All About Denmark

Technical diving skills can add range and bottom time, as well as provide added margins of safety, but they are not a requirement for most dives. Essentially, the seas are a submerged landscape that stretches from the brackish waters of the Baltic, over the straits between the main islands to Kattegat and Skagerrak, and on to the North Sea, which has almost the same salinity as the oceans.

Most parts of the seabed are soft, composed of sand or mud, but there are also extensive areas with a hard bottom made of gravel and pebbles, and what are locally called stone reefs, as well as some unique “bubbling reefs,” which are vertical sandstone structures formed by bacteria utilising methane gasses seeping up from deposits deep underground.

Due to the marked differences in salinity, which is significantly lower south of the Danish straits, there is also a pronounced difference in habitats and species across the waters, most notably along the north-south axis, which is the direction of water flows and the overall salinity gradient. Biodiversity is generally the highest in areas with the biggest salt content, whereas wrecks are much better preserved in areas where salinity is low. Once past the Danish straits and into the Baltic, the salinity drops and steadily decreases, eventually transitioning into brackish water with low biodiversity.

The Danish archipelago is a convoluted mosaic of islands, fjords, bays, inlets, estuaries and straits connecting the main bodies of water. Despite being only 368km (229mi) from the northernmost point to the southernmost, the country’s coastline is officially 8,754km long. There are 443 named islands and 1,419 islands bigger than 100m². In other words, one can nearly always find an alternate dive location nearby if the first one blows out.

Denmark does not offer much of an integrated hospitality-and-dive industry in which accommodation, meals and diving are offered as a combined package by an operator, but many of the larger dive centres will coordinate or have deals with nearby hotels, holiday homes or summerhouse rental bureaus. And fine dining, including 25 Michelin-star restaurants, can be found not far away in cities and towns across the country.

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Diving takes place all year, but activities are markedly lower during winter for obvious reasons; it is much colder, and the days are short and often murky. That said, winter and spring diving often rewards those who choose to venture out while the water is cool and therefore has very little or no algae, resulting in excellent visibility. Summers tend to be quite lovely, with pleasant temperatures above and under water, as well as long days and evenings with white nights around midsummer. During summer, there is no other place I would rather be.

Bottom topography, salinity and currents

The key to appreciating the nature and distinct qualities of Danish dive sites and diving is understanding how the bottom profile forms a saddle point going across the Danish Straits in an east-west direction. This ridge or shallow plateau divides the seas into waters of significantly different salinity, and as a result, also into different types of biotopes.

The three straits are named Øresund, Store Bælt and Lille Bælt (The Sound, Great Belt and Little Belt, respectively). The former two are busy shipping lanes connecting the Baltic Sea to the North Sea and Atlantic Ocean.

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South of the straits

South of the straits, we find the Baltic Sea, which is brackish. It was a freshwater lake, called the Ancylus Lake, only 8,000 years ago, and there is a continuous inflow of freshwater from groundwater, rivers and streams in its catchment area in Northeast Europe. The Baltic discharges through the Danish straits adjoining the North Sea; however, the flow is stratified and complex.

Because fresh and brackish water is less dense than oceanic seawater, as the Baltic water flows out, northwards, it does so on top of the saltier oceanic water, which creeps along the bottom in the opposite direction, wedging underneath in a southbound direction.

The straits are not just horizontal constrictions, they are also shallow, except for some winding deep-water channels, which serve as shipping lanes. This is where land bridges, not too long ago, separated the freshwater lake from the ocean and made southern Sweden contiguous with Central Europe.

As a result, the bottom profile of the straits constitutes thresholds, akin to a tall doorstep on the seabed, which obstructs deeper layers from flowing over and into the Baltic—not entirely, but to quite a limited extent. The significance of this inflow of salty water is, among other things, that it brings oxygen to the deeper layers of the Baltic, which are otherwise quite depleted.

South of the straits, biodiversity drops significantly because far less species live, or can even survive, in brackish water than in fresh or saltwater.

On the other hand, because salinity is low, wrecks are much better preserved, which is also due in part because shipworm, which devours timber, cannot thrive here. So, old wooden wrecks, often many centuries old, remain well preserved on the bottom.

