Changes in mixing depth reduce phytoplankton biomass in an Arctic lake: Results from a whole-lake experiment

Figures & data

Figure 1. Thermal structure of a lake in southwest Greenland during the first two weeks of July in an early (2016) and a late (2015) ice-off year. These data are based on thermistor strings collecting data at 1-m intervals in the water column. Colored contours represent temperature changes (°C) throughout the water column over time.

Table 1. Physical and chemical data from the epilimnia of the two lakes used in the whole-lake manipulation study during 2013 and 2014. The detection limit for DIN was 2 µg L⁻¹; otherwise, values were below detection (bd).

Lake	Date	Mean Temp. (°C)	pH	DO (mg L^−1)	Conductivity (µS cm^−1)	1 Percent PAR (m)	DIN (µg L^−1)	TN (µg L^−1)	TP (µg L^−1)	DOC (mg L^−1)	Chl-a (µg L^−1)
Control	14 June 2013	9.8	6.5	11.3	65	6.02	7	580	8	9	7.4
	24 June 2013	13.1	—	—	—	—	bd	578	8		3.8
	17 July 2013	14.7	—	—	—	—	8	677	18	10	3.8
	31 July 2013	14.5	6.5	10.2	67	6.85	5	610	11		4.1
	14 June 2014	18.0	6.8	11.4	63	6.72	19	585	4	9	5.6
	26 June 2014	13.5	7.7	10.8	64	6.61	bd	717	3	10	4.9
	15 August 2014	12.0	8.4	10.6	71	7.71	bd	602	4	11	5.8
Experimental	14 June 2013	6.1	6.4	10.5	69	5.83	bd	480	12	10	4.5
	24 June 2013	11.5	—	—	—	—	bd	439	8		4.8
	17 July 2013	14.4	—	—	—	—	95	611	10	11	3
	31 July 2013	14.0	7.4	10.3	71	6.44	65	443	7		3.2
	14 June 2014	18.6	6.7	11.2	62	7.13	bd	475	4	9	3.7
	26 June 2014	10.9	7.8	10.5	71	6.75	4	645	17	10	3.1
	15 August 2014	12.3	8.1	10.1	73	6.73	bd	540	5	11	5.1

Figure 2. Changes in control and experimental lake thermal structure during the open water seasons of 2013 and 2014. The experimental lake was manipulated in 2014.

Figure 3. Changes in the density (cells mL⁻¹) of major phytoplankton phyla in the sediment trap of a control lake (top panel) and an experimentally manipulated lake (bottom panel) in southwest Greenland in 2013 and 2014. The experimental lake was manipulated in 2014.

Figure 4. Changes in the biovolume (µm³ mL⁻¹) of major phytoplankton phyla in the sediment trap of a control lake (top panel) and an experimentally manipulated lake (bottom panel) in southwest Greenland in 2013 and 2014. The experimental lake was manipulated in 2014.

Table 2. Total zooplankton abundance (number of individuals per sample) and the relative abundance of major zooplankton groups in the two lakes used for the whole-lake manipulation in 2013 and 2014. Cladoceran zooplankton were not found in either lake at any time.

Treatment	Date	Zooplankton abundance	Copepods	Rotifers
Control	14 June 2013	630	0.07	0.93
	31 July 2013	238	0.01	0.99
	14 June 2014	1,110.4	0.17	0.83
	26 June 2014	275	0.06	0.94
	15 August 2014	324	0.08	0.92
Experimental	14 June 2013	231	0.03	0.97
	31 July 2013	342	0.01	0.99
	14 June 2014	1,329.75	0.08	0.92
	26 June 2014	845.6	0.04	0.96
	15 August 2014	228.5	0.02	0.98

Table 3. Proportions of explained variance and relative contributions of environmental variables to phytoplankton phyla distributions in southwest Greenland lakes as determined by CCA.

		CCA axis 1	CCA axis 2
Proportion of constrained variance explained		0.51	0.31
CCA axis loadings
Environmental variables	Temperature	−0.18	−0.66
	Conductivity	−0.74	−0.17
	DIN	0.42	−0.44
	TP	0.39	−0.43
	DOC	−0.51	−0.45
	PAR	−0.23	0.46
	Average light	−0.44	−0.84
Taxa	Chlorophytes	−0.04	−0.15
	Diatoms	0.52	0.30
	Dinoflagellates	0.26	−0.68
	Cyanobacteria	0.06	−0.74
	Chrysophytes	−0.70	0.47
	Cryptophytes	−0.16	0.04

Figure 5. Canonical correspondence analysis biplot relating physicochemical and biological data in southwest Greenland lakes across the summer season of 2013. Physical and chemical parameters are represented with vector arrows; phytoplankton phyla fall along vectors that best predict their distributions across the landscape of southwest Greenland.