Even very small doses of environmental toxins have huge effects on ecosystems. Imagine a small fish eating algae that contains a little bit of the toxin. A larger fish eats that fish and so on, up the food chain. As the toxin moves from the source to the small fish to the larger fish and eventually to the top of the food chain, the toxin becomes more potent, wreaking havoc on the top predators in the ecosystem. This is bioaccumulation.
One famous example occurred with the pesticide, DDT. As a result, one of the top predators in the food chain, the bald eagle, was more susceptible to the toxin. This caused the shells of the eagle’s eggs to shatter before the chicks hatched and the decline of this top predator led to an imbalance in the ecosystem.
Now, it’s happening again and this time, our favorite hand-holding marine mammals are the unsuspecting target — sea otters. Unlike the previous case, the toxin isn’t a human waste product like DDT, but a toxin produced naturally by algae in the ocean.
A research team led by Megan Moriarty at the University of California, Davis studied the effect of one specific algal bloom toxin, domoic acid (DA), and its effect on sea otters. DA usually affects the nervous system of mammals and has been shown to affect the hearts of southern sea otters. Moriarty and her team set out to understand the impact.
What Causes Toxic Algae Blooms?
Algal blooms occur when a large population of algae grows in the same location. Sometimes, these blooms are so big we can see them from space! When large numbers of algae gather in the same place they are all releasing communication chemicals which interact and can produce toxins, like DA.
The algae blooms that produce DA are formed by the diatom Pseudo-nitzschia. Marine heat waves and terrestrial runoff of chemicals like nitrogen and phosphorus, common in fertilizers, can trigger the formation of these blooms. During a heatwave in 2015, a bloom of Pseudo-nitzschia resulted in the largest recorded outbreak of DA along the North American west coast.
How does DA affect sea otters?
It is important to understand how this toxin affects sea otters because they are keystone species, meaning that their loss would lead to an ecosystem collapse since they keep the balance between different food chain levels. Their position on the food chain makes them vulnerable to DA poisoning. They eat invertebrates that easily uptake DA which is bioaccumulated in the sea otters. This, coupled with the fact that they live in a region that experiences frequent toxic blooms, makes them extra susceptible. DA toxins can lead to cardiomyopathy in sea otters which is a disease that can eventually lead to heart failure.
The research team used radio tracking to follow 186 sea otters from 2001-2017 to understand their behavior, such as their eating habits, and the environmental threats they face. When a sea otter died, they examined the body for signs of cardiomyopathy. They combined this information with datasets that monitored environmental DA in shellfish, dead otters, and water samples which allowed them to create a survival model and understand the connection between sea otter heart disease and DA.
Sadly, 48 of the otters they tracked died and 70% of them died of fatal cardiomyopathy. Sea otters were more susceptible if they were exposed to high levels of DA the previous year and ate a lot of crabs and clams which tend to bioaccumulate DA. The negative effects of DA were particularly noticeable in older otters.
What does this mean for the future?
Sea otters eat similar foods to humans and if otters are consuming high levels of DA in crabs and clams, DA exposure should also be considered when developing human seafood guidelines to avoid potential toxicity in humans.
This study was the first to show how sea otter behavior, eating habits, and environmental DA interact to cause heart disease in sea otters. Understanding how toxins and diseases are connected to all aspects of the ecosystem is important in developing tools and strategies to combat them.
Future research should focus on understanding the risks to humans and should address the causes of DA production such as reducing the size and frequency of toxic algae blooms. Current steps we can take include reducing greenhouse gases to slow ocean warming and reducing nutrient runoff into the oceans.
Source: Moriarty M.E. et al. Exposure to domoic acid is an ecological driver of cardiac disease in southern sea otters. 2021. Harmful Algae 101: 101973. https://doi.org/10.1016/j.hal.2020.101973