Microscopic plankton — primarily phytoplankton and zooplankton — form the foundation of the ocean’s food web and follow well-defined seasonal cycles in Arctic and subarctic waters, including those off the coast of Alaska. In spring, extended daylight and nutrient-rich waters trigger a surge in phytoplankton growth, followed by a rise in zooplankton populations that feed on them. These zooplankton, in turn, sustain fish, seabirds, and marine mammals, passing energy upward through the food web in a rhythm that structures the entire marine ecosystem.
Although long-term averages of plankton bloom timing have not shifted dramatically in all regions, recent studies have observed increased variability and altered bloom timing in response to sea ice loss and warmer ocean conditions, particularly in ice-influenced waters like the Bering Sea. This variability can disrupt the coordination between primary producers and grazers, change how much energy stays in the water column versus sinks to the seafloor, and ultimately affect the human communities and industries that depend on healthy marine ecosystems.
Phytoplankton Bloom Timing and Its Drivers
The seasonal growth of plankton typically begins in spring, when phytoplankton — microscopic, plant-like photosynthesizing organisms — bloom in large numbers. This bloom is especially intense in Arctic and subarctic waters, triggered by a short seasonal window of extended daylight, high nutrient availability, and increasingly stable surface conditions that results from stratification, or the separation of the water column into distinct and stable layers.
Stratification is driven by two primary factors: warmer surface temperatures and the addition of freshwater, both of which reduce water density, separating it from deeper and denser layers. This is critical for phytoplankton development, as unstable and mixing ocean layers can push them below the sunlit zone, depriving them of the energy needed to photosynthesize and grow.
The mechanisms that create stratification vary across regions. In subarctic waters like the Gulf of Alaska, where sea ice is sparse, stratification is primarily caused by the heating of surface water in the spring and summer, along with freshwater input from rivers and melting glaciers. In more northerly, ice influenced regions such as the northern Bering Sea and Chukchi Sea, melting sea ice can play a major role. As sea ice melts later into spring, it injects freshwater directly into the surface layer, creating a stable environment that enables early phytoplankton blooms — often occurring before zooplankton have emerged from overwintering. In contrast, when ice retreats early, stratification is delayed, and phytoplankton bloom occurs later in the season under warmer conditions.
Zooplankton feeding and Energy Transfer
While phytoplankton blooms form the base of the ocean food web, zooplankton — microscopic animals such as copepods — occupy the next trophic level. Many are herbivorous and depend on phytoplankton, timing their life cycles to coincide with spring and summer blooms. Some hatch from dormant eggs, while others ascend from deep overwintering stages, building energy reserves to last through the colder months.
For this dynamic to function effectively, the timing of phytoplankton blooms and zooplankton emergence must be closely aligned. When blooms occur significantly earlier than zooplankton emergence, zooplankton may miss the peak of food availability, limiting their growth and reproductive success. Conversely, a bloom that occurs later can mean newly emerged zooplankton face food shortages at critical early life stages. As discussed earlier, these scenarios are strongly influenced by the presence of sea ice. In colder years, melting sea ice promotes rapid stratification and earlier phytoplankton growth, whereas in years with minimal ice cover, delayed stratification and blooms can occur — a consequence that has been documented in the northern Bering Sea.
In addition to altering timing, warming can also shift the species composition of zooplankton communities. Following the 2014–2016 Pacific marine heatwave, researchers observed a marked decline in larger, lipid-rich copepods and an increase in smaller warm-water species. These smaller copepods generally offer less caloric value to predators, and may have contributed to the widespread die-offs and reproductive failures documented among juvenile fish, seabirds, and whales during this time — all animals that rely, directly or indirectly, on energy provided by zooplankton.
plankton and benthic ecosystems
Disruptions to phytoplankton bloom timing affect not only pelagic (open-water) species but also benthic organisms — those that live on or near the seafloor. Many benthic communities, including clams, crabs, and bottom-dwelling fish, feed on sinking phytoplankton that hasn’t been consumed closer to the surface. This relationship is an example of benthic-pelagic coupling (an energy exchange between organisms in the water column, and those on the seafloor), with plankton forming a critical link/
The role of plankton in benthic-pelagic coupling is particularly strong in the ice-dominated waters of the Bering and Chukchi Seas. As mentioned earlier, when sea ice lingers into spring, it can trigger early-season ice-associated blooms before zooplankton have fully emerged. With fewer grazers present, a greater portion of the bloom sinks to the seafloor, strengthening benthic food webs. In contrast, when sea ice retreats early and blooms occur later in open water, they are more efficiently consumed by zooplankton, reducing the amount of nutrients that can reach the sea floor.
As climate change alters the timing and nature of blooms, benthic-pelagic coupling is weakening, favoring pelagic or open-water species over benthic-dependent ones. In the northern Bering Sea, long-term monitoring has revealed declines in both the abundance and diversity of benthic prey, with cascading effects on species such as walrus, gray whales, seabirds, and diving ducks that rely on seafloor food sources.
Climate Trends and Future Shifts
As climate change accelerates, the composition and structure of planktonic cycles are expected to shift, and the conditions surrounding bloom timing are becoming more variable and harder to predict. These changes can ripple through the food web: warmer conditions can favor smaller, lower energy zooplankton, benthic ecosystems may receive less sinking phytoplankton, and species that depend on high fat prey may struggle to meet their energetic demands. The stability of commercial and subsistence harvests are also at risk. Coordinated, long-term monitoring of bloom dynamics, zooplankton composition, and ice conditions will be essential for understanding and responding to these shifts as Alaska’s ocean ecosystems continue to change.
Written by ASLC Communications Coordinator Peter Sculli
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