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Marine Primary Productivity: Measurements and Variability

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Marine Primary Productivity: Measurements and Variability Marine Primary Productivity: Measurements and Variability Matt Church Department of Oceanography MSB 612 Sunlight CO2 + 2H2OCH2O + O2 + H2O + heat Photosynthesis Gross Primary Production (GPP): The rate of organic carbon production via the reduction of CO2 inclusive of all respiratory losses. Net Primary Production (NPP): Gross primary production less photosynthetic respiration (RA): NPP = PN –RA Net Community Production (NCP): Gross primary production less all autotrophic and heterotrophic losses due to respiration (RA+H). NCP = PG –RA+H **If we are interested in carbon available for export or consumption by higher trophic levels, NCP is the key term. If we want to know how much total energy was captured by photosynthesis, we need to know GPP. What methods would you use to measure primary production in the sea? ∆O2 ∆CO2 ∆Organic matter Isotopic tracers of C and/or O2 What methods are most suitable for measuring aquatic primary production? • Typical rates of photosynthesis in the ocean range between 0.2-2 µmol C L-1 d-1 or -1 -1 0.3-3 µmol O2 L d -1, • Concentrations of DIC ~2000 µmol C L O2 ~220 µmol L-1, and TOC ~80-100 µmol L-1 • Analytical sensitivity of carbon and oxygen determinations: -1 –CO2 by coulometry = 1 µmol C L -1 –O2 by Winkler titration = 0.4 to 2 µmol O2 L –TOC by HTC = 2-4 µmol C L-1 **Measuring very small signals against large background pools** Commonly used methods for measuring aquatic photosynthesis • Changes in O2 concentrations – incubations (Gaarder and Gran 1927) and in situ dynamics. • CO2 assimilation: stable or radioisotopes of carbon (13C or 14C) – technique first applied by Steeman- Nielsen 1951. • Oxygen isotope disequilibria (18O, 17O, 16O) • Satellite remote sensing Primary production approach 1: 14C-bicarbonate assimilation •Examine assimilation of 14C (as bicarbonate) by plankton. 14 - •Add C labeled HCO3 to bottles containing seawater; incubate in the light. •Harvest plankton by filtration, acidify the filter, and count radioactivity (using liquid scintillation counter) assimilated into plankton biomass during incubation period. Typically PAR (400-700 nm) transparent •Rate of primary production is 14 polycarbonate bottles determined by the amount of C- are used for these label assimilated into particles experiments...but UV is relative to the total DIC pool excluded. What does the method measure? Average HOT program 14C-assimilation (µg C L-1 d-1) Overestimate (no UV light)? •Gross Or… Underestimate primary (static incubation production? at excessive irradiance)? •Underestimate due to •Net primary isotope dilution from production? respired CO2? •Overestimation due to exclusion of •Net grazers? •Underestimate due to community loss of DOC? production? 14C-based determinations of aquatic primary production abound… HOT program 14C-assimilation (µg C L-1 d-1) Equatorial Pacific iron addition experiment Assimilation of 14C-bicarbonate • 1000’s of ocean measurements • Relatively “easy” to measure • Estimates carbon fixation directly Several caveats: • 1) Always returns a positive result. • 2) Does not discriminate light and dark respiration. • 3) Typically measures something between NPP and gross production. • 4) Generally ignores organic carbon produced and excreted or lost during incubation. • 5) Requires incubation and confinement of samples Chlorophyll normalized rates of 14C-bicarbonate assimilation suggest phytoplankton in the open ocean approach maximal growth rates...even with very low nutrients. This assumes 14C-assimilation equals NPP. Primary production approach 2: O2 light-dark bottle •Measures changes in oxygen concentrations in light and dark bottles following incubation •Light bottle = net community production (photosynthesis and community respiration). •Dark bottle: community respiration. •Light + Dark = Gross primary production GPP = ∆O2 (light) - ∆O2(dark) Vertical profiles of GPP at Station ALOHA measured by the light/dark bottle technique Williams et al. (2004) We will revisit this technique… The light bottle/dark bottle O2 technique • Measures gross and net primary production • Relatively “easy” to measure Several caveats: • 1) assumes rate of respiration in dark = light. Various light dependent oxygen consumption pathways (Mehler reaction and photorespiration) • 2) requires incubation and confinement of samples. • 3) requires conversion of O2 to carbon (photosynthetic quotient, PQ). O2/C PQ values vary from 1.1 to 1.4 depending on nitrogen sources and end products of photosynthesis. Photosynthetic quotients Sunlight CO2 + 2H2OCH2O + O2 + H2O + heat Photosynthetic production of carbohydrate: 1 mol CO2 for 1 mol O2 PQ of 1 But more than carbon, water, and light are needed to produce biomass . 