Effects of atrazine runoff on Chesapeake Bay aquatic life: Risk assessment of atrazine on the blue crab

The EPA estimates that corn production accounts for 86% of the domestic usage of Atrazine. Approximately 75% of the field corn acreage in the U.S. is treated with Atrazine. Methods of application include groundboom sprayer, aircraft, and tractor-drawn spreader.

Atrazine binds well in a soil profile after initial application. Clay and loamy soils have more affinity to atrazine than sandy soil because of soil aggregate structures and particle size density. Soil binding of atrazine initiates degradation of atrazine in the soil environment. The degradation of atrazine into derivative metabolites depends on factors including the soil type, percent organic matter, clay content, soil pH, and soil structure (Stagnitti et al., 1998; Kookana et al., 1998). There are five processes that determine the rate of atrazine degradation: hydrolysis, adsorption, photodegradation, volatilization, and microbial degradation (Stagnitti et al., 2001). Each of the degraded metabolites has varying degrees of persistence and toxicity (Stagnitti et al., 2001).

Fate of atrazine in aquifers and soils has been well documented in the past and many researchers have indicated that significant amount of atrazine may be stored in soil after application (Kookana et al., 1998). Atrazine is moderately hydrophilic, and a significant proportion is found in groundwater and surface runoffs due to its solubility and leaching potential (Stagnitti et al., 1998). Some research has shown the persistence of degraded atrazine metabolites in aquifers for more than 20 years (Hayes et al., 2003). Recent studies on the fate of atrazine in surface waters indicate the widespread occurrence of atrazine year round (Dana et al., 1993). One study found the concentration of atrazine as high as 49,000μg/kg in eroded soil leading to concentrations in surface waters as high as 1000 μg/L (Douglas et al., 1993).

Atrazine usage around the Chesapeake Bay area has increased exponentially and the concentrations of Atrazine in surface waters of the Chesapeake watershed can reach up to 98 μg/L in one growing season (Hall et al., 1999). In some agriculturally dominated regions concentrations can exceed 100 μg/L and persist for at least 30 days (U. S. EPA 2007).

Research was conducted through the GoogleScholar literature service, using keywords “atrazine”, “Chesapeake Bay”, “blue crab”, “aquatic impacts”, “EPA”, “NRDC”, and using all two and three-word keyword combinations. ScienceDirect was also used, with the key words “atrazine,” “Chesapeake Bay,” “agriculture,” “blue crab,” “herbicide,” and “hypoxia.” ISI Web of Science was also used with keywords “blue crabs,” “mesozooplankton,” and “herbicide.” The U.S EPA website was searched using similar terminology, and considerable time was spent in tracking back the body of recent EPA decisions on atrazine regulation to the primary literature cited in those reports.

Blue crabs are important indicators of the Chesapeake Bay’s ecological health, and being both predator and prey, they serve important functions in the trophic cascades. Blue crabs are known for their complex migratory life cycle [Figure 4] and they exist over a wide range of habitats within the Chesapeake Bay (King et al., 2005).

Habitat shifts are dependent on a variety of factors including salinity, food source, habitat, and location of mates (Hines et al., 2008; Dittel et al., 2008). Due to these migrations, blue crabs serve as important source of energy transfer in aquatic ecosystems: they take in energy and nutrients as a top predator within the Chesapeake’s estuary, and act as important food source for larger benthic mammals in the nutrient depleted pelagic ocean (Aguilar et al., 2008, King et al., 2005). Blue crabs also serve as good indicators of ecosystem health because of their traceable changes in spatial distribution due to anthropogenic effects on food and habitat (King et al., 2005; Seitz et al., 2005). Blue crab populations do not flourish in watersheds associated with agricultural land use. Watershed land use has been linked to reductions in fish community biodiversity (Moerke & Lamberti, 2006, King et al., 2005). Shore line alterations that eliminate the habitat of the blue crab, including marshes and downed woody debris, change the spatial distribution of the species (Seitz et al., 2005).

Due to blue crabs importance as ecological indicators in the Chesapeake Bay habitat, it is important to understand how atrazine affects the vitality of this species. The effects of atrazine on blue crabs are indirect, and thus it is important to have a thorough understanding of blue crab’s life cycle at all stages, including habitat and food sources.
Blue crab: Life Cycle

Blue crabs (Callinectes sapidus) range from Nova Scotia to Northern Argentina. Within the United States blue crabs are most common south of Cape Cod and mate along the estuaries of the mid-Atlantic seaboard, most abundantly in the Delaware and Chesapeake Bay (Aguilar et al., 2008, Davis & Davis, 2008). As with all species of crabs (Brachyura), blue crabs must molt as their internal tissue and body size increases, several times throughout their life (Alaska Fish Science Center, 2008). Callinectes sapidus females molt 18 to 20 times and males molt 21 to 23 times throughout their life time, not counting larval molts (Zinski, 2006).

Figure 5: Patterns of migration of Blue Crabs (Hines et al., 2008)

Currents of the MAB exhibit a circulatory pattern in the summer months, transporting larvae southward close to shore, and northward, due to winds, further out to sea (Epifanio et al., 1989; Jones & Epifanio, 1995). Eventual transport into the natal estuary is controlled by downwelling and other wind-driven events that occur during autumn (Jones & Epifanio, 1995). Sanctuaries of null flow also exist with the MAB that prevent larvae from being transported long distances from the estuary (Tilburg et al., 2007). By the time Callinectes sapidus reenters the estuary it has reached its megalopae phase (Dittel et al., 2006).