Zooplankton are millimeter to centimeter scale organisms that live in water. A diverse and ecologically significant group of animals, they range in size from small copepods the size of a grain of rice to large jellyfish. 

In the Delaware Bay, zooplankton contain the larvae of commercially important species, including oysters, crabs, shrimp and various finfish, and are considered an important measure of fishery sustainability. They are also a key food source in the marine food web.

“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,” explains Cohen, assistant professor of marine science and policy in the College of Earth, Ocean, and Environment (CEOE).

Despite their ecological importance, zooplankton have been understudied in the Delaware region.

Cohen’s research team is using a brand new technology called Zooscan, an optical scanning system that resembles a flat bed scanner used to scan papers — only waterproof — to identify and characterize the zooplankton species present in the Delaware Bay. 

The work builds on an early zooplankton study in Delaware Bay conducted in the 1950s by Joanne Daiber, UD’s first female marine biologist. Cohen plans to compare his results to Daiber’s original data in order to understand the biological changes that have occurred in Delaware Bay over the past half-century. He also is looking at the physical and chemical properties of the water column.

Cohen’s research team recently completed the second of six planned cruises on CEOE’s newest research vessel named in her honor — the R/V Joanne Daiber — to collect zooplankton samples from 16 stations in the bay. They have created a library of zooplankton images specific to the Delaware Bay to identify and characterize the new samples. 

Once onboard, researchers transferred the animals into a solution and poured them onto the optical scanner, where a high-resolution picture was taken of the zooplankton present. Computer algorithms extracted the individual images, characterized specific species, including their size and biomass, and placed them in files, where identifications are later verified by the researchers.

Cohen’s study, which is supported through funding from the Delaware Sea Grant College Program (DESG), will establish a baseline for future zooplankton studies in the region in the context of climate change. Collaborators on the project include partners from Delaware Department of Natural Resources and Environmental Control, DESG’s UD Citizen Monitoring program and other DESG-funded researchers.

“Delaware Bay has improved a fair amount since the 1950s and ’60s, particularly in the upper bay where it previously had a lot of nutrient pollution from sewage and sediment,” he says.

Cohen also is curious about what unexpected new species the study will detect. He suspects increased ship traffic in the bay has brought some invasive species to the area. One example is an Asian shore crab called Hemigrapsus, thought to have come over in ballast waters discharged into the bay as ships come into port. 

“Hemigrapsus is now one of the most dominant crabs found scrabbling along the rocks near the Lewes campus,” he says

According to Cohen, replicating the work of Joanne Daiber on a ship named in her honor adds a unique, if unexpected, dimension to the project.

“It’s been especially fun for the students; looking at historical documents from the earlier research cruises puts the work we are doing into context, especially in thinking about how much harder it was to take measurements with less advanced equipment,” Cohen says.

Sharing the science with others

Cohen views zooplankton as an incredible teacher resource in addressing Next Generation Science Standards. 

“You can easily collect zooplankton in a pond, near the coast or in an estuary. They can be sampled with simple nets, even rudimentary ones made out of pantyhose, and can be seen with the naked eye when in groups, making them suitable for classroom learning,” Cohen said.

He is collaborating with DESG education specialist Chris Petrone to develop a website for teachers to learn how to effectively use zooplankton in their classrooms. The site will include lesson plans, directions on how to build nets, and images from Cohen’s zooplankton image library. In the future, he hopes to expand the content to include other species and size scales. The research team is also planning a training workshop for teachers. 

Similarly, he and graduate student Adam Wickline are partnering with the UD Citizen Monitoring program to explore whether citizen monitoring of zooplankton populations is a viable way to collect data. Collaborating with Joe Farrell and Ed Whereat, Cohen and Wickline are exploring whether local citizens can help take zooplankton measurements and analyze the data.