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Science Nugget – Spring Bloom, 2021

more planktonic organisms spring 2021

Massive, early spring bloom in the northern Gulf of Alaska, April-May 2021

Satellite measurements

Some of the highest chlorophyll concentrations recorded during the 24-year occupation of the Seward Line were seen in late April – early May 2021, in association with a massive spring bloom of diatoms. As an illustration, satellite imagery from NASA’s VIIRS (Visible Infrared Imaging Radiometer Suite) shows the bloom developing rapidly between April 18th and 25th.

Chlorophyll-a April 18 2021
Satellite measurements of Chlorophyll-a in the Gulf of Alaska, April 18, 2021 [Credit: Rachel Potter from NASA VIIRS data]
Chlorophyll-a April 25 2021
Satellite measurements of Chlorophyll-a in the Gulf of Alaska, April 25, 2021 [Credit: Rachel Potter from NASA VIIRS data]

Field sampling provided details

Our field sampling from R/V Sikuliaq coincided with this bloom. We found relatively cool ocean temperatures and, in places, fresher conditions than usual, as shown below for the GAK line. High chlorophyll was mostly confined to the upper 20 m of the water column (as seen in the fluorescence section below). Likely, this bloom was a response to intermittent sunny days and light winds. Late April chlorophyll-a concentrations reached almost 30 μg liter-1 on the inner MID line. In contrast, in some springs the peak concentrations are only one-tenth this level. Surface macronutrients (nitrate and silicic acid) had been drawn down to levels usually observed in summer (<1 μM liter-1).

temperature and salinity plot
Anomalies of temperature and salinity along the Seward Line, spring 2021 [Credit: Seth Danielson]
fluorescence plot
Fluorescence along the Seward Line, spring 2021 [Credit: Russ Hopcroft]

Microscopy conducted on board showed a diverse mixture of chain diatoms and colonial Phaeocystis comprising the bloom phytoplankton community. Concurrently, late-stage juveniles of the spring dominant Neocalanus copepod community had large amounts of lipid stores. At some stations, these copepods were already beginning their seasonal descent to deep diapause depths. Conversely, seabird abundances were some of the lowest ever observed during the >20-year time series, and few marine mammals were sighted.

The early, intense bloom of large, lipid-rich phytoplankton bodes well for survival and growth of krill and larval fish in 2021.

Planktonic organisms
Planktonic organisms in the Gulf of Alaska, Spring 2021

For further information contact Russ Hopcroft (rrhopcroft@alaska.edu) or Suzanne Strom (stroms@wwu.edu)