Seas are smaller than oceans and are usually partially enclosed by land – they often form an interface between the land and the ocean. The exception is the Sargasso Sea, a region within the Atlantic Ocean which is bordered by ocean currents.

We often refer to the “seas” around the UK – that is because we are surrounded by a number of different seas. Around Great Britain we have the North Sea to the East, the Irish Sea to the west and the English Channel to the South. Beyond this we have the Atlantic Ocean.

The sea often appears blue because of the way light interacts with the water. White light is made up of many different visible colours ranging from red to violet – red has the longest wavelength, blue light the shortest. As water molecules are better at absorbing light with longer wavelengths, they absorb much of the red, orange, yellow and green light. The bluer colours, with shorter wavelengths, are less likely to be absorbed, giving the sea its blue hues.

Shallow water often appears clear as there are fewer water molecules to absorb the light, so other colours are able to reach the sea floor and reflect. The deeper you go, the more other colours are absorbed and the deeper blue the light becomes, until you reach the point where no visible light can reach, where it is completely dark.

The colour of the water also depends on other factors, such as what particles are floating in it. Coastal areas can sometimes look murky and brown as they contain sand from the seabed that has been churned up by waves.

Living things influence the colour, too. Phytoplankton are tiny organisms that act a bit like plants, using chlorophyll to absorb sunlight and grow. They absorb red and blue light, reflecting green light and giving the sea a greener look. Generally speaking, the more phytoplankton there are in the water, the greener it is.

Phytoplankton are really important as they produce more than 50% of the world's oxygen (meaning every second breath you take comes from the sea!) and are the foundation of marine food webs.

Anyone who has tasted seawater will know that it’s incredibly salty – it actually has an average salt content of around 3.5%. Some of the salt in the sea comes from undersea volcanoes and hydrothermal vents, but most of it comes from the land.

Rain water dissolves minerals and releases salts from the rocks on land, which are then carried down to the sea by rivers. As the sun warms the sea, water evaporates but the salt is left behind, making the sea even saltier.

It’s estimated that 4 billion tons of salt enter the sea each year, but the ocean is not growing any saltier because a similar amount of salt is deposited on the ocean floor each year, so the salt level is fairly balanced.

The saltiness (called salinity) of the sea is not the same in all parts of the world. Near the equator, where temperatures are higher, more evaporation takes place and so the seawater has a higher concentration of salt. Near the poles, melting ice and heavy rain dilute the seawater, making it less salty.

Tides are created by a combination of gravity, the rotation of the Earth and the orbit of the Moon (and also the Sun, but I’ll go into that later).

The gravitational attraction of the Moon causes the water on the side of the Earth that is closest to it to bulge towards the Moon. On the opposite side, inertia causes the oceans to bulge away from the Moon.

As the Earth rotates, these bulges travel around the world’s oceans as very long waves, which produce the high and low tides that we see on our beaches. If the moon stayed in the same place, the 24-hour rotation of the Earth would mean there would be a high tide every 12 hours. But, because the Moon is rotating around the Earth in the same direction at a much slower speed, it takes a little longer – 12 hours and 25 minutes. As a result, we still see two low and two high tides on most days.

Now for the Sun’s role! The gravitational pull of the Sun has exactly the same effect on our seas as the Moon, creating two bulges, only they are much smaller as the Sun is so much farther away. When the Sun and the Moon are aligned (on the same side of the Earth, or on opposite sides) their combined pull creates larger tides called "Spring Tides" (nothing to do with the season or time of year). When they are at right angles to each other, we see the smaller tides (called "Neap Tides"), where there is a smaller height difference between high tide and low tide.

Our seas are home to around half of all our wildlife – that’s over 30,000 species! More than 330 species of fish have been recorded in our seas, including over 40 species of shark.

UK seas are great for cetaceans (whales, dolphins and porpoises), with more than 25 species recorded. 13 of these are found regularly, including bottlenose dolphins, killer whales and harbour porpoises. Rare visitors have included the blue whale, the largest animal on the planet.  

Dolphins (and other cetaceans) can’t sleep in the same way that humans do, mostly because of the way they breathe. Humans breathe automatically, so even when we’re unconscious we keep taking in air. Dolphins don’t breathe automatically, they have to control their breathing.

Dolphins are mammals and, like other mammals, they still need their rest. To do this, dolphins sleep with one half of their brain at a time, leaving the other half awake to keep an eye on their surroundings and keep breathing. They’ll switch sides every now and then, so that each half of the brain gets chance to sleep.

Whilst sleeping, they tend to float motionless at the surface of the water, or swim slowly. Occasionally, in shallow water, they may rest on the seafloor, periodically surfacing for air. One eye will close when a dolphin sleeps, the opposite eye to the side of the brain that is resting. 

Many fish are denser than the water they live in, so without a way to make themselves more buoyant (create a lifting force) they would not be able to float in place. Most fish control their buoyancy with an organ called a swim bladder, a gas-filled sac. They can release gasses into the swim bladder or absorb them back from it to vary the amount of lift it produces.

As saltwater is denser than freshwater, fish in the sea don’t need to be as buoyant as those in lakes and rivers, so they typically have a smaller swim bladder.

Other fish have different ways of creating lift. Some, like deep-sea sharks, store fats and oils in their body to make themselves more buoyant, whilst others rely on a process known as dynamic lift – this basically means the fish have to keep swimming constantly, or they would sink.

Whales, dolphins and porpoises all belong to a group of marine mammals called cetaceans, which comes from the Latin word ‘Cetus’ meaning ‘large sea animal’ or sometimes ‘sea monster’.

