Mexico was selected a destination because of the cenotes. Cenotes are natural pits or sinkholes in the calcareous ground of Mexico which has very many caves due to its porosity.
Clear water – blind fish
The freshwater sources of the Yucatan flow towards the sea through this cave system, the largest underwater cave system in the world, and are then washed back into the system in the opposite direction by the seawater. We were interested in the flora and fauna of these freshwater lakes (cenotes) and the attached caves with their inhabitants such as the Mexican blind cavefish (Astyanax mexicanus).
Our program included two days of extensive diving and snorkelling in the cenotes. The first day took us into the Ponderosa cenote, also known as El Eden (N 20° 29’ 29,1’’/ W 87° 15’ 29,1’’). The travel time from Playa del Carmen where we were staying was approximately one hour. Access to a cenote usually cost US$ 10 for one day.
Quelli di noi che facevano snorkeling, la metà esatta del gruppo, potevano entrare direttamente nelle acque cristalline ed esplorare la fauna del cenote. I sommozzatori in primis hanno ricevuto un’introduzione dettagliata delle immersioni in grotta, visto che queste differiscono da altri tipi di immersione in molti modi e non soltanto per come muovere le pinne. Hendrik, la nostra guida di Yucatec-Divers (http://www.yucatek-divers.com), è un sommozzatore di grotte fino in fondo. Siamo partiti con lui per fare immersioni ai margini della grotta. Il primo sguardo sotto la superficie dell’acqua è stato impressionante: il cenote ribolliva di pesci, in predominanza poecilidi come i Poecilia velifera e i ciclidi. Abbiamo visto anche qualche sporadico pesce gatto. Prima di entrare nella grotta, abbiamo trascorso una mezz’ora nelle acque basse ad osservare intensamente i “nostri” pesci d’acquario. Utilizzando la sagola guida, per una mezz’ora ci siamo fatti strada tra passaggi e stanze con stalattiti e stalagmiti. L’acqua era così limpida che ci sentivamo come se stessimo levitando. Ognuno di noi cercava di non far muovere troppo sedimento per non offuscare la visibilità dell’acqua. Ad una profondità di qualche metro ci siamo imbattuti in un aloclino, lo strato che divide l’acqua dolce più leggera e la sottostante acqua salata più pesante. La visibilità si riduceva notevolmente quando ci si immergeva attraverso l’aloclino. Visivamente, sembrava come se ci fossero delle strie nell’acqua. La maggior parte dei pesci evitava l’acqua salata sotto l’aloclino. Gli unici pesci che abbiamo visto nell’acqua salata sono stati vivipari e pochissimi ciclidi. I raggi di sole penetravano in qualche punto il tetto della grotta, creando incredibili giochi di luce nell’acqua. In una delle caverne, la luce e l’ampiezza dell’ambiente ti facevano sentire come in una cattedrale. Dopo essere usciti dalla grotta, abbiamo avuto il permesso di finire il resto dell’ossigeno nelle bombole stando nell’acqua bassa del cenote. Siamo quindi riusciti a documentare l’intera popolazione ittica del cenote Ponderosa.
Afterwards, the snorklers and the divers tested the water parameters together. While determining the general hardness, we thought that the water test was wrong when there was still no change in colour after 25 drops. However, mixing with the salt water provided the explanation: As salt water has a GH (general hardness) of 350, an extremely high GH of 35 ° dGH was found in the fresh water, which had partly become mixed with the salt water. The water temperature of the fresh water was 25 °C, around 2 °C below that of the salt water. As the entire bottom was covered with algae and plants, it would be interesting to know the oxygen content at night. The high water temperature and the oxygen depletion as a result of the algae could lead to problems. The pH level was almost 7 and the conductivity was 8180 µS/cm. The carbonate hardness of 0 - 1 ° dKH was barely measurable. The large amount of algae made us curious to find out whether there were an algae nutrients. Which there were: Despite all the algae, there was still a detectable amount of phosphate in the water, namely 0.05 mg/l.
The Caribbean – coral reefs offshore of Cancun
One should actually go to Cozumel in order to see the most beautiful reefs of the region, but we didn't want to waste the short precious time we had in Mexico driving back and forth. That is why we selected the Yucatec Dive Center for our cenote dives, as well as for the snorkelling and diving in the ocean.
