What Lives in the North Sea? From Plankton to Whales

The North Sea supports one of the most productive marine ecosystems in the North Atlantic, hosting everything from microscopic algae that bloom each spring to harbour porpoises, grey seals, and minke whales. Covering roughly 570,000 square kilometres between Britain, Scandinavia, and the European mainland, it is shallow by ocean standards, mostly less than 200 metres deep, and its waters are stirred by powerful tides that pump nutrients from the seafloor into the sunlit surface. That combination of shallow depth, strong mixing, and nutrient runoff from some of Europe’s largest rivers makes the North Sea remarkably alive at every level of the food chain.

The Spring Bloom That Fuels Everything

Life in the North Sea runs on a seasonal clock, and the alarm goes off in early spring. As days lengthen and surface waters warm, phytoplankton, single-celled algae, begin multiplying rapidly in what ecologists call the spring bloom. In the open North Sea, the bloom typically kicks off around early March and lasts roughly six to seven weeks, peaking at concentrations of about 1.2 milligrams of chlorophyll per cubic metre before nutrients run low and the burst fades.¹ In the shallower southern bight, the bloom pattern is more complex: a mid-spring bloom typically dominated by the colonial alga Phaeocystis globosa is followed by a diatom bloom driven mainly by Thalassiosira species.²

Tides play an underappreciated role in sustaining this productivity. In the shallow southern North Sea, tidal currents resuspend sediment and organic particles, which paradoxically limits light penetration but also mixes dissolved nutrients into the upper water column. The trade-off keeps production going through summer in areas that would otherwise run out of fuel. In deeper, stratified waters further north, summer nutrient limitation takes over, and surface productivity drops.³ The result is a patchwork sea: some zones are light-limited and turbid but nutrient-rich, others are clear but starved of nitrogen and phosphorus. This spatial mosaic is part of what makes the North Sea so biologically varied.

Copepods and the Invisible Grazers

The phytoplankton bloom feeds a vast community of zooplankton, tiny drifting animals that graze on algae and in turn become food for fish. Among the most important are copepods, rice-grain-sized crustaceans that exist in enormous numbers. Two species of Calanus copepod have historically dominated the North Sea’s zooplankton community, and their fortunes are diverging in ways that ripple through the whole food web. Calanus finmarchicus, a cold-water species, has been declining, while Calanus helgolandicus, which prefers warmer conditions, has been expanding northward.⁴ Modelling suggests that if North Sea temperatures rise by another two degrees, C. finmarchicus will continue to shrink and C. helgolandicus will keep gaining ground in parts of the basin.⁵

This matters because the two species differ in size, fat content, and seasonal timing. C. finmarchicus is fattier and peaks earlier in the year, which historically aligned well with the needs of larval fish. A swap toward the leaner, later-blooming C. helgolandicus can mean less food at the right time for species further up the chain.

Sandeels and the Middle of the Food Web

If any single animal holds the North Sea food web together, it is the lesser sandeel. These small, slender fish spend much of their time buried in sandy sediments, emerging to feed on copepods and other zooplankton in dense shoals. Sandeels are spectacularly efficient at converting zooplankton energy into fish tissue, which makes that energy available to predators higher up.⁶ They are the main prey of a long list of North Sea residents: puffins, kittiwakes, guillemots, harbour porpoises, grey seals, cod, and many others.⁷

When sandeel numbers drop, the effects cascade. Research in the northwestern North Sea found that black-legged kittiwake breeding success was directly tied to sandeel abundance, and that closing the industrial sandeel fishery in certain areas appeared to benefit these declining birds, though environmental conditions before and after closure also played a role.⁸ Sandeels are one of the clearest examples of a “waist” species: they sit in the narrow middle of the food web, and squeezing them affects everything above and below.

The Big Commercial Fish

The North Sea has been one of Europe’s most important fishing grounds for centuries. Atlantic cod, herring, plaice, sole, haddock, whiting, and sprat are among the commercially harvested species, each with its own habitat preferences, spawning grounds, and sensitivity to environmental shifts.⁹

Cod is perhaps the most iconic and most troubled. Over the past century, the centre of cod distribution has shifted dramatically: where cod were once concentrated off the coasts of England and Scotland in the western North Sea, they are now found mostly in the deeper, cooler waters of the north and northeast. Statistical analysis points to warming as the main driver of the northward and deepening shift, while fishing pressure appears responsible for the eastward component, having depleted the stock from its historical stronghold.¹⁰ Temperature also affects cod recruitment: warmer water in the northwestern North Sea correlates with fewer young cod entering the population.¹¹

