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New Research Tools Reveal Groundwater Role in Delivering Nutrients to Coastal Embayments
Excessive input of nutrients, or eutrophication, is a widespread problem throughout coastal ecosystems worldwide.
by Tracey Crago
Using stable isotope techniques, scientists can now detect the impacts of increased wastewater loading at the molecular level-before changes at the population and community levels.
In moderation, nutrient inputs can be beneficial, stimulating phytoplankton production and resulting in increased fish and shellfish production. Excessive nutrients, however, can result in an overabundance of phytoplankton, sometimes resulting in blooms of nuisance species, low oxygen conditions from decaying algae, or loss of eelgrass or other submerged aquatic vegetation that serve as important habitat for fish and shellfish populations.
Determining the right level of nutrients for a particular ecosystem is a challenge that ecologists and environmental managers must address. Ongoing research supported by Woods Hole Sea Grant is improving our understanding of the delivery and fate of nutrients to a coastal ecosystem that may, in turn, lead to better management strategies.
Coastal watersheds deliver nitrogen to estuaries from three main sources: wastewater, atmospheric deposition, and fertilizers. Wastewater-derived nutrients pose a serious threat to coastal ecosystems, especially as development within the coastal zone continues to expand.
One technique currently used to identify the fate of waste-derived nutrients in coastal embayments is the application of stable isotope analysis of nutrients. This technique relies on the assumption that different nutrient sources have distinct ratios of the naturally occurring isotopes of a particular element-a signature. This signature can be used to calculate the relative contribution from each source.
Boston University professor Ivan Valiela and his former graduate student, James McClelland, used stable isotope ratios of nitrogen to track wastewater from coastal watersheds into estuarine food webs. They found that the signature of groundwater-borne wastewater is elevated relative to the other sources of nitrogen. Thus, the wastewater acts as a signature-enriched tracer introduced to estuaries. Even with low levels of nitrogen loading, researchers were able to detect elevated levels of the nitrogen ratios in estuarine plants and animals, thus demonstrating a direct link between wastewater discharges and incorporation into estuarine food webs.
Another graduate student, Marci Cole, extended the McClelland work by conducting a more extensive field sampling program to gather data on nitrogen concentrations for groundwater and wastewater nitrogen signatures in a number of locations ranging from estuaries to freshwater ponds to a salt pond.
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Marci Cole's doctoral thesis work involved collecting samples of estuarine producers (particulate organic matter or POM, macroalgae, and macrophytes) at a number of freshwater and marine sites on Cape Cod and Nantucket. She found that, of the three producers, macrophytes are the best indicator of wastewater nitrogen in an estuary. Graphic Credit: Marci Cole, Boston University |
Cole compared her values to the modeled wastewater nitrogen load for each estuary. Her results showed a relationship between the isotopic signal of groundwater and producers, and the wastewater nitrogen load value derived from the model. This relationship can be used to predict what percentage of nitrogen-of all the nitrogen coming into an estuary-is coming from wastewater via groundwater.
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WHOI geochemist Matt Charette conducting a groundwater test in Waquoit Bay, Mass., a Cape Cod estuary, to study how anthropogenic sources of nitrogen adversely impact the water quality. Photo Credit: Tom Kleindinst, WHOI |
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WHOI geochemist Matt Charette and his research assistant, Craig Herbold, measure nitrate concentration in a Cape Cod estuary using a nitrate analyzer. They can track groundwater-derived nutrients and identify their sources by analyzing chemical tracer data from this instrument. Photo Credit: Tom Kleindinst, WHOI |
An ongoing project, conducted by geochemist Matt Charette at the Woods Hole Oceanographic Institution, uses radium isotopes to look at sub-surface groundwater pathways to embayments in southern New England. These pathways are thought to play a role in the delivery of nutrients, such as nitrate and phosphate, to coastal waters.
Charette's preliminary results indicate that his techniques and methodology may help identify nitrogen trouble spots on a small or localized scale. Currently he is working with another Valiela graduate student, Kevin Kroeger, to determine the source of groundwater-borne nitrogen using stable nitrogen isotopes.
The results of these Sea Grant supported investigations are promising indications that the early effects of nutrient enrichment can be traced through watersheds and into food webs. Furthermore, the results of these studies will be important considerations in the development of long-term management actions for coastal ecosystems threatened by excessive nutrients.
Related Links:
Introduction to the Nitrogen Problem (Marine Biological Laboratory)
http://ecosystems.mbl.edu/Research/Clue/nproblem.html
"Balancing the Water Budget" (from WHOI/MIT Sea Grant newsletter, Two if by Sea)
http://web.mit.edu/seagrant/2ifbysea/archives/fall01/waterbudget.html
"Fate of Anthropogenic Nitrogen in a Nearshore Cape Cod Aquifer" (pdf file, from Biological Bulletin, pp. 58-59)
http://www.biolbull.org/cgi/reprint/199/2/221.pdf
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The Woods Hole Oceanographic Institution (WHOI) Sea Grant Program supports research, education, and extension projects that encourage environmental stewardship, long-term economic development, and responsible use of the nation's coastal and ocean resources. It is part of the National Sea Grant College Program of the National Oceanic and Atmospheric Administration. For more information about WHOI Sea Grant, visit our website at www.whoi.edu/seagrant |
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