Regional variation in N vs. P limitation in the Baltic Sea the role of sediment mineralisation processes

Transcription

1 Regional variation in N vs. P limitation in the Baltic Sea the role of sediment mineralisation processes Petri Ekholm & Jouni Lehtoranta Finnish Environment Institute (SYKE) COST869 Mitigation options for nutrient reduction in surface waters and groundwaters Working group 2: Influence of nutrients on ecological processes in surface waters

2 Hypothesis The concentrations of P in surface water and nutrient limitation (N/P) are strongly impacted by the dominant anaerobic mineralization pathway (Fe reduction / SO 4 reduction) in marine surface sediments The dominant anaerobic mineralization pathway depends on the amount of settling labile organic C ( eutrophication) and hydromorphological features that control O 2 transport to nearbottom water

3

4 Available on-line

5 Microbial oxidation-reduction reactions Reduction reaction Formula Depth in sediment oxic Aerobic respiration CH 2 O + O 2 CO 2 +H 2 O mm anoxic Denitrification 5CH 2 O + 4NO H + 5CO 2 + 2N 2 + 7H 2 O mm anoxic Manganese reduction CH 2 O + MnO 2 + 4H + CO 2 + 2Mn H 2 O cm anoxic Iron reduction CH 2 O + 4FeOOH + 8H + CO 2 + 4Fe H 2 O cm anoxic Sulfate reduction 2CH 2 O + SO H + 2CO 2 + H 2 S + 2H 2 O m anoxic Methanogenesis CH 2 O 2CH 4 + 2CO 2 m

6 The Baltic Sea The Gulf of Bothnia The Bothnian Bay The Bothnian Sea The Baltic Proper The Archipelago Sea The Gulf of Finland Map: HELCOM

7 Depth profile A brackish-water basin (a large estuary) km km 3 mean depth 55 m Catchment > km 2 14 countries 95 million people

8 Salinity stratification varies between the subbasins Virtually no halocline in the Gulf of Bothnia Halocline at about 60 m depth in the Baltic Proper and the Gulf of Finland

9 Phosphate in the surface water External P load does not explain the year-to-year changes in the P concentrations Internal processes dominate Graph: Kiirikki (2002)

10 State of surface sediments Similar development may be achieved in laboratory by adding labile organic C to sediment columns Photos: Seppo Knuuttila, Jouni Lehtoranta/SYKE

11 Traditional paradigm on nutrient limitation Although there is regional and seasonal variation, it is generally assumed that Lakes mostly P limited (e.g. Vollenweider, Schindler) Coastal waters mostly N limited (e.g. Ryther & Dunstan) 1. Less N-fixing cyanobacteria in marine than in lacustrine phytoplankton 2. More effective N removal (denitrification, anamnox) in coastal waters 3. SO 4 reduction sequesters Fe and thereby weakens P removal The Baltic Sea? 1. Less N-fixing cyanobacteria in marine than in lacustrine phytoplankton Does not apply 2. More effective N removal (denitrification, anamnox) in coastal waters No data in favour/against 3. SO 4 reduction sequesters Fe and thereby weakens P removal

12 P vs. N limitation of Baltic Sea phytoplankton N:P ratio in terrestrial loading ca. triple the Redfield ratio Total N:P in water also suggests P limitation DIN and DIP both low 3-day bioassays with natural planktonic communities chl a and primary productivity response variables Most pristine areas P-limited, more eutrophic areas N- limited Bothnian Bay P-limited Gulf of Finland N-limited throughout the growing season Bothnian Sea N-limited for summer months Tamminen T & Andersen T Seasonal phytoplankton nutrient limitation patterns as revealed by bioassays over Baltic Sea gradients of salinity and eutrophication. Marine Ecology Progress Series 340: Open Access.

13 A new issue on Silicon cycling! The Gulf of Finland may become Si limited in near future (diatom bloom)

14 Hypothesis of Blomqvist et al. (2004) Fe:P in near-bottom water > 2, efficient trapping of P when in oxic conditions (freshwater) < 2, incomplete trapping of P (seawater) SO 4 concentrations x higher in seawater than in freshwater Microbial SO 4 reduction produces sulphide that reacts with Fe so that its P-binding ability is lost Coupled or uncoupled cycling of P and Fe Blomqvist S, Gunnars A, Elmgren R, Why the limiting nutrient differs between temperate coastal seas and freshwater lakes: a matter of salt. Limnol. Oceanogr. 49,

