The North Sea, a shallow shelf sea situated off the northwestern coast of Europe, represents a significant marine environment. Its waters are relatively shallow, averaging 90 meters, though reaching over 700 meters in the Norwegian Trench. This geography, with strong currents and freshwater inputs from major European rivers, creates nutrient-rich conditions that support a wide array of marine habitats. As a result, the North Sea serves as an important home and migratory pathway for a wide variety of marine life.
Fish and Other Open-Water Dwellers
The open waters of the North Sea contain diverse fish populations that form the basis of food webs. Pelagic fish, those that live in the water column away from the seabed, are abundant, often forming large schools. Species such as herring (Clupea harengus) and mackerel (Scomber scombrus) are examples, undertaking migrations through these waters. These schooling behaviors offer protection from predators and facilitate feeding on smaller organisms.
Smaller pelagic species, including sprats (Sprattus sprattus) and sandeels (Ammodytes marinus), also inhabit the North Sea’s water column in large numbers. Sandeels, despite their name, are small, slender fish that burrow into sandy sediments during periods of inactivity but spend much of their time in the water column feeding. They are an important food source for many larger fish, marine mammals, and seabirds. The abundance of these schooling fish supports the North Sea ecosystem.
Beyond fish, the pelagic zone is home to gelatinous organisms in the open-water ecosystem. Several species of jellyfish, such as the lion’s mane jellyfish (Cyanea capillata) and the compass jellyfish (Chrysaora hysoscella), drift through the water, preying on smaller zooplankton and fish larvae. Comb jellies (ctenophores), like the sea gooseberry (Pleurobrachia pileus), are common, using iridescent comb rows for propulsion and sticky tentacles for capturing prey. These invertebrates play a role in nutrient cycling and as a food source for some fish.
Marine Mammals and Seabirds
The North Sea supports a diversity of marine mammals and seabirds, with many species relying on its waters for feeding and breeding. Common seals (Phoca vitulina), also known as harbor seals, are often observed in coastal areas, often hauling out on sandbanks and rocky shores. Grey seals (Halichoerus grypus) are prevalent along the eastern coasts of the UK and parts of the Wadden Sea. Both species primarily feed on fish, acting as apex predators in the nearshore environment.
Several cetacean species, including whales, dolphins, and porpoises, navigate the North Sea. The harbor porpoise (Phocoena phocoena) is the most common cetacean, often seen individually or in small groups, preying on small fish like cod and herring. White-beaked dolphins (Lagenorhynchus albirostris) are regular visitors, known for energetic surfacing and social behaviors. Other larger whale species, such as minke whales (Balaenoptera acutorostrata), occasionally enter the North Sea, following prey concentrations.
The North Sea’s coasts and islands provide important breeding grounds and feeding areas for millions of seabirds. Kittiwakes (Rissa tridactyla), gannets (Morus bassanus), and gull species (e.g., herring gull, Larus argentatus; great black-backed gull, Larus marinus) are widespread, feeding on fish and discards from fishing vessels. Auks, including common guillemots (Uria aalge) and razorbills (Alca torda), are proficient divers pursuing pelagic fish. Puffins (Fratercula arctica) are iconic inhabitants, nesting in burrows on cliff edges and foraging for sandeels.
Life on the Seabed
The seafloor of the North Sea, known as the benthic zone, hosts a wide array of life adapted to living on or within the sediments. Demersal fish, those that live near or on the seabed, constitute a large portion of the North Sea’s fish diversity. Cod (Gadus morhua) and haddock (Melanogrammus aeglefinus) are important commercial species foraging on the seabed for smaller invertebrates and fish. Flatfish like plaice (Pleuronectes platessa), sole (Solea solea), and turbot (Scophthalmus maximus) are common, camouflaged against the sandy or muddy bottom.
A wide variety of invertebrates inhabit the North Sea’s seabed in its ecosystem. Crabs, such as the edible crab (Cancer pagurus) and shore crab (Carcinus maenas), scuttle across the bottom, while lobsters (Homarus gammarus) seek shelter in rocky crevices. Echinoderms like starfish (e.g., common starfish, Asterias rubens) and sea urchins graze on algae and detritus. Sea anemones, sponges, and worms, including polychaetes and bivalves like clams, burrow into or attach to the sediments, filtering food or consuming organic matter.
Marine plants anchor to the seabed, forming habitats in shallower, sunlit areas. Kelp forests, composed of large brown algae like sugar kelp (Saccharina latissima), thrive in rocky areas with light, providing shelter and food for invertebrates and juvenile fish. Seagrass meadows, composed of common eelgrass (Zostera marina), are found in sheltered, sandy or muddy coastal areas. These underwater flowering plants create nurseries for many species, stabilize sediments, and contribute to coastal biodiversity.
The Unseen Foundations of Life
Below the surface, the North Sea’s ecosystem is supported by microscopic organisms, primarily plankton, which form the base of the marine food web. Phytoplankton, microscopic plants like diatoms and dinoflagellates, are the primary producers. These single-celled organisms harness sunlight through photosynthesis, converting inorganic nutrients into organic matter, much like plants on land. Their numbers are important, as they are the initial energy source for nearly all marine life.
Following the phytoplankton in the food chain are zooplankton, microscopic animals drifting with currents. This group includes copepods, krill larvae, and the larval stages of larger marine animals like fish and crustaceans. Zooplankton graze on phytoplankton, transferring energy up the food web. They are an important link, a direct food source for small fish like herring and sprats, as well as larger filter feeders such as baleen whales.
Beyond plankton, the North Sea’s waters and sediments contain other microscopic life forms, including bacteria and archaea. These microorganisms play an important role in nutrient cycling, breaking down organic matter and returning nutrients like nitrogen and phosphorus to the water column. This recycling ensures a steady supply of nutrients for phytoplankton, sustaining the North Sea ecosystem’s productivity. Their unseen work supports the marine environment’s health and productivity.