What makes a “five-star resort” for Antarctic krill

The best habitat for Antarctic krill can show less chlorophyll on a glider’s sensors than a poor one. A NOAA Fisheries research biologist explains why, using a joke: what if krill left online reviews of their surroundings?

Krill biomass estimates say how much krill live around Antarctica’s South Shetland Islands each year, but not what the environment is like. Krill like comfortable temperatures and lots of food, mainly phytoplankton. To find out what makes a “five-star resort,” the researchers use sensors on ocean gliders that measure water temperature, salinity, oxygen concentration and chlorophyll concentration.

Maxwell Bay, King George Island, Antarctica, in summer. Credit: NOAA Fisheries.

The five-star year

In this scenario winter sea ice retreats early in spring. Sea ice on the surface stops wind from mixing the upper ocean, where krill spend most of their time. Once it recedes, the wind stirs the upper layer and brings cold water and nutrients to the surface. Nutrients help phytoplankton photosynthesize, though heavy mixing also suspends particles that cut the sunlight reaching the water. Cold, salty, murky water therefore seems an unlikely recipe for good food, but many species of diatoms, krill’s favorite phytoplankton, thrive in just these conditions.

A diatom under a scanning electron microscope. This one is from the Pacific Northwest but comparable to an Antarctic diatom. Credit: NOAA Fisheries.

The gliders do not identify phytoplankton species. They measure chlorophyll, the pigment that captures the sun’s energy, and when chlorophyll and oxygen are both high near the surface, the researchers can infer lots of phytoplankton photosynthesizing. The tricky part is that diatoms are giants of the phytoplankton world, which makes them a krill “superfood”, so a habitat full of them may show low chlorophyll. The author’s picture: two equal ball pits, one of ping pong balls and one of basketballs, each ball with the same green dot. The ping pong pit holds many more green dots. If diatoms are the basketballs, chlorophyll can look sparse exactly when the food is best.

The one-star year

In the other scenario sea ice melts later in spring, as the water warms with longer, sunnier days. The fresh meltwater forms a layer on top of the saltier, nutrient-rich water below. Being less dense, it hardly mixes with the layer beneath, which traps nutrients near the surface. The layer warms in the sun and phytoplankton bloom heavily. Gliders see warm, not very salty water with high chlorophyll and oxygen.

Maxwell Bay, King George Island, in winter. Credit: NOAA Fisheries.

This looks perfect for krill, yet it earns one star because of the menu. The phytoplankton in these conditions are small and not very nutritious, the ping pong balls of the phytoplankton world. Krill have trouble eating them and gain little energy from them. Chlorophyll and oxygen stay high precisely because krill are not eating the phytoplankton.

Between one star and five

These two habitats are examples of what the study area may look like in a given year. Because ocean conditions are unpredictable, there is room for two-, three- and four-star years. Storms, circulation patterns and weather patterns such as El Niño and La Niña may mix features of both. Each year the team considers its krill biomass estimates alongside the glider measurements, to see whether the habitat resembles a fancy resort, a middle-of-the-road hotel or a budget motel. As the author puts it, krill are harsh critics.

Sources

• NOAA Fisheries, Southwest Fisheries Science Center science blog, “What Is a ‘Five-Star Resort’ for Antarctic Krill?”, by a research biologist at the center; the author’s portrait is left out; rewritten in hubnx's own words.