Linda Benedict | 5/5/2005 6:40:24 PM
Knowledge of wetlands is becoming increasingly important to study and research about wildlife. Five areas of mutual concern are these:
1. Improving water quality is probably the most important wetland function.
It might seem strange that a wildlife biologist would not list wildlife habitat as the most important wetland function, but more people and dollars are affected by water quality than by wildlife. Many people use the word “filter” to describe how wetlands improve water quality, but “converting” is a more appropriate and descriptive term. Wetlands can convert pollutants into non-pollutants, and the converter is never used up. For example, excess nitrate fertilizer from urban lawns and farm fields can get into water bodies. This allows undesirable plants to out-compete desirable plants. These vegetative changes then cascade up the food web, harming other organisms.
Wetlands, however, can reverse this process. They can convert the nitrate back into nitrogen gas, which is then released into the atmosphere where it is harmless. Air is 79 percent nitrogen and 21 percent oxygen. Here’s how it works:
Soil microbes convert nitrate back into nitrogen gas in a process called denitrification. Denitrifying bacteria are active where oxic water (with oxygen) meets anoxic water (lacking oxygen). The zone of denitrification is tremendous in wetlands because it wraps around every tiny plant root and is there 24 hours a day, seven days a week unless frozen. The bottom sediments of some ponds, lakes and bays also can support denitrifying bacteria, but denitrification there does not approach rates observed in wetlands because there are no living plant roots releasing oxygen. Upland soils support trivial amounts of denitrification because conditions there do not favor denitrifying bacteria.
2. Wetland loss in coastal Louisiana is extensive.
Although coastal wetland loss is still significant, the rate of loss has slowed somewhat since the 1960s. In the 1960s coastal Louisiana lost an acre every 20 minutes, but by the 1980s it took 32 minutes to lose an acre. Data for the 1990s are not yet available to determine whether coastal Louisiana continued to improve.
3. Wetland loss is primarily interior loss rather than shoreline erosion.
Most people use the word “erosion” to describe wetland loss in coastal Louisiana and, indeed, Louisiana has some of the fastest eroding shorelines in the world. Little of Louisiana’s wetland loss occurs because of erosion, however. A comparison of the state’s legal shoreline and the state’s physical shoreline (Figure 1) shows that in many areas, some wetland loss has occurred on eroding shorelines, but most wetland loss occurs in the marsh interior, away from the eyes of hunters and fishers.
4. Some wetland loss is natural; some is artificial.
Wetland scientists, managers and landowners could probably exchange ideas for a week and yet not reach consensus on the proportion of wetland loss that is natural and the proportion that is artificial. Wetland loss is extremely complicated in the Mississippi River Deltaic Plain because of the Delta Lobe Cycle, which is a natural cycle of delta building and delta disappearance (Figure 2). Subsidence keeps this cycle going. The river has been building a new delta lobe about every 1,000 years since the end of the last Ice Age, about 6,000 years ago. This is a good time to start because then Louisiana’s coastal wetlands were all shallow parts of the Gulf of Mexico rather than wetlands.
The current delta lobe, upon which New Orleans sits, started about 1,000 A.D. The previous one, Bayou Lafourche, started about 0 A.D. The delta lobe before that built most of St. Bernard Parish about 1,000 B.C. Around 2,000 B.C., the river ran through the area now occupied by Bayou Teche. Before then, the river built several delta lobes that subsequently subsided.
The Delta Lobe Cycle can be classified into four stages. Stage 1 is the initiation of a new delta lobe, and in Stage 4 the delta lobe has completely subsided and is under water. In Stage 1, the delta lobe is initially tiny but it grows to cover hundreds of square miles. All four stages of the Delta Lobe Cycle can be seen today on Louisiana’s coast. Stage 4 can been seen in a 50-mile stretch from Tiger Shoal eastward. Stage 3 can be seen in the 50-mile sweep of the Chandeleur Islands. Stage 2 can be seen in the 80-mile stretch of coast from Grand Isle to Caillou Bay. Stage 1 is hardest to find because it is so small; it can be seen in the 5 miles of coast at the Wax Lake Outlet and at the mouth of the Atchafalaya River. Wetland loss is less complicated in the Chenier Plain, where the only natural cycle of erosion and gain occurs on the coast of the Gulf of Mexico. All wetland loss in the Chenier Plain is most likely artificial except that which occurs on the shore of the Gulf of Mexico, and some of that is probably accelerated by human activity.
5. Lack of significant wetland creation is completely artificial.
Wetlands have been subsiding and converting to shallow open water in southeast Louisiana for thousands of years, yet more than four million acres of wetlands accumulated in coastal Louisiana during that same time. Louisiana was able to accumulate wetlands despite natural wetland loss because wetland creation was always faster than wetland loss. Currently, wetland creation in coastal Louisiana is virtually insignificant. Before the flood of 1972, the river was creating wetlands (at the expense of lakes) in the Atchafalaya swamp. Since 1972, the river has created some new wetlands at the mouths of the Atchafalaya River and the associated Wax Lake Outlet, but too few wetlands are being created to offset wetland loss elsewhere. If humans did not manage the division of water between the Mississippi River and Atchafalaya River at Simmesport, Louisiana, then natural wetland creation at the mouths of the Atchafalaya River and Wax Lake Outlet would be extensive. Those missing wetlands, and the commercial activities associated with them, are real but invisible costs of keeping the river from making the switch that it has made about every 1,000 years since the end of the last Ice Age. Figuring out how to manage the river for wetland creation as well as navigation and flood control is therefore a worthwhile goal.