Herring tell a different cautionary tale. North Sea herring populations were relatively stable for centuries before collapsing in the late twentieth century under intense fishing pressure. Ancient DNA analysis has confirmed that the demographic disruptions of the twentieth century were historically unprecedented, and that the collapses left small but measurable reductions in the stock’s genetic diversity.¹² The stock eventually recovered after strict fishing restrictions, but researchers continue to debate how to prevent future collapses, given that separating the effects of fishing from environmental variability on reproductive success remains genuinely difficult.¹³

Plaice recruitment tends to suffer in warmer years in the German Bight, herring spawning biomass near Orkney-Shetland actually correlates positively with temperature, and sprat stocks are influenced by salinity in the central basin.¹¹ The take-away is that each species responds to environmental change in its own way and in its own corner of the sea, which makes North Sea fish management fiendishly complex.

Marine Mammals Above and Below the Surface

The North Sea is home to three commonly sighted cetacean species. Harbour porpoises are the most abundant, present year-round across much of the basin. White-beaked dolphins and minke whales are more seasonal visitors, with peak sightings in late summer, particularly August, and a preference for coastal areas adjacent to deeper water.¹⁴ Other cetaceans turn up less regularly: bottlenose dolphins inhabit certain coastal pockets, and humpback whales have been spotted with increasing frequency in recent years, though they remain uncommon.

Harbour porpoises face particular pressure from the North Sea’s heavy industrial use. Aerial surveys conducted between 2015 and 2022 covering more than 80,000 kilometres of transects recorded thousands of porpoise sightings and found that the animals avoided areas with frequent vessel traffic, staying away from busy shipping lanes at distances up to nine kilometres.¹⁵ In one of the world’s busiest seas, that kind of displacement can push porpoises into suboptimal foraging habitat.

Harbour seals and grey seals are the two pinniped species that breed around the North Sea’s coasts. In several regions, including Scotland’s Moray Firth, harbour seal numbers have declined over the past two decades while grey seal populations have remained stable or grown. Diet studies suggest competition is part of the story: grey seals in the Moray Firth maintained a narrow, sandeel-dominated diet even as harbour seals were forced to diversify into lower-energy prey such as gadids and flatfish, a sign that harbour seals were losing access to preferred food.¹⁶ Isotopic evidence from the broader southern North Sea shows increasing trophic overlap between grey seals and harbour porpoises over time, raising the possibility that growing grey seal numbers are ratcheting up competition not just with harbour seals but also with porpoises.¹⁷

Life on the Seafloor

Most of the North Sea floor is soft sand and mud, but scattered patches of rocky reef, biogenic reef, and gravel create hotspots of biodiversity. When rocky reef sits on an otherwise sandy bottom, local species richness roughly doubles.¹⁸ Even in pure sand, certain organisms act as ecosystem engineers. The tube-building polychaete worm Lanice conchilega creates dense beds that function as intermediate reef systems, stabilising sediment and providing shelter for other invertebrates. Blue mussels (Mytilus edulis) and sea urchins (Psammechinus miliaris) further add to habitat complexity on both natural and artificial hard substrates.¹⁹

The seafloor also acts as a massive biogeochemical engine. Organic matter that sinks from the water column is rapidly recycled in North Sea sediments, with carbon mineralisation rates highest in the nutrient-rich south and declining northward.²⁰ This recycling returns nutrients to the water column, helping to sustain the next round of plankton growth.

The Wadden Sea Nursery

Fringing the coasts of Denmark, Germany, and the Netherlands, the Wadden Sea is a vast intertidal system of mudflats, channels, and salt marshes. It has historically been the largest flatfish nursery in the North Sea, providing shallow, food-rich habitat where juvenile plaice, sole, and flounder settle and grow before moving offshore. In recent decades, however, rising water temperatures have altered the nursery’s functioning, affecting the timing of larval settlement, the suitability of habitat, and growth performance of multiple flatfish species.

How Climate Change Is Reshuffling Who Lives Where

Warming is not just affecting individual species in isolation; it is rearranging the North Sea’s biological community. During warm periods, southern-affinity species have tended to become more prominent while northern-affinity species decline. The routes of invasion are more complex than a simple push northward. Southern-water species can enter the North Sea from the south through the English Channel, but they also arrive from the north via the continental shelf west of Britain and Ireland, carried by ocean circulation patterns.²¹ That means warm-water newcomers can appear in unexpected places, and purely temperature-based predictions of where species will end up often fall short.

