The Problem
The freshwater faucet has been turned off to many coastal bays, estuaries, and wetlands to supply the voracious needs of humans. Freshwater extraction from rivers is increasing as human population grows along with the expansion of urban, agricultural and industrial development.1 The demand for water has become so high that freshwater from rivers in some cases no longer reaches the sea (e.g., Colorado, Indus, Nile, Murray).2 The unfortunate reality for animals and plants living on the coast is that salinity has risen so high that in some cases these habitats may collapse. Losing habitats leads to a loss of ecosystem services. For example, if we lose wetlands, then we lose the barriers that protect us from storms.1,3
This insatiable usage of freshwater along rivers by humans also can reduce water quality in estuaries. Decreasing freshwater inflow in the Caloosahatchee Estuary, Florida is related to declining water quality, and the status of water can be used as a bioindicator of estuary health there.4 With unaltered flow, freshwater dilutes saltwater as it moves toward the ocean,5,6 but few coastal rivers are unaltered. The coastal water supply can be greatly altered e.g., the city of Corpus Christi, Texas has reduced water supply to nearby coastal wetlands by 45% in the last 45 years.7
Saltwater in coastal wetlands is also increasing because along with freshwater extraction, many of the world’s coasts are also suffering from land sinking (subsidence) and sea level rise. As a consequence, salt water is creeping into the surface and groundwater from the sea, or sometimes from underground.8
Climate change may affect the availability of freshwater in rivers in the future because of increasing drought or flooding. Because freshwater flow modulates salinity and dissolved organic matter levels, these relationships are likely to be affected by further changes in water availability with climate change.9 Among other problems, the reduction of freshwater into estuaries can cause a reduction in organic matter and nutrients in the water.10 Less freshwater input and/or a drier climate along the Gulf Coast would lead to less organic matter input from pelagic phytoplankton, ultimately with effects on estuarine productivity.11 Because these organisms form the base of the food chain for important fish and other organisms, reducing freshwater inflow can ultimately damage organisms important to human economies. Therefore, there are a number of projects working to divert more water to estuaries in Texas, Louisiana and elsewhere to support the food chain.12,13
Drought years are of particular concern because of changes in water inflow regulation.1 During droughts, the lack of freshwater flow to coastal forests can cause the death of freshwater tree species.1,8 Such changes in regulated water inflow also have effects on plankton blooms. For example, the San Antonio River has high anthropogenic nutrient inputs and high phytoplankton blooms during dry years, in contrast to other rivers without such high nutrient input (e.g., Guadalupe River).143 Overall, the loss of freshwater to coastal wetlands has caused much damage to both natural systems and human economies, particularly during droughts.
Altered flow is the general problem, it is not just that high salinity is bad and low salinity is good. Flood control and water diversion structures can funnel more freshwater into estuary areas than under natural conditions, and when the salinity drops to near zero, this situation can also kill or inhibit estuarine species. This problem occurs, for example, when the Bonnet Carré spillway, which diverts flood waters from the Mississippi River to Lake Pontchartrain, is opened to reduce flooding.15 Oyster landings were as much as 28 times lower in Mississippi after the spillway was opened. In other words, it is possible that a fire hose can be just as bad as turning off the faucet.
The Solution
We must determine out how to meet human needs for freshwater, while still maintain the ecological health and sustainability of coastal regions. Mostly, this problem has been not resolved because of a perceived use conflict between water developers and environmentalists. The environment nearly always loses in these conflicts because it is difficult, if not impossible to reduce human use, or make new water. Moreover, states often place legal constraints on environmental flows because they may not be viewed as a beneficial use of water.
