Sewage and agricultural wastewater pollutes many coastal areas around the world. Although this wastewater is enriched in nutrients, its numerous deleterious effects include killing off coral reef communities and reducing the oxygen content of the water.
Mangroves, forests that grow on the freshwater-seawater boundary, are thought to be beneficial in this context. They exhibit increased growth under nutrient-enriched conditions.
In other words, they won't die in response to wastewater, and are therefore thought to be ideal water filters. However, this is a short-term assessment; the long-term consequences of mangrove growth under wastewater conditions are unknown.
Catherine Lovelock (University of Queensland, Australia) and coworkers have investigated the long-term growth of mangrove growth under nutrient-enriched (simulated wastewater) conditions. They have found that these conditions ultimately render some mangroves more vulnerable to environmental stresses, leading to enhanced mortality in the long-term.
Basis of the enhanced mortality hypothesis.
The scientists suspected that mangroves exposed to nutrient-enriched conditions would be less resilient in the face of drought and saline stresses. They based this hypothesis primarily on the observation that plants invest more in parts that are aboveground (such as shoots), relative to parts that are underground (such as roots), when more nutrients are available.
Consequently, under such conditions, plants should be more vulnerable to drought, in which heavy root investment is required. Mortality should be especially severe in locations that experience low humidity and high salinity, which puts even more stress on the mangroves.
Testing the enhanced mortality hypothesis.
The scientists tested this broad hypothesis at 12 sites worldwide, over the course of 3 to 12 years. Each tree was experimentally fertilized by adding between 200 and 300 grams of fertilizer into 30 centimeter deep holes on either side of the tree (or the hole was simply created and sealed, without nutrient addition, for control experiments).
The fertilizer was either enhanced in nitrogen or phosphorus. Nitrogen enhances shoot growth relative to roots, while phosphorus is less effective for these purposes.
Both seaward fringing mangroves and landward scrub mangroves were studied. Seaward fringing mangroves are regularly inundated by tides, and therefore do not have the opportunity to develop hypersaline conditions.
On the other hand, landward scrub mangroves are less frequently inundated by tides. These highly variable conditions enable hypersaline conditions to develop during a drought, which may render them more susceptible to drought-induced mortality.
Tree growth, water salinity, and tree death were measured for all of the trees in the study. There were between six and nine replicates at each study site.
Enhanced mangrove mortality.
The scientists found that none of the trees died in the seaward fringing mangroves, whether they were fertilized or unfertilized. This is because their soil water salinity was similar to that of tidal water, 37 parts per trillion on average.
In contrast, mortality was observed in landward scrub mangroves, when the trees happened to receive less rainfall. The effect was enhanced with the nitrogen-fertilized (disfavoring root formation), but not phosphorus-fertilized, trees.
Tree mortality in the landward scrub mangroves was positively correlated with the average salinity of the soil water, in a statistically significant manner. Similarly, there was a statistically significant negative correlation between tree mortality and average annual rainfall.
In other words, plants which on average typically receive fewer nutrients under normal conditions died when exposed to salinity and drought. This effect was enhanced when root growth was hindered via artifical nutrient deposition.
Mangroves which are not regularly inundated by tides are not ideal for wastewater treatment, because their enhanced growth comes at the expense of resilience in the face of adverse environmental conditions. Hurricanes and other tropical storms may also be among the adverse conditions to which these mangroves may be susceptible.
However, mangroves that are regularly inundated with tides still appear to be ideal wastewater processors. Regardless of their capacity to treat wastewater, mangroves should be viewed as beneficial components of the environment, due to their documented ability to mitigate the effects of flooding (e.g., from hurricanes and tsunamis) and the diverse array of life that they harbor.
for more information:
Lovelock, C. E., Ball, M. C., Martin, K. C., & Feller, I. C. (2009). Nutrient Enrichment Increases Mortality of Mangroves PLoS ONE, 4 (5) DOI: 10.1371/journal.pone.0005600