# Ecosystem Metabolism in Small Ponds: The Effects of Floating-Leaved Macrophytes
## 4 Discussion
### Ecosystem Metabolism  
We calculated various components of ecosystem metabolism (GPP, R and NEP) and found that there were significant differences in all of these measures among the ponds. We calculated the values of GPP, R and NEP for a number of other small ponds, similar in size to ours, that used the open water method of estimating metabolism (Table 5). Many of these studies provided metabolism estimates on an aerial basis, and we converted them to a volumetric basis to compare with our values by using the depths of the ponds reported. Thus, while there were differences among our ponds, the range of values was well within those found in the published literature for other comparable ponds. We hypothesized that the pond with few floating-leaved macrophytes (Stickleback) would have higher GPP due to the lack of shading from floating-leaved macrophytes allowing more production of phytoplankton and submerged macrophytes. We expected lower GPP in the ponds with high levels of floating-leaved macrophytes due to shading. We also hypothesized that R would increase with increasing rooted floating-leaved macrophytes due to the input of organic matter from the macrophytes which would provide a source of nutrition for decomposers ([PERSON], 2018). Stickleback had an intermediate level of GPP, and phytoplankton biomass compared to the other ponds. It did have the highest percentage of submerged macrophytes among our ponds which should have contributed to GPP. It also had the lowest R of all of the ponds, likely due the lower overall macrophyte production in that pond as indicated by the lower macrophyte biomass compared with our other study sites. One of the ponds (Elephant) had intermediate amounts of floating-leaved macrophytes, but had the highest level of phytoplankton biomass, GPP and R. It also had intermediate levels of dissolved oxygen. Apparently the floating-leaved macrophytes on Elephant were not dense enough to prevent light and oxygen penetration. In addition, the macrophytes (primarily _Potamogeton_ spp.) in this pond had much smaller leaves than the water lilies in the other ponds. The amount of light penetrating Elephant allowed for an intermediate level of submerged macrophytes to be present. Apparently the intermediate level of floating-leaved macrophytes allowed for a mixture of high levels of phytoplankton and submerged macrophytes resulting in high levels of GPP. The pond with the greatest cover of floating-leaved macrophytes (Warner) had the lowest average phytoplankton biomass and also the lowest GPP and a low R resulting in a low NEP. [PERSON] et al. (2018) found that sites with macrophytes within lakes had higher GPP and R than pelagic sites. They found that the sites with macrophytes also had higher NEP which is different from what we found for ponds that essentially had no pelagic zones. [PERSON] and [PERSON] (2018) found that areas of a small eutrophic lake with rooted floating-leaved macrophytes had higher GPP and R than sites with submerged macrophytes. Unlike our ponds, the study lake was much deeper (avg. depth 4.4 m) and was turbid, leading to an open water zone in the lake. The lake also had fish and thus would seem to be more similar to Stickleback. As they indicated, it is important to consider locale-specific characteristics of ecosystem metabolism in shallow lakes and ponds. A number of studies have examined the impact of various physical, chemical and biologic factors on GPP, R and NEP in lakes. [PERSON] et al. (2018) found that chlorophyll \(a\) was the most important