<-previous | Geol 117 Home | Lectures | Review | next ->
Processes
Gross and Net Primary Production
Cycling of organic matter
Controlling factors
Seasonal variations at different latitudes
Global distribution
FATE OF ORGANIC MATTER -- HOW PP IS UTILIZED & CYCLED
PP = Synthesis of organic matter by autotrophs
1. Photosynthesis . . by phytoplankton is most important
sunlight
CO2 + H2O -----------------> [CH2O] + O2
chlorophyll
2. Chemosynthesis (e.g., by sulfur bacteria in hydrothermal vent systems):
H2S + O2 ---------------> S0, SO42- + chem. energy
chem. energy then used...
CO2 + H2O ---------------> [CH2O] + O2
3. Biosynthesis . . of other essential organic molecules in autotrophs:
energy
[CH2O] + nutrients (P, N, S) --------> proteins, fats and oils, DNA and RNA
Gross Primary Production . . (GPP)
= Total amount of organic matter produced by primary producers
(phytoplankton)
Net Primary Production . . (NPP)
= GPP minus energy utilized by phytoplankton for life processes
Phytoplankton "respiration" 70-90 % of GPP
NPP 10-30 % of GPP
Phytoplankton biomass available for consumption by herbivores
Note: GPP and NPP are rates, with units of grams of carbon per square meter per yr.
Biomass, or "standing crop" is grams of carbon per square meter
Global balance: Total production about equal to Total respiration
Prod. > Consump. + Decomp. in the photic zone
Prod. < Consump. + Decomp. below the photic zone
Slight excess production (~0.1% of GPP) in the oceans
---> deposition and preservation of organic matter in sediments
Fate of organic matter in sediments?
1.Sunlight
2.Seasonal heating -- stratification of surface waters
3. Nutrients
4. Vertical mixing ...
... seasonal density stratification
... upwelling of deep waters
5. Proximity to land-derived nutrients
6. Grazing by herbivores
7. Phytoplankton biomass ("standing crop")
All these influence the rate of photosynthesis (i.e., productivity)
Polar oceans -- intense mid-summer "bloom"
Nutrients are abundant -- good vertical mixing
Bloom initiated by summer sunlight (low-intensity, but constant)
Warming --> density stratification
--> phytoplankton can remain in photic zone
Productivity controlled by sunlight
Tropical oceans -- relatively constant but low productivity
throughout the year
High-intensity sunlight the year around
Density-stratified surface waters
Little vertical mixing, thus low nutrient levels
Productivity controlled by nutrient availability
PP = Synthesis of organic matter by autotrophs
1. Photosynthesis . . by phytoplankton is most important
sunlight
CO2 + H2O -----------------> [CH2O] + O2
chlorophyll
2. Chemosynthesis (e.g., by sulfur bacteria in hydrothermal vent systems):
H2S + O2 ---------------> S0, SO42- + chem. energy
chem. energy then used...
CO2 + H2O ---------------> [CH2O] + O2
3. Biosynthesis . . of other essential organic molecules in autotrophs:
energy
[CH2O] + nutrients (P, N, S) --------> proteins, fats and oils,
DNA and RNA
Phytoplankton biomass available for consumption by herbivores -> NPP (~10-30% of GPP)
Important note: GPP and NPP are expressed as rates of carbon "fixation" by phytoplankton per unit area -- gmC / m2 - yr. "Biomass" or "standing crop" of phytoplankton (or zooplankton) are expressed as mass of carbon per unit area, i.e. a sort of areal density -- gmC / m2.
Organic matter "cycling" in the oceans. On a global average (continents plus oceans) there is a balance between production and "respiration" of organic matter (including consumption and decomposition by bacteria). In the oceans, the production/respiration ratio changes with depth:
Prod. > Consump. + Decomp. in the photic zone
Prod. < Consump. + Decomp. below the photic zone
In the oceans as well there is a slight excess production of organic matter (~0.1% of GPP) in the oceans, which is deposited and preserved in sediments. What is the fate of organic matter in marine sediments? It is returned eventually to the ocean-atmosphere system, but on a time scale of several 100 million years. The processes involved are:
Tectonic uplift of ocean crust at convergent boundaries
Exposure and oxidation of organic matter
Factors controlling primary productivity.
