Aug 8, 2012 - Denise A. Bruesewitz,1,2 Jennifer L. Tank,1 and Stephen K. Hamilton3. Received 2 .... habitats, a finding that is likely due to a shallow, well-mixed ...... Strauss, E. A., N. L. Mitchell, and G. A. Lamberti (2002), Factors regulat-.
JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 117, G00N07, doi:10.1029/2012JG002006, 2012
Incorporating spatial variation of nitrification and denitrification rates into whole-lake nitrogen dynamics Denise A. Bruesewitz,1,2 Jennifer L. Tank,1 and Stephen K. Hamilton3 Received 2 March 2012; revised 18 June 2012; accepted 23 June 2012; published 8 August 2012.
[1] Despite dramatic increases in nitrogen (N) loading to fresh waters and growing scientific attention on the changing N cycle, measurements of nitrification and denitrification rates in lakes are lacking. In particular, we know little about how these processes vary spatially within a lake, and how this potential spatial variation contributes to a lake’s N dynamics. We measured sediment nitrification and denitrification rates at 40 sites in Gull Lake, Michigan (USA), and found that the shallow edge sediments ( 0.05), and transformed to meet the assumption of normality when necessary. We used simple linear regression (SLR) and stepwise multiple linear regression (MLR) to determine environmental controls on nitrification and denitrification rates (a = 0.05).
3. Results 3.1. Physicochemical Depth Profiles [14] Representative thermal profiles for each transect illustrated that epilimnetic water was approximately 15 C warmer than hypolimnetic water (Table 1). The metalimnion occurred at 8–11 m on each of the two July sampling dates, and the hypolimnion remained oxic throughout the sampling period. There was little change in water chemistry from the epilimnion to the hypolimnion during the study period (Table 1). For both transects, water column NO3 concentrations were consistently 320 mg NO3 – N L 1 in the epilimnion and 300 mg NO3 – N L 1 in the hypolimnion. For Transect S, NH+4 was similar between the epilimnion and the hypolimnion at 25 mg NH+4 -N L 1. The difference in NH+4 between surface and bottom water was slightly greater 2 weeks later when Transect N was sampled, with an increase of 10 mg NH+4 -N L 1 in hypolimnetic water (Table 1). Lake water SRP concentrations remained consistently low, near detection limits, at 2 mg L 1 at all sites and depths on both sampling dates. 3.2. Water and Sediment Characteristics Across the Transects [15] Water depth at our sampling points ranged from 0.3 to 32.9 m and encompassed the full range of depths found in Gull Lake. Surface water NO3 was not variable across transects (t-test, df = 38, p = 0.275), with a mean concentration of 318 2 mg NO3 – N L 1, ranging from 285 to 347 mg NO3 -N L 1. Surface water NH+4 concentrations were much lower than NO3 and averaged 20.7 1.0 mg NH+4-N L 1 across both transects. However, surface water NH+4 was lower at Transect N with a mean of 15.1 mg NH+4 -N L 1 (t-test, df = 38, p < 0.001), which we sampled 2 weeks after Transect S with a mean of 26.3 mg NH+4 -N L 1. [16] All of the metrics describing sediment characteristics were related to water depth. In general, sediment organic matter and sediment % N increased with increasing depth (SLR, r2 = 0.41 and 0.35 respectively, p < 0.01 for each). In contrast, sediment C:N declined with increasing depth
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Figure 2. Principal components analysis (PCA) of the sediment characteristics including sediment organic matter, sediment nitrogen (N,%) and C:N measured at each sampling point. Open circles represent edge sites (10 m deep). (SLR, r2 = 0.11, p = 0.037). Sampling points from both transects span the entire range in sediment characteristics and depths, suggesting that each transect is a good representation of the variability of benthic conditions in Gull Lake. [17] We chose the 2 m and 10 m depths to divide shallow edge from littoral, and littoral from profundal zones, respectively, based on the principal components analysis (PCA) that separated these 3 groups using sediment characteristics (Figure 2). The depth categories separated out along PC axis 1, which explained 64% of the variation in sediment characteristics, with an eigenvalue of 4.451 or a broken stick eigenvalue of 2.593; sediment organic matter and sediment C:N were strongly related to PC axis 1. Less variation in sediment characteristics was explained by PC axis 2 (12% of the variation), with an eigenvalue of 0.828, or a broken stick eigenvalue of 1.593. Sediment %N was related to PC axis 2. Given the categories delineated with the PCA, combining data from both transects, we sampled 9 points
