Workshop and Conference on Biogeochemical Impacts of Climate and Land-Use Changes on Marine Ecosystems November 2009

Transcription

1 Workshop and Conference on Biogeochemical Impacts of Climate and Land-Use Changes on Marine Ecosystems 2-10 November 2009 Nutrient cycling in the Mekong River plume M. Voss IOW Germany

2 Nutrient cycling in the Mekong River plume Maren Voss, Deniz Bombar, Joachim Dippner, Julia Grosse, Doan Nhu Hai, Frederike Korth, Nguyen Ngoc Lam, Iris Liskow

3 NO 3 - River input and atmospheric deposition New N sources for the pelagic cycle Diffusion N 2 N 2 NO 3 - N 2 - Fixierung NO 3- - Aufnahme? Cyanobacteria Phytoplankton Zooplankton Grazing higher trophic levels Euphotic zone Atmosphere NO 3 - Diffusion / upwelling remineralisation sedimentation particulate organic nitrogen 1% light

4 NO 3 - River input and atmospheric deposition New N sources for the pelagic cycle Diffusion N 2 N 2 NO 3 - N 2 - Fixierung NO 3- - Aufnahme? Cyanobacteria Phytoplankton Zooplankton Grazing higher trophic levels Euphotic zone Atmosphere NO 3 - Diffusion / upwelling remineralisation sedimentation particulate organic nitrogen 1% light

5 radius of deformation Surface layer OSW+MSW mmkgtw Diffusion coast upwelling MSW MSW Thermocline PTW PTW

6 Nitrate concentrations ( µmol L -1 ) SWM N 0 m 40 m coast latitude 13 N MSW+OSW Upwelling area 20 km Offshore from upwelling 40 km 100 km 150 km mmkgtw Mixed layer 80 m 120 m MSW MSW

7 Expecations for the production radius of deformation coast New OSW+MSW production upwelling MSW In the IM regenerated mmkgtw production Diffusion Nitrogen fixation Thermocline production based on diffused nitrate MSW PTW PTW

8 Station Map Mekong- Flußdelta A1 A A A SW Monsun (SWM) Post El Nino Juli 2003 Breitengrad (N) a VT SWM 2003 SpIM SWM m Längengrad (E) Spring intermonsoon (SpIM) 21. April - 2. Mai 2004 SW Monsun (SWM) Normal year Juli 2004

9 Determination of fixation rates A) 2,3 liters of natural sample + 2,4ml 15 N 2 - gas C) End of experiments: Vacuum filtration of samples through GF/Ffilters. B) Incubations of 6-7 hr on deck, cooled by flowing seawater, different depths simulated with neutral density screening D) Combustion of dried GF/F-filters, determination of POM - δ 15 N in mass spectrometer, calculation of N-fixation rates

10 Comparison of SWM periods in 2003 and 2004 MLD Mixed layer depths (m) SST Sea Surface Temperature ( C) Contour intervals are 5m for MLD and 1 C for SST. Bombar et al. sub.

11 Clear differences in NO 3 concentraitions NO3 within the upwelling- The white boxes represent mixed layer depths, The grey boxes represent of euphotic zone depths. Bombar et al. sub.

12 Comparison of SWM periods in 2003 and 2004 (a) primary production (mmolc m -2 d -1 ) VG 3, 2003 VG7, 2004 (b) nitrate uptake (c) nitrogen fixation Bombar et al. sub.

13 N 2 -Fixation and new production in the Mekong river plume o 0.01 N 2 -Fixation 0.1 in mmol m -2 d in the upper 40 m o NO 3- -uptake in mmol m -2 d -1 in the upper 40 m SWM 2003 SWM SWM 2003 SWM 2004 Breitengrad (N) Längengrad (O) Längengrad (O)

14 Water masses and nitrogen fixation SpIM Temperature ( C) Temperature ( C) N 2 -fixation (nmoll -1 h -1 ) SWM 2003 SWM Salinity Salinity

15 The fate of nitrogen

16 Stable isotopes 14 N 15 N δ 15 N [ ] = 15 N/ 14 N (sample) 15 N/ 14 N (standard) -1 x 1000 Proton Neutron Cyanobacteria NO 3 - ocean NO δ 15 N ( )

17 Typical δ 15 N values in the ocean when N 2 -fixation occurs Cyanobacteria -2 to 1 N 2 0 POM 3 bis 10 Phytoplankton 4 to 10 Zooplankton 2 to 6 Zooplankton 5 to 12 NO NO Fish 7 to 14 sources: this study, Montoya et al. 2002, Owens 1987, Liu and Kaplan 1989

