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Using a Macroalgal δ15N Bioassay to Detect Cruise Ship Waste Water Effluent Inputs
Citation:
KALDY, III, J. E. Using a Macroalgal δ15N Bioassay to Detect Cruise Ship Waste Water Effluent Inputs . MARINE POLLUTION BULLETIN. Elsevier Science Ltd, New York, NY, 62:1762-1771, (2011).
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.) was 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. Comparing measured δ15N against δ15N predicted from a 2 end-member mixing model when 50% of the spike was depleted, indicated no evidence of fractionation for NO3. At 1500 µM NH4, measured algal tissue isotope values were about 3 ‰ lower than was predicted by the mixing model (55% of NH4 taken up), indicating that there may be some fractionation during NH4 uptake. Algae exposed to CSWWE (δ15N=10.7) 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 suggest that some fractionation may be occurring during N uptake. In contrast, algae exposed to field conditions uniformly exhibited a statistically significant decrease of about 0.5 to 1 ‰ over 6d in the observed δ15N indicating that algae were using a light N source. Isotope mass balance calculations indicate that the bioassay algae incorporated N with a δ15N = 1 ‰. This is consistent with the measured δ15N-NO3 of +1.5 ‰ in the Skagway River. 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.