Stable isotope studies have identified the benthic algae as the primary carbon source in some seagrass systems. However, it is still not known what factors determine the relative importance of phytoplankton, seagrass, and benthic micro- and macroalgal production in trophic dynamics. Fry et al. (1987) hypothesized that nutrient availability may determine which primary producers supply the bulk of the organic matter to food webs, suggesting that, under eutrophic conditions, ample nutrients would foster extensive growth by the benthic microalgae and phy-toplankton, leading to a food web driven by algal production. This prediction is supported by the responses of some primary producer to elevated nutrients (Short and Burdick, 1996; see also Walker et al., Chapter 23 and Ralph et al., Chapter 24). However, in view our lack of understanding of mechanisms that affect food web dynamics and the current threat of eutrophication to seagrass systems, it is essential that the much more thorough investigations be carried out.
The role of nutrients is also important because, with eutrophication, the growth of epiphytes and phytoplankton are favored at the expense of sea-grass production, typically resulting in the loss of seagrass cover at high nutrient inputs when phy-toplankton and epiphytes shade out seagrass systems (Kemp et al., 1983; Borum, 1985. However, the effects of nutrients on epiphytes and phyto-plankton are complex (see Borowitzka et al., Chapter 19 and Walker et al., Chapter 23). Analyzing the isotopic compositions of seagrass residents under various nutrient regimes may permit identification of differences in the flow of N and C through the food web, as changes in the nutrient dynamics may be expected to cause shifts in the relative contribution of organic matter by various primary producers to higher trophic levels. The isotope addition experiments magnify differences in the iso-topic composition among producers and allow better resolution of C and N flows from primary producers to consumers (Peterson et al., 1985, 1993). Employing an experimental manipulation to generate distinct S15N values for seagrass and its epiphytes, Winning et al. (1999) were the first to use isotope additions. By adding 15N-enriched potassium nitrate to mesocosms containing Z. marina and its epiphytes, they were able to produce significantly changed S15N values for these two primary producers. This demonstrated the potential of manipulating isotope values in the field to resolve trophic relationships and such an approach can actually solve another serious problem that limits the use of S15N as a tracer of organic matter through simplified food webs in mesocosms studies (see next three paragraphs).
Fry et al. (1987) observed that the sulfur and carbon isotopic compositions for consumers and their diets were similar, but the S15N values of consumers were on average 3.2%o greater than that of their diet (as a consequence of excretion of 15N-depleted nitrogen). In many subsequent studies, this average value was subtracted from consumers to infer potential diets; however, this practice is ill advised as the range of variation is very wide (from 0 to 6%o; Fry et al., 1987). While it is tempting to simplify, a careful study of seagrass associated food webs based on stable isotopes requires detailed knowledge of (i) fractionation phenomena associated with metabolic assimilation, (ii) seasonal variability in isotopic ratios, and (iii) variability in isotopic ratios between plant parts (see Vizzini et al., 2003 for the two last items).
Mutchler et al. (2004) have developed a field methodology for the isotope addition experiments, based on the mesocosm work of Winning et al. (1999). In these experiments, 14N-labeled (i.e. 15N depleted), slow-release fertilizer is used to both simulate eutrophication and generate differential iso-topic compositions of H. wrightii and its epiphytes. After only 20 days of exposure to water column enrichment, the S15N values of the epiphytes were significantly different from those of the seagrass (—78%o vs. —31%o, respectively, Fig. 11).
Although this methodology was developed to address food web dynamics under eutrophic conditions, the approach could easily be modified to generate isotopic tracers under ambient nutrient conditions. By actively 'labeling' organisms within seagrass beds, one can trace not only the flow of organic matter to higher trophic levels, but through creative isotope additions, investigate the degree of movement of 'labeled' organisms and assess the extent to which these organisms facilitate the retention or export of organic matter within and between seagrass beds and other habitats.
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