Tegnue (singular tegnua) are rock formations which have developed in the sea bottom of the Gulf of Venice, in the northern Adriatic Sea. They are bio-sediments which are very similar to coral reefs and for this reason they also are called Adriatic coral reefs. They are found especially off the coasts of Caorle, the Cavallino peninsula, the Lido di Venezia island and the Sottomarina peninsula at Chioggia. There are eight such formations. They are at a depth of -15/-40 m. The largest formation is off the coast of the Cavallino peninsula.
Etymology Tegnue is a Venetian dialect word which means "held back." The name was given by fishermen whose nets got caught in these rocks at the bottom of the sea, which held them back.
Research and mapping out ARPAV, the Agenzia Regionale per la Prevenzione a Protezione Ambientale del Veneto (Veneto Regional Agency for Environmental Prevention and Protection) carried out three research projects which were part of international, national and local programmes between 2003 and 2007. The first was a collaboration between the Veneto and the Friuli-Venezia Giulia regions and Slovenia for a regional oversight project called “Development of studies monitoring activities of the evolution of the sea coast ecosystem aimed at safeguard, integrated management and enhancement of the resources of the sea.” The second was part of the 6th three-year national plan for Fishing and Aquaculture called “The tegnue: studies of some areas of particular environmental interest for the enhancement of local halieutic resources and nature safeguard.” The third was part of an “Interventions for the safeguard and enhancement of a repopulation sea oasis called Porto Falconara Tegnua” project which was part of a plan for the Caorle area.
The aims of the projects were to establish the Upper Adriatic Observatory as a regional hub, to coordinate the activities of local authorities, organisations and research institutes, to provide technical, scientific and service support for the regional policies regarding the improvement and safeguard of the quality of the coastal sea waters, to establish a Veneto sea databank, to establish monitoring of pollution, to enhance and support sustainable tourist activities, to safeguard and enhance areas of natural interest, to promote and disseminate information for an environmental education culture, and to study some areas of particular environmental interest for the enhancement of local fishing resources and the safeguard of nature.
ARPAV carried out geomorphologic and granulometric, structural, mineralogical and biological research. The results were published by ARPAV in conjunction with the Natural History Museum of Venice in 2010 with the title “The Upper Adriatic tegnue: enhancement of marine resources through the study of areas of environmental value”. The printed edition is out of print, but it can be downloaded from the ARPAV website.
Eight tegnue have been mapped out off the Veneto shore of the Gulf of Venice:
1. Caorle - It has a rectangular shape, it is 2,2 km long and 3.5 km wide, its area is 7.7 square km and it lies c. 2–3 km form the coast At a depth of -8 to -12 m 2. D’Ancona – It has a rectangular shape, it is 3,4 km long and 2.0 km wide, its area is 6.9 square km and it lies c. 7.7–9 km form the coast, at a depth of -16 to -21 m 3. Malamocco – It has a rectangular shape, it is 3,4 km long and 2.0 km wide, its area is 6.8 square km and it lies c. 1.7 km form the coast at a depth of -8 to -11 m 4. Sorse – It has a rectangular shape, it is 3,42 km long and 2.1 km wide, its area is 7.2 square km and it lies c. 17.6-19.6 km form the coast at a depth of -19 to -24 m 5. Venezia – It has a rectangular shape, it is 3,35 km long and 2.1 km wide, its area is 7 square km and it lies c. 10.9-16.4 km form the coast at a depth of -18 to -21 m 6. Cavallino Vicina – It has a rectangular shape, it is 3,45 km long and 2.12 km wide, its area is 7.3 square km and it lies c. 3.3-4.5 km form the coast at a depth of -14 to -17 m 7. Cavallino Lontana – It has a rectangular shape, it is 1.6 km long and 850 m wide, its area is 1.4 square km and it lies c. 10 km form the coast at a depth of -16 to -23 m 8. Chioggia – It has a rectangular shape, it is 3.4 km long and 2.1 km wide, its area is 7.1 square km and it lies c. 10 km form the coast
The Caorle tegnua is off the shore of Caorle. The Cavallino Vicina, D’Ancona, Cavallino Lontana and Sorse tegnue are off the shore of Cavallino peninsula. The Malamocco and Venezia are off the shore of the Lido di Venezia island. The Chioggia tegnua is off the shore of Sottomarina peninsula, Chioggia.
