Saturday, May 30, 2009

Mangroves, Eutrophication and Dead Zones

Recent research from the University of Queensland, the Australian National University and the Smithsonian’s Environmental Research Center demonstrates that, contrary to previous studies, nutrient influx of Nitrogen and Phosphorous into coastal forests may contribute to mortality in mangrove trees. In a time of advancing Dead Zones, these findings hasten warnings about the dire consequences of poor-conservation efforts in regards to the world’s starkly limited water resources.

The name “mangrove tree” can refer to any number of species common to the world’s brackish habitats, however here in Florida, the term generally refers to one of three species, the “red mangrove” (Rhizophora mangle), “white mangrove” (Laguncularia racemosa) or the “Black Mangrove” (Avicennia germinans). In order to survive and prosper in the plant hostile environments found within coastal areas, natural selection has provided the mangrove trees with highly specialized morphological adaptations.

The evolutionary history of mangrove trees stretches back more than 100-million years to the Lower Cretaceous of Southeast Asia – a geographic area where they currently exhibit a species diversity and range greater than anywhere else on the planet. Having had emerged from tropical and subtropical intertidal zones with excesses of salinity, temperature and erosion - and having deficiencies of freshwater and aerobic soils - mangrove trees demonstrated tolerances and strategies that enabled them to quickly expand their range around the globe. These adaptations include a mode of viviparous reproduction in which energetically independent seedlings remain attached to the parent tree following germination; remaining there attached until sufficiently mature to drop to the waters below as propagules for dispersion to far-off localities. From the plants ability to turn its leaves away from the intense midday tropic sun, to its skill in fine filtrating sodium from available marine waters, the mangrove is a dynamo of the brackish way of life.

Another adaptation to intertidal existence was the advent of soil accumulating suberised prop roots, which elevate the tree, filter salts and permit mangroves to construct their own islands. The roots are critical to the mangrove’s ecology for several reasons. One is that being immersed in a saline hydrology requires balance of sodium between the plant’s internal environment and that of the surrounding marine waters; this is critical for maintaining osmotic pressures and nutrient uptake in a world where the potential for desiccation is omnipresent. The roots also provide physical stability and support to the tree during frequent and intense wave bombardment and during high wind events such as tropical cyclones. Though amazingly adapted, the roots of mangrove trees may have met their match in anthropogenic nutrient loading…

Nutrient enrichment of plants through the addition of Nitrogen and Phosphorus loads, rather occurring naturally - or as more common – of anthropogenic origins, leads to growth in most species. These are simply the principles of fertilization in action; “more nutrients equals more growth.” However, as an important aside, “more growth” does not necessarily mean that the entire plant from root-to-canopy grows uniformly. In fact, in most cases fertilization leads to expansion of above ground plant structures only. Thought of from the plant’s perspective, “if there is currently adequate, or more than adequate, nutrients available, why should I invest in additional nutrient up-taking roots – I already have what I need from the soil, so I’ll invest in stems and leaves instead!” Such a “plant’s perspective” appears to be the one implemented by mangroves in the presence of increasing levels of eutrophication.

Using Florida and the Gulf of Mexico as one example of many from around the world, run-off carrying excesses of fertilizer from the “Bread Basket” of the United States is finding its way into the tributaries of the Mississippi, and in turn, into the Gulf of Mexico where explosions of algae growth have resulted in hypoxic conditions and the creation of a massive Dead Zone. Surges in the growth of algae and other noxious plants resulting from Nitrogen and Phosphorous loading is called eutrophication; eutrophication leads to de-oxygenated environments, the death of all organisms requiring oxygen and the total loss of ecosystem function. To make matters worse, far from being stationary, the dead zones move or “creep” from their epicenters, corrupting ecosystems both far and wide. For Florida, the Gulf of Mexico Dead Zone may contribute to “Red Tide” and the death of everything from phytoplankton to manatees in the State’s waters. Now, in addition to all of these previously known adverse effects, eutrophication has the potential to fertilize Florida’s coastal mangrove forests, causing rapid tree growth – but only of canopy components, not of roots.

As described above, the roots of mangrove trees are essential for multiple reasons and any increase in above surface foliage that is not accompanied by a proportional increase in rooting not only makes the trees more susceptible to desiccation, but also makes them directly vulnerable to mortality by frequent onslaught of wind and wave. If the current research is accurate, additional Nitrogen in coastal aquatic ecosystems may translate to a short-term growth of tree canopy followed by a long-term increase in the rate of mangrove tree death.

Lovelock, C., Ball, M., Martin, K., & C. Feller, I. (2009). Nutrient Enrichment Increases Mortality of Mangroves PLoS ONE, 4 (5) DOI: 10.1371/journal.pone.0005600
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