Water Depth enhances quality, provides fish refugia in the Atchafalaya River Basin

Linda Benedict, Kelso, William E.  |  11/23/2011 2:58:39 AM

Michael D. Kaller and William E. Kelso

As the largest bottomland hardwood swamp ecosystem in the United States, the Atchafalaya River Basin offers a tremendous diversity of terrestrial and aquatic habitats that support many economically and recreationally important activities including oil and gas extraction, shipping, farming, timber harvesting, hunting, and commercial and recreational fishing. Although diverse and productive, the Atchafalaya River Basin has been increasingly altered over the last century by a number of natural and human-related factors. For example, the hydrology has been affected by the closure of distributaries (river branches) from the mainstem river and erection of levees. Also, the ecology has been invaded by several exotic aquatic plants, such as hyacinth, hydrilla and salvinia. Like much of the Gulf Coast, the Atchafalaya River Basin has been impacted by the passage of several hurricanes, notably Andrew (1992), Ivan (2004), Katrina and Rita (2005), and Gustav (2008). Although the physical damage resulting from passage of a nearby storm may be minimal, the short-term and long-term effects on aquatic habitats and organisms can be significant.

The storm track of a hurricane plays a vital role in the severity of its effects on marine and freshwater communities. The strong winds associated with hurricanes that pass near the Atchafalaya River Basin can significantly affect aquatic communities and result in extensive fish kills. Hurricanes Andrew and Gustav passed directly over south central Louisiana as Category 3 and Category 2 storms, respectively, resulting in estimated fish kills of more than 100 million fish each. For most of these storms, the most likely causes of Atchafalaya River Basin fish kills have been the mixing of oxygen-deficient bottom water throughout the water column, as well as the release of highly toxic hydrogen sulfide from bottom sediments. As the water column turns over, fish have no time to adapt to decreasing dissolved oxygen levels, resulting in widespread suffocation. Water is classified as hypoxic when dissolved oxygen levels drop below 2 milligrams per liter, at which point most fishes tested so far experience an oxygen debt and become stressed. Measurements taken after the passage of recent hurricanes indicate that dissolved oxygen levels of less than 0.4 milligrams per liter were common throughout the affected area.

The occurrence of toxicity-related mortality from hydrogen sulfide can be inferred from the presence of spotted, longnose and alligator gar among the moribund fishes because these species can gulp air and use their swim bladders for respiration when dissolved oxygen levels decline. In addition to short-term mortality caused by rapid dissolved oxygen decline and release of hydrogen sulfide, high levels of precipitation in the Atchafalaya River Basin watershed can increase water levels to the point where large floodplain areas become inundated. In late summer, the stagnant water on the floodplain quickly warms, which promotes decomposition of organic material and strips oxygen from the water. As this water moves back into the canals and bayous from the floodplain as water levels recede, dissolved oxygen levels can remain depressed for days to weeks, extending the period of poor water quality.

Within the Atchafalaya River Basin, the southeastern portion – bounded to the north by I-10, to the south by Morgan City, to the west by Atchafalaya River levees and to the east by the East Protection Levee – is managed as the East Grand Lake Water Management Unit (WMU). The WMU contains some of the most popular and productive Atchafalaya River Basin recreational and commercial fishing destinations in Bayou Sorrel and Bayou Postillion. The East Grand Lake WMU bore the brunt of recent hurricanes because its southerly location did not allow for wind speeds or storm surges to abate over land. To study the immediate and long-term effects to fish and water resources in East Grand Lake, LSU AgCenter scientists deployed recording water quality probes prior to Gustav’s landfall. The data collected suggested that, indeed, during Hurricane Gustav, the dissolved oxygen levels declined severely during and immediately after the storm. Monitoring efforts following the passage of Hurricane Gustav suggested poor water quality was widespread immediately following the storm. The flushing of this poor water from the Atchafalaya River Basin began in the north and moved southeast, resulting in East Grand Lake sustaining some of the longest periods of poor water in the Atchafalaya River Basin, from August 31 through October 6, 2008.

