Decomposition

Decomposition in situ seems to be the most probable fate for seagrass leaf detritus (Fig. 3E and F) and even more so for below-ground production. Sea-grass rhizomes and roots are consumed by few herbivores (Valentine and Heck, 1999) due to the compactness of the tangled web that these organs form and because they are usually buried (particularly in large bodied species) (but see Valentine and Duffy, Chapter 20) for another viewpoint on the recent geological past and evolutionary consdierarions). As a consequence, in the absence of sirenians, etc, most

Fig. 6. Formation and destruction of Posidonia oceanica 'banquettes' in the Mediterranean Sea. Leaf litter accumulates on the shore by the action of strong waves forming deposits more than 2 m high. Once the maximum height has been attained, moderate wave action erodes the base of the banquette that partially collapses, returning material to the water (from Mateo et al., 2003).

Fig. 6. Formation and destruction of Posidonia oceanica 'banquettes' in the Mediterranean Sea. Leaf litter accumulates on the shore by the action of strong waves forming deposits more than 2 m high. Once the maximum height has been attained, moderate wave action erodes the base of the banquette that partially collapses, returning material to the water (from Mateo et al., 2003).

below-ground production becomes detritus and decomposes within the bed, with a usually modest fraction being accumulated as refractory material (see section IVB).

In general, the total (the sum of above- and below-ground) seagrass production that is decomposed in situ is large. The absolute amount of seagrass detritus transferred to decomposers and detritivores tends to be much larger than for many other aquatic and terrestrial producers (Cebrian, 1999, 2002). These patterns suggest two important corollaries. First, assuming that seagrass production is in steady-state

(i.e. no noticeable changes across years), these results suggest that most seagrass production is supported through internal nutrient recycling. Second, seagrass beds seem to maintain high levels of secondary production by microbial decomposers and invertebrate detritivores. Thus, the abundant faunal populations that are normally associated with sea-grass beds are supported mainly through the detritus-based food chain (see Section II.D.1); notice, however, that this neglects herbivory on algal epiphytes and benthic micro- and macroalgae, rich sources of food for fish in their nursery stages (see below and Borowitzka et al., Chapter 19; Valentine and Duffy, Chapter 20; and Gillanders, Chapter 21).

Seagrass decomposition can be highly variable, accounting for 15-95% of plant production (Fig. 3E) and the absolute flux to decomposers varies from 55 to 1150 gC m—2 year-1 (Fig. 3F). Harrison (1989) examined the extent of decomposition variability within and among seagrass species and discussed some factors responsible for that variability. His analyses pointed to three major factors (see next sections).

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