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Using d15 N in Fish Larvae as an Indicator of Watershed Sources of Anthropogenic Nitrogen: Response at Multiple Spatial Scales
Citation:
HOFFMAN, J., J. R. KELLY, G. S. PETERSON, A. M. COTTER, M. A. STARRY, AND M. E. SIERSZEN. Using d15 N in Fish Larvae as an Indicator of Watershed Sources of Anthropogenic Nitrogen: Response at Multiple Spatial Scales. Estuaries and Coasts. Estuarine Research Federation, Port Republic, MD, 35(6):1453-1467, (2012).
Impact/Purpose:
The nitrogen stable isotope, 15N, is an effective tool to track anthropogenic N sources to aquatic ecosystems. It may be difficult to identify potential N sources, however, where 15N responds similarly to multiple, concurrent activities in the watershed that cause higher nutrient loadings to receiving waters. We measured the d15N signatures of fish larvae in the river mouth and adjacent coastal waters of three Lake Superior watersheds that expressed a large gradient in population densities but which have similarly low levels of agricultural activity to determine if we could isolate the effect from wastewater N alone. Across stations, mean fish d15N spanned a broad range: 2.7‰ to 10.8‰. Fish larvae had watershed-specific 15N signatures that were positively correlated to various metrics of population density and to [NH4+], indicating a response to wastewater inputs. In the two more impacted watersheds, differences were found within wetlands across the river-lake mixing zone. The data show that fish larvae respond on appropriate and useful spatial and temporal scales to function as indicators of anthropogenic N inputs. Comparison of our data to fish sampled from across the Great Lakes demonstrate that there are nested responses in d15N: across the basin, fish d15N is strongly correlated to agricultural activity, but within watersheds of similarly low agricultural activity, d15N is strongly correlated to human population density. This study demonstrates that incorporating land use into survey design is a potentially powerful tool; by stratifying samples based on watershed activity it is possible to isolate nested responses over a large geographic scale and thereby constrain potential N sources.
Description:
The nitrogen stable isotope, 15N, is an effective tool to track anthropogenic N sources to aquatic ecosystems. It may be difficult to identify potential N sources, however, where 15N responds similarly to multiple, concurrent activities in the watershed that cause higher nutrient loadings to receiving waters. We measured the d15N signatures of fish larvae in the river mouth and adjacent coastal waters of three Lake Superior watersheds that expressed a large gradient in population densities but which have similarly low levels of agricultural activity to determine if we could isolate the effect from wastewater N alone. Across stations, mean fish d15N spanned a broad range: 2.7‰ to 10.8‰. Fish larvae had watershed-specific 15N signatures that were positively correlated to various metrics of population density and to [NH4+], indicating a response to wastewater inputs. In the two more impacted watersheds, differences were found within wetlands across the river-lake mixing zone. The data show that fish larvae respond on appropriate and useful spatial and temporal scales to function as indicators of anthropogenic N inputs. Comparison of our data to fish sampled from across the Great Lakes demonstrate that there are nested responses in d15N: across the basin, fish d15N is strongly correlated to agricultural activity, but within watersheds of similarly low agricultural activity, d15N is strongly correlated to human population density. This study demonstrates that incorporating land use into survey design is a potentially powerful tool; by stratifying samples based on watershed activity it is possible to isolate nested responses over a large geographic scale and thereby constrain potential N sources.
URLs/Downloads:
Using d15 N in Fish Larvae as an Indicator of Watershed Sources of Anthropogenic Nitrogen: Response at Multiple Spatial Scales