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Introduction to Primary Production, Respiration and Nutrient Cycling. Why we care? Coupling of atmosphere and ocean Ocean carbonate system & importance to chemistry in the of sea water Processes in controlling distribution of oxygen & dioxide in the ocean.
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Introduction to Primary Production, Respiration and Nutrient Cycling • Why we care? • Coupling of atmosphere and ocean • Ocean carbonate system & importance to chemistry • in the of sea water • Processes in controlling distribution of oxygen & • dioxide in the ocean. • 4) Ocean circulation and oxygen, carbon dioxide and nutrients Oscar Schofield (firstname.lastname@example.org)
Processes/Platforms:Time and Space Scales t Dickey, 2001a
(from N.Gruber) Monthly mean sea level at San Francisco (1855-1990) Annual averages of sea level at Venice and Trieste (1875-1980) Stewart et al., 1998
Photosynthesis O2 O2 CO2 CO2 Respiration
O2 CO2 Diffusion or gas exchange CH3 He O2 CO2 Abundance of Gases in Air and Seawater and Gas Exchange In addition to dissolved salts, organic molecules and suspended solids, sea water contains dissolved gases. Most of these gases enter the sea from the atmosphere, but others are produced within the ocean by marine organisms or are derived from the Earth’s interior (e.g. helium).
time Bud Bud Bud There will be a net uptake (or loss) of a gas by sea water from the atmosphere until the sea water reaches saturation. At saturation, the gas exchange process is said to be in equilibrium, i.e. the rates of exchange in and out are equal.
Proportion in Air Proportion in saturated sea Gas at 15 C and 35‰ N2 0.780 0.626 O2 0.209 0.343 CO2 0.0003 0.014 Ar 0.009 0.016 Saturation values are gas concentrations when a solution (here we are concerned with sea water but the term applies to any solution) has reached equilibrium with its overlying gas mixture (here the atmosphere). That is, saturation values are the most chemically favorable conditions. Since some gases are more soluble than others, the proportion of gases dissolved in saturated sea water is different from the proportion in the atmosphere. The gas solubilities of CO2 > Ar > O2 >N2
A saturation value depends on: -temperature (boiling water) -salinity (boiling water) Generally, cold water can hold more dissolved gases than warm water of the same salinity, and fresh water can hold more dissolved gases than salt water of the same temperature.
Carbonate System Although CO2 is a soluble gas in sea water, it also reacts chemically with waterand is present in sea water as a one of two dissolved anions, bicarbonate and carbonate, and as carbonic acid. FORMS of CARBON DIOXIDE IN SEA WATER carbon dioxide CO2 (dissolved) carbonic acid H2CO3 bicarbonate HCO3- carbonate ion CO32-
Reactions 2 and 3 (below) are acid-base reactions. Bicarbonate ion which is one of the major ions in sea water can act as both an acid and a base. 1). CO2 + H2O H2CO3 2). H2 CO3 + H2O H3O++ HCO3- acid base acid base 3). HCO3- + H2O H3O+ + CO32- acid base acid base pH is a short hand scale for representing the acidity or alkalinity of a solution which depends on the concentration of hydrogen ions (H+) (or hydronium ions, H3O+ ) in the solution.pH = - log (H+)
The carbonate system is largely responsible for maintaining seawater pH close to a value of 8. i.e., slightly alkaline. This is largely because in the ocean, sea water is also in contact with sediments that contain carbonate minerals the most important of which is calcite (CaCO3). If excess acid is added to the deep ocean (say for example via hydrothermal vent emissions) the acid is neutralized by reacting with carbonate ions in solution, and these are replaced by dissolution of carbonates in the sediments. If excess base is added more carbonate minerals precipitate and and are removed to the sediments.
Photosynthesis O2 O2 CO2 CO2 Respiration
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
In the text, photosynthesis is represented very simply . It can be represented more completely, if we think of it as a process that generates the organic matter in phytoplankton cells. Phytoplankton organic matter is made up of a large number of organic compounds (e.g. proteins, lipids, carbohydrates), but on average it has atomic ratios of C to N to P of 106 to 16 to 1. Thus, the process of photosynthesis can be represented as: hv 106CO2 + 122H2O + 16HNO3 + H3PO4 (CH2O)106(NH3)16H3PO4 + 138O2 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 liberated.
