By Ana Júlia Alves de Lima
If you work in the fishery, you already know this: when the ocean changes, your livelihood feels it first.
But one of the biggest changes happening right now is something we cannot see, a shift in how efficiently energy moves through the marine food web.
Across the Northwest Atlantic, plankton communities are reorganizing in response to rapid climate change. Ice cover is retreating in the north, marine heat waves are becoming more frequent and intense, and species are shifting their distributions to follow their preferred temperature ranges. When the base of the food web moves, the entire system feels the ripple effect.
A general rule scientists use to describe how energy moves through the food web is the “Rule of 10 per cent.”
Only about 10 per cent of the energy at one trophic level is passed up to the next. A classic example illustrated by the trophic pyramid (Figure 1) is this: roughly 1,000 tonnes of phytoplankton will support only about 100 tonnes in herbivorous zooplankton. By the time that energy reaches large predators, such as sharks, only about 0.1 tonnes (or 100 kg) of that original weight remains.
This means the ocean must produce massive amounts of plankton just to support the larger species we depend on. It also makes clear how small changes in energy transfer efficiency can dramatically influence how productive an ecosystem is.
But the 10 per cent Rule is only an average. In reality, there is massive variability in this value; efficiency can range from 0.3 per cent to 52 per cent between different levels of the food web. Energy transfer efficiency can vary a lot across regions, depending on water temperature, productivity, food quality, species composition, community metabolism and even the body-size structure. These variations are critical because they determine how much energy ultimately reaches the fish and invertebrates that support our fisheries.
And this is where climate change becomes crucial. When warming pushes plankton communities toward smaller or less nutritious species, or when higher temperatures speed up metabolism and recycling at the surface, fish receive less usable energy. Growth slows, body condition declines and fewer individuals reach harvestable sizes. In some regions, the food web could become so inefficient that even healthy fish populations would struggle to sustain historical catches.
My research focuses on understanding how this process varies across the Northwest Atlantic. I’m comparing how efficiently energy moves through the food web from the Scotian Shelf to Baffin Bay. Northern ecosystems, for example, may be naturally more efficient than regions farther south, but warming could start eroding that advantage.
To do this, I combine multiple approaches. Ecosystem models map the flow of energy among species. Stable isotopes provide a biochemical fingerprint of who eats whom. Productivity estimates reveal how much energy enters the system and size-spectrum models show how body-size govern the structure and trophic pathways of the ecosystem. By lining them up, we will be able to pinpoint where the system is strong, where it is weakening, and why.
This work is important because every commercial fishery — shrimp, crab, groundfish, pelagics — depends on this flow of energy. If warming continues to force less-efficient plankton communities into certain regions, some fishing grounds that are productive today may naturally support fewer fish in the future, regardless of management actions.
This research is meant to serve as a base for an early warning system. By understanding where energy is being lost and where it is still flowing efficiently, we can anticipate which areas are likely to remain resilient, and which ones may enter a period of decline as the climate continues to shift. We cannot manage what we do not understand, and right now the most important changes in the Northwest Atlantic are happening at the very base of the food web.
Ana Júlia Alves de Lima is a graduate student at the Fisheries and Marine Institute of Memorial University of Newfoundland.
