake water samples used in the experiments

Lake	TN (µM N)	TP (µM P)	DIN (µM N)	SRP (µM P)	DOC (mM)
Kersey	7.4	0.2	0.8	0.02	250
Beauty	3.9	0.1	1.1	0.02	67

Table 3. Average temperature and irradiance in Beauty Lake for experiments incubated at different depths

Incubation zone	Temperature (°C)	Irradiance (μmol m^−2 s^−1)
Epilimnion (2 m)	12.5 ± 0.3	1,392
Metalimnion (8 m)	6 ± 0.1	525
Hypolimnion (15 m)	4 ± 0.02	97

Figure 5. Response of Aulacoseira pusilla to incubation depth (epilimnion, metalimnion, hypolimnion) and nutrients (control, N + P) in bioassay experiments: (A) cell density and (B) growth rate in Kersey Lake populations; (C) cell density and (D) growth rate in Beauty Lake populations. Box ends are 25th and 75th percentiles; bold bars within boxes are 50th percentiles. Letters inside plots denote the results of post hoc analyses. Treatments with different letters indicate a significant difference between those treatments.

Figure 6. The response of Aulacoseira pusilla from Kersey Lake populations to different levels of light expressed as a percentage of ambient (100 percent, 60 percent, or 25 percent) at 2-m depth: (A) density and (B) growth rate. Box ends are 25th and 75th percentiles; bold bars within boxes are 50th percentiles.

Table 4. Values of the results of ANOVA analyses for the Nutrients addition x incubation depths, and light access experiments for Kersey Lake and Beauty Lake populations

Experiment	Lake population	Factor	Parameter	Dfn	Dfd	F	p value	Effect size
Nutrient addition and incubation depth	Kersey	Interaction Nutrient^x incubation depth	Density	2	12	1.9	0.18	0.24
			Growth rate	2	48	1.26	0.3	0.05
		Nutrient	Density	2	12	8.1	<0.05	0.4
			Growth rate	1	48	18.16	<0.05	0.27
		Incubation depth	Density	2	12	18.2	<0.05	0.75
			Growth rate	2	48	34.3	<0.05	0.59
	Beauty	Interaction Nutrient^x incubation depth	Density	2	12	0.35	0.7	0.05
			Growth rate	2	48	1.25	0.2	0.05
		Nutrient	Density	1	12	0.01	>0.05
			Growth rate	1	48	5^x10-4	0.9
		Incubation depth	Density	2	12	6.85	<0.05	0.53
			Growth rate	2	48	18.5	<0.05	0.43
Light access	Kersey	Light level	Density	2	6	1.31	0.34	0.34
			Growth rate	2	24	3.4	<0.05	0.24

Saros, J. E., S. J. Interlandi, A. P. Wolfe, and D. R. Engstrom. 2003. Recent changes in the diatom community structure of lakes in the Beartooth mountain range, USA. Arctic, Antarctic, and Alpine Research 35 (1):18–23. doi:10.1657/1523-0430(2003)035[0018:RCITDC]2.0.CO;2.

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Kessler, K., R. S. Lockwood, C. E. Williamson, and J. E. Saros. 2008. Vertical distribution of zooplankton in subalpine and alpine lakes: Ultraviolet radiation, fish predation, and the transparency‐gradient hypothesis. Limnology and Oceanography 53 (6):2374–82. doi:10.4319/lo.2008.53.6.2374.

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