Marine food webs

Photo: great blue heron ©

Plankton are passively drifting plants and animals that move on ocean currents. Some species can reach very high densities (up to 20 million cells per litre, over very large areas (thousands of square kilometres), and their "blooms" can be captured by satellite. Planktonic plants, bacteria, and algae (phytoplankton) are the foundation of the marine food web. Planktonic animals (zooplankton) provide a key link between the phytoplankton, that they eat, and the fish, seabirds, and other marine species that eat them.2


Seasonal change in zooplankton bloom, Strait of Georgia

Date of peak bloom
Graph: change in zooplankton bloom, Strait of Georgia. Click for graphic description (new window).
Source: adapted from Fisheries and Oceans Canada (DFO), 20105

The timing and duration of the peak zooplankton bloom has changed over the past 40 years in all Pacific and Atlantic marine ecozones+. For example, the peak abundance of Neocalanus, the dominant zooplankton species in the Strait of Georgia, occurs approximately 50 days early in the 2000s compared to the 1960s to 1970s. This has created a mismatch in timing between small fish and their zooplankton prey. Juvenile salmon that enter the Strait early in the season, such as chum, pink, and sockeye, have benefitted, while species that arrive later in the season, such as chinook and coho, have declined.15 Neocalanus has also Photo: Saanich Inlet herring, predator of zooplankton © VENUS at UVicdeclined sharply since 2001 and the decline in abundance may be accelerating and affecting species that depend on it for food.15

Spring phytoplankton blooms start earlier, are more intense, and last longer on the Scotian Shelf than they did in the 1960s and 1970s.16


Decline in krill in the western North Atlantic and Scotian Shelf

Mean number of krill (log(x+1)) per 3 m3 of filtered sea water, 1961 to 2008
Graph: Decline in krill in the western north Atlantic and Scotian Shelf. Click for graphic description (new window). Photo: krill © VENUS at UVic.
Note: no data are available for 1979 to 1990.
Source: adapted from Johns, 201017


Several zooplankton species that are considered to have a key role in the marine food web are declining. Euphausiids, or krill, in the western North Atlantic and Scotian Shelf, feed on phytoplankton in their youngest stages and are preyed upon by juvenile groundfish, pelagic fish, and baleen whales. Their abundance has declined between the 1960s to 1970s and the 1990s to 2008.18

Population trends for northern shrimp and four of their predators

Population measures specific to each species, 1976 to 2000
Compound graphic: population trends for northern shrimp and their predators. Click for graphic description (new window).
Note: Measures are: catch per unit effort (CPUE) for shrimp, millions tonnes for cod and redfish, kilograms per tow for skate and snow crab.
Source: adapted from Fisheries and Oceans Canada (DFO), 20105

In the Newfoundland and Labrador Shelves Ecozone+, in the 1990s, a decrease in groundfish abundance was accompanied by a dramatic increase in invertebrates such as shrimp and crab. A combination of several factors has potentially led to these changes in the marine food web, including overfishing of groundfish, change in water temperatures, and decreased predation on the invertebrates. In response, the commercial fishery has shifted from groundfish to species lower on the food web, such as shrimp, snow crabs, and, more recently, sea cucumber, whelk, and hagfish. The shift from a higher to a lower trophic level fishery is a worldwide phenomenon often referred to as “fishing down the food chain”.5

An equivalent shift in ecosystem structure occurred in the Gulf of Maine and Scotian Shelf, and the Estuary and Gulf of St. Lawrence ecozones+ between 1985 and 1990. The shift is reflected in decreases in groundfish and zooplankton and concurrent increases in seals, small pelagic fish, and invertebrates. A moratorium on the commercial groundfish fishery was implemented in the Gulf of Maine and Scotian Shelf in 1993, with only limited recovery of some groundfish species.5

Diet of thick-billed murre at Coats and Digges islands

Graph: thick-billed murre's diet at Coats and Digges islands. Click for graphic description (new window).
Source: adapted from Gaston et al., 200919

Hudson Bay and James Bay, the small Arctic cod is recognized as a keystone species that plays a central role in food web dynamics. Arctic cod is important in the diet of seabirds and marine mammals such as ringed seals and belugas, although it does not appear to be the sole food of any one species.20 Arctic cod can be extremely abundant – densities of 11 kg cod per square metre were recorded in ice-covered Franklin Bay in the Beaufort Sea.21

The major food of thick-billed murre nestlings at Coats and Digges islands shifted from Arctic cod to capelin in the mid-1990s. The shift reflects a change in the relative abundance of Arctic cod and capelin. Photo: thick-billed murre © Garry DonaldsonAs the extent and duration of sea ice declines, the abundance of Arctic cod, which is a sea-ice associated species, is declining, while capelin, which prefers warmer waters, is increasing.19 In contrast to Hudson Bay, capelin is decreasing as a proportion of the diet for murres in the Newfoundland and Labrador Shelves,19 where capelin abundance and size has declined.22


Global Trends

Over the past 50 years there has been a decline in size, a change in species composition, and earlier onset of phytoplankton blooms worldwide.2