Seagrass beds trap particles suspended in the water column via direct and indirect mechanisms. Leaf canopy indirectly enhances deposition of suspended particles by its interaction with water flow (see Koch et al., Chapter 8), which favours sedimentation rate (Gacia et al., 1999) and decreases resuspension of deposited particles on seagrass sediments (Gacia and Duarte, 2001). Yet, seagrass communities directly trap particles suspended in the water column. Two processes are responsible for direct particle trapping within seagrass beds: active filtering of the partic-ulate material by the suspended feeders associated with the plant community (i.e. macro suspension feeders or epibionts associated with the seagrass leaves) and passive adherence of the suspended material onto seagrass surfaces (mainly leaves).
Abundance and biomass of macro suspension feeders (ascidians, sponges, and bivalves) tend to be higher in vegetated areas when compared to bare sand. This is due to a combination of factors, including enhanced rates of recruitment within plant canopies (Duggins et al., 1990; Bostrom and Bonsdorff, 2000), shelter from predators (Peterson and Heck, 2001), and higher abundance of food availability (Peterson et al., 1984). The abundance of epifaunal suspension feeders (hydroids, bryozoans, barnacles, amphipods, spirorbids, and protozoa) is also higher in seagrass beds than in bare areas, since seagrass canopy increases the available surface for colonization. Suspension-feeder communities have been shown to control phytoplankton populations in shallow-semi-enclosed environments (Buss and Jak-son, 1981; Alpine and Cloern, 1992). Thus, active trapping of particulate organic matter (POM) from the water column by suspension feeders is expected to be higher in seagrass vegetated areas than over bare sediments.
There is very little quantitative information for direct particle trapping within seagrass canopies.
Studies in Western Australia (Lemmens et al., 1996) show that meadows of Posidonia australis were able to remove particles from the water column at much faster rates (approximately once a day) than unveg-etated sand bottoms, where densities of macro suspension feeders and epibionts were found to be significantly lower. These removal rates of suspended particles were higher in P. australis and Anphibo-lis antarctica meadows than those of Heterozostera tasmanica. Species-specific differences in particle trapping rates may be due to differences in (a) canopy surface, which limits the area of substrate for epibiont colonization, and (b) leaf life-span, which sets the time window for epibiont colonization and, thus, constrains maximum epibiont biomass development (Cebrian et al., 1994). In addition, local environmental conditions may also restrict the development of filter feeding assemblages. In meadows of P. australis mentioned above, the epibionts were the dominant filtering community comprising 76% of the filtering activity of the system (Lemmens et al., 1996).
Direct particle trapping (including both passive and active mechanisms) has been reported for a seagrass mixed meadow in the Philippines (Agawin and Duarte, 2002). In situ incubations of seagrass and bare sediment areas with labelled phytoplankton and labelled inert particles demonstrated that water column clearance rate was four times faster within seagrass canopies than in bare sediments at 1.5 m depth. The authors estimated approximately 5% of the filtering capacity was due to the activity ofproto-zoan epibionts (ciliates and amoeba-like organisms), whereas the largest fraction of particles was trapped by passive particle adherence on leaf surfaces.
Direct passive particle trapping has been estimated in seagrass meadows from South East Asia by quantifying the inorganic particles adhered to seagrass leaf surfaces across a wide range of sediment deposition rates (Gacia et al., 2003). These data did not quantify the particulate organic fraction of the passive trapping mechanisms but it provided rates of non-carbonate mineral clearance from the water column ranging from 0.1 to 0.6 g DWm-2 d-1 across meadows. These rates, however, represented only a minor fraction (<1%) of the total non-carbonated inorganic material suspended in the water column, since the studied sites supported high siltation rates (Gacia et al., 2003). The clearance capacity of sea-grass meadows by passive particle trapping depends on, for instance, seagrass productivity, local hydro-
dynamic conditions, and biomass and composition of the epibiont community, which is expected to enhance passive particle trapping by increasing the excretion of exopolymeric substances.
The above-mentioned mechanisms of particle trapping increases the particulate organic matter pool in seagrass sediments, since the structure of the seagrass leafcanopy and rhizosphere prevent resuspension and erosion of the material deposited on sediments interface.
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