Adriatic Sea

• 1. Introduction

  ›

  The Adriatic Sea holds a record that most people overlook: on the 4th of November 1966, the water in Venice rose 194 cm above normal levels. That single number tells you almost everything about the Adriatic's character. It is a sea that amplifies. Narrow and enclosed, it takes the Sun's pull on the tides, the sirocco's push across the water, and the basin's own peculiar geometry, and turns each force into something larger than it would be anywhere else. What makes this particular arm of the Mediterranean so distinct from every other sea on Earth? Why does its eastern shore bristle with more than 1,300 islands while its western shore is nearly bare? How did a sea this size become the hinge of so many empires, wars, and modern border disputes? And what does the name Adriatic itself actually mean? The answers reach back hundreds of millions of years, to a limestone platform that was once part of Africa.

• 2. A Sea Built from Stone

  ›

  The Adriatic sits on the Apulian or Adriatic Microplate, a fragment of crust that separated from the African Plate during the Mesozoic era. That separation began in the Middle and Late Triassic, when limestone started accumulating in the region. Between the Norian and the Late Cretaceous, carbonate sediments built up to depths of as much as 8,000 m, forming the Adriatic and Apulia Carbonate Platforms. Remnants of the Adriatic Carbonate Platform are still visible today in the southern Alps and the Dinaric Alps; remnants of the Apulia platform survive as the Gargano Promontory and the Maiella mountain. In the Eocene and early Oligocene, the microplate moved north and north-east, pushing up the Dinarides and the Alps. Then the motion reversed. In the Late Oligocene the Apennine Mountains rose, and the Italian Peninsula took shape for the first time, separating the Adriatic Basin from the rest of the Mediterranean. The geology never settled into stillness. An active 200 km fault has been identified northwest of Dubrovnik, and the fault causes the Apennine peninsula's southern tip to creep toward the opposite shore at about 0.4 cm per year. If that movement continues, the seafloor will be entirely consumed and the Adriatic closed off in 50-70 million years. Meanwhile, in the Northern Adriatic, the coast of the Gulf of Trieste and western Istria has already sunk about 1.5 m over the past two thousand years. The 1979 Montenegro earthquake, measuring 7.0 on the Richter scale, is a recent reminder of the region's seismic pulse. Historical earthquakes in the area include the 1627 Gargano peninsula and the 1667 Dubrovnik events, both followed by strong tsunamis. In the last 600 years, fifteen tsunamis have struck the Adriatic.

• 3. Two Coasts, Two Worlds

  ›

  Croatia counts 1,246 of the Adriatic's more than 1,300 islands. That concentration along the eastern shore is not an accident. The Dinaric Alps press close to the coastline on the Balkan side, producing a rugged, karstified shore whose submerged ridges become the island chains. Karstification began in earnest after the Dinarides' final uplift in the Oligocene and the Miocene, when carbonate deposits were exposed to the atmosphere and to weathering. That process extended down to 120 m below today's sea level, reaching as far as it did because of sea level drops during the Last Glacial Maximum. The Croatian islands include the two largest, Cres and Krk, each covering about the same area of 405.78 km2, and the tallest, Brač, whose peak reaches 780 m. The islands of Cres and the adjacent Lošinj are divided only by a narrow navigable canal cut in classical antiquity; the Greeks once knew the original single island as Apsyrtides. The western, Italian shore tells the opposite story. There the Apennine Mountains stand further from the coast, the shore is smooth and alluvial, and major islands are almost entirely absent. Only the Gargano and Mount Conero promontories break the Italian coast's regularity. The best-known Italian island feature is Venice itself, built across 117 islands. The asymmetry shapes more than geography: sea currents move counterclockwise through the Adriatic, delivering cleaner water up the eastern coast and returning increasingly polluted water down the western side. Common bottlenose dolphins are found in the eastern coast's waters but not the western ones, and the Croatian coast provides shelter for the critically endangered monk seal and for sea turtles.

