As an essential part of the ecological approach to material cycles in the biosphere, the study of the fate of organic matter is of equal importance to the study of primary production itself. For instance, the amount of seagrass production that is consumed by herbivores and decomposers sets limits to the level of secondary production that can be maintained by the bed (Cebrian and Duarte, 1998). Similarly, the capacity of seagrass beds to act as sinks of organic matter depends on how much production is left over by herbivores and decomposers and subsequently accumulated as refractory matter in the bed (Duarte and Cebrian, 1996; Mateo and Romero, 1997).
The fates that seagrass production may endure are related by the following mass-balance equations:
where ABt (the living compartment) and ADMt (the detrital compartment) are the changes in seagrass biomass and degradable detrital mass per unit time, respectively, and the other terms are NPP—the temporal rates of net primary production, H—herbivory, DP—detrital production, I—import, D—decomposition, E—export, and RA—refractory accumulation. Units for all these fates are usually reported in gDW per square meter per unit time. Equation (1) corresponds to the living compartment, and it states that any temporal change in seagrass biomass corresponds to the difference between production and the loss processes of herbivory and detrital production. Detrital production, in turn, includes wasteful removal by herbivores, exudation of dissolved organic matter, natural mortality through senescence and infectious mortality. Equation (2) corresponds to the detrital compartment, and states that any temporal change in degradable detrital mass is the difference between the gain processes of detrital production and import and the loss processes of decomposition, export, andrefractory accumulation. The following sections contain a selective discussion of the variability, controls, and consequences of the routes listed in Eqs. (1) and (2). The two main objectives of this discussion are to (i) summarize some important patterns in our current knowledge and (ii) identify gaps that merit further research.
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