**4. Hydrology: Nutrient concentrations**

The increase of anthropogenic nutrients is attributed to be a major cause of increased algal cover in coral reef ecosystems, based on the theory that in some reefs bottom-up ecological control in food chains regulates ecosystems. In order to try to better understand the process‐ es that regulate ecosystem resilience we additionally characterized the hydrological condi‐ tions throughout the three marine parks studied here. Samples were taken from different stations distributed on the same reefs where benthic surveying was conducted (Figure 5a,b), additionally including the hydrologic (nutrient) characterization of the Nichupte Lagoon, bounded to the north by Punta Cancun and to the south by Punta Nizuc (Figure 5a). In or‐ der to characterize the concentration of nutrients, particularly from dissolved inorganic ni‐ trogen (DIN) species that involve inorganic nitrogen: nitrate (NO3 - ), nitrite (NO2 - ) and ammonium (NH4), water samples were collected at two different depths: at the water sur‐ face and near the substrate. Sampling was conducted in the months representing contrasting seasonal conditions (dry and rainy seasons) in the years 2006 and 2007.

The average concentration of DIN in the sampled coral reefs is shown in Table 2. By ap‐ plying a factorial ANOVA considering the different climatic seasons and parks there were no significant differences in the concentration of DIN (F = 0.422, *p* = 0.657) but be‐ tween the dry and rainy seasons (F = 9.280, *p* = 0.003). Consistently, one can observe a slight increase in the concentration of dissolved inorganic phosphate (DIP) in the three parks during the rainy season; this increase is mostly evident in the PNIMCN and PNAPM, because it almost doubled the average concentration measured during the dry season (Figure 6). As was expected for Nichupté Lagoon, it was characterized by a high‐ er concentration of DIN with respect to the levels seen at the reef sites. In addition, Ni‐ chupté lagoon presents an opposite behavior to reef sites as a function of seasonality: while during the rainy season the DIN increases reefs, the concentration decreases in the lagoon, probably due to dilution effect (Table 2, Figure 6).

**Figure 4.** Cluster analysis of the coral reefs of Northern Quintana Roo (data for 2006-2007).

trogen (DIN) species that involve inorganic nitrogen: nitrate (NO3

seasonal conditions (dry and rainy seasons) in the years 2006 and 2007.

The increase of anthropogenic nutrients is attributed to be a major cause of increased algal cover in coral reef ecosystems, based on the theory that in some reefs bottom-up ecological control in food chains regulates ecosystems. In order to try to better understand the process‐ es that regulate ecosystem resilience we additionally characterized the hydrological condi‐ tions throughout the three marine parks studied here. Samples were taken from different stations distributed on the same reefs where benthic surveying was conducted (Figure 5a,b), additionally including the hydrologic (nutrient) characterization of the Nichupte Lagoon, bounded to the north by Punta Cancun and to the south by Punta Nizuc (Figure 5a). In or‐ der to characterize the concentration of nutrients, particularly from dissolved inorganic ni‐

ammonium (NH4), water samples were collected at two different depths: at the water sur‐ face and near the substrate. Sampling was conducted in the months representing contrasting

The average concentration of DIN in the sampled coral reefs is shown in Table 2. By ap‐ plying a factorial ANOVA considering the different climatic seasons and parks there were no significant differences in the concentration of DIN (F = 0.422, *p* = 0.657) but be‐ tween the dry and rainy seasons (F = 9.280, *p* = 0.003). Consistently, one can observe a slight increase in the concentration of dissolved inorganic phosphate (DIP) in the three parks during the rainy season; this increase is mostly evident in the PNIMCN and PNAPM, because it almost doubled the average concentration measured during the dry season (Figure 6). As was expected for Nichupté Lagoon, it was characterized by a high‐ er concentration of DIN with respect to the levels seen at the reef sites. In addition, Ni‐


), nitrite (NO2


**4. Hydrology: Nutrient concentrations**

36 Environmental Change and Sustainability

**Figure 5.** Sampling locations in the PNIMCN and PNAPM (a) and in the PNAC (b), used for characterizing the nutrient levels in the coral reefs of Northern Quintana Roo during 2006 – 2007.


**Table 2.** Average nutrient concentration in the Nichupté-Lagoon and the coral reefs of three National reef parks of Northern Quintana Roo during 2006 – 2007.

**Figure 6.** Average DIN concentration (µM) in in the Nichupté-Lagoon and the coral reefs of three National reef parks of Northern Quintana Roo (PNAC, PNIMCN, PNAPM) during 2006 – 2007 in two climatically contrasting seasons (rainy and dry).