The copepod shift from C. finmarchicus to C. helgolandicus described earlier is one of the clearest signals, but parallel shifts are visible across the food web. Cod retreat to the north and east. Sole and bass gain ground. Warm-water jellyfish and comb jellies show up more frequently. Even phytoplankton timing is changing: blooms in the southern bight have shifted earlier in certain years, driven not by temperature directly but by changes in nutrient dynamics and salinity.² When plankton timing shifts but fish spawning schedules do not keep pace, the result can be a mismatch between larval fish and their food supply.

Wind Farms, Oil Rigs, and Accidental Reefs

The North Sea is one of the most industrialised marine environments on Earth, studded with thousands of oil and gas platforms, shipping lanes, pipelines, cables, and a rapidly growing fleet of offshore wind turbines. All of that hard infrastructure, paradoxically, creates habitat. Platforms, turbine foundations, and the rock armour placed around them to prevent scour act as artificial reefs, attracting algae, mussels, anemones, crabs, and fish.²²

Studies of offshore wind farms have found that the rocky scour protection around turbine bases supports a different and more species-rich epibenthic community than the surrounding sandy seabed.²³ Life-cycle assessments suggest that during the operational phase, offshore wind farms produce no net adverse impact on benthic communities in the original sand, and the artificial reef effect can lead to a doubling of species richness locally, along with dramatic increases in species abundance on the structures themselves.²⁴ Blue mussels use these offshore structures as stepping stones, colonising locations they could not reach in a single generation of larval drift.¹⁹ That connectivity effect could become increasingly significant as wind farm construction accelerates across the basin.

The ecological picture is not entirely rosy, though. Seabed occupation by turbine foundations displaces some soft-sediment habitat, and construction-phase noise can disturb porpoises and other marine mammals. The net effect depends heavily on how farms are sited and how much undisturbed seabed remains.

What Bottom Trawling Does to Carbon Storage

If artificial reefs are an accidental positive, bottom trawling is a well-documented negative. The North Sea floor stores substantial quantities of organic carbon in its sediments, and dragging heavy fishing gear across it disrupts that storage. Modelling work estimates that North Sea sediments hold roughly 550 metric kilotons less organic carbon than they would without trawling, and that the associated macrobenthos biomass is about 14 percent lower in trawled areas.²⁵ The carbon disturbed by trawling does not all go into the atmosphere, but a substantial fraction of the resulting aqueous CO₂ emissions is expected to accumulate in the atmosphere over decades.²⁶ Managing trawling intensity is increasingly discussed not just as a fisheries issue but as a climate one.

Bringing Back the Flat Oyster

One of the most ambitious restoration projects in the North Sea focuses on a creature most people would not associate with it: the European flat oyster, Ostrea edulis. Historical records show that the central North Sea once supported extensive oyster beds covering an area of roughly 35,000 square kilometres, about six percent of the entire sea. These beds were home to rich communities of other invertebrates and fish. They were largely wiped out by dredging before World War I.²⁷

Efforts are now underway to rebuild them, starting with pilot reefs in the German North Sea. Early results show a recognisable colonisation sequence: mobile species like fish and crabs arrive first, followed within a year by a broader range of invertebrates. Biodiversity on these pilot reefs is climbing but remains below what historical records suggest the original beds supported, a reminder that reef restoration is a slow process measured in years and decades rather than months.²⁸ Oyster reefs filter large volumes of water, stabilise sediment, and provide three-dimensional habitat in a sea that has lost most of its natural hard structure. If the restoration scales up, it could meaningfully change the biodiversity landscape of the central North Sea’s sandy floor.

Competition at the Top

With grey seal numbers rising and harbour porpoise populations still substantial, the North Sea’s top mammalian predators are increasingly bumping into each other. Diet overlap between harbour seals and grey seals is already high, with shared prey lists overlapping by about 70 percent. Grey seals and porpoises overlap less in prey species but their isotopic niches, a measure of what they actually assimilate from their food, have been converging over time.¹⁷ As grey seals expand, harbour porpoises appear to be losing access to high-energy prey items, potentially forcing them into less profitable foraging strategies.

Disease adds another layer of unpredictability. Phocine distemper virus (PDV) epidemics have periodically devastated harbour seal populations in the North Sea, most dramatically in 1988 and 2002. Modelling of the Skagerrak-Kattegat region suggests that higher frequencies of such outbreaks could weaken harbour seals’ competitive standing enough to allow Baltic grey seals to expand westward into new territory.²⁹ In a warming sea where prey composition is already shifting, the balance among top predators is genuinely uncertain, and the outcome will depend on how multiple pressures interact rather than any single factor acting alone.