106CO2 + 122H2O + 16HNO3 + H3PO4 → [(CH2O)106 + (NH3)16 + H3PO4] + 138O2 - The reduction of NO3 to NH3 consumes reductant at the expense of CO2 reduction to organic matter. Thus, growth on - NO3 results in a reaction stoichiometry of ~1.4 moles of O2 produced for 1 mole of CO2 fixed (photosynthetic quotient), while growth on NH3 has a photosynthetic quotient of ~1. Primary production approach 3: 18 O2 gross production 18 • Addition of H2 O: light bottle/dark bottle incubation approach. Photosynthetic 18 splitting of H2O yields O2. 18 • O2 produced during photosynthesis measured by mass spectrometry. • Only measures GPP; no measurement of R or NCP by this method. 18 Juranek and Quay (2005); Comparison of O2 production to 14C-bicarbonate assimilation at Station ALOHA Primary production approach 4: The triple oxygen isotope (16O, 17O, 18O) • Measure 3 oxygen isotopes (16O, 17O, 18O) in air and dissolved O2 • Photosynthesis has a characteristic oxygen fractionation during the splitting of H2O. • Unique O2 signature from photosynthesis can be distinguished from atmospheric exchange. • Requires information on air-sea exchange, which is not easy to determine • Ratio of the different isotopes of oxygen provides an estimate of gross primary production. – No containment of samples – Integrates over weekly time scales – Valid for mixed layer only • See Luz and Barkan (2000) Primary production approach 5: In situ variations in oxygen, nutrients, or carbon • Examine annual or seasonal scale - changes in O2, NO3 , and CO2 concentrations in the upper ocean. • As long as exchange and diffusive loss can be accounted for this approach should provide an estimate of NCP. Seasonal changes in oxygen can be used to examine net community production. **Note this approach is only applicable in ecosystems where a strong seasonal thermocline develops. Mixed layer O2 is in equilibrium with the atmosphere Rate of subsurface O2 accumulation provides information on NCP Riser and Johnson (2008) Satellites to the rescue…but we don’t measure production from space Temperature Satellites measure irradiance upwelling from the Earth’s surface. Can be used to define chlorophyll, temperature, and irradiance. Need models that relate these things to primary production. Chlorophyll Satellites “measure” chlorophyll, temperature, and light • ~1 km resolution • Need models that relate photosynthesis to these remotely sensed variables. • Nontrivial challenges with remote sensing: stability and accuracy of sensors, correction for atmospheric interferences, and conversion from ocean color to chlorophyll. • Depth-dependent descriptions of phytoplankton productivity generally include the following terms: vertical light attenuation, biomass normalized productivity, photoperiod length, and incident light flux. Photosynthesis-irradiance To quantify photosynthesis as a function of irradiance (and determine biomass normalized photosynthetic rates), a device called a photosynthetron is often used. 14C-bicarbonate is added to whole seawater samples, samples are placed in temperature and light controlled incubation. After short incubations (<2 hrs) rates of photosynthesis are derived. Photosynthetic responses to irradiance B P max α Ek Photosynthetic rate can be described as a function of light intensity: B B P = P max (1-exp(-E/Ek)) P= photosynthesis B P max = maximal rate of photosynthesis E= light B Ek = Saturation irradiance (light needed to obtain P max) Remember, predictive models are only as good as the data and assumptions that go into them… Different methods yield different answers…which is right? Location 14C-PP GPP NCP (mg C (mg C m-2 d-1) (mg C m-2 d-1) m-2 d-1) BATS 144 to 1490 620-1560 76-104 by changes in O2 72-213 by CO2 drawdown North Atlantic 686 to 1260 1400-2830 -270 to 1890 by in vitro O2 Station ALOHA 170-1077 560-1470 -640 to -14 by in vitro O2 55-180 by in situ O2 fluxes 28-164 by CO2 drawdown Arabian Sea 1165 1340-4080 -15 to 1690 by in vitro O2 •Maybe depends on time scale of interest: 14 • C and ∆O2 in vitro: hourly to daily 18 •H2 O: daily •18O, 17O, 16O: weekly - • ∆O2/NO3 /CO2 in situ: seasonal.
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