Powerhouse of the planet: why marine micro-beasts matter

Tuesday 12th December 2023

…a guest blog written by Marianne Wootton, Senior Plankton Analyst & NMBAQC Scheme Zooplankton Manager, at Marine Biological Association as part of our COP28 impacts of climate change on invertebrates series.

The microscopic world of microbes and mini-beasts are fundamental to our existence on the planet.  On land they consume and break down organic matter, recycling and releasing nutrients that are the building blocks of life.  In aquatic environments they do the same, sitting at the very bottom of the food web, supporting almost all complex lifeforms.

Our globe is a watery world with over 70% of its surface covered by ocean; this vast habitat is home to a wide range of sea creatures that we know and love, such as: starfish, seals, fish, sharks and of course the largest animal on the planet, the Blue Whale.  All of this life is dependent on a group of organisms called the plankton.  The term plankton is used to describe a wide range of free-living organisms which, because of their small size and lack of locomotory strength, are destined to drift, at the mercy of currents and streams.

Plymouth phytoplankton © Marianne Wootton/MBA
Plymouth phytoplankton © Marianne Wootton/MBA

Within the plankton are thousands of different species which can be broadly split into two main groups, the phytoplankton and the zooplankton.

The phytoplankton are singled celled algae, tiny plants, that photosynthesise (to make food), just like plants on land, and produce oxygen as a by-product.  As the marine environment is so huge and these microbes are so numerous, uncountable billions, the amount of oxygen they produce is huge too.  Approximately 50% of all oxygen on Earth is produced by the phytoplankton, so you could say that every other breath is a plankton breath!

As well as performing this essential life-giving function, the phytoplankton provide food for a diverse range of microscopic animals known as the zooplankton.  Many species of organisms that we are familiar with around our coasts, such as barnacles, crabs and sea snails, begin their life in the zooplankton as larvae; these we call meroplankton.  Just like on land when a caterpillar metamorphoses into a butterfly, very often the larval forms of these marine creatures can look very different to the adult form.

Crab larva © Marianne Wootton/MBA
Crab larva © Marianne Wootton/MBA

Many species of zooplankton however may never grow more than a few millimetres in size and spend their whole life as part of the plankton; these we call holoplankton. The crustaceans dominate this forever drifting portion of the plankton, which includes water fleas, krill and the mighty copepod. Copepods are arguably the most abundant multicellular animals on Earth, they are so abundant in our oceans that they outnumber insects by three orders of magnitude.

The zooplankton provide food for larger organisms in the marine environment, for example copepods are an essential prey item for juvenile fish, who are in turn food for larger fish, sharks, seals and dolphins and seabirds.  Gelatinous organisms, like jellyfish and salps, are part of zooplankton too and are an important food source for sea turtles.

Planktonic creatures, like all organisms, are made of carbon; also like all organisms they excrete carbon.  This continuous rain of excretions from trillions of planktonic bodies coupled with their dead carcasses, aggregate and sink down in the ocean as “marine snow”.  This marine snow functions as a carbon sink, drawing carbon dioxide from the atmosphere down into the deep ocean, potentially locking it away for millennia.

Plankton can be sensitive to their environment, with each species requiring a different set of optimal conditions in which to thrive and survive.  Temperature, salinity, pH, dissolved oxygen and water chemistry can all impact the geographic distribution of where we find certain species and affect their abundance.  This makes plankton not only an indicator of ocean health, but also of planetary health.

We know our planet is changing, global temperatures are on the rise and this is affecting our oceans.

Continuous Plankton Recorder Body © Marianne Wootton/MBA

The Continuous Plankton Recorder (CPR) Survey has been monitoring the health of our oceans for over 90 years and is the longest running, most geographically extensive marine survey in the world.  Currently based in Plymouth, UK, the CPR was invented by Sir Alister Hardy in the 1920s to further our understanding of fisheries around the UK.  The CPR collects phytoplankton and zooplankton on a moving band of silk gauze and is now in operation throughout the world.  Using an unchanged methodology of sample collection and plankton analysis for over 60 years, the CPR Survey maps the occurrence and abundance almost 800 planktonic taxa.

Throughout the history of the CPR Survey, hundreds of scientific reports have been published from its findings, however there is a re-occurring theme and that is linked to the poleward movement of temperate planktonic species, in response to warming sea surface temperatures; with some species having moved 1000km northwards in the last 50 years.

A recent study comparing 60 years of data from the CPR Survey together with datasets from 15 other plankton research institutes, has demonstrated a general decline in the abundance of plankton in the North East Atlantic.  The research shows how complex the marine system is, with some regions displaying resilience and others vulnerability; with winners and losers across the different key planktonic groups.

Changes in species distribution and abundance of plankton have been shown to affect the survival of larval fish, including Cod. Of course these impacts are not only limited to fisheries, but to other forms of marine life that might feed on the plankton or depend on these fisheries.

The plankton provide us with half of our oxygen, are involved in the diet of nearly all marine animals and are an essential component of the biological carbon pump.  Life on Earth would look very different without these marine micro-organisms.  Continued monitoring of planktonic species is critical if we are to try and understand how we are affecting our oceans and what consequences this may have on our planet.

Would you like to learn more about the CPR Survey?  Visit the Marine Biological Association website mba.ac.uk

Marianne Wootton © Cornwall Climate Care
Marianne Wootton © Cornwall Climate Care

Marianne is Senior Analyst for the CPR Survey. Having worked for the Marine Biological Association as a Plankton Analyst since 2002, she has a wealth of experience in phytoplankton and zooplankton identification from around the world. Marianne’s taxonomic expertise is regularly used to teach plankton identification to a wide range of learners, from university students to other research scientists.  She is co-author on a plankton guide to the North Atlantic and, in collaboration with the NE Atlantic Marine Biological Analytical Quality Control Scheme (NMBAQC), is a lead developer of an international zooplankton quality assurance scheme.  Since joining, Marianne has been involved in many aspects of the CPR survey, from Workshop Technician and Shipping Clerk to quality control of analysis data and plankton taxonomy.  Marianne is currently chair of an International Council for the Exploration of the Sea working group, EUROBUS, looking at the spread of the non-native copepod Pseudodiaptomus marinus.