• 4. Water That Never Stands Still

  ›

  The Adriatic's entire volume is exchanged through the Strait of Otranto in just 3.4 years, plus or minus 0.4 years. That replacement rate is strikingly fast; by comparison, around 500 years are needed to exchange all of the Black Sea's water. The speed matters because the rivers flowing into the Adriatic discharge up to 5,700 m3/s, amounting to 0.5% of the total sea volume, or a 1.3 m layer of water every year. The Po River is the largest single contributor, accounting for 28% of river discharge into the Adriatic, with an average flow of 1,569 m3/s. The Po ranks second in annual discharge into the entire Mediterranean; the Neretva and the Drin rank third and fourth. Freshwater also enters the sea from an unusual source: submarine springs called vrulja, which are estimated to supply 29% of the total water flux into the Adriatic. Thermal springs discovered offshore near the town of Izola have temperatures between 22 and 29.6 C and are rich in hydrogen sulfide, enabling specific ecosystems to develop around them. This constant inflow of fresh water makes the Adriatic a dilution basin for the Mediterranean as a whole, collecting a third of the fresh water that flows into that larger sea. The result is that Adriatic salinity, ranging between 38 and 39 PSUs, sits somewhat lower than Mediterranean averages. In the shallower Northern Adriatic, which rarely exceeds 100 m in depth and is only 15 m deep at its northwestern end, this freshwater influence is strongest. At the south, the permanent South Adriatic Gyre measures 150 km in diameter and drives bottom water out through the Ionian Sea all the way to the Levantine Basin, making the Adriatic the primary source of most of the East Mediterranean's deep water.

• 5. Venice and the Rising Sea

  ›

  In May 2003, then-Prime Minister Silvio Berlusconi inaugurated the MOSE project, or Modulo Sperimentale Elettromeccanico, an experimental model for testing inflatable gate technology. The project calls for laying 79 inflatable pontoons across the seabed at the three entrances to the Venetian Lagoon. When tides are predicted to rise above 110 cm, the pontoons will be filled with air to block incoming water from the Adriatic. The barriers were first deployed on the 3rd of October 2020. As of November 2020, completion was expected in 2021. The concern is not only flooding. Under a worst-case sea level rise scenario between 2050 and 2100, the barriers could need to be deployed up to 187 days a year, which would essentially cut the Venetian Lagoon off from the Adriatic and lower the lagoon's oxygen levels while trapping pollution inside the city. Venice's vulnerability is compounded by subsidence. The sinking slowed after artesian wells were banned in the 1960s, though recent studies have suggested the city is no longer sinking. Still, a state of alert remains in place. The same forces driving Venice's risk affect the Po delta and have historical precedents elsewhere along the coast: unusual tides causing flooding have been recorded in recent years in the towns of Koper, Zadar, and Šibenik. The amphidromic point, the pivot around which tidal movement rotates, sits at the mid-width of the sea east of Ancona. The tidal motion along the basin's minor axis also generates a seiche, an oscillating back-and-forth of water that adds to the overall variability. A geodetic benchmark using average Adriatic sea level at the Sartorio pier in Trieste was established by Austria-Hungary in the late 19th century and remained in use by Yugoslavia and its successor states; in 2016 Slovenia adopted a new benchmark tied to the upgraded tide gauge station in the coastal town of Koper.

• 6. Empires at the Water's Edge

  ›

  Settlements along the Adriatic dating to between 6100 and 5900 BC appear in Albania and Dalmatia, linked to the Cardium pottery culture. Greek colonisation began in the 7th and 6th centuries BC with the founding of Epidamnos and Apollonia. By 246 BC Rome had established a major naval base at Brundisium, now Brindisi, to block Carthaginian ships during the Punic Wars. That base drew Rome into conflict with the Illyrians and triggered the Illyrian Wars, fought from 229 to 168 BC. The first Roman intervention, in 229 BC, was the first time the Roman navy crossed the Adriatic to launch a military campaign. The sea then became a corridor for the Republic's civil wars. Caesar's campaign against Pompey suffered a three-month delay when winter storms and a naval blockade prevented Mark Antony from reaching him from Brundisium with reinforcements. Marc Antony and Octavian later crossed to Dyrrachium together to fight Brutus and Cassius, a campaign that ended at the Battle of Philippi. In 402 AD, during repeated Germanic invasions of Italy, the capital of the Western Roman Empire was moved to Ravenna because the surrounding marshes made it defensible and the Adriatic provided an escape route by sea. When the West fell in 476 AD, Ravenna became the capital of the Ostrogothic Kingdom. The Republic of Venice rose from this same turbulent period, receiving a Byzantine tax exemption in 1082 that helped it grow into a maritime power. In 1202 the Fourth Crusade was diverted to conquer Zadar at Venetian insistence, the first time a Crusader force attacked a Catholic city. The Ottoman Empire's defeats of the Hungarian and Croatian armies at Krbava in 1493 and Mohács in 1526 ended independent Hungarian rule and brought the Habsburg monarchy to the Adriatic shore, where it would remain for nearly four hundred years. Ottoman raids on the Adriatic coast effectively ceased after the Battle of Lepanto in October 1571.

