Krill: The Disappearing Backbone of Marine Ecosystems
Are we that close to krill-ing off biodiversity as we know it? Apparently so, because keystone species are feeling the pressure with every passing day.
Art by Joseph Elhardt
When humans think of the “great deep,” outlandish, alien sea creatures come to mind: National Geographic images of anglerfish, vivid apparitions straight out of 20,000 Leagues Under the Sea, timeless sea shanties depicting the fearsome giant squid. Not often is it that some of the most vital organisms in marine ecosystems, keystone species, are at the forefront of our attention. One such linchpin, krill, is a miniscule, hardly visible, invertebrate that often passes for a shrimp look-alike. These finger-sized crustaceans tend to go unnoticed in modern society, their tangerine hue not bright enough to attract the interest of humans. Despite not being one of the more appetizing types of seafood for mankind, krill are a crucial main dish for animals of the Antarctic. For decades, krill has been harvested by humans without a care for the vital role they hold within the Southern Ocean food webs. Now a new threat has been recognized, as climate change threatens the existence of Antarctic sea ice which krill rely upon for nesting grounds. The key to their gargantuan presence on Earth, krill risk losing more than ever before.
Krill dominate the Antarctic Ocean from the shadows with their massive numbers, grouped together in swarms so dense they can be spotted by satellites in space. With eighty-five currently identified species, researchers estimate that the combined biomass of krill — which are individually no larger than a paper clip — worldwide could range from anywhere between 125 million and 600 million tons. Such swarms can drift through the waters at lengths of four miles, boasting densities of over 10,000 krill per square meter. Naturally, as with any resource present at such a scale on Earth, you’d be inclined to think that the statistical stability of krill would be able to overcome any threats to its population size; how could humans even attempt to jeopardize an organism of such a scope? Unfortunately, even the most abundant organism on the planet hardly stands a chance of escaping the all-encompassing nature of climate change. Direct and indirect anthropogenic influences — reflected in commercial fishing practices and the accelerated melting of sea ice — have developed into two, potent sources of stress for krill populations, signifying the greater doom that awaits these crustaceans.
What exactly makes these pinky-sized invertebrates so irreplaceable within the vast oceans of this planet? To answer this question, we must step back into one of the most fundamental topics of ecology: ecological efficiency. Within a food chain, trophic levels quantify the different stages of energy movement between categories of lifeforms, separated into producers and consumers; notably, only 10% of energy is passed from one level to the next. Sub-categories place primary producers (organisms with photosynthetic capabilities) at the bottom-most level, while top predators take the spots of tertiary or quaternary consumers.
In a typical marine food web, phytoplankton replace terrestrial plants as primary producers, and are considered the most energy efficient. As one of the few species capable of directly feeding upon phytoplankton, krill — categorized under zooplankton — take the spot of primary consumer within the food chain. What makes krill so potent as a food source for all predators alike is (1) sheer numbers, making it available to every Antarctic predator, and (2) its tendency to swarm in densely packed groups, which makes feeding much less work for large predators. Krill is a superfood, allowing even normally tertiary consumers to adopt an energy efficient food source into their diet and essentially gain more for less.
A food web depicting the role of Antarctic krill in Southern Ocean ecosystems. (Image Courtesy of Cool Antarctica)
The most populous species of krill, Euphausia superba, serves as a primary source of food for not one, but seventeen different marine animals, such as baleen whales, seals, penguins, fishes, birds, squid, and cephalopods. If they manage to evade the predation tactics of nearly every Antarctic organism larger than them, krill can persist in the Southern waters for an impressive lifespan of up to ten years. To prevent the rapid depletion of a common food source, the species’ predators have likewise taken steps to ensure that their feeding patterns do not overlap. Baleen whales, for example, stop by plankton blooms in polar waters over the summer before continuing their migration towards warm, tropical regions of the ocean.
With so many organisms dependent on krill for sustenance, what does krill, in turn, depend on? That would be phytoplankton–microscopic, buoyant algae which photosynthesize using chlorophyll at the ocean’s surface. During winter months, live phytoplankton form layers within and underneath Antarctic sea ice, which doubles as both a shelter and constant food source for larval and juvenile krill. Fast forward six months, and the bright polar summers create the perfect set of conditions for phytoplankton blooms: a combination of nutrient-rich waters brought up from the deep via Antarctic upwellings, 24-hour sunlight, and ideal ocean temperature. When the surface sea ice melts, both phytoplankton and krill are free to multiply endlessly. The result? An explosion of krill clouds overtaking the sub-Arctic and Antarctic Oceans, and the perfect rest stop for migrating consumers.
Krill feeding on phytoplankton located on sea ice, grazing the underside of the ice cap to collect the phytoplankton as they go. (Image Courtesy of Ice Stories)
As much as they provide shelter, the presence of sea ice is a figurative Achilles heel for our star organism. In addition to the multitude of predators waiting to eat them, that is. Temperature especially stands out as a weakness in that a fraction of a degree Celsius can make a significant difference for these tiny creatures. In fact, krill provide a concrete example of what exactly the implications of “rising ocean temperatures” — a term loved by media coverage — are. The ideal conditions for phytoplankton survival require ice cover to protect them from the harsh, stormy oceans of the South, as well as cold water, which is richer in nutrients. If the surface of the ocean were to be warm instead of cold, upwelling — the phenomenon in which nutrient-rich water rises from the deep to the surface via ocean currents — would not occur and nutrients would be locked below the surface. The following summer, phytoplankton blooms would be smaller in size, and krill would emerge from the melting ice to a noticeable lack of food and a significant difference of 1-2°C. Though researchers have found it difficult to track increases and decreases in Antarctic krill population due to the sheer scale of the endeavor, studies have theorized that krill populations may have dropped 80% since the 1970s.
Krill are not the sole bearers of this insufferable fate that threatens the collapse of entire ecosystems. Sea otters — regulators of the sea urchin population in coastal marine habitats — have been deemed “climate change warriors”, tasked with keeping kelp and seagrass ecosystems in check and promoting carbon sequestration. Starfish, when removed from their ecosystem, directly resulted in the widespread takeover of the unrestrained mussel population. Alarmingly so, recent research has established a direct connection between the warming of the oceans and sea star wasting syndrome, a term for cases of sea stars dying of hypoxia due to aerobic bacteria buildup at high temperatures.
The effects of the presence and lack of presence of starfish in its ecosystem.(Image Courtesy of Institute for Research for Development, Montpelier)
Looking back on history, it’s always been our old, persistent habits that produce the greatest consequences, and it’s past time we pull the plug on this one — once and for all. Krill serve as a dark example for the extent of influence humans have on this planet. One of the most extensive species in the world, research now shows that krill may one day face the same endangerment as many other species. It’s up to us to ensure that climate change is mitigated before it can topple entire ecosystems and sweep biodiversity from the face of this planet.
In some ways, these global phenomena feel so far from us, a disconnect heightened by sheer distance and the differences between nature and civilization. That doesn’t mean, however, that we get to pretend they are not happening. Spreading awareness is always a safe and easy first step, making sure these issues are felt within the bubbles we place ourselves in before breaking out of them entirely. Climate change communication is difficult, unfamiliar, but so incredibly necessary if anything is to be accomplished. Otherwise, humanity’s insatiable greed and sheer disregard for the Earth’s required natural balance causes us to willfully blind ourselves to the impacts of the climate on the world around us — impacts inherently caused by us. It is essential that we open our eyes and face our actions, before their consequences grow to a size much too large to control.