Please enable JavaScript.
Coggle requires JavaScript to display documents.
Carbon Fixation as Blue Carbon and Carbon Sequestration by Seagrass &…
Carbon Fixation as Blue Carbon and Carbon Sequestration by Seagrass & Seaweed (L6)
Carbon Fixation (photosynthetic)
Carbon fixation: by photoautotrophic algae
has the potential to diminish the release of CO2 into the atmosphere
help alleviate the trend toward global warming (e.g. primary producers; phytoplankton, seaweed and seagrass)
Light energy -> chemical energy/organic molecules
Involves Calvin cycle(reduction)
carboxylation of ribulose 1,5-biphosphate (RuBP) catalyzed by the enzyme RUBISCO, a monophyletic enzyme.
Carbon Sequestration
the long-term storage of carbon in plants, soils, geologic formations, and the ocean
refers to: storage of carbon that has the immediate potential to become CO2 gas.
Mechanims of Fixation & Sequestration
Marine plants: have C3(3-phosphoglycerate with 3 carbon atoms) type of photosynthetic carbon fixation
Plants which employ the C3 pathway: use ribulose 1,5-bisphosphate carboxylase/oxygenase (RUBISCO) ~ primary carboxylating enzyme
In the chloroplasts, CO2 is bonded covalently to ribulose 1,5-bisphosphate, forming a six-carbon compound ~ that immediately splits to form two molecules of 3-phosphoglycerate.
Sediment brought in by tides or storm:
~ primarily inorganic matter
~ some organic matter (deposited & accumulate within wetland)
Allochthonous carbon:
~ carbon associated with organic matter that comes from outside sources of fixation/photosynthesis
Autochthonous carbon:
~ majority of the organic matter and carbon that accumulates within tidal wetland
Above & below ground plant biomass, contribute to soil organic matter and eventual carbon sequestration.
Carbon Fixation in Seagrass
Seagrass meadows
:
net autotrophic
act as net CO2 sinks - significant fraction of seagrass production occurs below-ground as roots and rhizomes (where this material can be preserved over long-time scales).
~ A large fraction of fixed carbon is not consumed by herbivores, seagrass tissue is relatively refractory and decomposes slowly.
~ Ratio between carbon fixed in photosynthesis and the consumption of organic carbon in respiration: determine whether the plants show a net positive carbon balance, allowing growth, or not.
~ A positive balance achieved at light levels higher than the light compensation point.
As leaf thickness increases, the respiratory burden of the leaf tissue increases faster than its ability for carbon fixation.
Amounts of chlorophyll a per unit of tissue weight decrease with leaf thickness.
As irradiance levels increase, rates of carbon fixation increase.
Until the light saturation point is reached.
At saturating light levels, the maximum rate of net photosynthetic carbon fixation may exceed respiratory carbon consumption of the leaf tissues some 20 times, typically around 5 times.
Functional equilibrium
:
root growth is limited by the rate of supply of carbon from the leaves
the growth of leaves is limited by the supply of water or nutrients by the roots.
Carbon Sequestration in Seagrass
Carbon accumulation may be seasonal
~ Summer: when seagrasses are at their maximum density
~ Winter: when resuspension (low water velocity) may be greater than accumulation when seagrasses are minimal
Epiphytes on seagrass leaves: foster the accumulation of sediment particles by increasing the roughness of canopy & increasing the thickness of boundary layer on leaf surface.
In highly wave-exposed locations: seagrasses do not accumulate fine sediments due to resuspension.
Sequestration Rates in Seagrass
Average 220.7 ± 20.1g Corg m^-2 year^-1.
Median 167.4g Corg m^-2 year^-1 with values ranging from 2094 to 2124g Corg m^-2 year^-1.
Carbon Sequestration in Seaweed/Kelp Forests
Kelp forests:
~ highly productive ecosystems.
~ produce large amounts of fixed carbon.
~ do not develop their own soft organic-rich sediments.
~ have limited capacity to act as long-term carbon sinks in traditional sense.
~ produce large amounts of detritus through incremental blade erosion, fragmentation of blades and dislodgement of whole fronds and thalli
Detritus:
~ as carbon donors to adjacent benthic
Estimated global average rate of detrital production by kelps: 706 g C m^-2 year^-1.
Rates of detrital production:
~ 8 to 2657 g C m-2 year-1, from blade erosion and fragmentation
~ 22 to 839 g C m-2 year-1 for loss of fronds and thalli.
Sequestration Rates in Seaweed
173 TgC year^1 (range, 61–268 TgC year^-1)
Factors influencing Sequestration
The impact of shading and simulated grazing 🡪 high-intensity shading and high-intensity clipping/grazing show significantly lower net community production and carbon content in belowground biomass 🡪 due to the removal of above-ground biomass.
High-intensity disturbances reduce the ability of seagrass meadows to sequester carbon.
Eutrophication: causes dramatic weakening (100-fold) of the C sink capacity of seagrass-dominated estuaries 🡪 which implies that nutrient runoff into estuaries should be managed so as to maintain C capture and storage by seagrasses
Impacts of common, large-scale environmental (i.e. floods, hurricanes, cyclones) and direct human disturbances (e.g. anchoring, trawling, dredging) on seagrass C stocks, particularly to test whether such disturbances can cause extreme CO2 efflux from seagrass meadows.