Introduction to Primary Production, Respiration and Nutrient Cycling

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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 (oscar_at_ahab.rutgers.edu)
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Processes/PlatformsTime and Space Scales
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Dickey, 2001a
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OCEANOGRAPHY IS FRIGGIN HARD
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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 Earths interior (e.g. helium).
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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.
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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 gt Ar gt O2 gtN2
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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.  
 
Conditions Saturation Conc. for O2 (mmol/kg)
Freshwater (25oC) 258
Seawater (25oC) 206
Freshwater (5oC) 398
Seawater (5oC) 308
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Carbonate System
Although CO2 is a soluble gas in sea water, it
also reacts chemically with water and 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-
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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)
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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.
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phytoplankton need   light CO2 nutrients wate
r   In the ocean, light and nutrient availability
may limit the rate of photosynthesis.
THE MAJOR FORESTS IN THE SEA ARE PHYTOPLANKTON
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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.  
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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.
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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.
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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.

Physical mixing processes
Biologically derived nutrients
Irradiance
Phytoplankton
Nutrients
Nutrients
Zooplankton
Sinkage Senescence
Higher Trophic Levels
Particle Dynamics
Carbon Flux
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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".  
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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.  
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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
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Geographic Differences in Nutrient Profiles
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Surface distribution of chlorophyll a using
SeaWiFS data sets Note physical forcing effects
Coastal, Equator, North Atlantic
SeaWiFS Team/GSFC/NASA
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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.   Nutr
ient profiles generally increase with depth.
Concentrations may be below detection in surface
waters, especially in the open ocean.  
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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.  
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New Jersey Coastal Upwelling
Barnegat
Cape May
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Geographic Differences in Nutrient Profiles
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