In recent years, often when transects are done prior to laying down cables or pipelines, several medieval shipwrecks have been discovered with their rigging still intact. The Vasa, now on display in its own museum in Stockholm, is arguably the most famous example of a perfectly preserved warship from the 17th century. Steel ships also suffer much less corrosion and remain intact for a much longer time thanks to the same low salinity as well as lack of oxygen in the Baltic Sea.

North of the straits

North of the straits, in between the peninsular part of Denmark called Jutland, and the Swedish west coast, we have a body of water known as Kattegat, which is saline, just slightly less than the ocean. Here, we find the usual complement of saltwater flora and fauna.

This salty water, being denser than the water in the Baltic, flows south along the bottom, underneath the outflowing brackish water from the Baltic, often resulting in a strata of currents going in different directions at different depths.

It is predominantly in the three straits that we find the best and most diverse dive sites, and surely the most popular. This is down to two main factors. Firstly, the straits have been the main shipping lanes between the Baltic and the Atlantic for centuries, if not millennia, so this is obviously also where many shipwrecks have occurred. The Danish waters are littered with them, including modern vessels, warships and planes from WWII, warships from WWI and earlier, tall ships and cargo vessels, Hanseatic kogges and Dutch flutes, Viking ships and canoes from all the way back to the Stone Age. There are tens of thousands of registered locations, but, granted, that count includes wrecks of which there is now barely anything left to see.

The second reason is the current, which in places—most notably in Lille Bælt (Little Belt), which is arguably the area with the best dive sites in the country—carves out steep banks and drop-offs close enough to the beach that you can just walk out from the coast and dive to, say, a depth 40m without much of a swim. We will get back to that in more detail in the following stories.

Where there are currents, there are also nutrients, and consequently, thriving life, and the banks along the straits or stone reefs often display prolific life.

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Free app forecasts currents

The Danish Maritime Authority has made a website and created an app that is simply a brilliant tool for divers.

It is called Sejladsudsigt, which roughly translates into “Forecast for Sailing.” It is available for iPhone, iPad and Android.

It comes with a fascinating and most useful feature forecasting the currents, even at different depths. The screenshots below show you how it looks.   

The interface and menu system are also in English. All one has to do is select a region and pick a depth, and then the app displays colour-coded graphs depicting the predicted strength and direction of currents, which may be different at different depths.

So, now one can know what to expect in advance and prepare accordingly for a planned dive. ■

Click the titles below to see more feature articles on diving and wrecks in Denmark:

Denmark: Diving & Dining
From the capital city of Copenhagen, across the Great Belt to Funen and the Little Belt to Jutland, travelling through the green fields of the Danish countryside, Scott Bennett describes his diving and dining adventure through Denmark, stopping along the way for a five-course meal at one of the 25 Micheline star restaurants found across the country.

Denmark's Wrecks: A Selection from WWII to the Age of Sail
The seas around Denmark have seen thousands of shipwrecks from ancient times until today. We take a look at a selection of wrecks from WWII minesweepers to WWI Battle of Jutland armoured cruisers to Age of Sail vessels with cannons.

Denmark's Øresund & Isefjord
Diving in Denmark, how does it really measure up? Since Morten Bjørn Larsen lives in Copenhagen, he talks about his favorite dives in and around the island of Zealand, where the capital city is located. Several wrecks in Øresund and a bridge in Isefjord top the list.

M/F Ærøsund: From Ferry to Artificial Reef
M/F Ærøsund is a former ferry that served the islands in the South Funen archipelago. It was scuttled in 2014 in a sheltered bay just 550m off Funen’s southern coastline where it now rests at a depth of only 19m. It is easily visible from the surface.

Denmark: Marine Archaeology
Stone Age settlements and canoes, Viking ships, medieval cogs, fluyts, tall ships, warships, defence systems, jetties, harbour installations and aircraft wrecks—Denmark has got it all.

Denmark: Slåensø – Freshwater Diving
Approximately right in the centre of Jutland, the western peninsular part of the realm, there is a region called Søhøjlandet, which translates to “The Lake Highland.” It is a sparsely populated and largely forested area with many lakes, a large part of which make up a reserve.

Dive Industry in Denmark: The JJ-CCR Story
Some 80km south of Copen­hagen, about one hour’s drive, in what appears to be just some ordinary and inconspicuous farm buildings surrounded by fields out in the countryside, we find JJ-CCR—manufacturer of world-renowned closed circuit rebreathers.