Mersey Plankton Project

Here at MGET we undertake a whole host of different surveys. From birds to plants we try to record as much of the biodiversity of this amazing habitat as possible. Thanks to funding received by The Environment Partnership in 2024, we have been undertaking plankton surveys in the river to try and learn more about these amazing mini-beasts. In this month’s blog we will talk about why planktonic animals are so important, how we go about surveying for plankton and we will also take a closer look at some of the species we catch during our surveys.

Why plankton?

First of all, it is useful to explain what plankton are. Plankton are tiny plants, animals and bacteria that float or drift through the water. They are not strong enough swimmers to move against tides or currents, so they are carried wherever the water takes them. For us in the upper estuary, plankton populations are particularly interesting because we get both freshwater and marine species, often in the same location! During our plankton surveys we also catch and record larger more mobile invertebrate species, so we include them in our plankton surveys too.

Plankton numbers are of particular interest to us at MGET as not only are there hundreds of species that depend on the plankton population for food, but they are also a useful indicator of how clean the Mersey water is.  By monitoring plankton diversity over the course of a year, we can learn more about how these species respond to changes in water quality.

Mersey Plankton projects

In 2024, we started our first plankton project alongside University of Salford student, Sophie. This research project investigated the diversity of plankton species in the Mersey estuary throughout different tide stages at four different locations. Sophie found that there are certain windows in a tide where plankton abundance and diversity is highest. This is during the flood of the tide when the currents sweep planktonic animals’ up stream. This study helped us to identify where the key survey windows are to ensure we are able to sample as many species as possible.

During Sophie’s study, and through our own surveys, we have detected really high numbers of certain species, I’ll discuss some of these in more detail later. It is these species that provide food for the hundreds of thousands of birds and fish that call the Mersey their home.

The diversity of benthic (bottom dwelling) and pelagic (drifting) species means that many different predatory species, with very different feeding behaviours, can come to the Mersey to feed. Species like shelduck (including our resident ruddy shelduck) and teal dabble in the thick mud sifting out food items like mud scud and brown shrimp. Filter feeding species like Shovelers are able to sift through the water to feed on the pelagic species. Ambush predators like grey herons and little egrets feed on the abundant estuarine prawns this can be found all through the estuary.

How do we do our plankton surveys?

Using the funding we obtained from the TEP Environment grant, we purchased a number of plankton nets and sampling trays. These nets are like large cones with a filter in the narrow end. The nets sample passively, that is that we throw them into the water, allow the current to carry them downstream and then wait for the plankton to come to us! We typically leave the nets in the water for fifteen to twenty minutes per deployment. This ensures that anything that gets caught in the net doesn’t have to be in there for very long. Once we have hauled the net back in, we unscrew the filet at the end of the net, pass the contents through a kitchen sieve to remove the fine sediment then transfer the remaining contents into the sampling trays for counting. We also have a small observation tank and aerator to make sure the animals we catch don’t get too stressed before we return them back to the river.

We use field microscopes and hand lenses to get an up close and personal look at our catch and to help with identification. For some species where dissection is required for identification to species level, we just record which genus the species is from.

We try to identify and count what we sample as fast as possible, and we aim to make sure that every animal we catch goes back into the river fit and healthy.

What species do we catch?

We catch lots of different types of animals during the surveys, from fish to jellyfish and many, many different types of shrimp. In fact, since we have been doing our surveys we have recorded over 30 different species! Here are the top three most common species we catch:

1: Copepods

Copepods (not a species but a group of species) are absolutely the most abundant plankton we find in the river. In fact, we catch so many it’s impossible to count them in the field! Despite their tiny size, they are able to thrive in the Mersey’s constantly changing conditions.

These little drifters graze on phytoplankton (plant plankton) and in turn become food for other invertebrates, fish and birds. As primary zooplankton, they form a crucial link in the food web. Copepods might be easy to overlook, but they play an essential role in keeping the Mersey a functioning ecosystem.

2: Mysis shrimp

The mysis shrimp is our most abundant macro (big but still little) species. Around the size of an earwig, this amazing free swimming shrimp is part of the family known as opossum shrimp as they give birth to live young! We often see the tiny juvenile shrimps wriggling around inside the female’s brood pouch, a specially adapted body part!

These species are filter feeders and help to break down the organic material in the river. In one net haul we can catch over 1000 of these shrimps, and they are a valuable food source for birds and fish.

3: Brown shrimp

The brown shrimp is another macro species that we catch lots of. Probably most famous for being the shrimp used for potted shrimp, these little flat shrimps sift through the sediment on the riverbed. Masters of camouflage, they blend into sandy riverbed, hiding from predators and ambushing prey.

We also find lots of them in our tidal creeks and pools. By feeding on worms, small crustaceans, and organic matter, they help recycle nutrients and keep ecosystems balanced. In turn, they are another vital food source for fish and birds.

What else do we find?

During our surveys we have recorded some species which we really did not expect to find so far up the river. As we mainly undertake our surveys at Spike Island (17 miles inland from the mouth of the river) many of the species we catch are freshwater and brackish species. However, during the project, we have found many species that I think many people would be shocked to see! Some of the real surprises are:

Moon jellyfish (Aurelia aurita)

Moon jellyfish are very common around the UK coast and are instantly recognisable from the four rigs you can see in their bodies. Moon jellyfish can swim, however they spend most of their time drifting around feeding on plankton like copepods and mysis shrimp, they use their long stinging cells to help catch their prey.

We usually catch a few moon jelly fish each year during our surveys from Spike Island and Widnes Warth. However, in a recent survey in June 2026, we caught five of them in just a few hours!

Juvenile European seabass

We were really excited about this one! During our monthly surveys in 2024 and 2025 we caught juveniles of many species of fish including dace, chub, perch, roach and common gobies. Incredibly during some surveys at Fidlers Ferry sailing club we also caught a small number of very small European seabass!

These amazing little fish were likely to have come from a nearby, un-recorded spawning ground. The presence of these little bass help to support the data from a recent scientific paper, published by researchers from the University of Bangor, which suggest warmer sea temperatures have resulted in an upward shift in the breeding range of bass.

Larval crabs

These guys have to be one of the cutest species we catch. Whilst we can’t really tell which species of crab they are, during the summer we find small numbers of free-swimming crab larvae.

These larva are between 1mm and 2mm long (about half a grain of rice or a quarter of a ladybird). The claws and legs are visible and recognisable as a crab, but they still have tiny tails that let them swim around up in the water column. As they become older this tail folds underneath them and they spend much more time on the riverbed.

It is amazing to see such a diversity of life in the Mersey estuary. These small species that we don’t see day to day, and may not even realise are there, are absolutely key in the estuary ecosystem.

If you would like to learn more and see the species for yourself, we will be holding another Mersey Plankton Open Day on Tuesday 11^th August.  Join us at the old transporter bridge slipway on West Bank Promenade to discover the secret world of River Mersey plankton! Keep an eye on our social media pages and the events page of our website for further information.