Zooplankton: A tiny creature with a big role in the cod crisis

One of the North Atlantic’s smallest ocean critters is making big waves in New England.

Centropages. Photo courtesy NOAA
Centropages. Photo courtesy NOAA

Over the last decade, we’ve seen the collapse of our iconic Atlantic cod fishery due to extreme overfishing. Now, a new study is showing a potentially disastrous link between the effects of climate change and the ailing species’ chance of recovery.

Warming waters are bound to be bad news for a cold water fish, but the problem goes much deeper than that, affecting the entire life cycle of the species. Some of this is due to tiny, microscopic creatures called zooplankton. So what are these little guys, and why are they so important?

Zooplankton is a categorization of a type of ocean organism that includes various species, including Pseudocalanus spp, and Centropages typicus. These two species happen to be the major food source of larval cod in the Gulf of Maine.

Zooplankton, which are usually smaller than 1/10 of an inch, play a major role in the Atlantic’s food web. When there are lots of them, things are pretty good. Young fish prey on them and grow to be healthy, adult fish.

But when there aren’t enough plankton to go around, species like Atlantic cod can suffer. When cod larvae aren’t easily able to find the food they need to grow, fewer of them make it to their first birthday.

And without lots of cod that survive to be at least 4 years old (the age at which females begin spawning), the recovery of the entire stock can stall. The stock needs larger, older, more productive females to thrive in order to have any hope for recovery.

Warming and shifting

But why would the plankton be in such short supply? This is where climate change comes in. According to NOAA, temperature changes can cause the redistribution of plankton communities. In the Gulf of Maine, scientists have found fewer plankton in the same areas where cod populations have been found to be struggling. The shifts in temperature lead to the displacement of a critical food source, making it difficult for young cod to survive.

Larval cod. Photo courtesy NOAA
Larval cod. Photo courtesy NOAA

With the Gulf of Maine is warming faster than 99% of ocean areas, this is an enormously alarming problem. More temperature changes and the shifting of plankton populations could make it even harder for New England cod populations to return to healthy, sustainable levels.

While the cod crisis is the result of many factors – but the loss of tiny zooplankton is a big problem. When considering how to best help cod stocks recover, fishery managers must take into account the effects of climate change, or else risk the total collapse of the species.

New England’s Corals

When most of us think of coral, we picture a scene not unlike that found in Pixar’s Finding Nemo: a vast multicolored reef in the warm shallow waters of the tropics, inhabited by a multitude of equally colorful fish. But did you know that many intricate and colorful species of coral can be found right here in our own New England waters? Growing along the ridges of underwater canyons and seamounts off the Atlantic coast, the New England version of a tropical reef plays host to our own aquatic flora and fauna, more suited to the chilly waters of the northwest Atlantic.

Though they resemble undersea plants, corals are in fact colonies of tiny, soft-bodied invertebrates whose secreted exoskeletons form, over time, the large and intricate structures that we recognize as coral. In the warm waters of the tropics, groups of these exoskeletoned colonies form extensive reefs in the clear, shallow waters close to the shore.

Though snorkelers may appreciate the clear waters of the tropics, the water is so clear in these areas because it contains few nutrients or plankton. Very little mixing of the water column occurs in these uniformly warm waters, so nutrients remain trapped on the bottom of the sea, preventing the multiplication of plankton, and leaving the water empty of food. As a result, tropical corals get their nutrients through a symbiotic relationship with photosynthetic algae, which grows inside the coral and lends it energy from the sun.  Large animals like whales, however, are unable to sustain themselves by hosting algae. Instead, whales like humpbacks and right whales breed in tropical waters but return to New England to feed in the summer. The constantly mixing warm and cold waters of New England bring nutrients to the surface, encouraging plankton growth, and are thus a veritable soup of life. New England corals enjoy this soup just as much as the whales do, and most of them filter feed instead of relying on algae to do the work for them.

A striking purple coral, Clavularia sp., seen in Nygren Canyon. Image via NOAA/Okeanos Explorer
A striking purple coral, Clavularia sp., seen in Nygren Canyon. Image via NOAA/Okeanos Explorer

New England corals live not in the near-shore shallows but along underwater canyons and seamounts. Last summer, NOAA’s Okeanos mission documented some of the wide array of marine life in the Northeast’s canyons, including Oceanographer Canyon, a deep underwater channel that cuts into the southern edge of Georges Bank. You can see images and video from the mission here. The seamounts are part of the New England Seamount Chain and often rise to within 100 feet of the ocean surface, ensuring a rich habitat for undersea creatures due to the high concentration of particulates in the water and the nearness of sunlight.

Unfortunately, cold water corals grow slowly and are very susceptible to the effects of trawling, which is why Fishery Management Councils along the east coast have begun to take action to protect areas like canyons and seamounts with rich deep-sea coral populations.

New England’s corals are surrounded by towering kelp forests, fish and mammals of all kinds, and even sea turtles. So if you’ve ever wanted to dive the Great Barrier Reef, but balked at the idea of that plane ticket to Australia, consider exploring the underwater scenery right in our own backyard!

Feature Image via NOAA/Okeanos Explorer

Not Your Average Drifter – Plankton Part II

Last week we met some of our most important New England residents – the phytoplankton. Now, we are happy to introduce their animal counterparts – the zooplankton. These animals drift around in the sea in truly astonishing number and form . If you missed our post on their plant partners – the phytoplankton – you can find it here. Go ahead and read it, I’ll wait.

Okay? Well, those phytoplankton are extremely productive, and they’re eaten by many animals, most of which fall into the category of “zooplankton.”

Microscopic or massive, if you’re an animal that can’t swim against the current you’re part of the zooplankton. Some of these animals drift in the water their entire lives. These are what we scientists (who enjoy inventing and using large words) call the holoplankton (holo = entire, plankton = wanderer). The copepods are a perfect example of this.

