Do water plants clean up water?Submitted by editor on 2 April 2019.Get the paper!
Anyone who has had an aquarium or a garden pond is likely to be familiar with the concept of water plants (macrophytes) helping to keep the water clean. In freshwater lakes, macrophytes are doing exactly that. Recall the lakes that you may have visited—you might notice that some have clear waters and lots of macrophytes, while others are turbid and with few or no plants. These may be examples of alternative stable states of shallow lakes, and macrophytes are known to stabilize the clear-water state (1).
This concept, however, has mainly been developed from observations in temperate areas, particularly Europe and North America. What about tropical and subtropical areas? Researchers previously thought that the effects of macrophytes in improving lake-water quality might be weaker in the tropics because of the vastly different community structures (2). Recent studies in low-latitude areas provide an opportunity to reassess and compare the effect of macrophytes on lake-water quality globally (Fig. 1).
Global distribution of the studies included in the meta-analysis. The study design is indicated by the shape and color of the symbols. The number of pairwise comparisons (n) in each study is indicated by the opacity of the symbols.
The effects have been measured in various ways, including comparing water samples from within and outside macrophyte stands; enclosing parts of lakes and manipulating the density of macrophytes; and comparing samples from the in-flow and out-flow of macrophyte wetlands. We synthesized the findings of 47 such studies by calculating and summarizing the Hedges’ g effect size, a quantitative measure of the magnitude of an effect.
We found, in general, that the positive effects of macrophytes on lake-water quality such as reducing the biomass of phytoplankton and lowering nutrient levels, hold true even when tropical and subtropical studies are included—despite not necessarily directly enhancing the clarity (measured by Secchi depth) (Fig. 2). Interestingly, when comparing the effects along a latitudinal gradient, we found that the macrophytes may be similarly effective or even more so towards lower latitudes (Fig. 3). We also found that the effects depend on the macrophyte growth form and the study design.
Mean Hedges’ g effect sizes and 95% confidence interval of the analyses of five metrics describing the effects of macrophytes on water quality. Chl: chlorophyll a concentration; TN: total nitrogen concentration; TP: total phosphorus concentration; SD: Secchi depth; TSI: trophic state index. Total numbers of pairwise comparisons (n) are denoted in parentheses. Each dot represents a pairwise comparison, and larger dots represent lower intra-study variance, which were used as a weighting factor in the analyses.
Effects of macrophytes on total phosphorus concentration across latitudinal gradients. Each dot represents a pairwise comparison, and larger dots represent lower intra-study variance, which were used as a weighting factor in the analysis. The regression line represents the fitted values from the meta-regression, and the grey ribbon represents the 95% confidence interval.
Why do these matter? The role of macrophytes in promoting the clear-water state is not only of theoretical interest, but highly useful in real life situations, namely the restoration of turbid lakes. While there have been several examples of using submerged macrophytes in temperate (3) and subtropical (4) lake restoration, our finding now encourages more investigation and experimentation in tropical and subtropical lakes.
Our mesocosm experiments in Singapore represent some of the most recent efforts to apply these findings, assessing the potential of macrophyte species in clearing water bodies dominated by cyanobacteria (5) (Fig. 4). The findings add to the growing line of evidence that macrophytes could be of great use in the restoration of (sub)tropical lakes.
The authors through Yiluan Song