Recent Nitrogen Isotope Data from Massachusetts and Cape Cod Bays

1. Recent Nitrogen Isotope Data from Massachusetts and Cape Cod Bays Joseph P. MontoyaGeorgia Institute of Technology Charles S. MayoCenter for Coastal Studies Georgia Tech Biological Oceanography

2. Nitrogen and its Isotopes • Nitrogen is a critical plant nutrient • Nitrogen availability typically limits biological production in marine systems. • Nitrogen injected into a system is consumed rapidly and converted into biomass. • Nitrogen has two naturally occurring stable isotopes • Isotopes differ in mass but have the same chemical behavior. • Most of the nitrogen on Earth has an atomic mass of 14, but about 0.367% of the global pool of nitrogen has an atomic mass of 15. • We can use these isotopes as tracers to follow the movement of nitrogen through ecosystems. Georgia Tech Biological Oceanography

3. d15N Notation • The natural abundance of 15N in a sample can be expressed as the per mil (‰) deviation from the isotopic composition of a reference compound: where R is the isotope ratio 15N:14N and the reference compound is atmospheric N2. Georgia Tech Biological Oceanography

4. Isotopic Fractionation • Many biological processes discriminate between molecules containing the two isotopes of nitrogen. • This discrimination can be quantified using a discrimination factor, which is effectively equal to the instantaneous difference in d15N between the substrate and product of a reaction: • Uptake of DIN by phytoplankton is accompanied by isotopic fractionation with e = 5 - 10‰. Georgia Tech Biological Oceanography

5. Effects of Fractionation on Substrate and Product Georgia Tech Biological Oceanography

6. Nitrogen Isotopes in Food Webs Georgia Tech Biological Oceanography

7. Sewage Produces an Isotopic Signature in PN Georgia Tech Biological Oceanography

8. The Isotopic Signature of PN Propagates into Zooplankton Georgia Tech Biological Oceanography

9. Center for Coastal Studies Monitoring Program • Goals • To define and monitor critical chemical and biological parameters in the water column of Massachusetts and Cape Cod Bays. • To assess the impact(s) of the Boston Outfall on the Bays ecosystem of the Bays. • Field Program • Monthly sampling of stations in Cape Cod Bay supplemented by quarterly sampling of stations extending northward to the outfall in Massachusetts Bay (since summer 2001). • Samples collected include suspended particles (PN), zooplankton, and dissolved nutrients. Georgia Tech Biological Oceanography

10. So Far, So Good … • The d15N of zooplankton in Cape Cod Bay has not changed appreciably in the last decade. • This implies that nitrogen injected into Massachusetts Bay by the new Boston outfall has not yet had a significant impact on the nitrogen supply to Cape Cod Bay. • An obvious question is how far does the influence of the outfall actually extend. • New stations in Massachusetts Bay extend the CCS surveys to the region of the outfall. Georgia Tech Biological Oceanography

11. Survey: Summer 200115N of samples collected between 28 July and 4 August 2001 (cruises SW216, SW217, and SW220) as a function of distance southward from the outfall along the mean flow path through Massachusetts Bay and into Cape Cod Bay. A smoothed trend line is shown for each data set. A: zooplankton from the 250 – 500 µm size fraction (circles) and zooplankton from the 500 – 1000 µm size fraction (squares). B: Surface particulate nitrogen (open circles) and deep particulate nitrogen (filled circles). Georgia Tech Biological Oceanography

12. Survey: Autumn 200115N of samples collected between 30 October and 18 November 2001 (cruises SW229, SW230, SW21, and SW232) as a function of distance southward from the outfall along the mean flow path through Massachusetts Bay and into Cape Cod Bay. A smoothed trend line is shown for each data set. A: zooplankton from the 250 – 500 µm size fraction (circles) and zooplankton from the 500 – 1000 µm size fraction (squares). B: Surface particulate nitrogen (open circles) and deep particulate nitrogen (filled circles). Georgia Tech Biological Oceanography