18 Relationships between animal and diet isotopic compositions for carbon and nitrogen stable isotopes 30 Carbon 30 Nitrogen number of experiments δ 13 C = Food Source Indicator number of experiments δ 15 N = Trophic Level Indicator Δ (δ consumer -δ diet ) Δ (δ consumer -δ diet ) from Peterson and Fry 1987

19 Biomass decreased in 5 zooplankton size fractions ( µm) with increasing trophic level log 2 (normalized biomass) Increase of δ 15 N values With the size of the plankton Sargasso Sea 1 Sargasso Sea 2 Georges Bank Gulf of Maine In areas with high nitrogen fixation the δ 15 N values are lower δ 15 N ( ) Quelle: Fry und Quinones 1994

20 Plankton ID (all seasons) C:N ratio Food? C:N = 6.6 Phytoplankton Zooplankton n = 40 n = 40 n = 38 n = 31 n = 33 n = 34 n = 30 > POM in µm Loick et al. 2007

21 Quantification of the importance of nitrogen fixation SWM 2003 & 2004 N from NO 3 - uptake δ 15 N ( ) N from N 2 - Fixation Upwelling area offshore SpIM 2004 N from NO 3 - δ 15 N ( ) uptake N from N 2 - Fixation > > Loick et al µm-scale

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23 Quantification of the importance of nitrogen fixation SWM 2003 & δ 15 N ( ) Upwelling area offshore SpIM 2004 δ 15 N ( ) > > 2500 Loick et al in µm

24 In the offshore region nitrogen fixers are an important food source for higher trophic levels

25 Summary Due to El Niño influence, upwelling was weaker in July 2003, average primary productivity ± 18 mmol C m-2 d ± 25 mmol C m-2 d-1 Upwelling nitrate fluxes of 17 ± 2 mmol N m-2 d-1 in July 2004 was consistent with N-demands of primary productivity, but not in Nitrogen fixation was a significant N-source in the area, (up to 375 µmol N m-2 d-1 in offshore waters) - equal to 2% - 25% of diffusive nitrate fluxes. Nitrogen from fixation is a significant source for higher trophic levels.

26 The Mekong River estuary

27 A semienclosed coastal body of water, which has a free connection with the open ocean and within which sea water is measurably diluted with the fresh water dervied from land drainage Pritchard, 1967

28 Estuarine Circulation freshwater Marine water Outflow is characterised by the amount of outflowing water And the bottom topography.

29 Estuarine Circulation Δ h Level surface fresh water ρ 1 h A ρ 2 B 1. ρ 2 > ρ 1 2. The pressure upon A is the same as upon B, from the difference in height Δh canbecalculated no horizontal movement of the water. 3. All points above and A have a higher pressure than above B the pressure gradient moves the freshwater over the salt water. Mann & Lazier, 1991

30 Saltwedge estuary Mekong Tidally mixed estuary Fjord type estuary Mann & Lazier, 1991

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32 Hydrography-seasonality Southwest monsoon Northeast Monsoon 12.0 Vietnam Mekong estuary 12.0 Vietnam Mekong estuary Breit engrad (N) latitude (N) southern anti- Cyclonic gyre 200 m Breit engrad (N) latitude (N) Cyclonic gyre 200 m Längengrad (O) longitude (E) Längengrad (O) longitude (E) nach Hu et al. 2000

33 Mekong estuary m 3 s -1 in March/April up to m 3 s -1 in September/October True Colour Satellite image (NASA)

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37 Latitude (N) N 2 -Fixation in the Mekong river plume o µmol m d µmol m d 10 µmol m-2 d µmol m -2 d µmol m -2 d d -1 Breitengrad (N) Auftriebsgebiet 9.5 Salinität < 33.2 im SWM 2003 Salinität < 33.2 im SWM Längengrad (O) SWM 2003 SWM 2004 SpIM 2004 Voss, Bombar, Loick, Dippner GRL 2006

38 Summary The plume extension depends on the monsoon season and rainfall Nutrient concentrations are not extremely high in the Mekong (like some eutrophied European Rivers), but moderate except fo silica The Mekong plume seems to support various nitrogen fixers - even as far offshore as the upwelling zone. DDAs seem to grow well in a certain band of the Mekong plume. We are only beginning to understand the biogeochemistry of the estuarine waters.

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