Nutrient fluxes are essential components of the material and energy flow that sustains ecosystems. Nutrient-seagrass interactions can be viewed under, at least, three perspectives: (i) nutrient dynamics in seagrass beds (how nutrients enter, how they are used and recycled, and how they leave these ecosystems), (ii) influences of nutrient abundance on seagrass beds (how changes in nutrient supply alter functional aspects of the plant and modify ecological interactions), and (iii) influence of seagrass beds on nutrient cycles (how seagrasses modify the general nutrient fluxes in coastal waters). This chapter has focused mainly on the first two, and it is clear that a good understanding of them requires a multilevel approach, encompassing perspectives ranging from physiological, and even molecular, to ecological, and even bio-geochemical. The effects of nutrient additions provide a good example of this hierarchical nature of seagrass-nutrient interactions, which includes from changes in physiology to changes in the community interaction network.
A large body of work has been done in this field, which is, certainly, a favorite topic in seagrass biology. In this chapter, which was not aimed at providing a thorough and exhaustive literature review, around 150 references (strictly concerning seagrasses) have been cited, evidence in itself of a sustained research effort. Although this research effort has allowed a progressive understanding of the different issues expressed above, large gaps, concerning almost any of the subjects addressed (e.g. within-plant transport mechanisms, microbial processes in the sediment, organic and inorganic nutrient accumulation, etc.) are evident, and it is clear that sustained effort is still needed to fill these gaps. It has also to be acknowledged that despite the work done by seagrass biologists, we are at least a step behind what is known concerning nutrient-plant interactions in terrestrial ecosystems. The latter is a great source of new ideas and techniques. It is rather difficult to foresee the best direction for fruitful research in the coming years. Most probably, several approaches will produce significant advances. However, one has the impression that the most exciting endeavor in this field would be the attempt of a large, ecosystem-level, experiment in which multidisciplinary teams would address different aspects of seagrass-nutrient interaction from the three perspectives listed above. This is more a dream than a guess. In any case, basic research should aim to explore the less known aspects identified in this chapter (and, desirably, also others). Moreover, there is an increasing demand for knowledge to be applied to seagrass conservation (see Kenworthy et al., Chapter 25). Since anthropogenic eutrophication is one of the main threats to seagrass ecosystems, providing society with this knowledge falls within the scope of the topic of this chapter; and is a basic duty of the scientific community.
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