What can zooplankton teach us about ecosystem health?

In the Delaware Bay, the estuary where the Delaware River meets the Atlantic Ocean, zooplankton are giving scientists new ways to think about species diversity and how it relates to climate change. Zooplankton's short reproductive cycles and adaptability provide a microcosm for observing the effects of climate change, as well as useful indicators of ecosystem health.

Jonathan Cohen, an assistant professor of marine science and policy for the College of Earth, Ocean, and Environment at the University of Delaware, explained his team's research in a recent release. Cohen and his colleagues hope to classify the Bay's zooplankton species using a sophisticated scanning system called Zooscan.

Compiling a more complete list of local zooplankton species will allow the researchers to track the organisms more closely. Cohen explained that the region's zooplankton have yet to be studied to their fullest potential – an opportunity that lends itself well to examining the effects of climate change on local marine ecosystems.

Zooplankton and the food web
Zooplankton populations are useful indicators of food web stability. Thanks to their massive numbers and small size, they are a primary food source for many larger marine organisms. According to a study published in the Journal of Plankton Research, zooplankton are the most abundant multicellular animals on the planet – their numbers even outrank those of insects.

This prominence coupled with their status as an important food source makes them vital to the ocean ecosystem. In short, alter zooplankton populations, and you risk throwing off the balance of the entire oceanic food web.

Copepods are small crustacean zooplankton.Copepods are small crustacean zooplankton.

Cohen's team also noted that zooplankton do more than just provide sustenance. They also carry the larvae of crabs, shrimp, finfish and oysters, a trait that scientists find useful for examining wider ecosystem stability

"People forget that marine organisms aren't just adults; they are eggs and larvae that are dispersed in the water column, then become juveniles and adults," Cohen said.

Zooplankton, carbon sequestration and climate change
Biologists know that every component of ecosystem health is interlinked, and in few scenarios is this more evident than a rapidly warming planet. Though zooplankton's short reproductive cycles make them more adaptable than larger organisms, it's difficult to predict how warming ocean temperatures might affect their populations – and consequently, the vast number of species that depend upon them for food.

"Zooplankton are also an important component of preventing global warming."

Zooplankton are also an important component of preventing global warming. Two particular types of zooplankton – foraminifera and pteropods – have shells made of calcium carbonate, according to the Smithsonian Museum of Natural History. These shells store carbon that eventually forms into sediment on the ocean floor when the organisms die. In this way, zooplankton are an important part of the oceanic carbon cycle, which sequesters carbon from the atmosphere and helps prevent climate change. Though zooplankton may be a critical part of this process, excessive atmospheric carbon levels still threaten to cause temperature increases throughout the world's oceans.

Warming ocean temperatures also threaten to decrease diversity within zooplankton populations. According to a study published in the Ices Journal of Marine Science, colder water lends itself to nutrient diversity, as it tends to be more turbulent and therefore "well-mixed." Remove that variety, and certain species – such as doliolids, ctenophores and salps – will take center stage, providing little in terms of biodiversity.

How will ocean acidification affect zooplankton?
As the ocean absorbs more carbon into its ecosystem, its pH balance decreases. This process, known as ocean acidification, is one of the biggest known threats to ocean sustainability.

Early research into the relationship between ocean pH and zooplankton may have given scientists premature hope that these important organisms would be able to thrive in acidic conditions, but more recent speculation has shaken some of that confidence. A study published in Global Change Biology stated that although zooplankton initially showed insensitivity to changes in pH, scientists discovered major variability upon closer examination. The researchers found that organisms at certain stages of the life cycle were more vulnerable to acidic conditions than those in other seasons of life, with larvae showing the most sensitivity.

What can scientists take away from zooplankton research?
Cohen's systematic studying and categorizing of local zooplankton species will help provide clues into the Delaware Bay's ecosystem health. Cohen said that although the area has improved in some capacities – including a reduction in nutrient pollution – there are other challenges to be faced, such as the spread of invasive species.

In addition to helping scientists navigate planet-wide challenges related to climate change, such as warming temperatures, ocean acidification and food web disturbances, studying organisms like zooplankton can help researchers support small-scale ecosystems. In the Delaware Bay's case, researching local zooplankton populations provides information about local food webs, fish farm sustainability, biodiversity and the effects of human activity on the ecosystem.

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