13. Survey: Spring 200215N of samples collected between 14 - 25 March 2002 (cruises SW258, SW259, and SW260) as a function of distance southward from the outfall along the mean flow path through Massachusetts Bay and into Cape Cod Bay. A smoothed trend line is shown for each data set. A: zooplankton from the 250 – 500 µm size fraction (circles) and zooplankton from the 500 – 1000 µm size fraction (squares). B: Surface particulate nitrogen (open circles) and deep particulate nitrogen (filled circles). Georgia Tech Biological Oceanography

14. Interpretation: Propagation of the Isotopic Signature of PN into Zooplankton Georgia Tech Biological Oceanography

15. Spatial Extent of Sewage Impact • The d15N of PN and zooplankton in Massachusetts and Cape Cod Bays clearly show the impact of nutrient inputs associated with the Boston outfall. • The extent of the minima in d15N of PN and zooplankton mark the spatial penetration of sewage-derived N into the Bays ecosystem. In effect, these data provide us with a biogeochemical index to the spread of sewage N into the coastal ecosystem. • The limit of sewage spread appeared to be ca. 40 km ion summer 2001, and ca. 60-70 km in autumn 2001 and spring 2002 • To our knowledge, this is the first time that the spread of sewage N into the biota has been traced over tens of kilometers in a coastal ecosystem. Georgia Tech Biological Oceanography

16. Contribution of Sewage to PN • A rigorous estimate of the contribution of sewage N to plankton biomass would require measurement of the spatial distribution of DIN and its isotopic content. • We can, however, use a simple mixing model to estimate the importance of sewage N within the isotopic minima we observed near the outfall: • d15NMarin = 5‰, a value consistent with deep nitrate in the North Atlantic as well as pre-outfall d15N values in Massachusetts Bay • d15NDIN = 6‰, a conservative value consistent with recent measurements of Deer Island effluent. • We used e= 5 and 10‰ for phytoplankton uptake of DIN. The higher value of e yields a conservative estimate of the contribution of sewage DIN to phytoplankton biomass. Georgia Tech Biological Oceanography

17. Estimated Contribution of Sewage to PN e = 10‰ Georgia Tech Biological Oceanography

18. Contribution of Sewage to Zooplankton • We can extend our mass balance approach to the zooplankton using deviations from a reference zooplankton d15N as a measure of the incorporation of sewage N into animals: • d15NZplRef = 7.8 - 9.0‰, reflecting seasonal variation in the mean d15N of zooplankton well removed from the outfall. We used the mean value for stations within Cape Cod Bay to define d15NZplRef for each survey. These values are consistent with measurements of zooplankton collected before the outfall went on line. • As before, we used e = 5 and 10‰ to account for fractionation during phytoplankton uptake of DIN. • We used a “trophic effect” of 3.5‰ to account for the shift in d15N between an animal’s food and its own tissues. Georgia Tech Biological Oceanography

19. Estimated Contribution of Sewage to Zooplankton Summary of Previous Isotopic Measurements e = 10‰ Georgia Tech Biological Oceanography

20. Conclusions… • The d15N of PN and zooplankton vary significantly in the region of the outfall. • The d15N minimum south of the outfall provides a biogeochemical index to the penetration of sewage N into the planktonic ecosystem. • The spatial spread of sewage N into the plankton appears to vary seasonally. • The boundary of the zone of measurable sewage input to the plankton varied between ca. 40 and 70 km south of the outfall in our three surveys to date. Georgia Tech Biological Oceanography

21. Conclusions (contd.) • A simple mixing model suggests that sewage makes a significant contribution to both PN and zoooplankton biomass in the region of the outfall. • Sewage may account for over half the nitrogen in suspended particles and roughly a third of the nitrogen in zooplankton. • Isotopes are a Leading Indicator of Sewage Impact • Shifts in the isotopic boundary may be the first indicator of impending ecosystem-level changes associated with sewage inputs. • But they don’t tell us everything… • Ecosystems are highly complex networks • Community level changes may extend beyond the isotopically defined zone of sewage impact. Georgia Tech Biological Oceanography