While there has been intense study of the refractory biomacromolecules in terrestrial and freshwater environments (e.g. Derenne and Largeau, 2001), in seagrass sediments this aspect has received little attention. Moreover, what little information there is refers to carbon fractions and not to nutrients associated to refractory particulate or dissolved organic matter. There is evidence on the abundance of refractory compounds in seagrass sediments (Posi-donia oceanica, Danovaro, 1996), and the quantitatively most important is probably lignin. Nevertheless, these compounds, formed by long chains of phenolic compounds, are basically a carbon sink as no nutrients, apart from C, are linked to them. However, both N and P appear (or are likely to appear) bound to refractory organic compounds. In effect, available data indicate that organic pools represent a substantial part of sediment P (>50% in Florida Bay, Koch et al., 2001), and most of it is probably in refractory forms. Concerning N, it has been found recently that nitrogen from intrinsically labile amino acids can be preserved in the sediments of continental margins over thousands of years in refractory networks of peptide-like material (Grutters et al., 2002). Since one-third of all the amino acids found in sediments could be directly derived from bacterial cell walls, given the relatively high bacterial activity observed in seagrass sediments (Lopez etal., 1995; Danovaro, 1996), it is reasonable to think that this could be a relevant mechanism for nitrogen immobilization in seagrass sediments.
Factors controlling the burial rate of nutrients in these refractory pools are far from understood; species-specific differences in biochemical composition, oxygen availability, bacterial activity (that can be in turn nutrient-limited), origin of the material (since part can be allochthonous, i.e. of terrestrial origin, or from nearby ecosystems such as man groves), and temperature are just some of the potential controls to be considered (Henrichs, 1993; Lopez et al., 1995; Mateo and Romero, 1997; Danovaro et al., 2002; Mateo et al., Chapter 7). A very peculiar case of burial occurs when the seagrass below-ground organs are stored with no or only slight decomposition, and accumulate as peat-like deposits. This fact has been reported only for a very small number of seagrass species. Such species are, as far as we are aware, Thalassodendron ciliatum (Lipkin, 1979), Posidonia australis (Shepherd and Sprigg, 1976), and Posidonia oceanica (Boudouresque et al., 1980; Romero et al., 1994; Mateo et al., 1997). Only for this last species has the magnitude and dynamics of the deposits been studied, and key aspects concerning their importance for carbon are summarized elsewhere in this book (Mateo et al., Chapter 7). The buried dead materials (rhizomes, leaf sheaths, and roots) of P. oceanica have a nutrient content of 20-50% N and 5-20% P, compared with the original living material. This decrease in nutrient concentration takes place very quickly (3-5 yr, Romero et al., 1992). No further changes seem to occur, since the N and P concentration in material 10-20 yr old does not differ from that found in material more than 1,000 yr old. Since bulk decay rates of this detritus have been estimated to be extremely low, between 0.00008 and 0.00036 yr-1 (Mateo et al., 1997), it is concluded that nutrient release from it is very low, and that these deposits are long-term nutrient sinks, representing net losses of 0.8 and 0.044 g of N and P m-2 yr-1, or, roughly, 7% and 5% of the total plant N and P, respectively, annual requirements for plant growth.
Future efforts in the study of the burial of seagrass production should be devoted to the characterization of the refractory organic matter in general but, more specifically to those compounds containing nitrogen and phosphorus. Nevertheless, the general conclusion so far is that the amount of nutrients lost in terms of burial is modest in terms of the ecosystem budget. However, the role of seagrass beds as nutrient sinks should not be disregarded, especially in the context of coastal waters facing problems of eutrophication (see Marba et al., Chapter 6).
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