Export

The export of materials from seagrass beds has many important implications for surrounding communities and ecosystems (Romero et al., Chapter 9; Kenworthy et al., Chapter 25; Bell et al., Chapter 26). Since most exported detritus is decomposed in downstream systems (Mann, 1988), the quantity of detritus exported sets the limits to the levels of secondary production that the bed can support beyond its boundaries (i.e. allochthonous secondary production; see Chapters 25 and 26). Since export represents a nutrient loss for the bed, these losses must be compensated by exogenous nutrient inputs (Duarte and Cebrian, 1996; Mateo and Romero, 1997; Romero et al., Chapter 9).

Despite the importance of export in seagrass-dominated coastal ecosystems, few reports exist on its impact on bed economy. This oversight may be due to three inherent technical difficulties.

First, and most importantly, seagrass beds are often open systems and have widespread exchange with offshore waters, driven by the interaction of several forces, including wind, tides, and waves. This renders measurement of detri-tal export difficult. Most measurements of de-trital export are limited to specialized systems connected to open waters through narrow outlets (e.g. coastal lagoons).

Second, the boundaries of seagrass beds, which define the location at which export measurements are taken, are sometimes difficult to define with certainty, making measurements somewhat arbitrary.

Third, detrital traps used to derive direct estimates of export are difficult to deploy.

These methodological limitations have discouraged researchers from quantifying detrital export from seagrass beds and have resulted in a scarcity of publications on the issue. These problems emphasize the importance of developing alternative methods. Romero et al. (1992) proposed an indirect method: in the hypothetical absence of export, litter stocks in the bed depend on inputs from leaf fall (the main source of variation being depth) and outputs due to remineralization. Leaf fall rates can be estimated as the difference between primary production (using the method described in Zieman, 1974) and biomass increase. Decay rates can be approximated using the classical in situ litter bag incubations. This can be expressed mathematically in order to predict litter accumulation in a given moment and a given place in the bed.

where L i is the predicted standing litter at time i, Fj is the weight of leaf fallen between times i and i — 1, t is the time interval between consecutive samplings, k is the decay rate for this period and area (e.g. depth), and Li—t is the standing litter observed in situ at time i — t (i.e. before the initiation of the period). Knowing the standing litter stock at the end of the period Li (sampled in the field), export (Ei) can be calculated as the difference

Such an approach entails intensive field effort, requiring estimates of leaf input, litter decay, and litter stocks throughout the year, and it possibly underestimates discontinuous export events. Nevertheless, all the methods required are robust, easy to apply, and integrate changes over long time. Therefore, this should be a useful integrative approach for future studies.

The few reports indicate that export can vary from 0 to 100% of total production (e.g. Bach et al., 1986; Hemminga and Nieuwenhuize, 1990; Stapel et al., 1996; Mateo and Romero, 1997; Ochieng and Erfte-meijer, 1999; Hemminga and Duarte, 2000). This large variability results from the high variability of the intensity of physical energy in the bed, the major driving force (Josselyn et al., 1983; Bach et al., 1986; Fry and Virnstein, 1988; Mateo et al., 2003; Mateo and Rossi, submitted; Section II.A of Koch et al., Chapter 8). Weather, tides, and the degree of bed exposure (i.e. area of open water or fetch and openness to offshore waters) dictate this intensity. The crucial role of physical energy is shown by supra-littoral deposits in different ecosystems. The largest accumulations of seagrass leaf litter cast on beaches have been reported in a small Mediterranean exposed bay (Tabarca Island, Alicante, Spain) for the species P. oceanica (Mateo et al., 2003; Fig. 5, left and top right); the distribution and height of the deposits ('banquettes') accurately described the water energy reaching the perimeter of the bay, with leaf litter accumulation in amounts from 18 to 500 kg of dry wt.(m shoreline)-1, at both ends and in the center of the bay, respectively (see also Kuo and den Har-tog, Chapter 3 for P. australis examples). The authors estimated for the P. oceanica example that the total supralittoral deposits represented 50.7, 71.0, 27.2, and 8.7% of the annual bed dry weight, carbon, nitrogen, and phosphorus production, respectively. They concluded, however, that the deposits were only temporary sinks because the accumulated material can eventually return to the water (Fig. 6). At the other extreme, export figures for C. nodosa leaf litter in a semi-enclosed estuarine bay (Alfacs, Tarragona, Spain) were found to be almost negligible due to the rapid wave energy dissipation against the embayment shore (the relative proportions be ing 0.26, 0.27, 0.27, and 0.16% of the annual bed dry weight, carbon, nitrogen, and phosphorus production, respectively; Mateo and Rossi, submitted). Some seagrass species have long, bulky leaves that sink soon after shedding, whereas others produce light, thin leaves that can float for long periods before sinking and are thus, more likely to be exported. Zieman et al. (1979) provided the first example of the importance of leaf buoyancy. They compared adjacent beds of the relatively broad-leaved turtle grass (T. testudinum) with beds of the thin-leaved manatee grass (Syringodium filiforme), and showed that, whereas turtle grass exported 1% of its leaf production, manatee grass exported 75%.

