Download Lithosphere (solid materials, soil and rocks) Seagrass

Lithosphere (solid materials, soil and rocks) Seagrass
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Lithosphere (solid materials, soil and rocks)
Morphology: The morphology of an embayment or other potential seagrass
locations plays a role in determining limiting or favouring hydrodynamic
conditions related to nutrient fluxes and sediment dynamics. Similarly, the
morphodynamics may determine whether a location is favourable or
morphologically too variable for seagrass establishment.
Fig. 5. Habitat requirements seagrass
lithosphere
Slope: Seagrasses usually do not occupy steeply sloping coastal zones in large densities. This is probably related to the angle of light incidence
and seagrass growth strategies.
Sediment type: Sediment grain size and properties (mineral or organic; cohesive or non-cohesive) play a role in settlement possibilities, rooting,
aeration, sediment nutrient availability (diffusion), resuspension (erosion, turbidity) and biogeochemical processes. Sediment characteristics are
usually more critical for initial seagrass settlement than for the development of existing (healthy) seagrass beds.
Biogeochemisty: Organic C, Phosphate, Ammonium, Nitrate, Ph, Redox, Oxygen, microbial activity, Sulphate, Sulphide, Ferric (Fe3+) and ferrous
(Fe2+) Iron are important biogeochemical factors.
The ability to maintain a positive net carbon balance over time determines the survival of seagrass. Large respiratory demands are involved in
maintaining a large heterotrophic biomass (roots, rhizomes). Enabling colonization of sulphate-rich submerged anoxic sediments requires a large
amount of energy. Seagrass is able to grow on anoxic soils, and due to the high energy required, this is what explains why seagrasses are
amongst the flowering plants with the highest light requirements.
In the sulphate-rich marine environment, sulphide toxicity is a major threat. Important aspects in this respect are:
1.
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3.
4.
5.
6.
Sediment particle size
Sediment organic content
Sediment carbonate content
Redox
Fe3+, phosphate, sulphate and inorganic-N concentrations
Temperature.
In healthy seagrass beds there is a delicate balance between biogeochemical processes preventing sulphide toxicity. This balance may be
disturbed when light availability decreases, when the input of organic matter and nutrients changes or when sediment type is altered. Seagrasses
transport oxygen, which is produced by photosynthesis under the influence of light, into the rhizosphere, keeping redox levels high. The extent to
which the rhizosphere may be oxygenated depends on the rate of photosynthesis, sediment porosity (particle size) and microbial and chemical
oxygen demand. Microbial activity is directly related to organic substrate, temperature and the availability of electron acceptors.
The most important electron acceptors in this respect are, in order of preference: oxygen, nitrate, Fe3+ and sulphate. The higher the organic
content and microbial activity, the larger the demand for electron acceptors and the probability of sulphide production. Nitrate is formed by an
oxygen-consuming process of nitrification. Ammonium needed for nitrification may result from microbial mineralisation of organic matter or from
nitrogen fixation by sulphate reducing bacteria. Ferrous iron is formed from ferric iron and binds with sulphide leading to insoluble black iron
sulphide, thus reducing sulphide toxicity. However, phosphate also reacts with iron and depending on its concentration may prevent the formation
of Fe2+. In carbonate sediments, phosphate in its turn is bound the calcium carbonate. This illustrates the complexity of biogeochemistry.
As for nutrient concentration, the amount of organic matter present is not necessarily related to the amount available for the microbial community.
A tight coupling between organic matter input and fast microbial decomposition of organic matter (expedited by high temperature) may result in
low organic matter contents and high sulphide levels. In general, sufficient aeration of the sediment (biologically by bioturbation or photosynthesis;
mechanically by wave- and current-induced pressure variations) in balance with organic matter availability and microbial activity leads to healthy
conditions for seagrass settlement and development.