Morphology The sizes of the tegnue can be very different, from a few to several thousands of square metres, with elevations from the bottom form a few tens of centimetres in the low formations, sometimes called "lastrure" (slab formations) and a few metres in the higher ones, which are often found at greater depths. There are three types of formations:
Clastic sedimentary rocks formed by the hardening of sediments (sands) or organogenic debris (probably linked to variations in sea level in geologically recent epochs) called “beachrocks.” They look like sub-horizontal slabs protruding from the bottom with very variable thickness and usually a very slight inclination, which causes an easy submersion by sediments, whereas the prevalence of the erosive action of the sea currents can cause new structures to surface or the formation of depressions and cavities along the perimeter areas. Sedimentary rocks from chemical deposition whose origin is linked to the rise of methane gas from the sea bottom and its reaction with sea water which activates a process which can cause the precipitation of carbonates and the hardening of sediments. Subsequent erosive action causes the rising of the structures. Organogenic rocks, structures produced by the action of constructor organisms, vegetable and animal, whose calcareous skeleton stratifies and can form structures with a moderate thickness. Such a formation process leads to varied and irregular morphologies; micro and macro cavities caused by different speeds and irregularities in the growth of the various constructor organisms. They are carbonate rocks formed by marine organisms superimposed on hard substrates which had previously formed though the consolidation of sands. They are reefs which are different from tropical reefs in that the main bio-constructor organism is not coral, but Corallinaceae, red calcareous algae, bryozoa (filter feeders), Serpulidae (annelid worms) and encrusted Cnidaria.Boldrin A., Aspetti ecologici delle formazioni rocciose dell’Alto Adriatico. Atti Convegni. Scientifici Nazionali, Programma Oceanografico, 1979: 1197-1207Mizzan L., Malacocenosi e faune associate in due stazioni altoadriatiche a substrati solidi. Bollettino del Museo Civico di Storia Naturale di Venezia, 41, 1992: 7-54. The hardening of the sand incorporated calcareous shells of molluscs which stratified to form a certain thickness.Tegnue, Environment, Organisms and Curiosities, 2007
Often the different typologies coexist. In various cases the organogenic formations can develop on top of structures of sedimentary nature, covering them with layers of variable thickness in the same way in which they can colonise artificial substrates of human origin.
The importance of the vegetable part in organopgenic construction over the animal ones is highly variable and subject to water brightness factors; that is, linked to the depth and turbidity of the waters. The morphology is very irregular. There are surfaces very rich in micro crevices and have high diversity. They are often called reefs, but compared to tropical sea coral reefs, which are derived from the action of hermatype organisms (reef-building coral) which need hot and clear water, the tegnue, given the turbidity of the local water, do not substantially depend on photosynthesis processes at a benthos level. They lie in highly turbid waters due to a high quantity of suspended material of fluvial origin which reduces brightness of the bottom. However, this is also a rich source of nutrients for filter feeders and detritivores (which obtain nutrients by consuming detritus, decomposing plant and animal parts and faeces) which constitute the dominant element of the tegnue population.Casellato S., Sichirollo E., Cristofoli A., Masiero L., Soresi S., Biodiversità delle “Tegnùe” di Chioggia, zona di tutela biologica nel Nord Adriatico. Biologia Marittima Mediterranea 12 (1), 2005: 69-77.
Many reefs, particularly the larger ones, are surrounded by a course belt of debris with variable sizes, up to several metres, accumulated by the currents and characterised by populations which mainly pertains to coastal debris. It cannot be ruled out that the reefs can even be derived from bioconcentration phenomena activated on large accumulations of debris deposited in the gaps between fossil dunes, as it has been noted off the Cavallino peninsula coast, or caused by a particularly dense presence of Pinna nobilis (noble pen shell) populations in the past.Newton S.R., Stefanon A., 1976 - Primi risultati dell’uso simultaneo in Alto Adriatico di side scan sonar, sub bottom profiler ed ecografo. Memorie di Biogeogeafia Adriatica 9: 33-66.Magistrato Alle Acque – Mizzan, Studio B.6.78/I. Attività di monitoraggio alle bocche diporto. Controllo delle comunità biologiche lagunari e marine. Relazione finale. Prodotto dal concessionario Consorzio Venezia Nuova, 2006 In the reefs in this area there are structures of consolidated debris in small oranogenic patches scattered irregularly in a wide shell debris area. Various hundreds of Pinna nobilis were sticking out of the bottom for over 30/40 cm, causing an accumulation of coarse shell debris rich in encrusted organisms and widely colonised by venus clams (Venus verrucosa). Because of the uniqueness of these areas, after their discovery there was intense fishing with turboblowers which probably had strong negative impact.
Research on the origins of the tegnue The Istituto di Science Marine (Marine Science Institute) of the CNR, the Consiglio Nazionale delle Ricerce (National Research Council), coordinated a research on origins of the tegnue in conjunction with the University of Padua, OIGS, Istituto nazionale di oceanografia e di geofisica sperimentale (National Institute of Experimental Oceanography and Geophysics), ISPRA Istituto superiore per la protezione e la ricerca ambientale (Higher Institute for Environmental Protection and Research) and the Argentinian CONICET, Consejo Nacional de investigaciones científicas y técnicas (National Council of Scientific and Technical Investigation). The results were published as an article called "Paleochannel and beach-bar palimpsest topography as initial substrate for coralligenous buildups offshore Venice, Italy" in 2017.
Venezia Today provided a brief coverage of the report. These coralline formations are an anomaly compared to the dendritic bottom of sand and loam on which they rest. To improve the understanding of these formations researchers carried out more than 200 dives to map the morphology and bathymetry of the sea bottom on which they rest, took samples of loose rocks and sediments and placed wells to analyse the water in the depths of the sea. Previous genetic models were not satisfactory, so the samples were analysed in a laboratory. This has shown that the tegnue developed along elongated morphological structures of ancient fluvial channels present in the plain of the bottom of the sea which was above water during the last glacial period some 20,000 years ago. The sediments have been dated to 9,000 years ago and their hardening on which the first organisms became attached occurred 7,000 years ago.
Ecology