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Marine food webs transfer energy from phytoplankton (primary producers) through zooplankton, small fish, and up to top predators. Trophic efficiency is typically ~10%, meaning 90% of energy is lost at each trophic transfer. Short food chains (e.g., phytoplankton → krill → whale) are more energy-efficient than long ones. The microbial loop — decomposers and bacteria recycling dissolved organic matter — also plays a critical role in nutrient cycling. Pelagic (open water) and benthic (seafloor) food webs are coupled through the sinking of organic particles.
Calculate biomass at each trophic level given a known primary production rate and 10% trophic efficiency. Compare the relative fish yields of upwelling zones (short food chains) versus oligotrophic gyres (long food chains).
From your study of marine primary productivity, you know that phytoplankton at the ocean surface fix carbon using sunlight — they are the base of nearly all marine life. A food web maps how that energy flows upward through the ecosystem: who eats whom, and how much energy survives each transfer. Unlike a simple food chain (a single linear sequence), a food web is a network with many branching and overlapping pathways, reflecting the reality that most marine organisms feed at multiple levels and switch prey depending on availability.
The central quantitative fact about marine food webs is trophic efficiency — roughly 10% of the energy at one level passes to the next. The other 90% is lost as metabolic heat, waste, and incomplete digestion. This has enormous practical consequences. Consider a productive upwelling zone where phytoplankton fix 10,000 units of carbon. If krill eat the phytoplankton (one step) and whales eat the krill (two steps), the whales receive about 100 units — a short, efficient chain. Now consider an oligotrophic gyre where tiny phytoplankton are eaten by nanoflagellates, then by copepods, then by small fish, then by tuna — four transfers, leaving only about 1 unit at the top. This is why the world's most productive fisheries cluster around upwelling zones and continental shelves, not the vast open ocean, even though the open ocean covers far more area.
Running alongside the classical food web is the microbial loop, a parallel pathway that was only recognized in the 1980s. When phytoplankton and other organisms release dissolved organic matter (through excretion, sloppy feeding, or cell lysis), bacteria consume it and convert it back into particulate biomass. These bacteria are then grazed by protists, which are in turn eaten by larger zooplankton — funneling dissolved carbon back into the classical web. In nutrient-poor waters, the microbial loop can process more carbon than the direct phytoplankton-to-zooplankton pathway, making it the dominant energy conduit in much of the ocean.
The pelagic (open water) and benthic (seafloor) food webs are not independent systems — they are coupled by the rain of organic particles sinking from the surface, called marine snow. Dead phytoplankton, fecal pellets, and aggregates drift downward, delivering food to organisms on the deep seafloor that never see sunlight. The efficiency of this biological pump determines how much surface production reaches the deep ocean, influencing both deep-sea ecosystem richness and the ocean's capacity to sequester carbon. Where surface productivity is high, the seafloor community beneath is richer; where it is low, the deep benthos is sparse. Understanding marine food webs thus connects surface ecology to deep-ocean biogeochemistry and, ultimately, to the global carbon cycle.