Fish sampling in the East Grand Lake WMU in 2009 revealed low numbers and diversity of fish, presumably because of low dissolved oxygen caused by Hurricane Gustav. Importantly, fish numbers and diversity have been low in this region since hurricanes Katrina and Rita compared to data from 2000-2005, suggesting the 2005 hurricanes had a similar effect. Similar water quality data are not available for hurricanes Andrew, Ivan, Katrina or Rita. However, comparisons of sport fish abundance in the years before (2000-2005) and after hurricanes Katrina and Rita (2006-2007) suggest substantial reductions in recreational fishes in the WMU after passage of these hurricanes, possibly through suppression of dissolved oxygen (Figure 1). Therefore, because fish kills can occur when dissolved oxygen is suppressed, it is reasonable to assume that the reduced fish numbers following Hurricanes Katrina and Rita were the result of low dissolved oxygen.

LSU AgCenter research also indicates that many fishes survive hurricane impacts and thrive in lower Grand Lake. Fish sampling in 2011 suggest fish numbers may be recovering and fish that survived the hurricanes appear to be in good condition (see photos). The concept that fish that survived a bottleneck in population numbers would have excellent growth and condition is termed a compensatory response. Fish exhibit indeterminate growth, unlike mammals, and may continue to grow throughout their lives given adequate food, space and release from predation. When competition with other fish and predation pressures are reduced, the compensatory response is often increased growth and better overall condition. Evidently, the large, well-conditioned fish in recent sampling, many of which are old enough to have been alive during Hurricane Gustav, suggest the successive hurricanes did not affect all fish equally in the East Grand Lake WMU. Research in the neighboring Lake Verrett system by Nicholls State University indicates that fish only need access to small areas of good water during poor water events to survive and maintain populations.

Lower Grand Lake is one of the deeper areas of the Atchafalaya River Basin, and AgCenter research suggests that deeper water in the Atchafalaya River Basin is more resistant to turnover, depression of dissolved oxygen and release of hydrogen sulfide. For example, AgCenter scientists demonstrated that an area in lower Grand Lake with 1 meter greater depth would support 1.1 milligrams per liter more dissolved oxygen than a comparable shallower area in other parts of the Atchafalaya River basin. Although the entire Atchafalaya River Basin and within it the East Grand Lake WMU are accumulating sediment and converting from water to land, lower Grand Lake is deeper and holds more oxygen averaging 0.4 meter deeper and supporting a mean of 5.4 milligrams per liter dissolved oxygen compared to a mean of 4.7 milligrams per liter in other parts of the Atchafalaya River Basin. Therefore, it is possible that fish found in deeper waters with higher dissolved oxygen survive the hurricanes and subsequent periods of poor water. Although deep water and higher dissolved oxygen refugia may not be the only reason for fish survival and subsequent success, the concept seems reasonable based on water quality measurements taken throughout the Atchafalaya River Basin over the past decade.

In conclusion, the lower Grand Lake represents one of the Atchafalaya River Basin’s most productive and most resilient regions. Although located in the path of several hurricanes over the past 20 years, the lower Grand Lake continues to support strong recreational and commercial fishing opportunities, as well as supporting the bottomland hardwoods and swamp emblematic of the Atchafalaya River Basin. AgCenter research suggests that water depth is an important component to the productivity of the lower Grand Lake and, further, that water management activities in the East Grand Lake WMU focus in part on maintenance of deeper areas as storm refugia for Atchafalaya River Basin fishes.

Michael D. Kaller, Assistant Professor, and William E. Kelso, F.O. Bateman Distinguished Professor and Associate Director, School of Renewable Natural Resources, LSU AgCenter, Baton Rouge, La.

(This article was published in the fall 2011 issue of Louisiana Agriculture magazine.)

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