Reduced Nitrogen in Coastal Waters. Maren Voss Leibniz Institute for Baltic Sea Research Warnemünde, Germany

Transcription

1 Reduced Nitrogen in Coastal Waters Maren Voss Leibniz Institute for Baltic Sea Research Warnemünde, Germany

2 Topics of this talk N-cycling of coastal zones and human impact Eutrophication in the Baltic Sea a vicious cycle? The role anoxia in N- transformations The role of benthic life for N-removal Summary

3 N-Budget for the Ocean (10 6 t a -1 ) Atm. Dep. 33 N 2 -Fix N 2 Denitrif N 2 O Denitrif./Nitrif. 4 River 48 Ocean inputs: losses: Sediment PON 14,8 (Galloway et al., subm.)

4 Land Ocean Interaction in the Coastal Zone LOICZ-domain Coastal shelves m (cover 5% of global ocean surface) Loicz webpage

5 The role of coastal seas Traffic and transport, recreation and housing 35% increase of population living within 60 miles of the coastlines by 2025 compared to This means that 2.75 billion will life within a narrow coastal strip in 20 yrs. from now. ( Nourishment of people since coastal seas are the most productive fisheries zones in the world. At the same time these zones are endangered through: Sea level rise (esp. southern hemisphere) Storms and hurricanes Human activities (constructions etc.) Eutrophication Anoxia

6 Eutrophication cascade from Gray, 1992 Enrichment phase Macro algae Phytoplankton Benthos Fish biomass biomass biomass biomass Initial effect Changed species composition Secondary effect Shading depth reduction Hypoxia Toxic / nuisance blooms Behavioral effects Extreme effect Mass growth Ulva, Cladophora Toxic effects Mortality of species Ultimate effect Anoxia / Mass mortalities

7 NO 3- concentration and export Note! Neither the length of the rivers, water flow and size of drainage basin correlates to nutrient river loads! But the population in the drainage basin does correlate! Peierls et al. 1991

8 Links between coastal hypoxia and mankind (after Diaz & Rosenberg, 2001) Coastal population with rising standards of living. nutrient input from STP and direct land runoff atmospheric deposition N from combustion and from fertiliser/manure use on agricultural land. Coastal nutrient input leads OM production (=eutrophication) anoxia can develop when stratification minimizes vertical exchange processes this is the case in estuaries halocline or in summer thermocline

9 Zones with oxygen depletion Oxygen depletion Annual Episodic Periodic Persistent

10 The N- cylce: N 2 O NO - 3 Dinitrogen Prim.prod. N PON 2 N 2 -Fixation N 2 H 4 Ammonification Denitrification NO N-monoxid Anammox Ammonium NH + 4 O 2 DNRA Occurs with anoxic conditions DNRA: Dissimilatoric Nitratreduction to ammonium Anammox: anaerobic ammoniumoxidation Nitrit NO - 2 NO - 3 Nitrat Nitrifikation N 2 O Hannig 2006

11 More consequences of O 2 depletion Other oxidation agents than O 2 are used by bacteria e.g. SO 4 2- H 2 S Nitrification and Denitrification produce N 2 O Metal speciation changes oxic anoxic e.g. Fe III (OH) 3 PO 3-4 Fe II + PO 3-4

12 Chesapeake Bay Susquahanna Patuxent Potomac Rappahannock York James

13 Hypoxia below the mixed layer: Chesapeake Bay surface O 2 (%) NH NO (µmol) Surface waters % 100% Bottom waters bottom N S 10 0% after Horrigan et al. 1999

14 Hypoxia below the mixed layer: Data from the Indian Shelf distance from the coast (km) distance from the coast (km) here denitrification is suggested to produce the N 2 O Naqvi et al. BGD, 2006

