# Ecosystem Metabolism in Small Ponds: The Effects of Floating-Leaved Macrophytes
## 4 Discussion
### Issues with Measuring Ecosystem Metabolism  
We found that there were some concerns with using the changes in dissolved oxygen to assess levels of pond metabolism. First, the dissolved oxygen model of assessing metabolism does not detect anaerobic metabolism ([PERSON], 1991; [PERSON], 1992), which likely plays a key role in productive, shallow systems. Next, it is also likely the method does not detect oxygen changes due to photosynthesis and respiration of floating-leaved macrophytes, because they can exchange gases directly with the atmosphere ([PERSON], 1992). [PERSON] et al. (2018) were able to estimate respiratory carbon loss from macrophytes, phytoplankton, macrozoobenthos and microbial communities utilizing Electron Transport System activity. They found that that in macrophyte dominated lakes, macrophytes account for as much as 80% of the summer respiratory carbon loss, while in phytoplankton dominated lakes, microorganisms in the sediments account for the vast majority of summer respiratory carbon loss. The model we used to calculate ecosystem metabolism included the use of the [PERSON] (2018) empirical model optimized for low wind speeds to estimate reaeration. [PERSON] et al. (2018) indicated that there are significant difficulties in assessing the reaeration coefficient (\(k\)) and found that different models of estimating \(k\) resulted in different estimates of lake metabolism making the determination of whether a lake was autotrophic or heterotrophic on an annual basis especially difficult. [PERSON] et al. (2018) found that wind speed influences gas transfer velocity in small ponds with small ponds having lower levels of reaeration. [PERSON] et al. (2018) found that the [PERSON] (2018) model that we used tended to underestimate reaeration rates. Our values of the reaeration coefficient (\(k_{600}\)) were similar to those that [PERSON] et al. (2018) estimated using the same model and these were 2-3 times lower than estimates made using measured changes in gas concentrations (CO\({}_{2}\)) between the atmosphere and water and 3-6 times less than values estimated by [PERSON] et al. (2018) using a standard propane tracer method. Consequently, our estimates of pond metabolism could be inaccurate, underestimating GPP and R.  
Not only could reaeration values be subject to error, but the calculation of ecosystem metabolism assumes that the water column is well mixed, allowing changing O\({}_{2}\) levels to be representative of the entire water column. On the dates we measured vertical profiles of temperature and O\({}_{2}\) we found that the ponds were, to varying degrees, stratified. In the two ponds where we measured oxygen levels at two depths throughout the sampling period, the differences we found in oxygen levels between the surface and 0.75 m below the surface were more pronounced in the pond with large amounts of floating-leaved macrophytes (Warner) than that in the pond with very few floating-leaved macrophytes (Stickleback), suggesting that the presence of the macrophytes restricts reaeration. [PERSON] and [PERSON] (2018) indicated that neglecting transient stratification may be responsible for many of the errors found in models of high frequency time series of dissolved oxygen used to estimate ecosystem metabolism. [PERSON] et al. (2018) found that ponds with greater amounts of macrophytes, especially if they are non-rooted floating macrophytes and cover a large percentage of the pond surface, resulted in lower oxygen levels. [PERSON] et al. (2018) described the impact of floating-leaved macrophytes on oxygen exchange with the atmosphere indicating that they can vent O\({}_{2}\) to the atmosphere greatly reducing water O\({}_{2}\) level. [PERSON] et al. [81] found stratification of temperature and oxygen in urban ponds, while [PERSON] et al. [82] found that daytime temperature stratification was common in small ponds with convective mixing occurring during the night. [PERSON] et al. [83] indicated that dense submerged vegetation can lead to reduced mixing in shallow ponds.