Both of our boats set out in the morning, one for our 8 snorklers, and one for the 8 divers. The travel times to the offshore reefs were between 10 and 30 minutes. The reefs are patch reefs on a sandy bottom at a depth of 5 - 25 m. Compared to the Indo-Pacific Ocean, the Caribbean has relatively few species and has only 1/3 of the coral species found in the region, for example. Nevertheless, it is very popular, and this is surely due to the clear water and the numerous magnificent experiences with a Caribbean flair that can be enjoyed above water among other things. It is only after rain fall or during the hurricane season that the visibility deteriorates, sometimes dramatically. Rainfall also forced us to postpone our dives in the ocean for one day.
The reefs revealed most of the typical Caribbean reef dwellers, ranging from the dwarf/pygmy cherub angelfish to the French grunt. Sharks and barracudas were the only ones we didn't get to see, unfortunately. Instead, the gorgonians were extremely beautiful and we started measuring the water parameters at noon: 26 °C, pH 8.1, KH 9 °dKH, 53.5 mS/cm, Ca 500 mg/l, Mg 1100 mg/l. We were able to fit in four dives that day and thus catch a glimpse into the Caribbean reef world.
Sac Actun – the second largest cave system in the world
The second cenote diving day brought us to two other cenotes in the south near Tulum: The Gran cenote forms part of the second largest underwater cave system in the world, the Sac-Actun System with a total length of 215 km! The location of the cenote was picturesque with steep cliff walls in a beautiful tropical dry forest. A stairway brought the visitors and divers several meters below the ground. Wooden catwalks led to the water at different places in the open part. We saw turtles in the water for the first time in addition to the fish species of the Ponderosa cenote. Unfortunately, due to all the tourists, the water was not as clear in the shallow part as in the Ponderosa cenote. Nevertheless, we were able to test the water, observe fish and watch the turtles swimming. While diving, we noticed that we did not dive through a halocline. This was reflected in the water parameters: "Only" 350 µS/cm conductivity at a GH of 30 ° dGH and a KH of 15 ° dKH. The pH level was 7.4. Barely any algae, and characins for the first time, which we probably did not see in the Ponderosa cenote because of the high salt content there.
Aktun Ha Cenote, known as Car Wash because taxi drivers had washed their cars here for many years, was the last cenote in our itinerary. That was certainly a good idea back then, because the lake in this cenote is not located in a hole, and instead is at ground level. The special thing about this cenote was a crocodile that was peaceful and patient while we photographed it.
GPS data of the two cenote:
N 20° 14’ 46,7’’
W 87° 27’ 53’’
Actun Ha (Car Wash)
N 20° 16’ 28,2’’
W 87° 29’ 10,8’’
Round about the cenotes
Everyone interested in more than reptiles got their money’s worth. There was a tropical dry forest with many large rocks around the cenotes. The largest, therefore most readily visible, lizards were the Black Iguanas (Ctenosaurus similis + C. melanosterna). But there were also skinks and other small lizard species living there. A nonvenomous Mexican Parrot Snake (Leptophis mexicanus) was discovered by our Polish participant, Piotr. His wife, Anna, was inspired by the plants of the tropical dry forest, and Klaus was untiring in walking around with his tripod to document birds for his ornithology friends back home.
Lux measurements showed values of 93,000 lux with a cloudless sky and an air temperature of 30.5 °C. UV-B measurements yielded: 0.166 mW/cm2 in the midday sun and 0.063 mW/cm2 in the underbrush. The PCE UV radiation metre, which measures UV radiation between 290 and 390 nm (thus UV-A and B), showed 4.13 mW/cm2 in the sun and 0.168 mW/cm2 in the shade. Wavelengths between 315 and 380 nm are referred to as UV-A radiation. UV-B radiation is between 280 and 315 nm.
The Tulum Mayan temple ruins
On our last day, we used the late afternoon hours when the cenotes were closed anyways to visit the famous Mayan temple ruins located right on the seaside. There were barely any tourists there that late in the day. Our guide explained many interesting details that we wouldn't have noticed otherwise such as the construction of the archways, which the Mayans had invented. We then took our group photo against the picturesque backdrop of the ruins and the ocean. Klaus, who was off hunting bird photos again, though, missed the shooting.