Nine years of phosphorus management with lanthanum modified bentonite (Phoslock) in a eutrophic, shallow swimming lake in Germany

Figures & data

Table 1. Main morphometric and physicochemical (before application of LMB) characteristics of Lake Baerensee.

Characteristics	Unit	Value
Watershed area	ha	10
Surface area	ha	6
Mean depth	m	2.63
Maximum depth	m	3.8
Volume	10³ m³	156
Trophic state	—	hypertrophic
Conductivity	µS/cm	260–370
pH	Std. units	8–9
Carbonate	mg/L	0.8–4
Bicarbonate	mg/L	123–155
TOC	mg/L	14
DOC	mg/L	12
TP	mg/L	0.08
TN	mg/L	2.1

Table 2. Annual mean values of samples from 1 and 3 m depth. Variables in italics were considered for principal component analysis (PCA). Number of samples (n), annual mean values (mean), minimum (min), and maximum (max) values are shown. The n of total inorganic nitrogen (TIN), ammonium nitrogen (NH₄-N), dissolved organic carbon (DOC), total nitrogen (TN), soluble reactive phosphorus (SRP), calcium (Ca), and dissolved silica (SiO₂) is identical to the n shown for total phosphorus (TP). Values were set as limit of detection when they fell below. Capital letters (A, B, C, and D) indicate time periods used in tests for significant differences and classification in PCA. LMB reapplications are represented with a slash and were conducted in May 2010 and in March 2013.

		2007	2008	2009	2010	2011	2012	2013	2014	2015
		A	B	B	B/C	C	C	C/D	D	D
	n	9	7	4	6	6	5	5	5	5
Chl-a (µg/L)	mean	35.9	19.6	19.2	9.3	15	14.3	8.6	6.8	14
	min	6.2	10.1	9.8	1	1	1.8	1	1.2	8.2
	max	76.3	27.3	35	19.4	31.1	30.2	15.4	11.3	25.8
	n	9	7	5	6	6	5	5	5	5
TP (mg/L)	mean	0.061	0.033	0.042	0.028	0.034	0.037	0.033	0.036	0.052
	min	0.024	0.025	0.028	0.023	0.026	0.029	0.029	0.031	0.029
	max	0.089	0.065	0.057	0.039	0.043	0.047	0.036	0.049	0.078
TIN (mg/L)	mean	0.65	0.42	0.24	0.13	0.08	0.10	0.15	0.18	0.17
	min	0.2	0.14	0.04	0.04	0.04	0.05	0.04	0.1	0.06
	max	1.53	0.79	0.45	0.35	0.15	0.13	0.27	0.3	0.26
NH4-N (mg/L)	mean	0.55	0.32	0.14	0.1	0.05	0.06	0.11	0.07	0.08
	min	0.11	0.14	0.04	0.04	0.04	0.04	0.04	0.04	0.04
	max	1.4	0.62	0.27	0.35	0.09	0.10	0.25	0.19	0.15
DOC (mg/L)	mean	10.7	8.74	8.52	8.25	8.40	7.69	7.71	9.22	8.9
	min	9.1	8.15	7.7	7.6	7.25	7.2	7	8.25	8.25
	max	12	9.45	10.5	9.2	9.5	8.35	8.45	9.95	10.4
TN (mg/L)	mean	1.81	1.22	1.06	1.03	0.76	0.89	0.89	0.87	0.94
	min	1.25	0.94	0.75	0.4	0.59	0.85	0.55	0.55	0.6
	max	2.85	1.5	1.5	1.95	1	0.95	1.25	1.05	1.45
SRP (mg/L)	mean	0.013	0.005	0.005	0.005	0.005	0.005	0.005	0.005	0.016
	min	0.005	0.005	0.005	0.005	0.005	0.005	0.005	0.005	0.005
	max	0.037	0.005	0.005	0.005	0.005	0.005	0.005	0.005	0.022
Ca (mg/L)	mean	43.5	56.1	53.6	54.8	55.6	51.7	51.5	53.2	54.4
	min	37.3	52.5	49.0	51.5	50.5	48.5	49.0	47.5	48.5
	max	50.5	58.5	58.0	56.5	62.0	55.0	55.0	59.0	60.5
SiO2 (mg/L)	mean	8.4	3.0	3.9	2.8	5.0	2.5	4.0	4.1	6.0
	min	3.1	0.4	1.7	0.4	0.4	0.9	1.4	0.4	0.4
	max	12.0	5.4	6.0	8.3	11.0	5.5	6.3	7.3	9.9

Figure 1. Range of (a) total phosphorus concentrations, (b) chlorophyll a concentrations, and (c) total inorganic nitrogen concentrations among temporal subgroups (A–D). Samples in period A (n = 9) were taken in 2007, the year of the initial LMB application. B (n = 14) comprises samples from 2008 to the first reapplication in June 2010. C (n = 16) covers the period between the first and the second reapplication of LMB in March 2013. Samples in period D (n = 14) were taken after the second reapplication of LMB up to end of 2015. Lines within the boxes mark the median. Lines below and above the median indicate the 25^th and 75^th percentiles, respectively; whiskers below and above the boxes indicate the 10^th and the 90^th percentiles, respectively; and dots mark minimum and maximum concentrations of the range. Groups that do not share the same small letter (a, b, c; above the boxes) are significantly different (Mann–Whitney U-test; P < 0.05).

Figure 2. Results of principal component (PC) analysis for selected lake water characteristics depicted as a biplot along first (PC 1) and second (PC 2) principal component. Samples are classified with time periods (A–D); hatched areas cluster samples from period A and B visually. NH₄-N = ammonium nitrogen concentration; Chl-a = chlorophyll a concentration; DOC = dissolved organic carbon concentration; SiO₂ = dissolved silica concentration; TP = total phosphorus concentration; Ca = calcium concentration; EV = eigenvalue; CV = cumulative variance explained. Variables were ln transformed before analysis.

Table 3. Classification of ln transformed variables with the axes by Pearson's correlation coefficients of principal components (PC) scores and variable scores. Asterisks denote significant correlations (α < 0.01).

	Chl-a	TP	NH4-N	DOC	Ca	SiO2
PC 1	0.60^*	0.76^*	0.55^*	0.80^*	−0.81^*	0.75^*
PC 2	0.53^*	−0.41^*	0.67^*	−0.07	0.32	−0.13

Figure 3. Development of mean annual chlorophyll a (Chl-a) concentrations and mean annual total P concentrations depicted in a scatter plot. Pre (n = 3) is only considered as a separated group in this plot and relates to available data from 2007 before application. Trophic states were set with reference to Vollenweider and Kerekes (1982).

Vollenweider R, Kerekes J. 1982. Eutrophication of waters. Monitoring, assessment and control. OECD programme on monitoring of inland waters (Eutrophication control), Environment Directorate. Paris (France): Organisation for Economic Co-operation and Development (OECD).

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