In the tropical and subtropical shallow coastal areas where Thalassia occurs, water-column dissolved inorganic concentrations are generally low (PO^- < 1^M and NH+ + NO- < 3^M; Tomasko and Lapointe, 1991; Agawin et al., 1996; Ziegler and Benner, 1999), with exception of areas with high river inflow or which are frequently exposed to the air during low tides (Erftemeijer and Herman, 1994; Stapel et al., 1997). The majority of the N and P pools in Thalassia meadows are present in the sediments, adsorbed to sediment particles or bound in organic matter and therefore largely unavailable to the plants (Erftemeijer and Middelburg, 1995; Stapel et al., 1996a; Koch et al., 2001). Jensen et al. (1998), for example, reported that total sediment P in the upper 20 cm in subtropical carbonate sediment in Bermuda was 500-fold larger than the pool of P dissolved in pore waters. Pore-water NH+ concentrations in meadows of both Thalassia species usually vary between 2 and 200 |xM (Erftemeijer, 1994; Lee and Dunton, 1999b; Table 1). But exceptionally high figures (of up to 800 |xM) have been reported in T. hemprichii meadows from the East African coast (Erftemeijer and Herman, 1994; Stapel et al., 1996a; Marba et. al., 2002), and in T. testudinum beds in the Laguna madre (Lee and Dunton, 2000). Pore-water PO4- concentrations are generally higher in T. hemprichii beds (up to 35 |xM) compared to T. testudinum meadows (<1.5 |xM; Fourqurean et al., 1992a; McGlathery et al., 2001). The grain size of the sediments, determining diffusion rates of nutrients, as well as the sediments' origin (terrigenous or marine) and the so far poorly studied biogeochemical interactions, may be an important factor controlling nutrient availability in pore water, as has been put forward by Erfte-meijer et al. (1994), Erftemeijer and Middelburg (1995) and Stapel et al. (1996a) for T. hemprichii. The pore-water chemistry is also affected by current velocity of the overlaying water column. Under stagnant water conditions, sulfide levels may increase in pore water, while high flow rates may lead to reduced nutrient concentrations (Koch, 1999a). Nutrient concentrations in Thalassia beds show annual fluctuations in sediment pore water, in the water column, as well as in the plant material, which are due to seasonal influences (Erftemeijer and Herman, 1994; Lee and Dunton, 1999a,b; Ziegler and Benner, 1999).
The oligotrophic waters in which Thalassia proliferates have very low concentrations of dissolved inorganic nutrients, but in these waters, total dissolved nutrient pools (including dissolved organic nitrogen and phosphorus) may be up to five times higher than the total concentration of the dissolved inorganic nutrients (Hansell and Carlson, 2002). The availability of the dissolved organic nutrients to Thalassia is not well known and is likely to be variable because it depends on the composition and molecular size of the compounds. Little is known concerning the uptake of organic nutrients by Thalassia or any other seagrass (see Romero et al., Chapter 9), but other submerged primary producers such as marine algae and bacteria and various macrophytes are known to assimilate organic nutrients (Lipson and Nasholm, 2001).
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