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Introduction to Marine Primary Productivity and Carbon Cycle

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Introduction to Marine Primary Productivity and Carbon Cycle Introduction to marine primary productivity and carbon cycle Satya Prakash [email protected] 400 Law Dome Ice Core, Antarctica 380 Mauna Loa, Hawaii Slope: 360 1970 - 1979: 1.3 ppm y-1 -1 340 2000 - 2006: 1.9 ppm y 320 Concentration (ppm) 2 CO 300 280 1820 1840 1860 1880 1900 1920 1940 1960 1980 2000 2020 Year CO2 – Temperature Relationship CO2 Concentration Temperature 380 4 2 340 0 300 -2 (ppmv) 2 260 -4 Degree C Degree CO -6 220 -8 180 -10 0 50 100 150 200 250 300 350 400 450 Age (Kyr) VOSTOK Ice Core data How much is 100 ppm?? 1 ppm = 2.12 * 1015 gm = 2.12*109 tonnes 100 ppm = 2.12 thousand crore tonnes 1m. 1m. = 1000 kg = 2.44 टन 1m. CO2 1m. Water or one tonne 1m. 1m. Partition of Anthropogenic Carbon Emissions into Sinks [2000-2006] 45% of all CO2 emissions accumulated in the atmosphere 55% were removed by natural sinks Ocean removes ~ 24% Land removes ~ 30% Upper Photic Layer Photosynthesis O2 O2 CO2 CO2 Respiration Deeper Aphotic Layer The Ocean Euphotic zone light - ~little N Aphotic zone no light - lots N Photosynthesis is a process that generates the organic matter in phytoplankton cells. The process of photosynthesis can be represented as: hv 106CO2 + 122H2O + 16HNO3 + H3PO4 (CH2O)106(NH3)16H3PO4 + 138O2 Available solar energy in the waveband 400-700 nm. This reaction illustrates the need for the nutrients: nitrate and phosphate. It also shows that for every 106 CO2 molecules taken up, approximately 138 O2 molecules are produced. Primary production is the rate of synthesis of organic material from inorganic compounds such as CO2 and water. It is significant as it provides the base of most of the entire marine food chain. The formation of organic carbon compounds from inorganic carbon (e.g. carbon dioxide) involves a reduction reaction; the reducing power) comes from either the absorption of light (photosynthesis), or the oxidation of other compounds (chemosynthesis). It is a rate, hence involves dimensions of time: mg C m-3 d-1, or in a depth integrated sense, mg C m-2 d-1 The Biological Pump • Plankton grow, mature and die—taking carbon with them to the deep ocean • They have a larger effect on climate than any single other process or group of organisms • 99% of marine life relies on plankton—they form the base of the marine food chain. •About 10% of the carbon fixed by photosynthesis in the surface layer, escapes this layer by sinking into the deep ocean. This flux is called New Production or Export Production. phytoplankton need: light CO2 nutrients water In the ocean, light and nutrient availability may limit the rate of photosynthesis. THE MAJOR FORESTS IN THE SEA ARE PHYTOPLANKTON Major Nutrients - 2- + • Nitrogen (NO3 , NO4 , & NH4 ) –Limiting in marine systems 3- • Phosphorus (PO4 ) –Limiting in freshwater systems • Silica (SiO2) –Important to diatoms • Redfield ratio 106 : 15 : 16 : 1 C Si N P Nutrient sources to surface waters are: rivers and land runoff upwelling atmosphere The most productive regions of the oceans are the coastal regions because this is where upwelling is strongest and where river and land runoff meet the sea. Here nutrients result in high productivity rates, which in turn result large fisheries. Regions with upwelling represent the productivity Equatorial upwelling Coastal upwelling Water turbidity Components of primary production Total Production = New production + Regenerated production New Production Regenerated Production Ammonium Urea Photic Photic Zone Nitrate Recycling New Production is defined Regenerated Production is as production due to newly uptake of recycled borne nitrate into surface nutrients such as layer ammonium and urea f-ratio = New Production / Total Production New Production ~ Export production Excess nutrients Excess aquatic plants Fish kills Dead plants decay Low dissolved oxygen Vertical distribution of Nutrients Photic Zone and Compensation Depth Biomass Nutrients Photosynthesis Irradiance Intensity Ik Z (meters) Atlantic & Pacific nutrient and oxygen distribution Sources and sinks of dissolved Oxygen Sources: Physical exchange between atmosphere and Ocean, mainly diffusion By product of photosynthesis Sinks: Community respiration Bacterial degradation of organic matter Leads to formation of oxygen depleted zone in the sub-surface layer (100 – 1000m) Dissolved Oxygen in Sub-surface water Oxygen profiles in the Northern Indian Ocean Major Oxygen minimum zones around the world’s Ocean. A map showing the annual mean dissolved oxygen levels at a depth of 200 m. Source: Levitus Climatology Oxygen Minimum Zone Oxygen Minimum Zones (OMZs) are the intermediate-depth layers characterized by very low oxygen concentrations OMZs occur in regions of low dynamical supply and high demand of oxygen i.e., in regions of low ventilation by subsurface currents and productive upwelling systems, where intense biological uptake of oxygen associated with bacterial respiration and remineralisation occur [Resplandy et al., 2012] Low oxygen levels have dramatic implications for the ecosystem, coastal environment, and economics In