Ammonium (NH4) is the most important DIN species in the reef waters. Its concentration is consistently higher than nitrates and nitrites, except for the PNAC during the dry season in which ammonium and nitrate have approximately the same concentration (Figure 7). It was expected that ammonia were the most important DIN species, since the success of high pro‐ ductivity of coral reefs results from the high nutrient recycling that takes place within the ecosystem. Most of the new nitrogen entering the reef is through N2 fixation, so it is absor‐ bed and converted to organic nitrogen which can be later consumed and passed through the food webs or returned to the system as ammonia by passing through the process of ammo‐ nification, so it is excreted in the urine of organisms [21].

During the dry season, there is no significant difference among the three species of DIN in the three parks, however the average concentration of nitrate is higher in the PNAC. During the rainy season nitrate concentration is high in all parks but ammonium is sig‐ nificantly higher, being PNAPM the park with the highest ammonium concentration (Ta‐ ble 2, Figure 7).

The hydrographic information generated in this study indicates that there is no evidence of eutrophication in reef areas in the Mexican Caribbean. In general, nutrient concentrations are low, typical of the reef zones of the Wider Caribbean. The general average of DIN (1.3 ± 1.6 μM) in the Northern Quintana Roo reef parks is lower than that reported for the Florida reef tract (4.3 ± 7.4 μM; [22]) and within the observed range of the coral reefs of Tobago (1.6 ± 1.1 μM; [23]).

It has been shown that groundwater seepage into the coastal area of this region can sup‐ ply significant amounts of nutrients to the water column [24-25]. However, a study in seagrass meadows of *Thalassia testudinum* in Puerto Morelos reef lagoon has shown that water seeping through the springs (locally called 'ojos') can enrich the water column - and seagrasses-- with phosphorus, but not with nitrogen [25]. These authors found that in *T. testudinum* meadows of Puerto Morelos, the pore water contains extremely low lev‐ els of nutrients (1.2 - 3.42 μM of ammonium and 1 - 1.5 μM of phosphate) compared with the world average for seagrass meadows (~ 86 μM of ammonium and 12 μM of phosphate). This low DIN concentration in the water column of Puerto Morelos suggests that nitrogen could be limiting the growth of seagrass meadows. The nitrogen content in the tissue *T. testudinum* in the reef lagoon (% N> 1.8), however, is high enough to not show this limitation [25].

**Figure 6.** Average DIN concentration (µM) in in the Nichupté-Lagoon and the coral reefs of three National reef parks of Northern Quintana Roo (PNAC, PNIMCN, PNAPM) during 2006 – 2007 in two climatically contrasting seasons (rainy

Ammonium (NH4) is the most important DIN species in the reef waters. Its concentration is consistently higher than nitrates and nitrites, except for the PNAC during the dry season in which ammonium and nitrate have approximately the same concentration (Figure 7). It was expected that ammonia were the most important DIN species, since the success of high pro‐ ductivity of coral reefs results from the high nutrient recycling that takes place within the ecosystem. Most of the new nitrogen entering the reef is through N2 fixation, so it is absor‐ bed and converted to organic nitrogen which can be later consumed and passed through the food webs or returned to the system as ammonia by passing through the process of ammo‐

During the dry season, there is no significant difference among the three species of DIN in the three parks, however the average concentration of nitrate is higher in the PNAC. During the rainy season nitrate concentration is high in all parks but ammonium is sig‐ nificantly higher, being PNAPM the park with the highest ammonium concentration (Ta‐

The hydrographic information generated in this study indicates that there is no evidence of eutrophication in reef areas in the Mexican Caribbean. In general, nutrient concentrations are low, typical of the reef zones of the Wider Caribbean. The general average of DIN (1.3 ± 1.6 μM) in the Northern Quintana Roo reef parks is lower than that reported for the Florida reef tract (4.3 ± 7.4 μM; [22]) and within the observed range of the coral reefs of Tobago (1.6

It has been shown that groundwater seepage into the coastal area of this region can sup‐ ply significant amounts of nutrients to the water column [24-25]. However, a study in

nification, so it is excreted in the urine of organisms [21].

and dry).

38 Environmental Change and Sustainability

ble 2, Figure 7).

± 1.1 μM; [23]).

**Figure 7.** Average DIN concentration in the coral reefs of three National reef parks of Northern Quintana Roo (PNAC, PNIMCN, PNAPM) during 2006 – 2007 in two climatically contrasting seasons (rainy and dry). ANOVA statistical test (bars represent Std. Error).

In contrast to the typical oligotrophic conditions that characterize coral reef areas, nutrient concentration inside the Nichupté Lagoon system is much higher. The weighted average concentration of DIN inside this lagoon system (14.7 ± 11.6 μM for winter and 4.18 ± 1.98 μM 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‐ drographic characteristics.

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 commonly very scarce.