The living cetaceans can be divided into two groups: the toothed whales (Odontocetes) and the baleen whales (Mysticetes). As the name suggests, toothed whales have teeth, as well as a single blowhole. Baleen whales have two blowholes on top of their head, and instead of teeth they have plates of baleen. Made from keratin (like fingernails and hair), baleen is bristle-like and acts as a strainer, filtering food from the water.  

So what about dolphins and porpoises? They both belong to the toothed whale group, but are in separate sub-groups (known as families). All oceanic dolphins belong to the family Delphinidae, whereas porpoises are in the family Phocoenidae.

The main difference between dolphins and porpoises is that dolphins have conical teeth, but porpoises have spade-shaped teeth. Porpoises also tend to be much smaller, with less of a pronounced dorsal fin.

Yes! The deeper waters of the UK are home to reefs of cold-water coral, growing from the seabed at depths of over 400m. Just like their tropical counterparts, they’re a living structure built from thousands of individual creatures, creating a home for lots of other species.

We also have soft corals living in shallower waters all around the UK. These soft corals are a bit different to the corals that form coral reefs, but really interesting nonetheless! They too are made up of many little creatures called polyps, which capture food from the water with their tentacles. We also have sea fans living in our seas – a type of slow growing coral that can live for 50 years or more.

Many of the animals recorded in our seas are seasonal visitors, passing through or arriving at certain times of year. These include seabirds, sharks, cetaceans and even turtles! Here are a few examples:

Arctic terns have the longest migration of any animal, travelling around 96,000km. These small seabirds travel to the UK in summer to nest on coasts and offshore islands, fishing in the sea for sand eels. At the end of summer, they fly back south, making it as far as Antarctica!

Leatherback turtles, the largest of the world’s seven sea turtles, sometimes visit UK seas in the summer months, feasting on jellyfish. They are more tolerant of cold water than other sea turtles, as they can keep their body temperature higher than the water around them.

Basking sharks are the second largest fish in the world, reaching the size of a bus! They’re seen in our seas throughout the summer, perhaps arriving to breed – despite their size, these sharks are mysterious and there’s lots we still don’t know about them. Other seasonal sharks include the shortfin mako and the blue shark, which is known to travel over 9,000km in a single trip.

There are lots of issues threatening our seas. Plastic pollution gets the most attention, and it is definitely a serious issue. Globally, 12.2 million tonnes of plastic enter the sea each year, with devastating effects on the life in our oceans: 90% of all seabirds have plastic in their stomachs, with approximately 1 million dying each year from ingestion or entanglement in plastics. It affects all kinds of wildlife, from the tiniest creatures to the largest.

But there are other issues that are just as important. A lack of safe havens means species don’t have chance to recover – things are improving, with the creation of more Marine Protected Areas, but there is still lots of work to be done to ensure these sites provide real protection for our marine wildlife.

Commercial fishing has a huge impact on our seas, not only by reducing the number of fish but by damaging the seabed itself – destroying fragile habitats and reducing the availability of homes for our precious marine wildlife. Development at sea can also be a problem. Both fishing and development need to be manged sustainably, ensuring they work as much for the wildlife as they do for us.

You can learn more about the threats to our seas, and how we can protect them from these threats, here.

80% of the litter in our seas comes from the land, and this includes an awful lot of plastic. It doesn’t matter whether litter is dropped by the coast or in the middle of the country, it can still end up in the sea (though obviously it gets there much more quickly if it’s dropped at the coast).

Wind and rain carry litter into rivers and down drains, which then flow into the sea and take the rubbish with them. Even rubbish that goes in the bin isn’t safe, as it can blow away during transportation to landfill, or even from the landfill site itself.

Plastics are also washed down sinks and plugholes in the form of microbeads, found in cosmetic products and many other household items. Many of these products are now banned in the UK, but some still exist.

Plastic can also end up in the sea in the form of nurdles. Nurdles are the pre-production pellets that are used to make many of our plastic products. Billions of them are lost every year through spills and mismanagement and are often found washed up on beaches. Nurdles are lentil-sized, smooth plastic pellets that come in many colours. If you find one, please report it to The Great Nurdle Hunt.

Only around 1% of the plastic in our seas shows up on beaches. The rest is floating at sea or sunken to the seabed. Remember that plastic doesn’t go anywhere – it only breaks up into smaller and smaller pieces which are more easily eaten up by marine creatures. Beach cleans help, but they are not the solution on their own – we need to turn off the plastic tap.

There are lots of things you can do at home to help save our seas. You can help stop plastic pollution by reducing your use of plastics, and recycling or reusing the rest. Think carefully about whether you actually need a plastic item, particularly single-use plastic items, and opt for reusable items instead. Don’t flush anything but pee, poo and toilet paper – facewipes and cotton buds that go in the toilet can easily end up in the sea, so put them in the bin.

Plastic microfibres from synthetic clothes (such as fleeces) are a growing threat to our oceans, with up to 700,000 microfibres released per wash! You can combat this by using special bags or balls in your washing machines to catch these tiny microfibres.

If you eat seafood, make sure your dinner is sustainable. Some species should be avoided altogether, whilst others can only be sustainably caught using certain fishing methods. Cornwall Wildlife Trust have produced a ‘Good Seafood Guide’ to help choose the right seafood.

Climate change is another significant threat to our oceans. By reducing your carbon footprint, for example by using public transport, cycling or just making sure to turn off electronic items that aren’t being used, you can help fight one of the world’s most pressing environmental issues.

For more ideas on how you can help wildlife, visit our simple actions page.