A JRC-led study published in Nature Scientific Reports describes a first satellite-based indicator of food available to fish, revealing a close link between ocean productivity fronts and zooplankton biomass.
The proposed indicator describes the daily distribution of suitable feeding habitat of mesozooplankton, the medium-sized class of zooplankton. Since mesozooplankton are essential for the feeding of marine food chains, this new monitoring indicator provides critical information for research and policy, and particularly with regard to sustainable fishing.
Tracking mesozooplankton - an essential element of the food web
The article investigates the association between the biomass of mesozooplankton and satellite data on phytoplankton activity in the North Atlantic.
Mesozooplankton are small organisms (0.2 to 20 mm) that drift in oceans and seas. They form the most important food base for larger animals, including fish. Known as primary consumers, they mostly graze on the plant-like plankton (phytoplankton) near the surface of the oceans.
Phytoplankton - the first element of marine food chains - carry out photosynthesis using chlorophyll-a, a common pigment of plants that acts as a solar panel, capturing energy from the sun. As chlorophyll-a changes the way water absorbs and reflects sunlight (showing as blue and green, respectively), it can be observed from space by tracking the green features at the ocean surface. This chlorophyll-a data is taken as a proxy for phytoplankton biomass.
Based on a 15-year experiment carried out over the whole North Atlantic and using more than 50 000 zooplankton field samples, this study found, for the first time, a close link between abundance of mesozooplankton and the importance in size of chlorophyll-a fronts.
Fronts are oceanic features such as eddies that occur at the interface between two water masses, often resulting in continuous phytoplankton productivity. This is the main point on which the approach is based: as these features of oceanic productivity can last for weeks to months, they create ideal conditions for the development of mesozooplankton populations, which they need three to four weeks to peak. The longer these productivity features, the better feeding opportunities for the food chain.
Feeding hotspots and climate change
Small fish and top predators are shown to aggregate towards productivity fronts, with the result that these fronts represent feeding hotspots for the whole food chain of the surface ocean.
The direct observation by satellite allows these feeding hotspots to be monitored on a daily basis. Such tracking over the past 15 years reveals a slight increase in the amount of mesozooplankton available to fish, both at regional and global scales, despite the observed warming of the surface ocean due to climate change.
Climate change can influence oceanic productivity in two opposing ways: the warming of the upper layer of the ocean reduces productivity (as the nutrient-rich deeper waters are impeded from reaching the sunlight of the upper layer), but this is offset by the increased mixing from winds and the variable distribution of precipitation and evaporation events. The balance so far appears to favour increased productivity.
Sustainable management of fishing
The information provided by this observation-based indicator is essential for estimating the environmental variability of fishing yields and, more generally, the level of food we could sustainably extract from our oceans. It represents a major step towards the adaptation of the fishing effort to climate-induced marine productivity, and is an important indicator to help advances in marine food web modelling, fisheries science and the sustainable management of oceans.