Nitrogen fate and transport in a watercress-dominated stream

Figures & data

Figure 1  Mesocosm experimental set-up. A, In-stream acclimation (before incubation). B, Re-circulating incubation system.

Table 1  Mesocosm incubation summary.

Dates	Duration (h)
2 Apr 2003	3.5
4–6 Apr 2003	43.5
8–12 Apr 2003	91
14 Apr–18 Apr 2003	87.5
29 April–3 May 2003	85
6 May–9 May 2003	76.5
16 Dec–18 Dec 2003	53
19 Dec–22 Dec 2003	72

Table 2  Release tracer solutions.

Date	Release type	KNO3 mass (g)	NaCl mass (g)	Water volume (l)	Release duration (min)	Release rate (l/min)
5/21/02	Continuous	108.5	1900	26	101	0.21
5/21/02	Pulse	−	1915	6	c. 20 s	c. 18
6/7/02	Continuous	281	2800	26.5	62	0.21
6/7/02	Pulse	−	2454	9	c. 20 s	c. 27
12/11/02	Continuous	1140	5366	24	100 min	0.22
12/11/02	Pulse	−	2981	15	c. 20 s	c. 45
12/16/02	Continuous	1249	5197	26	86 min	0.20
12/16/02	Pulse	−	3036	15	c. 20 s	c. 45

Figure 2  Nitrogen uptake rates vs water column nitrate concentration: mesocosm experiments. A, Autumn. B, Early summer.

Table 3  Summary of watercress uptake model parameterisation: mesocosm experiments.

r max (mg-N/g dw/day)	K N (mg-N/l)	Early summer G r (unitless)
3.0	1.7 (1.2–2.4)^a	1.0 (0.81–1.23)^b
^aUncertainty range based on 10% deviation from optimal RMSE in curve fitting; ^buncertainty range based on range of calculated summer:autumn uptake rate ratios.

Figure 3  Nitrate to chloride ratios measured during tracer releases. A, 21 May. B, 7 June. C, 11 December. D, 16 December.

Table 4  Summary of model transport parameters: chloride releases.

Parameter	21 May	7 Jun (cleared)	11 Dec	16 Dec (cleared)
Q (l/s)	2.9	10.3 (pulse), 11 (continuous)	8.9	6.9
u (m/s)	0.058	0.176 (top), 0.170 (bottom)	0.123, 0.117	0.101, 0.126
D (m^2/day)	6000	10500	7500	4200
α (day^-1)	9	100	60, 25	70, 70
A S (m^2)	0.02	0.0045	0.0070, 0.019	0.0057, 0.010

Figure 4  Autumn nitrate releases: modelled vs measured. A, 21 May. B, 7 June (cleared).

Figure 5  Early summer nitrate releases: modelled vs measured. A, 11 December. B, 16 December (cleared).

Figure 6  Root mean squared error vs uptake rates: modelled vs measured nitrate profiles. A, 21 May. B, 11 December.

Table 5  Summary of quantified mean daytime nitrate uptake rates (mg-N/g dw/day).

	Autumn	Early summer
Mesocosm^a	1.8±0.6	2.0±0.4
Field release modelling^b	1.4±0.5	8.4±2
^aError estimates equal to 1 standard deviation; ^berror estimates obtained from visual assessments of Fig. 6.