the Indian Ocean, OMZs are found in both the Arabian Sea (AS) and the Bay of Bengal (BoB) The Arabian Sea OMZ (ASOMZ) is the second most-intense OMZ of the world ocean and is usually observed between 100-m and 1000-m depths, with oxygen concentrations less than or equal to 20 μmol/L The oxygen concentrations in BOBOMZ are more or less constant --------------------------------------------------------------------------------------------------------------------- - Oxic Zone : Region in where dissolved oxygen is abundant (O2 more than 100 µmol/kg) Hypoxic zone : A typical threshold for hypoxic zone is approximately 60 µmol/kg (~10- 60μm/kg) Suboxic zone : The suboxic zone is defined as a region which experience nitrate reduction but not sulphate reduction (Suboxic range : O2 < 2-10 µmol/kg) Anoxic zone : region which experience complete depletion of oxygen and are a more severe condition of suboxia (~0μm/kg) ----------------------------------------------------------------------------------------------------------- Nitrogen Cycle N2 Nitrification - PON NH + NH OH NO - NO3 Oxic 4 2 2 Suboxic - - NO2 NO3 Denitrification Remineralisation - NO2 Organic N NO N2O Anammox + N NH4 2 Climate Change and Dissolved Oxygen Global Surface Warming Heating Less Upwelling Increased Less Ventilation Stratification Less export production Reduction in supply from Increase in surface residence time of deeper water Less Oxygen demand Decrease in O2 inventory in the sub-surface/deeper layer What are the controllers on Export Production? If macronutrients are unavailable then the CO2 flux is reduced! Nitrogen appears to be the most important controlling factor that limit the primary productivity of ecosystems. 1) Ocean nutrient inventory 2) Utilization of nutrients in HNLC condition 3) Change of Redfield Ratio What is “HNLC”? • High Nutrients Low Chlorophyll • Mainly in Southern Ocean, Equatorial and sub- Arctic Pacific Ocean • First defined by Minas et. al 1986 as a region having potential for high production but lower observed productivity • Several hypothesis have been proposed to explain this condition HNLC regions of World Ocean HNLC Hypothesis to explain HNLC Condition 1. Low specific growth rates • Nitrate and phosphate concentrations are 2. Low temperature high year round but standing stocks of phytoplankton are always low (0.2-0.4 µg/L; 3. Deep mixed layer normal yield is 1 µg /L) 4. Grazing hypothesis • Iron concentrations in these waters are sub- nanomolar: the same as those that are 5. Fe limitation known to limit growth of phytoplankton, particularly large species such as diatoms. • Addition of low levels of Fe promotes growth of large phytoplankton. -bottle experiments -in situ fertilization experiments Different Iron experiments done so far Redfield ratio (stoichiometry carbon, nitrogen and phosphorus in phytoplankton. Redfield (1963) described the remarkable congruence between the chemistry of the deep ocean and the chemistry of living things in the surface ocean (i.e. phytoplankton). Both have N:P ratios of about 16. When nutrients are not limiting, the molar element ratio C:N:P in most phytoplankton is 116:16:1. Redfield thought it wasn't purely coincidental that the vast oceans would have a chemistry perfectly suited to the requirements of living organisms. He considered how the cycles of not just N and P but also C and O could interact to result in this match. Figure 13.8 Modern Time N2 fixation De-nitrification N = 25790 N* = N – 16 P Redfield Ratio of 16:1 for particulate organic matter is an upper bound for N:P in the dissolved inorganic phase. Today’s ocean has an average dissolved inorganic N:P ratio of ~14.7:1 Implies an imbalance between nitrogen fixation and denitrification •CO2 exchange is dependent on limiting nutrient Ratio of nitrogen fixation to denitrification is crucial to CO2 •Dissolved inorganic nitrogen exchange limits productivity Nutrient flux and dead zones: correlated?? Increase of Nitrogen influx into river Distribution of dead zones in the world Excess nutrients Excess aquatic plants Fish kills Dead plants decay Low dissolved oxygen UNCE, Reno, NV The biggest concern with excess nutrients is eutrophication Results in: • impacts on lake/stream ecology webs; • toxins; • drinking water treatment problems; • other changes in lake chemistry How global warming affects ocean productivity? Effects of Climate Change on Oceans 1. Melting sea ice, stratification 2. Warming –productivity-changes in community structure 3. Precipitation regimes-nutrient transport from the landLong- eutrophication time series are needed to get a clear 4. Ocean acidification picture Schematic model illustrating the effect of sea-surface warming on upper-ocean processes in low (Upper) and high (Lower) latitudes. Graph depicts the effects on stratification and mixed-layer depths, nutrient supply (yellow arrows), plankton biomass, and particle flux (green arrows). Source: Riebesell, Kortzinger and Oschlies , PANS, 2009 The Efficiency of Natural Sinks: Land and Ocean Fractions • Part of the decline is attributed to up to a 30% decrease in the efficiency of the Southern Ocean sink over the last 20 years. • This sink removes annually 0.7 Pg of anthropogenic carbon.
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