Sunlight
Sunlight is obviously required for phytoplankton potosynthesis.
Solar radiation also controls mixing and density stratification in the upper part of the water column.
Nutrients
Vertical mixing
Upwelling of deep water is the largest source of nutrients.
In regions where there is a strong density stratification of surface waters, upwelling is limited.
Proximity to land-derived nutrients
Grazing by herbivores effects the biomass (or standing crop) of phytoplankton, and hence the capacity for photosynthesis.
Polar oceans -- intense mid-summer bloom.
Nutrients are abundant in surface waters because of good vertical
mixing throughout most of the year.
The bloom is initiated by summer sunlight (low-intensity, but
constant).
Solar radiation warms surface waters. This leads to stability
and density stratification, which allows phytoplankton to remain
in the photic zone.
Productivity is controlled by sunlight
Tropical oceans -- relatively constant but low productivity
throughout the year
High-intensity light the year around
Density-stratified surface waters
Little vertical mixing, thus low nutrient levels
Productivity controlled by nutrient availability
Mid-latitude oceans -- spring and autumn blooms. To
see how this more complex pattern of seasonal productivity occurs,
let's follow an annual cycle.
Winter:
Surface-water mixing (cooling, storms, waves)
Nutrients are available, but sunlight is limiting
Spring:
Increased sunlight and density stratification
Phytoplankton remain at surface, and an intense "bloom" results
Summer:
Zooplankton grazing reduced plant biomass
Nutrients are released, leading to a second, less intense bloom
Fall:
Nutrients become depleted and sunlight diminishes
Productivity is controlled both by sunlight and by nutrient availability
Open oceans
o Limited nutrient supply results in low rates of primary productivity
per unit area (i.e., low [gm C / m2 - yr]
o Because open oceans make up 90% of total ocean area, total production
rate is high, i.e., primary production at low rates over large
area.
Polar and equatorial upwelling zones in open ocean
o Upwelling is due to Ekman transport and thermohaline circulation.
o High nutrient supply results in moderate to high rates of primary
production per unit area.
Continental shelves
o Runoff from land and vertical mixing in shallow oceans results in higher nutrient supply and high rates of primary production per unit area.
Coastal zones of intense upwelling at low latitudes
o Combination of prevailing winds, surface currents, Ekman transport,
and low-latitude sunlight results in very high rates of primary
production per unit area.
Estuaries and shallow coastal waters
o Photic zone extends to the bottom.
o Nutrient regeneration by (1) intense vertical mixing and (2)
nutrient supply from land supports abundant benthic plants and
algae as well as phytoplankton.
o Very high rates of primary production per unit area.
-Define "primary production."
-Write the generalized chemical reaction for photosynthesis. Which
group of organisms are the most important photosynthesizers in
the world ocean?
-What is the source of food (i.e., the base of the food chain)
for communities living around hydrothermal vents on the deep-sea
floor? What organisms are responsible? How do the "synthesize"
organic matter?
-Define the terms "gross primary production (GPP) and "
net primary production (NPP). In general terms, approximately
what fraction of GPP is used by primary producers for their own
life processes? What fraction remains as biomass of primary producers?
-Describe the balance between production and destruction (consumption
and decomposition of organic matter (a) in the photic zone (b)
at greater depths, and (c) for the ocean as a whole.
-In contrast to the terrestrial environment, there is a slight
excess of organic-matter production in the oceans. What is the
fate of this organic matter? How is it returned to the ocean-atmosphere
system?
-Identify and describe briefly the three factors that control
primary productivity in the oceans.
-Which factor or factors control seasonal variations in primary
productivity in (a) polar oceans and (b) tropical oceans. Briefly
justify your answer.
-Describe how a combination of sunlight and nutrient availability
controls seasonal variations in primary productivity in mid-latitude
oceans.
-What factor limits primary productivity (per unit area) in the
open ocean, 90 % of the total ocean area?
-Primary productivity (per unit area) is moderately high in polar
and equatorial upwelling zones in the open ocean. What processes
are at work here?
-Coastal zones of intense upwelling at low latitudes are regions
of particularly high productivity (per unit area). As before,
explain why this is so.
-What particular processes and properties of estuaries make them
the most productive regions of the world ocean?
<-previous | Geol 117 Home | Lectures | Review | next ->