Understanding trophic links between marine plankton to consumers to improve assessments of UK pelagic habitats

Phytoplankton are responsible for 50% of global photosynthetic productivity, recycling nutrients to sustain life in our oceans. Even within the microscopic size of phytoplankton there is a huge size range, from 2 microns to 100 microns. The smallest phytoplankton are called Picophytoplankton are are less than 2 microns in size and can contribute up to 55% of the phytoplankton biomass in UK marine waters and are very adaptable. They are made up of thousands of different varieties of species, but some types, especially cyanobacteria are well studied using a method called flow cytometry based on their size and photosynthetic pigments. Cyanobacteria can thrive in low nutrient environments and associated with weaker energy transfer at each stage of the food chain. The next size is nanophytoplankton which range from 2-20 microns in size and are more complex, again made up of thousands of species and some types can be studied using flow cytometry whilst others are counted using a microscope. Larger phytoplankton are called microphytoplankton. Zooplankton are tiny animals or protozoa that eat phytoplankton and known as primary consumers. They are eaten in turn by young fish. Microphytoplankton and Zooplankton are easier to count on a microscope because they are easier to see and are used in official government assessments of marine health for sustainability to ensure larger marine organisms are getting enough food from phytoplankton or zooplankton. Currently small phytoplankton are not included in these assessments because their monitoring is less universal.

Several recent studies show Phytoplankton and Zooplankton are changing in UK waters. In the Western English Channel, small-sized cyanobacterial phytoplankton have been increasing in the summer seasons over 50 years. At the same time, key zooplankton such as small crustaceans called copepods, have declined by 50% over the same period in summer. They normally feed on phytoplankton and is a concern. It is known that size of plankton can affect how much energy moves up the food web to fish and marine mammals, with smaller phytoplankton associated with less energy transfer. One possible link between smaller phytoplankton and zooplankton is the timing of their appearance. If larger phytoplankton are growing and peaking earlier this may not match the timing of zooplankton or young fish key growth stages. Our team has already found time-based relationships within smaller phytoplankton and environmental variables and found that smaller phytoplankton growth characteristics are quite consistent across different UK water bodies using a novel method called Continuous wavelet transformation(CWT). Our earlier exploratory studies of interactions even within small phytoplankton aggregate groups and bacteria have confirmed relationships within these biological groups and also with environmental variables like temperature and nitrogen-based nutrients.

Our aim now is to see if CWT can identify time-based relationships across the marine food web from smaller phytoplankton and larger phytoplankton, zooplankton and young fish. This approach has not been used before. Ultimately we want to create a statistical model to see if smaller phytoplankton affect the growth of larger marine organisms. Additionally we want to use this approach to find key relationships that can easily be used to measure food webs in official governmental assessments of marine health, such as OSPAR.