Ocean acidification, a byproduct of increasing carbon dioxide (CO2) in the atmosphere that results in a reduced pH in the ocean, has been the subject of a lot of research over recent decades to figure out how animals will respond. A lowered pH is expected to have different effects on different species – some species who can adapt quickly to changing conditions are going to be “winners” and others, who can’t, are expected to be “losers”. Often, the distinction between winners and losers is mysterious; some animals in one system can be winners while closely related animals in a different system can be losers.
The authors of this review article synthesized a great deal of the available research on the effects of ocean acidification on marine animals, focusing on a particular category: “nuisance” species. They defined a nuisance species as any marine animal that has an undesirable effect from a human perspective, such as invasive species or species that interfere with aquaculture. Their findings suggest that a lot of these nuisance species will be winners under ocean acidification.
First, the winners of ocean acidification (and the ones likely to cause humans the most grief): algae, jellyfish, and invasive molluscs.
In general, algae are going to be the gold medal winners in the competition to adapt to ocean acidification – adding more carbon dioxide to the water benefits them greatly because CO2 is one of the ingredients for photosynthesis. While increased growth is good for species we as humans want to keep around, like seagrasses, it’s bad for species we don’t want to keep around, like invasive species and species that cause harmful algal blooms. Invasive algae have all the ingredients necessary for adapting quickly to lower pH: they grow quickly, are tolerant to a wide range of water quality conditions, and reproduce quickly. If herbivorous fish can’t keep up with all this algae, invasives have the ability to alter ecosystems, reducing biodiversity by outcompeting native species (Figure 1).
Other nuisance algae are the species that cause harmful algal blooms, which have adverse effects for both people (e.g. shellfish poisoning) and fish (e.g. mass mortality events). Many experiments on harmful algae species revealed that they grow faster and sometimes produce more toxins under higher temperatures and higher CO2 conditions. With temperature increases happening alongside ocean acidification, this finding is a cause for concern and could inspire many shellfish lovers to lower their carbon footprints. (For more info on shellfish poisoning, read this great oceanbites article!)
Jellyfish have been in the news lately as the canary in the coal mine of poor water quality: when jellyfish bloom, it’s often a sign of a degraded ecosystem. Blooms can cause people all sorts of problems, from clogging fishing nets to stinging beachgoers (Figure 2).
There have been few studies that have focused on how jellies respond to ocean acidification, but they have noticed two major effects. The first, a positive effect from the human perspective, is that a lower pH reduces polyp formation, which means that the jellies aren’t able to reproduce as quickly. But the second effect is a negative from the human perspective: another study found that jellyfish increased production of nematocysts (stinging cells) under ocean acidification, meaning that each sting packs more of a wallop than under present day conditions.
Many marine molluscs have been shown to be negatively affected by ocean acidification, but most studies are focused on the ones that people love to eat like clams, mussels, and scallops. The less popular molluscs focused on here often disrupt the growth of those delicious shellfishes, like the slipper limpet and mussels (Figure 3).
There’s good news and bad news with these nuisance encrusters: adult slipper shells are hardy and increase their shell growth under higher CO2 conditions, but their larvae don’t fare as well and show reduced growth under the same conditions. Delayed larval development doesn’t bode well for the population, but increased growth does, so it’s difficult to predict the ultimate response of this species to ocean acidification.
Other nuisance molluscs are feasting on the shellfish we like to eat: there’s a predatory snail invading the Pacific Northwest aquaculture facilities, eating its way though oyster and scallop farms. A study on these snails showed that they ate more oysters in acidified water, possibly because the oysters themselves showed decreased growth – the snail ate more oysters to get the same amount of nourishment.
Next up, let’s hear it for the losers of ocean acidification that are currently causing problems in various ecosystems that may be deterred with a lower pH: echinoderms (sea stars) and fish.
Echinoderms (Sea Stars)
The crown-of-thorns starfish is one of the nastiest marine villains out there: it has invaded Australia’s reefs and feasts on the coral there, causing more than 50% coral mortality in affected reef sites (Figure 4).
The good news for coral (and the bad news for the starfish!) is that higher CO2 reduces the ability of the starfish’s sperm to swim around and fertilize eggs by as much as 75% under low pH conditions.
The lionfish, native to the Caribbean but expanding northward, is the poster child for invasive species: it eats everything in its path and nothing eats it in return (Figure 5).
While no one has done any research on lionfish particularly, the authors of the review looked at other papers studying the effects of ocean acidification on fish. Studies so far have shown that fish are resilient to physiological effects (growth rate and respiration stay the same), but what fails under ocean acidification are their sensory systems. Fish under elevated CO2 swim towards predator chemical cues, rather than away from them. As a result, these fish are more vulnerable to predation. Hopefully, that will also happen to the lionfish, whose juveniles are more vulnerable to predation than the full-grown adults.
The authors point out that all of the studies that currently exist measure the effects of ocean acidification in somewhat of a vacuum: the best way to study ocean acidification is putting together an experiment that looks at the effects throughout the entire food web. Researchers are working on that currently, making the field of ocean acidification an exciting one to follow. Even though there are nuisance animals that will experience negative effects, making our lives easier, the majority of effects are negative for both animals and humans alike. If anything, it gives us one more reason to reduce our CO2 emissions: more CO2 may lead to the spread of harmful or nuisance marine animals.
Have you noticed any changes in ecosystems in your life? Are there suddenly more jellies now than there ever were, or lobsters are no longer in your state’s waters? Tell us about it in the comments!