Turning on the faucet of hydrologic remediation is the most effective way to revive ailing coastal environments. The process is easier said than done, because adaptively addressing the myriad of social needs and situations along rivers to free water for coastal bays, estuaries, and wetlands is complicated.1 These actions are urgently needed, however, because damage to coastal habitats, especially vegetation, is becoming extensive in many estuaries. However, programs to manage coastal vegetation with freshwater are emerging in the Australia, Europe, South Africa, and the United States.16
As salinity increases for coastal vegetation, strategically timed freshwater remediation may become an important restoration approach if the damage from climate-induced drought, hurricanes and sea level rise expand on the coast.8,17,18 Managed flood releases could be used to improve vegetation health by increasing flood and sediment flow and/or reducing salinity.8,19,20 For emerging hydrologic remediation programs to be effective, environmental indices are sorely needed to signal managers and ultimately decision makers of critical markers of declining estuarine health.21 Ultimately, these projects cannot be successful without the careful consideration of the water requirements of humans balanced with those of the natural environment.3
Freshwater delivery to coastal wetlands can be accomplished in a variety of ways. A common way on the northern Gulf Coast of the United States is through existing river diversions.13 In Louisiana, these diversions were created to restore the function of coastal marshes, with a focus on sediment delivery.13 Sea level rise does not necessarily need to drown coastal vegetation if plant growth is high.5 Diversions on large river systems, for example, the Mississippi River via Davis Pond Diversion to Barataria Estuary can modulate salinity, dissolved organic matter distribution in estuaries and improve vegetation growth to help maintain elevation.22,23 In particular, changes in salinity can affect the production of bacterioplankton and freshwater forest species.22,23 Therefore, maintaining lower salinity levels can be useful in management. Such coastal management can be accomplished through river inflow, which can be regulated to change water circulation, salinity gradients and sediment dynamics.24
Hydrological restoration of bays and estuaries is also important and becoming increasingly more common.25 The Nueces Estuary system, including a marsh, bayou, and bay, historically produced abundant shellfish that required brackish salinities.26 When a new dam was constructed, the reduction of freshwater inflows caused the system to shift to a hypersaline state with the loss of shellfish. Two hydrological restoration projects were completed: a restoration of flow from the river to the marsh, and a pipeline to pump flows directly into the marsh and bayou system that leads directly to Nueces Bay. The project increased environmental health and sustainable water supply to humans.
Beyond coastal conservation, the maintenance of optimal production levels in estuaries is important to the fish and oyster industries. Underlying these human economic pursuits is phytoplankton production; the supply of freshwater flow to the coast determines estuarine phytoplankton productivity as related to the effects of frequency, duration, timing, and magnitude of inflow. Also of note is that the relationship of seasonal freshwater flow, as driven by El Niño Southern Oscillations (ENSO), has a bearing on the management of water resources and fisheries.27
Hydrological remediation should be responsive to shifts in vegetation health.1 Models of freshwater inflow and biological responses explore the biotic limits of coastal ecosystems.28 Models were helpful in freshwater remediation efforts deployed to revive, for example, Eucalyptus trees along the Murray River in Australia during a long-term drought. There, the release of freshwater was only effective if the trees were still relatively healthy, which was why it was critical to detect early signs of tree stress.8 Another example of the response of freshwater trees to salinity reduction was observed in coastal freshwater forests in Louisiana. Salinity levels were reduced there when river diversions were operated at a 6 times their normal level. During this remediation period, there was a 3 fold increase in the production of coastal forest trees at Jean Lafitte National Historic Park and Preserve. In addition, the growth of tree trunks was much higher in the lowest salinities along the Gulf Coast from Texas to Florida.23
Freshwater flow has contributed to the health of other types of coastal wetlands. For example, freshwater flow and elevation contribute to an increased size of salt marsh extent in Batis and Borrichia dominated communities in the Nueces Delta in Texas.3 Upstream water extraction projects degrade downstream marsh vegetation.
One of the most famous projects to restore flow to coastal wetlands has been undertaken in the Everglades, where water flow driven by rivers has been essential in the original patterning and ecosystem dynamics of the ridge and slough wetlands. Historical hydrologic alteration has shifted water depth, flow of surface and ground water, and phosphorus supply. Certain vegetation types are disappearing in the Everglades because of hydrologic alteration by humans. For example, reduced freshwater flow and sea level rise have increased salinity levels in the southwestern part of the Everglades, so that sawgrass (Cladium jamiacense) is disappearing there.29
Additionally, water on flooded coasts may not be the beneficial fresh water coming from rivers, but instead damaging salty water from the sea. One technological need is for managers to have a better idea of where the water is coming from in various parts of an estuary. The source of water can be determined by identifying the type of stable isotopes of carbon, nitrogen and sulfur in the water (?13C, ?15N and ?34S).11
Another angle that may catalyze the conservation of coastal habitats is the idea that many people are willing to put their money where their mouth is. One survey found that residents along the Rio Grande would be willing to pay $129 per person or $9.9 million to have flowing water in the river channel.30 While we do not know exactly what benefits people have in mind when they say that they will pay for the river to flow; some people just have an intrinsic appreciation for simply seeing the water flow in the river. At least some people are willing to pay for this benefit. These diversions can also be effective in modulating salinity to keep plant production high with the added benefit of managing water elevation through increased plant growth.23
Water extraction and coastal re-engineering have challenged conservationists to supply adequate freshwater to conserve the world’s coastal estuaries. Coastal wetlands are vitally important to humans for their ecosystem services, and flow management techniques are helpful to keep coastal vegetation healthy.
Acknowledgements
Funding came from the U.S. Geological Survey Ecosystem Program. The work is part of Middleton’s project with the National Conservation Leadership Institute with thanks for the mentoring of Dale Caveny, Ann Forstchen, and Gina Main. Also thanks to Ken Rice and Scott Wilson for logistical support. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.
References
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