Deep-Sea Corals Record Human Impact on Watershed Quality in the Mississippi River Basin
Over the past 200 years, the Mississippi River watershed has seen the onset of industrialized agriculture and undergone intense land-use changes—a few examples include widespread plowing, drainage, irrigation, and fertilizer use. Related changes have occurred downstream, in shorelines, ecosystems, and coastal waters in the Gulf of Mexico. For example, the Mississippi carries excess nutrients, such as nitrogen and phosphorus, into the Gulf of Mexico, fueling algal blooms that deplete dissolved oxygen in coastal waters, producing a low-oxygen, or “hypoxic,” zone every summer off the coast of Louisiana and Texas. The hypoxic zone—commonly called the “dead zone” because it stresses or kills organisms that cannot move out of it—is a heavily studied phenomenon (for example, see the USGS Science Features Top Story “Dead Zone: The Source of the Gulf of Mexico’s Hypoxia”) whose upstream influences are closely monitored (for example, see “Nitrate Levels Continue to Increase in Mississippi River; Signs of Progress in the Illinois River,” this issue). In contrast, little information has been available until now about whether land-use changes upstream affect the whole-scale biology and chemistry of the Gulf of Mexico, including the deep sea and its ecosystems.
New research by the U.S. Geological Survey (USGS) shows that upstream land-use changes do affect deep waters of the Gulf of Mexico, as evidenced by the chemistry of long-lived deep-sea corals whose skeletons incorporate material from the surrounding water as the corals grow. Analyses of these coral skeletons (analogous to tree-ring studies) reveal a history of land-use change and provide baseline information that can assist efforts to improve water quality in the Mississippi watershed and the Gulf of Mexico.
The new research was reported in the journal Global Biogeochemical Cycles. To conduct the study, USGS oceanographer Nancy Prouty teamed up with national and international colleagues to analyze a suite of chemical components in deep-sea black corals. These corals build their skeletons of protein and chitin (the material in crustacean and insect shells) rather than the more familiar calcium carbonate (CaCO3) of most shallow-water corals. Prouty and her coauthors performed numerous chemical analyses on layers of coral skeleton dated in an earlier study (see “Long-Lived, Slow-Growing Corals in Deep Waters of the Gulf of Mexico,” Sound Waves, March 2011) which showed that some of the black corals live for at least two millennia. The earlier study also confirmed that black corals make their skeletons mainly from particulate organic matter that sinks from the ocean surface. Thus, black corals can capture and record in their skeletons the history of changing constituents in surface waters, including elements delivered to the Gulf of Mexico from the Mississippi River Basin.
The authors introduced a new tracer of agro-industrialization: the trace metal rhenium (Re), which is believed to be released by coal burning and smelter emissions as well as by erosion of organic-rich sediment. They combined Re analyses with nitrogen isotope analyses to produce an extremely sensitive new toolset for understanding how agro-industrialization and land-use changes directly affect ocean biogeochemical cycles.
“Our coupled analysis of trace metals and bulk and compound-specific nitrogen isotopes in corals from the Gulf of Mexico capture the rapid sensitivity of deep-sea corals to upstream changes in watershed quality,” explains Prouty. “These records provide not only a temporal perspective over the past one or two millennia, but also a quantitative context to evaluate the effects of future and ongoing land-use and climate change on nutrient loading and downstream biogeochemical cycles.”
In progressively younger layers of the deep-sea corals’ proteinaceous skeletons, the researchers discovered increasing levels of both Re and bulk nitrogen isotope (δ15N) values. By analyzing compound-specific nitrogen isotopes in amino acids from the coral skeletons, the authors were able to conclude that the δ15N increase was due to a change in source nutrients, such as nitrate, rather than other causes, such as a change in the structure of the food web. The enrichment in both nutrients and Re was particularly strong over the last 150 to 200 years.
“Rhenium concentrations in major rivers worldwide suggest that the enrichment is largely due to human activity, but no pre-anthropogenic measurements of Re exist to validate this observation,” said Prouty. “With our coral analyses, we have the first-ever record of pre-industrial Re variability—allowing us to quantify the anthropogenic contribution of Re in the Mississippi River and to the Gulf of Mexico: about 30 percent.”
Rhenium may seem like an obscure element, but naturally derived Re comes from the same source as naturally derived organic carbon—the erosion of organic-rich sediment. Because of this link, the authors were able to use the coral Re data to make estimates of organic carbon released by erosion in the watershed and compare those to estimates made by other researchers using other techniques (for example, isotopic composition of dissolved inorganic carbon).
“I love this work because it feels so much like a detective story,” said Prouty. “We spend a lot of time checking the evidence, following multiple leads, ruling out suspects, comparing findings from different lines of inquiry. And it’s gratifying to achieve results that not only advance our knowledge but can also be used to restore watersheds and coastal ecosystems.”
One group likely to benefit from the new work is the multiagency Mississippi River/Gulf of Mexico Watershed Nutrient (Hypoxia) Task Force, established in 1997 to reduce and control hypoxia in the Gulf. Conservation goals of the Hypoxia Task Force include reduction in fertilizer use, nitrate transport, and contaminants through modifying agricultural practices. Without baseline information, however, these targets remain elusive. Results from the novel suite of tracers developed by Prouty and her coauthors provide critical—and until now unavailable —baseline information about trace metals and nutrients. Ultimately, this information can assist efforts to reduce future fertilizer use and nitrate transport and enhance watershed restoration.
Nancy Prouty is a research oceanographer at the USGS Pacific Coastal and Marine Science Center in Santa Cruz, California, where she focuses on projects to enhance understanding of climate variability on human-relevant timescales and the impact of human activities on climate. She is involved in both shallow coral studies with the Pacific Coral Reefs Project and deep-sea coral studies with the Diversity, Systematics and Connectivity of Vulnerable Reef Ecosystems (DISCOVRE) team. She is part of a multiagency team that received a U.S. Department of the Interior 2013 Partners in Conservation Award for its investigation of the ecology of deepwater canyons off the U.S. mid-Atlantic coast (see “USGS Among Recipients of Prestigious DOI Partners in Conservation Award,” this issue, and “Deepwater Canyons 2013: Pathways to the Abyss).
The full citation for the new article is:
in this issue:
Deep-Sea Corals Record Human Impact on the Mississippi River Basin