Glacier Bay’s Waters in 2020: Warmer, Fresher and a Weaker Plankton Bloom

Glacier Bay’s oceanographic monitoring runs year-round from the research vessel Fog Lark.

The glacial fjords of Glacier Bay National Park and Preserve were once filled with ice. Now cold, nutrient-rich seawater fills them and fuels extraordinary productivity: humpback whales, brown bears, salmon and puffins all depend on the ocean. This National Park Service resource brief reports the 2020 results of a monitoring program that has measured the bay every year since 1993. Oceanography became a “vital sign” of the Southeast Alaska Inventory & Monitoring Network in 2009.

The 2020 findings

• Glacier Bay’s waters are warming at 0.1°C per decade and freshening at 0.28 psu per decade.
• Surface waters were 0.5-2°C warmer than normal in summer and fall 2020, except in the East Arm. The bay’s annual average temperature was the 10th warmest in the 28-year record.
• The phytoplankton bloom that powers summer productivity was about 15% weaker than usual.
• Salinity stayed near normal despite one of southeast Alaska’s wettest years on record, especially summer.
• Long-term trends in the park mirror the Gulf of Alaska, where surface temperatures returned to near average early in 2020 after recent marine heatwaves.
• Other research suggests biological recovery from the 2014-2016 marine heatwave continues.

How it is measured

Scientists sample 22 stations along the length of the bay, from Icy Strait outside the mouth to the heads of the East and West Arms. Eight stations are sampled March through October to follow the seasons, and all 22 in mid-summer and mid-winter. At each, a metal frame of instruments is lowered from the research vessel, and the sensors record continuously from surface to seafloor: temperature, salinity, light, turbidity (sediment in the water), dissolved oxygen and chlorophyll fluorescence, a sign of phytoplankton.

Map of Glacier Bay and the monitoring stations (red triangles), with the Lower Bay, Central Bay and East and West Arms outlined (National Park Service).

Three regions

The Lower Bay opens to the Gulf of Alaska through Icy Strait and Cross Sound. Comparatively warm, salty Pacific water is mixed hard by tidal currents as it crosses rocky sills left by the ice. The narrow, deep East and West Arms are ruled by their glaciers: from spring to fall they receive huge quantities of cold, fresh, sediment-laden meltwater, which makes a freshwater layer with sharp temperature and salinity differences, and cloudy water that blocks light and cuts productivity. Glacier signals reach about 40 miles to the Central Bay, which is broad and moderately deep, mixes the two and has large, long phytoplankton blooms.

In July 2020 the upper East Arm (5.5°C on average) and West Arm (6.2°C) were much colder than the Central and Lower Bay (10.1°C). Summer and fall surface temperatures in the West Arm and Central Bay ran 0.5-2°C above normal, but the East Arm stayed near its 1993-2019 average. With over 20 inches of rain from June to August, the freshwater layer was deeper than usual across both arms, and the sediment plumes that normally reach the Central Bay were seen only in the far north of the arms.

Seasons and the food web

The seasons matter because everything depends on food. Phytoplankton feed zooplankton such as krill, which feed small schooling fish, the favorite prey of many marine mammals and seabirds. In the long Alaskan winter the bay is cold, salty and mixed from top to bottom, with short days, little sun and frequent storms, and the ecosystem is relatively dormant. In spring the sun returns and snowmelt begins to layer the water, with lighter fresh water on top of denser salty water, trapping phytoplankton where light and nutrients are abundant. They bloom in April and May, followed by zooplankton and forage fish. By midsummer, with nearly 18 hours of sun a day, surface waters are warm, fresh and teeming with life. In 2020, phytoplankton were about 15% lower and less sustained from May to October, with the East Arm an exception, having an unusually strong spring bloom. Opportunistic observations suggest zooplankton and forage fish were more available as prey during the recent heatwave years.

Long-term trends

After nearly 30 years of data, only about 25% of the variability in the record is explained by regional climate patterns and 75% by local weather, according to an analysis not yet published. Except for a few months in 2017-2018, bay temperatures have been above average since 2014. Park waters are warming 0.1°C per decade, while the Gulf of Alaska warms 0.2°C per decade, consistent with global climate change. During the 2014-2016 Pacific marine heatwave the park’s waters reached 1.5°C above normal, and effects included fewer humpback whales and a lower calving rate, seabird die-offs and smaller populations of important commercial fish. Surface salinity is declining at 0.28 psu per decade, faster than the 0.06 psu per decade of the Gulf of Alaska; 2020 had the 5th wettest summer on record, and the drought years of 2018 and 2019 showed saltier readings.

Only seven tidewater glaciers remain, and all but one are retreating or thinning, so how far their influence on park waters changes is a central question for the program. Despite extreme summer rain and slightly warm water, 2020 conditions were close to normal in all seasons. The program depends on monthly cruise volunteers from the park community, and park mechanics keep the boat running.

Sources

• National Park Service, Glacier Bay National Park and Preserve, Southeast Alaska Network, Oceanography 2020, a resource brief by Craig Murdoch and D. Goodwin; rewritten in hubnx’s own words.