Glacier Bay’s marine world: what drives its food web, from plankton to sea otters

Almost one-fifth of Glacier Bay National Park and Preserve is marine water, none of it more than 30 miles from the coastline, which makes the park one of the few protected areas in the world with extensive saltwater habitat. Its marine waters hold three major environments: continental shelf, coastline and fjord estuaries. Nutrient-rich water from seasonal runoff and tidal turbulence, plus long spring days, produces booming communities of marine life.

The colorful world just beneath the surface is filled with life in Glacier Bay. Photo: NPS.

Why the bay is so productive

The bay is a network of large U-shaped valleys once filled by glaciers and now flooded by the ocean and fed by streams and rivers. As the glaciers retreated, the ocean moved in and made a wide array of underwater environments with overlapping biological communities. That complexity, in a small area, is one reason for the unusual variety of life. Most of the productivity is driven by phytoplankton, tiny plant-like organisms too small to see that float in a shallow, sunlit band, mainly in the Middle Bay. They form very dense populations through spring, summer and fall.

Several factors decide how productive the water is:

• Nutrients. Nitrogen, phosphorus, carbon and sulfur come from the decay of dead organisms that sink to the seafloor. Turbulence and vertical mixing carry them back up, where phytoplankton use them to grow and reproduce.
• Temperature. Surface water is warmer than the water below and changes with the seasons. The animals are most productive in the temperature range that suits their growth and reproduction.
• Salinity. Salt concentration varies with area and depth. It is highest where the water is coldest and heaviest, and lowest in the warm surface layer, where rain and evaporation change it. In estuaries, river water dilutes the sea, which affects where fish are found and so how seabirds feed.
• Seabed shape. The contours of the ocean floor steer the flow and mixing of water. Where deep currents meet a shallow shelf, cold nutrient-rich water is forced upward. In Glacier Bay the tides, working with the bottom topography, do the mixing. The balance of layered water and mixing keeps adding nutrients from the depths while holding phytoplankton in the lit zone, which the park describes as the key to the bay's productivity.
• Light. Phytoplankton need enough light to photosynthesize quickly and a stable water column to stay near the surface. All these factors come together in spring and early summer.

Sea urchins and giant kelp are common features of the marine environment. Photo: NPS.

Who lives there

The marine community includes plankton, fish and shellfish, seabirds and marine mammals, all dependent on one another. Producers make their own food, and consumers eat the producers and each other.

• Plankton. Phytoplankton are producers. In the right conditions of sunlight, stable water and nutrients they bloom, growing and reproducing rapidly for anything from hours to months. Zooplankton, small invertebrates such as krill, larval fish, protozoans and jellyfish, feed on them, and several species time their reproduction to the spring and early summer blooms. Fish eat zooplankton and are then eaten by larger fish, birds and mammals such as sea lions and whales.
• Seaweeds and salt-marsh plants. They start growing in early spring and are eaten by birds, and even mountain goats, at low tide and by snails at high tide. What decomposes sinks as detritus to feed worms, sand fleas, barnacles, anemones and clams. Kelp includes bull kelp and giant kelp, which grow in large forests.
• Fish and shellfish. More than 200 species of fish live here, including five species of Pacific salmon, pollock, Pacific halibut and rockfish, along with crabs, clams, shrimp and scallops. The intertidal zone has barnacles, sea cucumbers, sea urchins and mussels.
• Seabirds. Thousands nest on cliffs and shore rocks, among them terns, puffins, gulls and jaegers.
• Marine mammals. Humpback, minke and orca whales feed here, as do porpoises, sea lions and harbor seals, which breed and care for their young on the ice floes.

Sea otters are a common sight in Glacier Bay waters. Photo: NPS.

Sea otters are considered a keystone species and an indicator of the health of the nearshore ecosystem. They keep sea urchin numbers in check, since urchins out of control destroy kelp forests, and the forests are fish nurseries, foraging grounds and shelter from severe winter storms.

Sea anemones in Glacier Bay waters. Photo: NPS.

Seasons

Productivity explodes in spring and early summer, and the other animals follow the same rhythm. Herring and cod release their eggs into the abundant plankton, and skates make large yolk-rich eggs from energy stored earlier. Marine birds and mammals put on fat to last through the lean times. Summer is short, and as winter takes hold several species migrate south while most of the rest slow down and wait for spring.

Sources

• National Park Service, “The Glacier Bay Marine Environment,” https://www.nps.gov/articles/glba-marine-environment.htm; rewritten in hubnx's own words.