Historical Trends

 

Trends through recent history tracking the extent and severity of hypoxic masses in both the Chesapeake Bay and The Gulf of Mexico share many similarities (Figures 1 and 2). Overall, research has found that in both bodies of water, significant hypoxia was not present prior to the early 1900’s and the severity and spatial extent of hypoxia has increased rapidly since the 1950’s (Rabalais et al. 2002). The volume of water affected by hypoxia has expanded dramatically, especially since 1980 (Hagy et al. 2004). In addition, while in the past, summertime anoxia has occurred only when spring flooding was significantly excessive, summer anoxia currently occurs regardless of the volume of springtime river flow.

The growth and intensification of hypoxic waters has been attributed to a significant increase in nitrogen influx from river discharge. A number of indicator tests support this conclusion. In the Chesapeake, Zimmerman and Canuel (2002) measured total organic carbon content in sediments dating back to the late 18th century to estimate the progression of eutrophication. The researchers concluded that primary productivity in the estuary is correlated with historical increases in hypoxia. In addition, past trends of dissolved oxygen levels have been traced using geochemical indicators, with results suggesting that the Chesapeake has experienced increasing oxygen depletion since the beginning of the 20th century, but especially after 1960 (Adelson et al. 2001). Bacteriopigments in Gulf sediments reveal similar trends in Louisiana (Chen et al. 2001). The only other explanation for a growing trend of hypoxia is century-long increases in river flow, which would lead to increased nutrient input and water column stratification. However, studies have found that this trend is not evident, leading to the conclusion that the quality of river discharge, rather than quantity, causes the occurrence of hypoxic zones (Rabalais et al. 1999).

Figure 1. Long-term hypoxia trends in the Chesapeake Bay. Source: CBF

Historical data describing activity within Mississippi River watershed has been compared to long-term changes in hypoxia to determine which factors have changed over time that are most strongly correlated with the growing hypoxia indicated by sediment cores (Rabalais et al. 2002). First, a dramatic increase in fertilizer application occurred between the 1950’s and 1980’s. In addition, population in the Mississippi basin has steadily increased since the 1950’s, resulting in increased nitrogen inputs from municipal wastewater discharge. Landscape alterations, such as deforestation, conversion of wetlands to agricultural fields, and loss of riparian zones, have also taken place, resulting in the loss of buffer areas that would normally remove some nitrogen in runoff before entering the river.

Figure 2. Long-term hypoxia trends in the Gulf of Mexico. Source: EPA

In the Chesapeake watershed, land clearance activity has been documented since the 17th century, yet incidence of hypoxia is not evidenced until the early 19th century when agricultural application of organic fertilizer was first introduced (Hagy et al. 2004). In correlation with the use of organic fertilizer, periodic eutrophication and hypoxia events in the deepest parts of the bay occurred. In the 1950’s, application of synthetic fertilizers became widespread, and the estuary started experiencing more extensive hypoxia events. Between the 1950’s and 1980’s, human population within the Bay watershed approximately doubled, leading to increased atmospheric deposition and municipal discharge of nutrients (Boesch et al. 2001). In addition, the use of inorganic fertilizer tripled during this time, resulting in more extensive and severe hypoxia in the Chesapeake Bay. Thus, as anthropogenic sources of nutrients have increased over time, so has the extent and severity of hypoxia.