Investigating nutrient sources and impacts for offshore reefs in Mexico

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Many reefs along the Yucatan coast of Mexico have shown a recent decline in health

Hydrographic and water quality analyses reveal inputs and cycling of wastewater-derived nutrients in two Yucatan lagoons. Isotopic analyses provide a means for tracking nutrient export and point-source impacts on offshore reefs.

Like the rest of the Mesoamerican Barrier Reef, many reefs along the Yucatan coast of Mexico have shown a recent decline in health. Multiple factors may contribute, but off the Yucatan a leading suspect is the potentially lethal combination of rapid coastal development, inadequate wastewater treatment and the region’s unique hydrogeology.

Porous carbonate rocks result in near-complete absence of surface waters and, instead, a groundwater regime carried by a vast network of caves. Access to fresh underground water, through ‘cenotes’ (collapsed cave ceilings), was the lifeblood of Mayan civilisation. However, the same regime presents a modern-day problem for coastal environments – through offshore groundwater discharge.

Development

Over the last two decades there has been exponential development along the Yucatan coast – The ‘Riviera Maya’– but no parallel development of wastewater treatment. The potential consequence is offshore export of nutrients that can fuel expansion of coral-smothering algae. However, isolating the effects of groundwater discharge is not straightforward.

Akumal Bay presents a particularly pressing and valuable study site. Akumal is a seaside resort with multiple hotels and a workers’ town (Akumalito). Its tourism industry depends heavily not only on its reefs, but on an exceptional population of endangered sea turtles, which rely both on the reefs and seagrass beds, as well as on beaches for nesting.

The lagoons are serving as local nutrient sources to adjacent reefs

The little wastewater in Akumal that is treated is simply injected at depth on land, with little knowledge of potential offshore transport. Akumal Bay has multiple groundwater vents, and both its reefs and seagrass beds, and therefore its turtles, are at risk from nutrient discharge. The locations of offshore vents, as well as circulation within the bay, are not well known. Further, there is already considerable reef health decline around Akumal, but it is patchy. Thus, it is difficult to pin down local discharge as the culprit, as opposed to a more regional effect (discharge along the entire Riviera Maya combined with longshore transport) or other factors.

Fieldwork

To address the problem of working with elusive offshore vents, two groundwater-fed lagoons, Yalku and Yalku Chico, were selected for study. These present constrainable groundwater point sources for assessing effects on fringing reefs immediately offshore. Both are shallow (<3m) and fed by groundwater at their heads. However, Yalku is larger and more complex than Yalku Chico, with a broader mouth that allows more extensive interaction with offshore waters.

Fieldwork was conducted over six weeks during the summer of 2013, with assistance from staff at the Centro Ecologico Akumal (CEA, a NGO charged with environmental management of the Akumal area), and formed the basis of dissertation projects for four University of Edinburgh students.

Objectives were firstly to assess inputs and cycling of nutrients in the two contrasting lagoons, alongside hydrographic surveys (water column structure, circulation and freshwater-seawater interaction). Further objectives were to determine freshwater and nutrient export from the lagoons, and ultimately to assess whether nutrient discharge could be traced on the seafloor at sites offshore.

Hydrographic surveys were conducted by Conductivity-Temperature-Depth (CTD) profiling and current meter deployments. These provided clear pictures of shallow and deepwater circulation, and density cross sections that highlighted the extent of freshwater/seawater mixing and freshwater export (see sample section from Yalku). A key finding was that, due to its narrow mouth and limited tidal exchange, Yalku Chico showed less freshwater/seawater mixing, and therefore a clearly stratified water column. However, both lagoons showed distinct freshwater plumes (distinguishable 100s of metres offshore).

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Density cross section (early falling tide) for Yalku lagoon and offshore sites. A pool of fresher water is evident in the upper lagoon. Extensive mixing occurs across the lagoon, but a surface plume of fresher water is observed offshore, with a more subtle gradual increase in bottom-water density (figure generated courtesy of Ocean Data View)

Nutrient levels

Water quality analyses included a full suite of nutrients as well as chlorophyll (a measure of phytoplankton growth and nutrient uptake). These permitted assessment of groundwater nutrient inputs, nutrient distributions (see example for nitrate in Yalku) and the amount of nutrient cycling within, and export from, each lagoon. Clear nutrient enrichments were found in both lagoons. However, especially in Yalku Chico, where residence time is short, little nutrient cycling was observed within the lagoons, and thus most or all nutrients appeared to be exported.

Distribution of nitrate (M) in Yalku lagoon in bottom waters (left) and surface waters (right) and during rising (upper) and falling (lower) tides. Clear signs of nutrient enrichment from groundwater inputs (at head of lagoon) are evident in surface waters, with decrease towards the mouth due to mixing with offshore water (image produced with Ocean Data View)

Distribution of nitrate (μM) in Yalku lagoon in bottom waters (left) and surface waters (right) and during rising (upper) and falling (lower) tides. Clear signs of nutrient enrichment from groundwater inputs (at head of lagoon) are evident in surface waters, with decrease towards the mouth due to mixing with offshore water (image produced with Ocean Data View)

Because of intense dilution, nutrient levels dropped sharply offshore, and in bottom waters soon reached detection limits. We therefore turned to stable nitrogen isotopes as a possible tracer of seafloor wastewater impact. Seafloor plants, including seagrasses and macroalgae, have isotopic compositions that reflect the signature of the nitrogen they assimilate. Because wastewater nitrogen is distinctly enriched in 15N relative to 14N (more positive δ15N) when compared to nitrogen in uncontaminated seawater, plant nitrogen isotope signatures may serve as sensitive indicators of wastewater contamination.

While previous spot measurements of algal δ15N in the lagoons and Akumal Bay had suggested evidence of wastewater contamination, results were not fully conclusive because of difficulties with comparing species, and because no high-resolution site study was conducted. We therefore collected macroalgae along a detailed transect ranging from a contaminated pond (Laguna Lagartos) upstream of the lagoons, to multiple sites along each lagoon, and at sites at increasing distance offshore. Our findings (see figure) confirmed evidence of wastewater contamination in Laguna Lagartos and the lagoons (positive δ15N) values. However, the key finding was a clear and systematic decrease in algal δ15N with increasing distance offshore, spanning local reef sites. Moreover, two different species (Lobophora and Dictyota) showed almost identical trends (with a consistent offset).

Stable nitrogen isotopic compositions (15N) of seafloor algae from an upstream contaminated pond (Laguna Lagartos) and from sites within and offshore of Yalku lagoon. The vertical dashed line indicates the approximate location of the lagoon mouth. Blue and red dashed trend lines are for two different species of algae at sites offshore of the lagoon

Stable nitrogen isotopic compositions (δ15N) of seafloor algae from an upstream contaminated pond (Laguna Lagartos) and from sites within and offshore of Yalku lagoon. The vertical dashed line indicates the approximate location of the lagoon mouth. Blue and red dashed trend lines are for two different species of algae at sites offshore of the lagoon

Together, our results strongly indicate that the lagoons are serving as local nutrient sources to adjacent reefs, and that algal nitrogen isotopic signatures serve as reliable indicators of this process, with potential to be used in other local sites. We are currently working with CEA to relate our results to reef survey data from sites offshore of the lagoons, to determine whether links can be established between indices of nutrient impact and reef health. Further studies will build on this research as part of a study of groundwater vents in Akumal Bay; identifying locations, tracking groundwater flow across the bay, and assessing impacts on seagrass beds and reefs.

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