Reduced Nitrogen in Coastal Waters Maren Voss Leibniz Institute for Baltic Sea Research Warnemünde, Germany
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
N-Budget for the Ocean (10 6 t a -1 ) Atm. Dep. 33 N 2 -Fix. 87-156 N 2 Denitrif. 150-450 N 2 O Denitrif./Nitrif. 4 River 48 Ocean inputs: 168.0 237.0 losses: 168.8 468.8 Sediment PON 14,8 (Galloway et al., subm.)
Land Ocean Interaction in the Coastal Zone LOICZ-domain Coastal shelves 0-200 m (cover 5% of global ocean surface) Loicz webpage
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 1995. This means that 2.75 billion will life within a narrow coastal strip in 20 yrs. from now. (http://www.livescience.com/) 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
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
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
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
Zones with oxygen depletion Oxygen depletion Annual Episodic Periodic Persistent http://upload.wikimedia.org/
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
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
Chesapeake Bay Susquahanna Patuxent Potomac Rappahannock York James
Hypoxia below the mixed layer: Chesapeake Bay surface O 2 (%) NH NO 4+ 3- (µmol) Surface waters 60 30 30 20 150% 100% Bottom waters bottom N S 10 0% after Horrigan et al. 1999
Hypoxia below the mixed layer: Data from the Indian Shelf 80 60 40 20 0 distance from the coast (km) 80 60 40 20 0 distance from the coast (km) here denitrification is suggested to produce the N 2 O Naqvi et al. BGD, 2006
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 13,400-1 13,924 t yr -1 (2) (3) DON Atm. dep. 300 kt yr1,990-1 2,566 t yr -1 (3) Ammonium 1,206 4.650 t yr -1 importance
Population density [People km -2 ] 0-1 1-5 5-12 12-38 N 38-11,000 Σ 85 million
Winter DIN concentration in the Baltic Sea Model run from February 1985 µmol Neumann, 2005
Voss et al. 2006 Relationship between DIN load landuse and δ 15 N-NO 3 cultivated land in catchment [%] 80 60 40 20 0 r² = 0.968 n =11 p<0.001 DIN load [µmol/l/a] 0 50 100 150 200 250 DIN load [µmol/l/a] 250 200 150 100 50 0 r² = 0.841 n =11 p<0.001-2 0 2 4 6 8 10 flow weighted δ15n-no3 [ ]
Voss et al. 2006 Land use and isotope signatures 0 100 80 60 40 nitrate from atmosph. deposition. [%] 20 80 Kemijoki Neva 60 nitrate from pristine soils [%] 40 Kokemäenjoki Paimionjoki 20 Peene Vistula 100 Oder 0 20 40 60 80 100 nitrate from agricultural land [%] 0
Baltic Sea salinity distribution 0 Kattegat Bornholm Sea Gotland Sea Gulf of Bothnia 30 psu 20 psu 40 Water depth (m) 80 120 160 10 psu ANOXIA 200 240 marine fresh
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
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
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
Cyanobacteria
Anoxia in the Baltic Sea area in km² recently laminated sediments Naturally laminated sediments Johnsson et al. 1990
Filtration capacity M. arenaria 6 4 2 1
Nutrients in pore waters of sandy sediments NH4 (µm) Sediment depth (cm) z (cm) -4 0 4 8 12 16 20 24 0 100 200 300 400 500 17.01.1999 12.03.1999 22.04.1999 06.04.1999 26.05.1999 20.07.1999 Highly variable between 500 and almost 0 µmol L -1
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
Succesion of benthic life under less and less oxygen Sedimentdepth Sedimentdepth 1 2 3 4 5 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).
Succesion of benthic life under less and less oxygen Sedimentdepth Sedimentdepth 1 2 3 4 5 O 2 redrawn after Pearson and Rosenberg, 1978 There is hardly a way back from stage 5 to 1!
Extension of sandy sediments 66 latitude 65 64 63 62 61 60 59 58 57 56 55 0-80 -160-240 -320 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 54 10 12 14 16 18 20 22 24 26 28 30 longitude -400
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.