Particulate Organic Matter in Seagrass Sediments

The positive effects of seagrass beds on particle deposition, together with the retention of most particulate organic matter produced by the bed itself (see Mateo et al., Chapter 7), enrich seagrass sediments with particulate organic matter (POM) compared to unvegetated areas. The percentage of POM in sea-grass sediments often accounts for approx. 4% of sediment DW (e.g. Morse et al., 1985; Boschker et al., 2000; Hemminga and Duarte, 2000; Enriquez et al., 2001). However, the content of POM in seagrass sediments, in general, is lower than the organic content of coastal sediments colonized by other communities (e.g. mangroves and coral reefs; KampNielsen et al., 2002; Kennedy et al., 2004).

The sources of POM in seagrass sediments are seston, macroalgae, epibionts, and seagrass detritus. The relative importance of one or another source depends on different processes including meadow production, export, and decomposition rates (see Chapter 7 for extensive explanations), eutrophication, coastal erosion, and overall water flow in the area. Studies of the contributions of different sources of organic matter in seagrass sediments indicate that a significant fraction of the organic matter is refractory (i.e. 56-84% in a P. oceanica meadow; Danovaro, 1996), while only a minor proportion (18%) is recovered in the biopolymeric fraction (lipids, carbohydrates, and proteins). Danovaro (1996) traced the origin of the labile fraction of particulate organic matter and found that 25% of this fraction derived from ben-thic microphytoplankton. Other studies, however, revealed the importance of seston as organic matter source in seagrass sediments. Gacia et al. (2002) estimated that 43% of the organic carbon in the sediment of a 15 m depth P. oceanica meadow derived from the seagrass and epibionts, while the remaining fraction was provided by seston. Similar percentages

O Seagrass A Macroalgae ■ Sediment

Zm1 Zm2 Zm3 Zm4 Zn1 Zn2

Fig. 2. Results of field studies of carbon sources used by bacteria in different seagrass beds. Stable carbon isotope ratios (513C) of phospholipids fatty acids of sedimentary bacteria were compared with <513 C ratios of the potential carbon sources found at the locations (seagrass, benthic macroalgae or sediment organic matter). Shown are the differences in the stable carbon isotope ratio between sediment bacteria and the potential carbon sources. Four studies of Zostera marina (Zm1-4), two studies of Zostera noltii (Zn1-2) and one study of Cymodocea rotundata (Cr) and Thalassia hemprichii (Th) are given. Carbon ratios of benthic microalgae were also analyzed at all sites, and were found to be important at the Zostera sites, but not for C. rotundata and T. hemprichii. Data are compiled from Boschker et al. (2000) and Holmeret al. (2001).

of seagrass material in sediment carbon pool (up to 30%) were observed in sediments colonized by Thalassia hemprichii in Kenya (Hemminga et al., 1994). Thus, seagrass material represents an important fraction of the organic matter accumulated in vegetated sediments. Seagrass-derived carbon, however, is not always the most important component in seagrass POM sediment pool, since seagrass meadows represent an important sink of organic matter from nearby ecosystems (Gacia et al., 2002; Kennedy et al., 2004).

The POM in seagrass beds enhances sediment mi-crobial activity when compared with that in bare sediments (Danovaro and Fabiano, 1995; Danovaro, 1996; Donnelly and Herbert, 1999; Nielsen et al., 2001). Analysis of the isotopic composition of bacteria-specific lipids (phospholipid fatty acids, PLFA's) has shown that the microbial utilization of seagrass organic matter is important in olig-otrophic sediments (Jones et al., 2003; Holmer et al., 2004), such as in Cymodocea rotundata and Tha-lassia hemprichii meadows in low-nutrient tropical sediments (Fig. 2). The isotopic carbon fractiona-tion in the bacteria specific PLFA's extracted from the rhizosphere sediment in the two seagrass meadows were similar to the isotopic carbon fractionation of both seagrasses suggesting that seagrass detritus was an important organic carbon source for the bacteria (Holmer et al., 2001). There was no correlation between the benthic microalgae or other primary producers present in the seagrass meadows and the bacterial isotopic signal, suggesting that these carbon sources were not utilized to a large degree by bacteria. On the other hand, under more eutrophic conditions, seagrass detritus does not appear to be the most important bacterial carbon source. Under these conditions benthic microalgae, macroalgae, and phytoplanktonic detritus appear to be preferentially utilized by the bacteria probably due to their increased abundance under eutrophic conditions and higher lability compared to seagrass detritus (Fig. 2; Boschker et al., 2000; Holmer et al., 2004).

A fraction of seagrass detritus, mainly leaves with epiphytes, can be found accumulating in either depressions of the seafloor, near seagrass meadows, or on marine sediments further away, such as beaches, and small harbors (Hemminga et al., 1991; Romero et al., 1992; Fabiano et al., 1995; Hemminga and Duarte, 2000). Seagrass detritus has also been found in the sediments of emergent dunes in Mauritania (Hemminga and Nieuwenhuize, 1990) and Western Australia (Kirkman and Kendrick, 1997), and in the sediments of adjacent ecosystems, such as mangrove forests (Hemminga et al., 1994). Hence, seagrass beds provide POM to adjacent coastal areas, which is expected to influence biogeochemical processes and the structure of systems in the vicinity (see Bell et al., Chapter 26).

The effect of seagrass beds on POM sediment enrichment, however, changes during meadow development. Because production of seagrass detritus and seston deposition rate increase during seagrass colonization (Cebrian et al., 2000; Barron et al., 2004), and the slow mineralization (Mateo et al., Chapter 7) of the seagrass detritus retained in seagrass sediments, seagrass sediments become POM enriched during the life-span of the meadow (Pedersen et al., 1997). For instance, total POM accumulated in the sediments colonized by C. nodosa at Alfacs Bay (E Spain) increased to 96.9 ± 37.9 gm-2 year-1 (Pedersen etal., 1997) during meadow development. Similarly, it has been demonstrated that export of seagrass detritus to adjacent coastal systems also increases as meadows develop (Cebrian et al., 2000). Detrital C. nodosa leaf export in established

meadows was three-fold higher than that in young meadows (Cebrian et al., 2000). The role of sea-grass beds as a sink, or a source of organic matter to adjacent sediments, therefore, varies during the colonization process.

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