15 Fe oxides bind P in soils by a specific ligand exchange reaction An example +1 0 Fe H 2 O H 2 O + H 2 PO - 4 Fe H 2 PO 4 + H 2 O H 2 O Not necessarily so in sediments Fe:P = 2 may refer to the dimers of Fe(III) and PO 4 formed when Fe(II) is oxidized

16 We modify the hypothesis so that The decisive factor is not SO 4 but the amount of labile organic matter (= C) that settles on the bottom, i.e. eutrophication Marine areas showing P limitation exist, if Fe reduction rather than SO 4 reduction is the dominant mineralisation pathway

17 The Baltic Sea Nutrient level high, formation of organic C high High O 2 consumption Energy flows to sediments, triggering microbial mineralisation processes Does it matter which mineralisation pathway dominates? Low nutrient loading High nutrient loading O 2 Termocline O 2 O 2 Sediment O 2 consumption does not exceed transport of O 2 from upper layers a few mm oxic layer in sediment surface O 2 Halocline Sediment O 2 consumption exceeds transport of O 2 from upper layers sediment surface anoxic Sediment Anoxic world where anerobic oxidation of organic matter prevails Sediment Anoxic world where anerobic organic matter oxidation prevails

18 Fe and P cycling Oligotrophic marine system Microbial reduction dominates Fe reduction Eutrophic marine system Chemical reduction dominates Fe reduction Water O 2 Fe(III)oxides Fe(III) bound P org-p Fe(III) Oxic sediment binds PO Anoxic sediment Burial of Fe(III) bound P Fe(III)oxides Fe(III) bound P org P 3-4 O 2 Efficient Fe and P cycling Fe(III) reduction Fe(III) reduction by H S 2 Fe:P>2 Fe(II) Solid FeS FeS 2 State 1 Low efflux of P PO 4 3- A Water O 2 Anoxic sedim Fe(III)oxides Fe(III) bound P org P Fe(III) binds PO 3- partly 4 Fe(III)oxides Fe(III) bound P org P Little Fe(III) bound P buried O Inefficient Fe and P cycling Fe(III) reduction by H S 2 2 Fe(III) reduction High efflux of P Fe:P<2 Fe(II) Solid FeS 2 FeS State 2 PO 4 3- B Lehtoranta (2003)

19 Fe and SO 4 reduction Fe reduction may dominate when there is bioavailable Fe(III) oxides oxic sediment surface enabling re-oxidation of reduced Fe(II) bottom animals causing bioturbation SO 4 reduction may dominate when bioavailable Fe(III) oxides are consumed there is labile organic matter in sediments sediment surface is anoxic (no renewal of Fe(III) oxides)

20 Phosphate in near-bottom water (µmol l -1 ) Lehtoranta J, Ekholm P, Pitkänen H Eutrophication-driven sediment microbial processes can explain the regional variation phosphorus concentrations between Baltic Sea sub-basins. Journal of Marine Systems (in press).

21 Regional pattern in the Fe:P in near-bottom water Intensive monitoring of water quality Monitoring of harmful substances in tissues of fish and evertebrates Zoobenthos monitoring State 1 State State N km Tot-Fe to DIP ratio (mol:mol) ,1 0,01 State Station Lehtoranta J, Ekholm P, Pitkänen H Eutrophication-driven sediment microbial processes can explain the regional variation phosphorus concentrations between Baltic Sea sub-basins. Journal of Marine Systems (in press).

22 External load Atmospheric N P Spring phytoplankton bloom P N 2 Aphanizomenon sp. Nodularia spumigena S C org FeOOH P CO 2 O 2 O 2 Aerobic mineralisation P Fe reduction Fe(III) (s) Fe(II) (aq) FeS P SO 4 reduction Photos: J. Turkia, R. Jokipii, P. Kokkonen, SYKE SO 4 2 (aq) H 2 S (g)

23 Summary N or P? Oligotrophic Gulf of Bothnia P-limited Eutrophic Gulf of Finland (as well as the Baltic Proper) N- limited Why? Regional variation in N/P limitation may be due to the differences in microbial mineralisation pathway Fe reduction gives way to SO 4 reduction upon increasing flux of organic C to sediments, i.e. eutrophication SO 4 reduction is also favoured when O 2 transport to the nearbottom water is prevented, e.g. by a halocline To improve the state of the most eutrophied areas, we should cut down both P and N load Probably the response will still be slow How to activate Fe in anoxic sulphidic sediments?