In the photic zone, photosynthesis leads to high oxygen concentrations and low total-CO2. When photosynthesis rates are high, oxygen concentrations can rise above saturation. This is a state called supersaturation. Marine bacteria, fungi, protozoans and animals that can not get energy from photosynthesis, decompose organic matter. This process is called respiration. It can be represented as the reverse of photosynthesis. (CH2O)106(NH3)16H3PO4 + 138O2 106CO2 + 122H2O + 16HNO3 + H3PO4 Respiration puts CO2 and nutrients back into the water. Respiration depletes O2 , making deep waters undersaturated with respect to the oxygen in the atmosphere. Respiration occurs throughout the entire water column and in sediments, but its effect on the distributions of oxygen and total-CO2 are usually not seen until depths below the euphotic zone.
Depth Distributions of Oxygen and Total-CO2/ Light/ Photosynthesis and Respiration In the upper ocean, these two profiles appear almost as mirror images. This is because both oxygen and carbon dioxide are involved in the production and destruction of organic matter, i.e. the soft tissues of marine plants and animals. CO2 is also taken up by some plants and protozoa to make calcium carbonate (CaCO3) hard parts. Since these shells dissolve at depth in the ocean, total-CO2 profiles may not co-vary as closely with oxygen profiles at depth. The greatest changes in oxygen and total-CO2 occur in the uppermost ~80 m of the ocean. This depth range corresponds to euphotic zone, the zone where there is sufficient light for phytoplankton (single celled plants that have chlorophyll) to grow through the process of photosynthesis.
Regenerated production New production Export production The Biological Pump About 10% of the carbon fixed by photosynthesis in the surface layer each year, escapes this layer by sinking into the deep ocean. This flux is called New Production or Export Production. Biologically derived nutrients Physical mixing processes Irradiance Phytoplankton Nutrients Nutrients Zooplankton Sinkage & Senescence Higher Trophic Levels Particle Dynamics Carbon Flux
Sequestration of Atmospheric Carbon The biological pump is an important mechanism for removing fossil fuel CO2 from the atmosphere into the ocean because Nearly all of the sinking particulate organic matter is converted back to CO2 through respiration in the deep ocean. Photosynthesis followed by a) the transport of carbon into the deep ocean and b) the respiration of the majority of this carbon, is called the "biological pump".
The biological pump is an important mechanism for removing fossil fuel CO2 from the atmosphere into the ocean because 1. it lowers surface CO2 concentrations, and 2. it transports particulate carbon into the deep-ocean, where even if it is oxidized back to CO2, it is removed from contact with the atmosphere for on the order of 500 years. Models show the pump is doubly important at high latitudes because here the waters of the deep ocean are formed. First of all cold waters have higher saturation values for gases than warm. Then if primary production rates are high too, even more CO2 will exchange from the atmosphere to the cold surface waters of the arctic and antarctic regions. The sinking of this water "captures" the CO2 and removes it from contact with the atmosphere.
Ocean circulation eventually brings the respired CO2 back to the surface, but the net effect is to keep the deeper ocean enriched in dissolved inorganic carbon. CO2 off gas (bud) CO2 rich water Pacific Ocean
Net CO2 flux (Takahashi et al 1995)
Surface distribution of chlorophyll a using SeaWiFS data sets: Note physical forcing effects: Coastal, Equator, North Atlantic SeaWiFS Team/GSFC/NASA
Nutrient Limitation Many elements are necessary for life, but only those in short supply are limiting to photosynthesis. Oceanographers consider nitrate, phosphate, silica, iron and several other trace metals to be the most biolimiting elements. Silicon is important for the growth of diatoms. Iron is required for photosynthetic electron transport and the synthesis of chlorophyll. Nutrient profiles generally increase with depth. Concentrations may be below detection in surface waters, especially in the open ocean.
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.
New Jersey Coastal Upwelling July 6, ’98 - AVHRR July 11, ‘98 - SeaWiFS Chlor-a (mg/m3) Temperature oC 19 20 21 22 24 .1 .3 .5 1 2 4 40N 40N Historical Hypoxia/Anoxia Field Station Field Station LEO LEO 39N 39N 75W 74W 75W 74W Barnegat Cape May