Production and Decay of Organic Matter

Organic matter can be produced in sediments by the photosynthesis of macrophytes and algae attached to sediment particles in the surface layers of sediments, provided sufficient light reaches the sediment. In shallow sandy sediments, wave action and currents may disturb the surface sediments, but attached algae may move toward the light if they have been buried. Organic matter is also produced by bacteria living on dissolved organic carbon and through the benthic food chain. Extensive production of organic matter takes place, for example, in mussel and oyster beds.

The dead and living organic matter in the sediment also decay by endogen respiration and mineralization after grazing. In general, the decay processes dominate in sediments. This is shown by the profiles of organic matter which show a declining concentration of organic matter with sediment depth. The decline indicates a production and import of organic matter at the sediment surface and a net decay in the deeper sediments. In the upper sediment, we may have oxygen penetrating millimeters or centimeters and allowing an aerobic decay. In the deeper sediments, oxygen is lacking and the decay is by anaerobic processes, where nitrate, sulfate, and oxidized iron and manganese compounds act as hydrogen acceptors instead of oxygen. Such anaerobic processes are slower and less efficient than aerobic processes.

The overall decay in sediments can be described by a first-order process:

dC/dt = - kC with the time-dependent solution, where Co is the initial concentration of an element, Ct is concentration at the time t, and k is a decay constant. The decay rate increases with temperature and the dependence can be described by an Arrhenius-type expression, k = k20 T-20)

where t is a constant.

Or, if a maximum and an optimum temperature are applied, k kopt e ^ P ^ (^max t)/ (┬┐max ^c

where kopt is the optimum rate constant, tmax is a maximum temperature, topt is the optimum temperature, and a is a constant.

The mineralization in the sediment is a complex mixture of many different components being metabolized by a variety of organisms. Consequently, a distribution of kt

C, = Co e decay rates and substrates should be applied rather than average values. This involves the identification of an overwhelming amount of substrates and decay rates and such an approach is rarely applicable. Overall decay rates can be determined from turnover of sedimentary organic matter calculated from the depth distribution and the sediment age. A seasonal time resolution can be achieved from temporal variations in oxygen uptake or anaerobic metabolism or simply by expecting decay rates to follow temperature.

Since sediment depth is a function of time and the decay rates are expected to decrease over time, the variability can be approached by use of partial differential equations and sediment age:

where ifi = J Co(i)e-ifi -&ij J Co(i)e-i/3di where C is the concentration of the decaying element, is the decay constant for the total element, g(t, ox) is the functions for influence of temperature t and oxygen concentration ox on biological age relative to chronological age, Z is the displacement of sediment surface relative to t= 0, z is the sediment depth expressed as a depth coordinate, and C0(i) is the distribution ofrate constants or the various fractions of the element C, when the element arrives at the sediment surface. The model can contain further equations allowing the decay and other processes to switch between aerobic and anaerobic processes according to threshold values for oxygen.

A more simple separation in three compartments can be made: an easy-degradable part of freshly settled sediment, a slow-degradable part of older sediments, and a third part of nondegradable sediment.

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