The loss of nutrients from a Thalassia meadow, like that from other seagrass meadows, occurs primarily through denitrification (Shieh and Yang, 1997), sediment resuspension (Koch, 1999b), the export of Thalassia leaves (Zieman et al., 1979) and her-bivory (Heck and Valentine, 1995; Valentine and Duffy, Chapter 20). These processes are strongly related to leaf senescence (Hemminga et al., 1991), water flow and turbulence (Koch and Gust, 1999) and herbivore action ('sloppy feeding'; Valentine and Heck, 1999). Fauna can consume up to 25% of the annual leaf production (Klumpp et al., 1993), which is a substantially larger loss than the water flow export of leaf material from Thalassia meadows, estimated to be maximally 10% (Greenway, 1976; Stapel et al., 1996b). Nutrients in Thalas-sia seagrass meadows are replenished by the accumulation of sestonic particles in the meadow (Agawin and Duarte, 2002), sedimentation of allochthonous material (Almasi et al., 1987) and nutrient uptake by seagrasses and other primary producers (Wear et al., 1999), especially diatoms (Ster-renburg et al., 1995). These processes are all controlled by water movements (Koch, 1994; Thomas et al., 2003). N2-fixation, which is positively related to the belowground biomass (Capone and Taylor, 1980), may contribute up to 8% of the N requirement of Thalassia (Hemminga et al., 1991; Moriarty and O'Donohue, 1993). Patriquin (1972) argued that essentially all the fixed nitrogen requirements of T. testudinum meadow could be supplied by heterotrophic nitrogen fixation within the rhizospheric sediments.
While processes of nutrient loss are relatively well defined, those involved in nutrient gains and retention in Thalassia meadows are far less clear. The persistence of often highly productive Thalassia beds in oligotrophic environments has therefore been considered as rather paradoxical. In these environments, Thalassia should possess efficient nutrient conservation mechanisms, but Hemminga et al. (1999) reported that T. hemprichii appeared to be rather wasteful with nutrients, and the low efficiency of utilization of internal nutrient pools seemed to be more characteristic for species of nutrient-rich areas rather than of nutrient-poor habitats. The main strategies for efficient internal nutrient conservation are considered to be a prolonged leaf lifespan (Hemminga et al., 1999) and internal resorption of nutrients from senescent leaves (Nienhuis et al., 1989; Hemminga et al., 1991). The average leaf lifespan for Thalassia is ~50 days, which is rather short compared to other (sub) tropical seagrass species, like Enhalus and Sy-ringodium (~100 and 90 days, respectively; see review by Hemminga et al., 1999). Stapel and Hemminga (1997), furthermore, found for T. hemprichii that the maximum potential resorption of nitrogen and phosphorus from senescent leaves was equivalent to 18-28% of the plants' N- and 31% of the plants' P-demand, respectively. Usually, only 5676% of these maximum values could be realized, because of loss of leaf tissue due to leaf fragmentation and premature detachment. Stapel et al. (2001) reported evidence for efficient external re-use of nutrients via the detrital pathway. The uptake of nutrients by the leaves probably plays a very important role in this process by efficiently recapturing the nutrients that are released in the water column from decomposing Thalassia leaves, and thereby contributing to nutrient conservation for the Tha-lassia meadow as a whole. This external re-use of nutrients is closely related to nutrient acquisition via the trapping of seston particles in the meadow and nutrient uptake by epiphytic and epibenthic primary producers. These organic nutrient pools are eventually regenerated in the sediments and the liberated nutrients then become available for root uptake and, after diffusion, for leaf uptake. The capturing and retention of these particulate organic nutrient pools depends on Thalassia bed size. Increasing Thalassia bed size may therefore also coincide with increasing nutrient retention, which, especially in nutrient-poor environments, may increase chances of survival. The highly dynamic conditions of the marine coastal environment, causing premature loss of leaves and leaf fragments, may have favored the development of an efficient nutrient conservation strategy outside the living plant over internal conservation strategies.
Research into the nutrient dynamics of T hemprichii and T. testudinum meadows are, like their geographic distribution, strikingly isolated from each other. For a better understanding of the nutrient dynamics of Thalassia, comparative research should be carried out, including studies along the East African coast. Important aspects that are less well understood in the nutrient dynamics of Tha-lassia and that should receive considerable attention are the role of the microbial community in the regeneration of nutrients from Thalassia tissue, the role of other primary producers, especially microalgae (diatoms), in fixing nutrients in the system, and the role of microelements, especially iron. The sediment compartment with its complex biogeochemical interactions remains, so far, under investigated, and only recently has received serious attention (Holmer et al., 1999; Terrados et al., 1999b; Enriquez et al., 2001; Holmer et al., 2001). Also, considerably more research is needed to link the sedimentary and nutrient regeneration processes to nutrient absorption by the living seagrass.
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