**5. Isotope systematics of δ15N in macroalgae**

In order to differentiate potential sources of nitrogen to the reef zones we analyzed the nitrogen isotopic composition (δ<sup>15</sup>N) of the tissue of several species of macroalgae collect‐ ed from reefs studied. The δ<sup>15</sup>N of macroalgae has been a widely used as tracer of nu‐ trient dynamics [27] This approach has been applied mainly in areas where nutrient sources are diffuse or little obvious, but also in areas where sources are very different, such as nutrient inputs from sewage. However, the spatial extent of its influence is not clear [28-29].

The δ15N values of macroalgae from the different reefs of the three national marine parks are shown in Figure 8. The more common genus found in the study area were *Dyctiota* spp. and *Halimeda* spp., followed by *Penicillus pyriformis*, *Ulotrix* spp. and the seagrass *Thalassia testu‐ dinum*. We grouped all species by location and compared the average-δ<sup>15</sup>N (± 1SD) of macro‐ algae between locations and sampling period (Figure 8a for winter-2006, and Figure 8b for summer-2007). For reference, we have included in this figure the average nitrogen isotopic composition of nitrate (δ15NO3 = 4.37 ± 2.5 ‰) for the three parks (Carriquiry unpublished data). The validity of this reference comparison rests on the assumption that nitrate is a ma‐ jor source of nitrogen for macroalgae.

Conservation and Sustainability of Mexican Caribbean Coral Reefs and the Threats of a Human-Induced Phase-Shift http://dx.doi.org/10.5772/54339 41

for summer) is between 2-7 times higher than the DIN average measured in Puerto Morelos, the coral reef area with the highest average DIN concentration (2.15 ± 0.84 μM) of our study area. These results, however, were expected because this lagoon system receives wastewater from the surrounding developments. After a few decades of continuous supply, there are now evident signs of eutrophication [25-26]. In spite of this situation, our results indicate that reef areas developing outside this lagoonal system are not affected, so far, in their hy‐

Favorably, the reef systems along the Mexican Caribbean coast still thrive under low nu‐ trient concentrations. However, the low concentrations of DIN in the coastal waters and the evident overgrowth of macroalgae on the reefs studied suggest the existence of diffuse nitro‐ gen sources fueling their growth. Nitrogen fixation could be a major source for these reefs (see further evidences of this in the isotopic section),and if this nitrogen source dispersed through the water column, it would raise the DIN up to 0.3 μM day-1[25]. This assumption is reasonable, especially when considering that the nitrogen isotope values (δ15N) in the tissues of macroalgae growing on these reefs (see below) are very close to the isotopic composition of atmospheric nitrogen (δ15N2 = 0‰). This new nitrogen, however,may pass "undetected" in our monitoring sampling because it may be immediately assimilated by the macrophytes upon entering the coastal zone where the coral reefs develop. In this regard, the actual mac‐ rophyte biomass itself may be the best evidence of large nitrogen inputs into the otherwise oligotrophic environments that characterize coral reefs, where macrophytes' occurrence is

In order to differentiate potential sources of nitrogen to the reef zones we analyzed the nitrogen isotopic composition (δ<sup>15</sup>N) of the tissue of several species of macroalgae collect‐ ed from reefs studied. The δ<sup>15</sup>N of macroalgae has been a widely used as tracer of nu‐ trient dynamics [27] This approach has been applied mainly in areas where nutrient sources are diffuse or little obvious, but also in areas where sources are very different, such as nutrient inputs from sewage. However, the spatial extent of its influence is not

The δ15N values of macroalgae from the different reefs of the three national marine parks are shown in Figure 8. The more common genus found in the study area were *Dyctiota* spp. and *Halimeda* spp., followed by *Penicillus pyriformis*, *Ulotrix* spp. and the seagrass *Thalassia testu‐ dinum*. We grouped all species by location and compared the average-δ<sup>15</sup>N (± 1SD) of macro‐ algae between locations and sampling period (Figure 8a for winter-2006, and Figure 8b for summer-2007). For reference, we have included in this figure the average nitrogen isotopic composition of nitrate (δ15NO3 = 4.37 ± 2.5 ‰) for the three parks (Carriquiry unpublished data). The validity of this reference comparison rests on the assumption that nitrate is a ma‐

drographic characteristics.