• 7. Borders Drawn and Redrawn

  ›

  The Congress of Vienna, meeting days before the Battle of Waterloo, awarded the Illyrian Provinces, spanning from the Gulf of Trieste to the Bay of Kotor, to Austria. That single act concentrated control of the eastern Adriatic in Austrian hands for a century. The collapse of Austria-Hungary in 1918 set off a cascade of competing claims. A private force of demobilized Italian soldiers seized Rijeka and established the Italian Regency of Carnaro, a short-lived state that observers described as a harbinger of Fascism. The 1920 Treaty of Rapallo transferred Zadar and the islands of Cres, Lastovo, and Palagruža to Italy while securing Krk for Yugoslavia. The Free State of Fiume created by the same treaty was abolished in 1924 by the Treaty of Rome, which gave Fiume, the modern Rijeka, to Italy and Sušak to Yugoslavia. After World War II, the 1947 Treaty of Peace with Italy created the Free Territory of Trieste as a city-state; it was partitioned in 1954 and the arrangement made permanent by the 1975 Treaty of Osimo. During the Cold War, the Adriatic became the southernmost flank of the Iron Curtain, with Italy in NATO and the Warsaw Pact establishing bases in Albania. Yugoslavia's dissolution in the 1990s produced new disputes that persist today. Croatia and Slovenia began negotiations over the Gulf of Piran in 1992 and have not fully resolved the matter. Croatia is still not implementing the resolution of the international arbitration for the Gulf of Piran and continues fining Slovenian fishermen who follow the agreed economic zone boundaries. The maritime boundary in the Bay of Kotor between Croatia and Montenegro remains disputed at the Prevlaka peninsula; a United Nations observer mission remained there until 2002. Following Croatia's accession to the European Union, the Adriatic became an internal sea of the EU.

• 8. Living Sea, Pressured Ecosystem

  ›

  The Croatian National Biodiversity Strategy Action Plan has identified more than 7,000 animal and plant species in the Adriatic. At least 410 species and subspecies of fish live there, representing roughly 70% of Mediterranean fish taxa, with at least 7 species found nowhere else. Sixty-four known species are already threatened with extinction, largely because of overfishing. In 2000, the total weight of landings from all Adriatic fisheries reached 110,000 tonnes. Among the overexploited species are common dentex, red scorpionfish, monkfish, John Dory, blue shark, spiny dogfish, Norway lobster, European hake, and sardines. Tuna has been fished in the upper Adriatic for thousands of years, but increasing pressure eventually stopped the migration of large schools northward. The last major tuna catch in the Gulf of Trieste area was made in 1954 by fishermen from Santa Croce, Contovello, and Barcola. The Central Adriatic is particularly rich in plant life, with 535 identified species of green, brown, and red algae. Four of the five Mediterranean seagrass species grow in the Adriatic, though Zostera marina and Posidonia oceanica are comparatively rare. Fin whales, sperm whales, and Cuvier's beaked whales have all been confirmed as migrants into the sea, with fin whales being the most common. Basking sharks and manta rays also visit. Several marine protected areas now guard these habitats. Italy's Miramare reserve in the Gulf of Trieste was established in 1986 and covers 30 ha of coast and 90 ha of sea. Croatia's Kornati national park, established in 1980, covers approximately 220 km2 and includes 89 islands and islets. Albania established its first marine protected area, the Karaburun-Sazan National Marine Park, in 2010, covering a total of 12,570 ha where the Adriatic and Ionian seas meet. The Port of Trieste, the largest Adriatic cargo port, is where the Transalpine Pipeline begins, supplying 100% of southern Germany, 90% of Austria, and 50% of the Czech Republic with crude oil, a reminder that this enclosed sea carries industrial weight well beyond its coastlines.