Hot spots and hot moments of planktonic nitrogen fixation in a eutrophic southern reservoir

Figures & data

Table 1 Characteristics of sampling sites in Lake Waco, TX (mean ± SE, n = 19, 2003-2004).

	Station
	A	B	C	D	E
Depth (m)	2.0	7.6	19.4	6.4	3.4
Turbidity (NTU)	12.2 ± 1.4	6.6 ± 0.6	6.3 ± 0.7	9.3 ± 1.2	13.2 ± 1.4
zeu (m)	1.5 ± 0.2	2.8 ± 0.2	3.0 ± 0.3	2.2 ± 0.3	1.4 ± 0.2
Temperature (C)	23.5 ± 1.5	23.2 ± 1.5	22.4 ± 1.4	22.4 ± 1.5	22.5 ± 1.5
NO3 (μg/L)	235 ±71	408 ± 110	489 ± 124	579 ± 143	641 ± 157
NO3:PO4 (molar)	9.5 ± 4.2	23.1 ± 10.4	22.2 ± 10.9	30.3 ± 12.7	32.1 ± 12.7
TN:TP (molar)	5.4 ± 1.2	10.6 ± 2.7	11.8 ± 3.1	8.1 ± 1.5	10.3 ± 2.3

Figure 1 Waco Reservoir, TX. Black circles with letters A–E indicate routine monitoring stations sampled monthly from March 2003 through September 2004.

Figure 2 Seasonal pattern of volumetric nitrogen fixation (filled circles, mean ± SE, n = 5), mean lake water temperature (C, open triangles) on Lake Waco, TX, and 15-day cumulative discharge from the North Bosque River (shaded bars).

Figure 3 Average (± SE) maximum (light-saturated) rates of nitrogen fixation during each season in Lake Waco, TX. N = 35, 30, and 30 for the fall/winter, spring, and summer periods, respectively. Bars with different letters were significantly different at the 0.05 level (SNK separation test). Statistical analyses performed on natural ln-transformed data to meet variance assumptions of data analysis.

Figure 4 19-month (2003–2004) average (± SE, n = 19) rates of nitrogen fixation expressed as volumetric, biomass-specific or areal rates at each of the 5 sampling stations on Lake Waco, TX. Bars with different letters were significantly different at the 0.05 level (SNK separation test). Statistical analyses performed on natural ln-transformed data to meet variance assumptions of data analysis.

Figure 5 Volumetric rates of N₂ fixation in relation to numerous environmental factors in Lake Waco, TX (2003–2004).

Figure 6 Monthly nitrate levels by station. Heavy dashed line is the identified midpoint of the 90% confidence intervals of the threshold values of NO₃ identified in the changepoint analysis.