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Land-Ocean Connectivity - from Hydrological to Ecological Understanding of Groundwater in the Coastal Zone

Final Report Summary - EGOMARS (Land-Ocean Connectivity - from Hydrological to Ecological Understanding of Groundwater in the Coastal Zone)

Project Objectives
The coastal zone, where fresh and saltwater meet, hosts some of the most dynamic, diverse and productive ecosystems on Earth. Ecological and hydrological land-ocean connectivity are important drivers of these ecosystems. This project aimed at advancing the understanding of hydrological-ecological coupling, by studying relationships of key fauna with coastal groundwater hydrology in the subtropical realm on the Yucatan Peninsula in Mexico. High-resolution sclerochronology and concurrent acoustic telemetry of the iconic Queen Conch Strombus gigas (a marine snail) were used to understand habitat usage of this endangered and socio-economically important species in the Caribbean Sea.

The study was carried out in collaboration between the European Institute of Marine Science (Institut universitaire européen de la mer IUEM, Plouzané, France), Centro de Investigación y de Estudios Avanzados del Instituto Politécnico Nacional (CINVESTAV, Mexico) and Parque Xel-Ha (Mexico), a commercial tourist park facility where field studies were carried out. Xel-Ha comprises a small, typical ground-water fed coastal inlet (‘caleta’), which provides a protected home for a large population of queen conch. We have learnt new aspects of the physical environment of groundwater-fed coastal ecosystems, how animals respond to it and how they use this ‘special’ habitat.

Main Results
(1) Xel-Ha – ‘the place where waters are born’. Xel-Ha means ‘the place where waters are born’ in the local Mayan language. Wa¬ters are ‘born’ in the southern arm of the caleta, where an underground spring discharges into Xel-Ha. The freshwater flow results in a vertical stratification between low-sali¬nity river water in the upper layer and saltwater on the bottom. The spring is flowing strong all year round, and even though the wind and the tides affect the water level and the mixing of water in the caleta, this stratification is maintained throughout the year. The salinity of the bottom layer where the conchs live is similar to the salinity of the ocean, but once every day during the early morning hours, its oxygen concentration is very low. This is due to the large oxygen demand by organisms living below the freshwater layer on one hand, and the inhibited flux of oxygen from the atmosphere to the bottom layer on the other hand. Despite these periodic low oxygen levels, a large population of conchs live on the seafloor underneath the freshwater ‘blanket’.

(2) How do queen conchs use their habitat? These marine snails - like all snails - move slowly. But where do they go over longer periods of time, for example over a one month or longer period? We answered these ques¬tions by tagging animals with small acoustic transmitters, and by recording their movement with acoustic receiver stations installed throughout the caleta. This ‘big-brother-is-watching’ study re¬vealed two important behavioural patterns.
For most of the time, an individual queen conch has a small home range (the space it uses). Over a six month period, the typical home range is 2000 m2 (or an area of 40m x 50m). This has impor¬tant ecological implications: a small home range of a fast-growing animal indicates that it does not need to go far to find food, in other words its movement is not limited by food availability. Over longer periods of one or two years, the conchs’ story changes. Even though these snails are slow, they are capable of moving large distances. As they grow older, they move from the more protected inshore regions towards the mouth of Xel-Ha, and many of them leave to the offshore ocean. This has important implications for fisheries management: Xel-Ha is a nursery ground for offshore populations of this threatened species.

(3) What can the shells tell us about the life of the animals? Shell-building animals like Strombus gigas incorporate chemical and isotopic signatures of the water they live in in their shell as they grow. By sampling these signatures along the growth axis of the shell, we can unravel the changes of environmental conditions during the life of an animal. For example, the oxygen isotope ratio embedded in the shell depends on both the temperature and the salinity of the water at the time of carbonate precipitation.

On the bottom of the caleta where the conch live, variations in ambient water salinity are small, and therefore (annual) variations in temperature dominate the oxygen isotope signature. We can match the shell isotope record with measurements of water temperature, and from this comparison we can calculate the age of an animal at the time of collection (how many summers and winters it has lived for). The isotopic records show that the animal shells grow in their conspicuous spiral pattern for 1.5-2 years, after which they cease to grow in length and instead develop a thick shell lip. This growth pat-tern is similar to that of conch living in ocean environments, suggesting that the somewhat stressful low-oxygen conditions in Xel-Ha due to persistent groundwater flow do not significantly affect their growth.

Conclusions and socio-economic impact
Throughout the Caribbean region, the queen conch is a highly important and endangered fisheries resource. Successful fisheries management, e.g. in the form of marine protected zone design depends on a good scientific understanding of the habitat usage of the target species. This study clearly demonstrates for the first time, that groundwater-fed estuaries are nursery grounds for Strombus gigas, which efficiently supply offshore populations. This suggests that these previously unrecognised habitats of this iconic species under the influence of groundwater discharge deserve special consideration in fisheries management in the region. The results of this study contribute to an improved understanding of coastal resources on Mexico’s Yucatan coastline, and are now taken up by the scientific and commercial partners of this project in Mexico in cooperation with local fisheries management authorities.

Contact: Dr Thomas Stieglitz, Tropical Water & Aquatic Ecosystem Research TropWATER, James Cook University, Townsville, Australia. email: thomas.stieglitz@jcu.edu.au