15 The Baltic Sea Surface area: 415,266 km² Catchment area: 1,720,270 km² Population: Within 10 km: 85 million 15 million Fresh water: 15,190 m³ s -1 Six largest: 6,565 m³ s -1 Kemijoki Neva Daugava Nitrogen Sources (Baltic Sea) Nemunas (1) Rivers 1000rivers-DE kt yr -1 total Vistula Oder (1) (2) Nitrate N 2 -fixation 400 kt yr 13, ,924 t yr -1 (2) (3) DON Atm. dep. 300 kt yr1, ,566 t yr -1 (3) Ammonium 1, t yr -1 importance

16 Population density [People km -2 ] N 38-11,000 Σ 85 million

17 Winter DIN concentration in the Baltic Sea Model run from February 1985 µmol Neumann, 2005

18 Voss et al Relationship between DIN load landuse and δ 15 N-NO 3 cultivated land in catchment [%] r² = n =11 p<0.001 DIN load [µmol/l/a] DIN load [µmol/l/a] r² = n =11 p< flow weighted δ15n-no3 [ ]

19 Voss et al Land use and isotope signatures nitrate from atmosph. deposition. [%] Kemijoki Neva 60 nitrate from pristine soils [%] 40 Kokemäenjoki Paimionjoki 20 Peene Vistula 100 Oder nitrate from agricultural land [%] 0

20 Baltic Sea salinity distribution 0 Kattegat Bornholm Sea Gotland Sea Gulf of Bothnia 30 psu 20 psu 40 Water depth (m) psu ANOXIA marine fresh

21 Stratification in the Baltic Sea salinity oxygen Oxic zone Nitrite NO 2 - nitrification halocline subox. zone chemocline anaerobic degrad. of OM sulph. zone denitrification Salinity [psu] from Hannig, 2006

22 ph dependency of the NH 3 /NH 4 + equilibrium (20 C, 35 psu) Preindustrial seawater value (4% NH 3, 96% NH 4+ ) 100,00 80,00 % 60,00 40,00 Ocean acidification high prim.produc. NH3 NH4+ 20,00 0,00 6,00 7,00 8,00 9,00 10,00 11,00 12,00 13,00 14,00 Direct toxicity for animals is hardly given through ammonia ph

23 T PO 4 3- S O 2 /H 2 S NO 3 - NH 4 + Typical changes in the Baltic Proper deep waters after an inflow event

24 Cyanobacteria

25 Anoxia in the Baltic Sea area in km² recently laminated sediments Naturally laminated sediments Johnsson et al. 1990

26 Filtration capacity M. arenaria

27 Nutrients in pore waters of sandy sediments NH4 (µm) Sediment depth (cm) z (cm) Highly variable between 500 and almost 0 µmol L -1

28 Physical oxygenation: Pore water flow and current induced pressure field oscillating flow pressure field intrusion release intrusion each m² of coarse grained sediment can filter up to 850mg C org. d -1 Precht and Huettel, 2004 Rusch and Huettel, 2000

29 Succesion of benthic life under less and less oxygen Sedimentdepth Sedimentdepth redrawn after Pearson and Rosenberg, 1978 The situation in the western Baltic Sea has worsened from 1932 to 1989 years by at least 1 stage (Ruhmor 1996).

30 Succesion of benthic life under less and less oxygen Sedimentdepth Sedimentdepth O 2 redrawn after Pearson and Rosenberg, 1978 There is hardly a way back from stage 5 to 1!

31 Extension of sandy sediments 66 latitude These sediments seem to guarantee: rapid removal of large quantities of organic carbon high nitrification capacity removal of river N- loads before these can enter the open Baltic Sea longitude -400

32 Summary Human activities have lead to enhanced N-input into coastal zones (not so much reduced N compounds). This N enters coastal seas mainly in oxidized forms. Under anoxic conditions NH 4+ is produced and not further nitrified and may accumulate under statified conditions. Coastal seas release N 2 O in rather unknown quantities. Biological activity of benthic life forms aerates the sediments (sandy, muddy) efficiently. When benthic life has died off it cannot easily return. Coastal sandy sediments are an efficient filter system for the organic loads, they even enhance nitrification and presumably also denitrification.

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