Export of below-ground parts is rarer and only strong storms can carry significant amounts out of the bed or throw them onto the beach (Bach et al., 1986; Fig. 5, bottom right).

In many temperate systems, autumn is characterized by high absolute amounts of litter export because many seagrass species shed most of their leaf biomass at this time (Cebrian et al., 1997; Mateo and Romero, 1997; Hauxwell et al., 2003). Accordingly, Bach et al. (1986) surveyed leaf export from an eelgrass bed in Phillips Island (NC, USA) monthly over 1 year and found the greatest levels of absolute export in late August, which was also the period of maximum leaf shedding. However, ecologically meaningful export rates are those relative to detritus production or to plant requirements. In a seasonal study of leaf litter export in a P. oceanica bed, Mateo and Romero (1997) found that the highest export losses relative to detritus production occurred from February-May although maximal litter stocks were recorded during July-October.

The nutritional quality of seagrass leaf litter is often strongly correlated with decomposition rates, which in turn influence export (Mateo and Romero, 1997; Perez et al., 2001). Thus, seagrass leaf litter nitrogen content is often positively correlated with decomposition rates although contradictory results abound (see Section III.C): if decomposition is slow there is the greater likelihood of export or burial (see Section IVB.1). Two P. oceanica beds, one off Medes Islands (NW Mediterranean, Spain) and another off the Island of Ischia (Naples, Italy), both located in open areas and at similar latitudes, had a three-fold difference in export rates (higher at Ischia). The effect of waves and currents affecting both beds seemed to be different, and the different export rates were most probably associated

Fig. 5. Supra-littoral deposits of Posidonia oceanica. Left: Old 'banquettes' of P oceanica leaf litter along the coast of Nueva Tabarca Island (Alicante, Spain). The banquettes shown are ca. 1.5 m high (photograph by J. L. Sanchez-Lizaso). Top right: Recently formed banquettes 0.5 m high from Corsica, France (photograph by M. Manzanera). Bottom right: Beach-cast detritus from P. oceanica below-ground organs. Rhizomes and a sheath-derived aegagropile can be distinguished (photograph by M. Manzanera).

Fig. 5. Supra-littoral deposits of Posidonia oceanica. Left: Old 'banquettes' of P oceanica leaf litter along the coast of Nueva Tabarca Island (Alicante, Spain). The banquettes shown are ca. 1.5 m high (photograph by J. L. Sanchez-Lizaso). Top right: Recently formed banquettes 0.5 m high from Corsica, France (photograph by M. Manzanera). Bottom right: Beach-cast detritus from P. oceanica below-ground organs. Rhizomes and a sheath-derived aegagropile can be distinguished (photograph by M. Manzanera).

with the different nutrient content of the leaf litter (Table 2). Nitrogen and phosphorus contents of the coarse leaf litter in Ischia were on average 0.42 and 0.039%, respectively. Equivalent figures for Medes were 1.24 and 0.067%, respectively, which are 3.0 and 1.7 times higher than those at Ischia, suggesting that leaf litter 'palatability' may be an important factor governing export rates in seagrass beds.

In comparing nutrient-rich and nutrient-poor C. nodosa stands growing in a semi-enclosed bay (Al-facs, Ebro River estuary, Spain), Perez et al. (2001) obtained differences in export rates that largely support the previous contention (Table 2). Around 53% of the total annual production of plant biomass was exported in poor stands, while in rich stands this value was 3.4 times lower (15.5%). Nutrient losses were much higher in nutrient-poor stands when compared to plant nutrient requirements (Table 2).

The large variability found in the percentage of leaf production exported suggests that seagrass beds may also vary widely in their levels of dependence on imported nutrients, from negligible (i.e. beds that export <10% of leaf production) to high (i.e. beds that export >80% of leaf production) levels. On the other hand, when export is regarded as an absolute flux (Fig. 3D), another important corollary arises: in spite of substantial variability, most values of absolute export tend to be large when compared with the amount of seagrass biomass that is consumed by herbivores (Fig. 3B).

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