Copepods may be the most abundant animal group on the planet, and although they contain considerable diversity, most of them are holoplanktonic. They are also usually gonorchoristic (I know, again with the huge words), which means they come in both the male and female variety. When two of them get together, so to speak, the fertilized egg will develop through many larval stages, until they finally metamorphose into the adult form. But all the way through this life cycle – egg to adult – the copepod will remain drifting along in the water.

The same is true for countless other animals, from the familiar jelly to the bizarre Phronima. All of them spending a life adrift, in a world that seems more like science fiction than reality.

Contrast this to members of the meroplankton (meros = partial), who spend only a portion of their lives in the water column. These animals may not be so foreign to you, as most of the meroplankton are the larval forms of animals that we know and love (and love to eat!). These animals drift around as larvae until they metamorphose and become large enough to swim against the current (at which point they are said to be “nektonic” – like a fast-swimming fish), or until they settle to a life on the sea floor (these animals are “benthic” – like a snail or mussel).

Most fish have larvae, as do barnacles, urchins, lobsters, mollusks, and many others. Some have giant spikes coming out of their heads, others look like flying saucers. But the fact that free-living larval stages exist in most marine animals means that they are (or were) evolutionarily important. Perhaps they evolved for dispersal – to avoid competition or inbreeding. Or maybe larvae evolved as a means to temporarily avoid predation on the sea floor… in truth, we don’t know for sure why the larval form evolved.

What we do know is that they are extremely abundant, and together with the holoplankton they make up an undeniably important and enormous group of animals. If the phytoplankton are at the base of the food chain, then the zooplankton are at the first rung. They are so massive in number that they can sustain huge populations of larger animals, some as large as our own North Atlantic right whales, which filter copepods, krill, and other zooplankton out of the water. But some zooplankton are eaten by their tiny buddies (other carnivorous plankton, like some fish larvae), which can make the marine food web a bit complicated.

And though they can’t swim against the current, they’re on the move. Their ecological importance makes the news of a study showing that climate change has caused dramatic shifts in the distribution of many planktonic species troubling. In the study, the investigators found that phytoplankton and zooplankton were two of the groups whose distribution was changing the quickest. As the authors’ of the study state, “species’ interactions and marine ecosystem functions may be substantially reorganized at the regional scale, potentially triggering a range of cascading effects.”

Translation: As the great drifter Bob Dylan said, “The times they are a changin’.” But that doesn’t mean you have to sit idly by… “If your time to you is worth savin’” then find out how you can help.

Up next in our plankton series – we’ll talk about a really cool citizen science plankton project you can get involved in using little more than your smart phone. Stay tuned!

Casey Diederich is a 5th year PhD candidate in Tuft’s University’s Biology Department, and is conducting his research on slipper-shell snails. We are thrilled to have Casey guest blogging for us about some of the more fascinating plants and animals in our ocean. – Ed.

Images copyright of Dr Richard Kirby, Plymouth University. These and other images can be found in the book on plankton, “Ocean Drifters, a secret world beneath the waves.”
www.oceandrifters.org

North Atlantic Right Whale Mysteries – the Plot Thickens

North Atlantic right whales – our critically endangered New England natives – are making more waves this week in the news. A mother and calf, like the ones Brian photographed above, were spotted off the coast of Plymouth, MA on Saturday. For an imperiled population of less than 500 individuals, in which every animal counts, this birth is a great thing. But the timing of the mother’s return to the Gulf of Maine with her calf is extremely curious. Our right whales head south to have their calves, and don’t usually return until April, possibly to take advantage of spring plankton blooms. These two are among the growing ranks of right whales who buck tradition and turn up early. This particular pair is so early that scientists have called it “mind-blowing.” What’s going on? Scientists are still investigating these early arrivals, and have speculated that the whales are simply following the food, which may be available at different times in our warming ocean. We’ll keep you posted as they work to unravel the mystery and, hopefully, help these endangered whales recover.

A Movable Feast

When you think about grazing, you picture a big mammal with hooves eating a helpless plant – placidly chewing and digesting, right? Well, there’s another kind of grazing that is much more dynamic. Researchers at the University of Rhode Island have, for the first time known to science, discovered a plant that “runs away” to avoid being munched. The tiny Heterosigma akashiwo you see above on the right, not only makes tracks when predators like the Favella favella are after it (the big, mouthy guy on the left), but it will even avoid areas where there used to be predators, but no longer are. They are, in effect, fleeing the scent of danger.

Before you stand up and cheer for the little guy, you should know that this particular plant is one of the “red tide” phytoplankton that can cause major fish kills when it blooms. Dr. Susanne Menden-Deuer, an oceanography professor who studies the plankton at the University of Rhode Island, speculates that its ability to flee may be one of the mechanisms that allow Heterosigma to grow prolifically enough to kill fish. The plant, a type of algae, will take refuge in areas with lowered salinity – places where its predators cannot survive. As the algae move into these areas of refuge, they are free to reproduce with little to check their population explosion.

These harmful algal blooms can cause major devastation and millions of dollars in economic damage to fisheries – killing not only finfish like salmon and herring, but also harming oysters, copepods, and sea urchins.

Usually, though, Heterosigma are not harmful at all, blooming only occasionally in spring and fall. In fact, they provide many benefits to us – they are an important food source at the bottom of our productive ocean food chain. Not only that, but phytoplankton are responsible for giving us the oxygen in half the air we breathe.

Menden-Deuer plans to conduct further research to find out what the connections are between the fleeing behavior and harmful algal blooms, and if other plants might be up to this evasive behavior.

Photo credit: Elizabeth Harvey (URI-GSO). Digital enhancement by Cynthia
Beth Rubin (RISD).