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Using macroalgal δ15N bioassay to detect cruise ship waste water effluent inputs in Skagway, AK
Citation:
KALDY, III, J. E. Using macroalgal δ15N bioassay to detect cruise ship waste water effluent inputs in Skagway, AK. Presented at Coastal Estuarine Research Federation 20th Biennial Conference, Portland, OR, November 01 - 05, 2009.
Impact/Purpose:
Nitrogen stable isotopes are a powerful tool for tracking sources of N to marine ecosystems.
Description:
Nitrogen stable isotopes are a powerful tool for tracking sources of N to marine ecosystems. I used green macroalgae as a bioassay organism to evaluate if the δ15N signature of cruise ship waste water effluent (CSWWE) could be detected in Skagway Harbor, AK. Opportunistic green macroalgae (Ulva spp.) samples were collected, cultured under nutrient depleted conditions and characterized with regard to N content and δ15N. Samples of this stock algal material were used in controlled incubations to evaluate the direction of isotope shift expected by exposure to CSWWE. By comparing measured δ15N against δ15N predicted from a 2 end-member mixing model no evidence of fractionation was found for NO3. At 1500 µM NH4, measured algal tissue isotope values were about 3 ‰ lower than was predicted by the mixing model, indicating that there may be some fractionation during NH4 uptake. Algae exposed to CSWWE exhibited an increase of 1 to 2.5‰ in δ15N values indicating that the CSWWE had an enriched isotope signature. Departures from the 1:1 mixing line may be related to fractionation or differential uptake of NO3 and NH3 with different isotope signatures since both compounds exist in the CSWWE. This issue will be resolved with isotopic analysis of the NO3 and NH4 and use of a 3 end-member mixing model. Fractionation will be further evaluated based on a Rayleigh Fractionation experiment. In contrast, algae exposed to field conditions uniformly exhibited a decrease of about 0.5 to 1 ‰ over 6d in the observed δ15N indicating that algae were using a light N source. Nitrogen derived from N2-fixing red alder (Alnus rubra) in the watershed of the many streams and rivers providing freshwater to Skagway Harbor may be a N source utilized by the algae. The green algal bioassay experiments indicate that the δ15N CSWWE signature was not detectable in Skagway Harbor under the CSWWE loading conditions of this experiment.