# Ecosystem Metabolism in Small Ponds: The Effects of Floating-Leaved Macrophytes
## 4 Discussion
### Ecosystem Metabolism
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Study [Reference]** & **GPP (mgO\_2/L/day)** & **R (mgO\_2/L/day)** & **NEP (mgO\_2/L/day)** \\ \hline [PERSON] et al. (2018) & 2.9 & –3.2 & –0.3 \\ [PERSON] et al. (2018)1 & 0.9–9.1 & –4.1—0.4 & 0.5–5.0 \\ [PERSON] et al. (2018) & 3.1 & –2.9 & 0.3 \\ [PERSON] (2018)2 & 3.6–7.1 & –5.3—4.2 & –0.6–1.62 \\ [PERSON] et al. (2018)3 & 0–15 & 0—15 & –0.38 \\ [PERSON] et al. (2018)4 & 5.0–8.8 & –48.0—38.9 & –39.2—33.9 \\ This Study5 & 2.2–5.5 & –6.8—3.6 & –1.5—0.1 \\ \hline \hline \end{tabular}  
* Range for 9 ponds. 2 Range for 2 ponds. 3 Values for Mirror Lake—NEP is average annual—were positive during summer. 4 Range for 2 ponds. 5 Range for 4 ponds.  
\end{table} 
Table 5: Values of GPP, R and NEP from a number of published studies. driver of GPP, R and NEP in the two lakes they studied and that temperature, CDOM (chromophoric dissolved organic matter), precipitation, wind speed, precipitation, mixed layer depth and water column stability were less important. There was a great deal of variability in measures of ecosystem metabolism at the daily level, and while aggregating the data by week increased the fit of some multiple regression models, the results were not consistent. [PERSON] et al. (2016) also found that a variety of factors, especially P concentrations, were more important than temperature in influencing GPP and R in aquatic systems. [PERSON] (2016), [PERSON] et al. (2017) and [PERSON] et al. (2018), among others indicate that P levels control GPP and result in enriched systems being autotrophic with high levels of chlorophyll \(a\). In our ponds chlorophyll \(a\) content did seem to correlate with GPP, but not necessarily with R or NEP. In our ponds there were no clear correlations between GPP and P concentrations, with Stickleback having the highest P concentration, but intermediate GPP. We did, however, see correspondence between high phytoplankton biomass and GPP. Our results are surprising, since P concentrations are often correlated with chlorophyll \(a\) concentration. This may indicate that the presence of floating-leaved macrophytes and the shading effect they produce, as well as the possibility of P uptake and storage in macrophytes, influence this relationship. There were seasonal variations in GPP, R and NEP in our ponds. In all of the ponds, except Railtracks, there were low levels of GPP and R in May with all of the ponds being slightly heterotrophic. Temperature and light levels were low during this period as was the amount of chlorophyll \(a\). Again, in all ponds other than Railtracks GPP and R increased in late summer and early fall. The reasons for Railtracks being an outliner are not clear, but it is the shallowest pond with the lowest volume and in May had the highest average temperature and light input and in late summer and early fall it had lower temperatures and light input. Other studies have shown seasonal changes in ponds or small lakes in GPP and R associated with changing levels of phytoplankton and submerged macrophytes ([PERSON] et al., 2017; [PERSON] et al., 2018; [PERSON] et al., 2018). Switching between autotrophy and heterotrophy has been noted in some ponds and small lakes ([PERSON] et al., 2017; [PERSON] et al., 2018) associated with peaks of phytoplankton production or decomposition, especially of macrophytes. [PERSON] et al. (2018) found that 25 lakes in Denmark, especially small lakes and forest lakes, had negative net ecosystem production. Total phosphorus, chlorophyll and dissolved organic carbon were correlated with lake trophic status, with lakes having high TP and chlorophyll \(a\) more likely to be autotrophic while those with high levels of DOC (likely from allochthonous carbon sources) more likely to be heterotrophic. Similarly, [PERSON] and [PERSON] (2018, 2019), [PERSON] et al. (2017), [PERSON] et al. (2018) and [PERSON] et al. (2018) suggested that the input of allochthonous matter results in small ponds and lakes being heterotrophic, especially if P levels, limiting production are low.