40 Environmental Change and Sustainability

commonly very scarce.

clear [28-29].

jor source of nitrogen for macroalgae.

**5. Isotope systematics of δ15N in macroalgae**

**Figure 8.** Average δ15N composition (± 1SD) of macroalgae collected at each site during the winter of 2006 (a) and the summer of 2007 (b). The average isotopic composition of nitrate (δ15NO3) in the coastal waters (Carriquiry, unpublish‐ ed data) of the three coral reef national parks studied here is included as a thin horizontal line in each diagram.

A common feature for both collection periods is the low δ15N values of macroalgae. During winter-2006, with the exception of Chankanaab reef at PNAC (δ<sup>15</sup>N of 5.0 ± 2.2 ‰), the δ15N of macroalgae is less than 3 ‰ and is markedly smaller than the isotopic value of nitrate (δ15NO3) (Figure8a). As in the winter of 2006, the average δ15N of macroalgae during the summer of 2007 varied from ~ 0.5 to 2.5 ‰, being always below the average δ15NO3 (Figure 8b).

It is noteworthy to highlight the results obtained from Chankanaab reef (at PNAC) which shows the greatest contrast in the average δ15N of macroalgae between sampling seasons: while the average δ15N of macroalgae was the most positive of all sampling stations during the winter of 2006 (Figure 8a), during the summer of 2007 Chankanaab reef presented the most negative δ15N values (-0.34 ± 1.43 ‰) of all the studied sites (Figure 8b). One factor that may explain this discrepancy is the difference in species composition between sampling sites. While in the winter of 2006 the δ15N was measured in *Dyctiota* spp. and *Penicilluspyri‐ formis*, during summer 2007 the species analyzed were *Lobophoravariegata*, *Dictyosphaera cav‐ ernous,Anadyomenestellata*and *Ulotrix* spp. The results for species indicates that the δ15N of *Dyctiota* spp. and *P. pyriformis* were characterized by positive values in both sampling peri‐ ods, while the species *L. variegata* always showed low δ15N values, including *A. stellata* whose measured δ15N was the most negative (-1.67 and -1.42 ‰ for Chankanaab and Dalila, respectively) in both sampling periods.

There are several alternatives to explain the low δ<sup>15</sup>N values of macroalgae in this re‐ gion. N2 fixation in coral reefs is regarded as a major component of the nitrogen cycle that provides new nitrogen for these ecosystems. It has been estimated that fixed nitro‐ gen can supply from one quarter to one half of the nitrogen requirements for the pri‐ mary producers in these oligotrophic environments [30]. It has been shown in coral reef areas where nitrogen fixation is predominant that δ15N value of macroalgae is close to 0 ‰, or even negative [31,32]. In agreement, the range of δ<sup>15</sup>N values measured in the dif‐ ferent species of macroalgae (<0 to <2.5 ‰) in our study, indicates that N2 fixation may be playing an important role in fulfilling the macrophytes' nitrogen demand. This con‐ clusion is also supported by [25] who found that the δ<sup>15</sup>N values of 1.9 ‰ in the sea‐ grass *Thalassia testudinum,* in Puerto Morelos lagoon, were the result of nitrogen fixation.

Alternatively, another possibility is the low concentration of dissolved inorganic nitrogen (DIN) in the studied reefs. Different studies have shown a positive relationship between the δ15NO3 *versus* nitrate concentration [NO3], both in temperate and tropical areas. It was found in a Massachusetts estuary that the δ15NO3 values approach 0 ‰ when the nitrate concentra‐ tion is reduced to levels <1 uM [33]. Similarly, other results obtained by [34]from the Mexi‐ can Caribbean reported that δ15NO3 decreases linearly with the concentration of nitrate in the water column.Thus, consideringthe low DIN concentrations that characterizeour study area it would be expected that the δ<sup>15</sup>N-DIN available for macroalgae should be character‐ ized by lower values.

Lastly, changes in the proportion of the different nitrogen species available for photosyn‐ thesis could explain the lower values in δ15NO3. In our study, ammonia accounts for 60 to 76% of the DIN levels. This implies that ammonium, but not nitrate, could be the main source of nitrogen to seaweeds in the region. Hence, the δ15N of macroalgae largely reflects the isotopic signature of ammonia. This hypothesis, although plausible, depends on the metabolic capacity of each species for using ammonium, depending on its availa‐ bility in the environment.
