# Ecosystem Metabolism in Small Ponds: The Effects of Floating-Leaved Macrophytes
## 4 Discussion  
### Physical/Chemical/Biologic Conditions of the Ponds  
All of the ponds in our study, regardless of the amount of floating-leaved macrophytes were relatively clear. The chlorophyll content for all ponds was within the typical range for lakes in the central hardwood area of Minnesota [47] and small ponds more generally [48]; however, they were all low compared to a large number of both small clear (\(<20\)\(\mathrm{\SIUnitSymbolMicro g}\)/L) or turbid (\(>31\)\(\mathrm{\SIUnitSymbolMicro g}\)/L) lakes in Minnesota [37; 49]. Elephant was the only pond that had phytoplankton chlorophyll biomass levels \(>31\)\(\mathrm{\SIUnitSymbolMicro g}\)/L, which [PERSON] [49] considered turbid, and then only in late August. The pond with fish (Stickleback) historically had high levels of floating-leaved macrophytes similar to Warner. With the introduction of fish, a new state was established: one with few floating-leaved macrophytes, a reduction in amphipods and a shift in the _Chaborous_ zooplankton assemblage ([PERSON], Warner Nature Center, personal communication, [PERSON], unpublished data). Stickleback are known to graze on zooplankton and can potentially bring about an increase in phytoplankton due to top-down control [50; 51]. However, unlike other studies where the introduction of fish led to a loss of macrophytes and increased amounts of phytoplankton [24; 25; 26; 27; 29], Stickleback had the greatest percentage of submerged macrophytes and an intermediate level of phytoplankton compared to other ponds in our study. Stickleback also had the highest level of nutrients (P and N) which again would lead to an expectation of increased levels of phytoplankton and low amounts of both submerged and floating-leaved macrophytes. [PERSON] and [PERSON] [52] indicated that rooted floating-leaved macrophytes lower P concentrations in the water column because of their high uptake of P through the roots resulting in lower sediment P concentrations. [PERSON] et al. [53] also pointed out rooted macrophytes may influence the level of nutrient retention in small ponds by allowing N and P to be stored in plant tissues. This process could account for the pond with the highest level of floating-leaved macrophytes (Warner) having among the lowest P levels, and the pond with lowest biomass of floating-leaved macrophytes and lowest overall macrophyte biomass (Stickleback) having the highest P levels. The differences in the amounts and types of macrophytes dominant in the ponds could be due to slight differences in the surrounding vegetation and land use (Elephant having higher amounts of housing and deciduous forest), the depth of the ponds (Railtracks much shallower than the others) and as mentioned above the presence of fish in Stickleback. In June there was little difference in phytoplankton populations, as assessed through chlorophyll \(a\), among ponds. As the summer progressed, chlorophyll \(a\) content remained low in the pond with the highest levels of floating-leaved macrophytes (Warner), while chlorophyll \(a\) content was higher in the other two ponds. Stickleback had intermediate levels of chlorophyll \(a\). Light penetrated farther into the water column in Stickleback indicating that even though chlorophyll \(a\) level were intermediate, something other than light was controlling phytoplankton production. While Elephant had the highest average phytoplankton biomass, it had more floating-leaved macrophytes than Stickleback. The smaller size of the leaves of the floating-leaved macrophytes in Elephant (_Potamogeton_--leaf size length generally 5-10 cm, versus waterlily--leaf size length generally 7-40 cm) led to a fairly high level of light penetration into the pond (small extinction coefficient) and thus may be responsible for its fairly high level of chlorophyll \(a\) compared to the other ponds with greater amounts of floating-leaved macrophytes. However, this does not explain the higher amount of phytoplankton in Elephant than Stickleback.