Dive in the river Jadro spring – surprising speleological discovery

Members of the Croatian Biospeleological Society on 9th February 2016 visited the river Jadro spring to obtain material for the 3rd volume of the book “The Cave Type Localities Atlas of Croatian Fauna”. In proximity to the river Jadro spring, three species of underground water snails (family Hydrobiidae) and one species of amphipod (Niphargus salonitanus) were described for science. Because of that, and the information on the morphology of the entrance of the spring area provided by the Slovenian biologist Teo Delić, the site had to be inventoried again. The river Jadro spring has strategic importance for the water supply of the city of Split and is under strict control, so access to the spring is not possible. For this opportunity, access to the spring was enabled by Mr. Mario Banal, Water Supply and Sewerage Ltd. Split worker.

During the dive, cave diving research was performed with the purpose of collecting underground fauna. To the great surprise, a submerged cave channel (cave) was discovered in the spring. It is interesting that the existing literature does not cite any knowledge of the existence of the cave at this spring. Information about the submerged part of the spring we could not find in discussions with geologists and hydrogeologists, who are acquainted with the situation in this part of Dalmatia. Even among divers, who previously dived into the spring of Jadro, the submerged cave channel is unknown. All of the above testifies, unfortunately, little public interest and severity in performing this kind of research. Despite a strong water current, Branko Jalžić managed to explore a sunken channel to a depth of 16 m and length of 25 m in the first dive.

Return to the Jadro spring was on 12th August 2016, during the lower water level. Cave diving research was performed by Petra Kovač Konrad, an associate in the association Hyla and member of association SO Željezničar, and Branko Jalžić, a member of the Croatian Biospeleological Society from Zagreb. Behind a large cave entrance, the entrance in a submerged channel is through the crack between collapsed rock blocks. Despite the low water level, there was a strong water current through the crack. After collapsed blocks divers have descended from 4 m to 12 m depth, where a spacious channel dimension of 10 m x 4 m is located. Aslope channel continued to a depth of 20 m, obtaining the same dimensions. At its bottom, there are numerous rock blocks of larger and smaller sizes. Strong water current complicated the positioning of a security thread and topographic survey of the area. After 80 m, the channel continues in several directions through the narrow cracks. A review of those cracks found that through some of them water is flowing in, while through the other ones the water is flowing out taking sediment away.

During the dive, numerous and diverse underground fauna was observed, so far unknown to live here. These are cave shrimp from genus Troglocaris, isopods from genus Sphaeromides and Monolistra, crayfish from genus Niphargus and tubes of polychaetes from genus Marifugia.

The river Jadro spring is a habitat of olm, which was not observed at this time, and it is needed to dive in even lower water levels to find it.

Branko Jalžić and Peter Kovač Konrad