# Ecosystem Metabolism in Small Ponds: The Effects of Floating-Leaved Macrophytes
## 3 Results
### Pond Metabolism--Variation between Ponds and Depths  
Figure 10 shows the seasonal changes in GPP, R and NEP among the ponds. Using mixed model ANOVAs we found significant differences among the ponds for the various components of ecosystem metabolism (GPP: F\({}_{3,455.2}\) = 41.5, \(p\) \(<\) 0.0001; R: F\({}_{3,464.6}\) = 21.0, \(p\) \(<\) 0.0001; NEP: F\({}_{3,472.4}\) = 13.6, \(p\) \(<\) 0.0001). GPP for all of ponds were significantly different from one another with Elephant having the highest average GPP and Warner having the lowest average GPP (Table 3). R in Elephant was significantly greater (values were more negative) than the other ponds (Table 3). R values in the other ponds did not differ significantly from one another (Table 3). NEP was highest in Warner and Elephant and lowest in Stickleback and Railtracks (Table 3). On average, all ponds were heterotrophic (R \(>\) GPP (Table 3)), although there were a number of days when the ponds were autotrophic (NEP \(>\) 0; 32%, 27%, 48% and 28% of the days for Stickleback, Warner, Railtracks and Elephant, respectively (Figure 10)).  
We found that there was a correlation between R and GPP and that these relationships varied by pond (Pond: F\({}_{3,627}\) = 15.2, \(p\)\(<\) 0.0001; GPP; F\({}_{1,627}\) = 1022.8.3, \(p\)\(<\) 0.0001; Pond \(\times\) GPP: F\({}_{3,627}\) = 10.7, \(p\)\(<\) 0.001). The slope of the relationships was greatest for Warner and lowest for Stickleback, and the difference was significant (t\({}_{620}\) = 5.5, \(p\)\(<\) 0.0001). The slope for Stickleback was not significantly different from the other two ponds.  
For the two ponds where we had oxygen probes deployed at the surface and 0.75 m below the surface, Stickleback had significant differences in R and NEP, but not GPP between the depths (mixed model ANOVAs, GPP: F\({}_{1,153.5}\) = 2.9, \(p\) = 0.09; R: F\({}_{1,161.5}\) = 17.1, \(p\)\(<\) 0.0001; NEP: F\({}_{1,161}\) = 22.5, \(p\)\(<\) 0.0001); in contrast, for Warner we found that GPP, R and NEP were significantly different between depths (mixed model ANOVAs, GPP: F\({}_{1,156.4}\) = 116.0, \(p\)\(<\) 0.0001; R: F\({}_{1,174.4}\) = 115.6, \(p\)\(<\) 0.0001; NEP: F\({}_{1,173}\) = 51.9, \(p\)\(<\) 0.0001). For Stickleback, surface measurements of GPP, R and NEP were higher than at 0.75 m (mean surface values for GPP, R and NEP: 3.2, \(-\)3.6 and \(-\)0.44 mgO\({}_{2}\)/L/day; mean values at 0.75 m: 2.8, \(-\)2.2 and 0.57 mgO\({}_{2}\)/L/day). These differences result in Stickleback being classified as heterotrophic based on surface measurements and autotrophic based on measurements at 0.75 m. For  
Figure 9: Percentage of ecosystem metabolism values that were impossible (GPP \(<\) 0 or R \(>\) 0) for a variety of models and reaeration coefficients from [PERSON] et al. [43] for four ponds from east-central Minnesota, USA.  
Warner, 58.9% of the values were \"impossible\" values (GPP < 0 and R > 0) making comparisons of average GPP and R between depths meaningless.  
Figure 10: Seasonal variation in GPP, R and NEP for four ponds in east-central Minnesota, USA. Fitted curves are cubic splines with a \(\lambda\) of 0.5. The shaded areas are bootstrapped confidence limits.