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All Plankton Great and Small

Photo of various plankton.

Although it may be the three quadrillion gallons of water in Lake Superior that takes your breath away, it’s the small things that keep the lake alive. Tiny bacterio-, phyto-, and zooplankton eek out their short lives as members of a vast but practically invisible community that we are only beginning to appreciate.

Several Sea Grant-funded researchers at the University of Minnesota Duluth (UMD) have been looking into the lake’s smallest residents. Their results, published in three articles in the Journal of Great Lakes Research, reveal food web secrets and compelling reasons to appreciate the scum of the Earth. The researchers believe that spiny waterfleas (an invasive zooplankton) lowered historic population levels of two native species of zooplankton in Lake Superior’s western arm. They also discovered genetic differences between Lake Superior picoplankton communities and those from the other Great Lakes.

Helpful Definitions

Tiny animals, bacteria, and plants floating in lakes and eaten by fish and other aquatic animals
bacterial plankton
plant (alge) plankton
simple-celled, extra-small plankton
Thermal stratification:
The layering of warmer waters over colder waters that can occur in lakes, usually in summertime. This layering occurs because as surface waters are warmed they become less dense than the underlying colder waters
animal plankton

Spiny Waterfleas Eat Natives

Like biologists scanning the Serengeti for lions and wildebeests, Meghan Brown, graduate student in Water Resources Science, and Donn Branstrator, assistant professor in UMD’s Biology Department, plumbed Western Lake Superior to estimate the population sizes of zooplankton species. Their results indicate all is not peaceful in this animal kingdom.

“There are changes afoot in the makeup of the zooplankton assemblage in Lake Superior that could potentially reduce energy leading from phytoplankton to young fish,” said Branstrator.

Comparing their survey results to data collected in 1973, the researchers suggest that the arrival of invasive spiny waterfleas may be linked to declining populations of two species (Bosmina longirostris and Daphnia retrocurva) and the increasing presence of another (Holopedium gibberum). The observations are consistent with other studies conducted on lakes where spiny waterfleas have invaded.

Spiky predators that are not above cannibalism, spiny waterfleas slipped into Lake Superior by 1987. They present a dual problem for young fish: their lengthy tail spines make them undesirable fish food and they compete with fish for the same zooplankton meals such as Bosmina longirostris, which are noticeably scarcer than they were 30 years ago. Daphnia retrocurva were not detected at all in the 2001 survey. Despite their helmeted heads, these zooplankton have also declined in other lakes when spiny waterfleas moved in.

As spiny waterfleas eat up the competition, more armored and evasive grazers thrive. Unprecedented numbers of Holopedium gibberum “shells” washing to shore on Minnesota’s Park Point made headlines and baffled biologists in 2001. Many Duluthians called Sea Grant with ideas about what these goo balls might be, suggesting the tapioca-like blobs were material from disposable diapers. In reality, a fleck-sized Holopedium lives inside a pea-sized mucous mantle. When these grazers are ready to reproduce, they abandon their transparent mantles, which can then wash to shore.

“Shifts in the lower parts of the food chain have the potential to change the abundance or diversity of the species farther up,” said Branstrator. “In an aquatic environment, Daphnia and Holopedium are like the gazelles and wildebeests of the African plains; they’re major herbivores. The timeframe is too short to know what the outcome of the current changes will be, but we shouldn’t allow ourselves to be too cavalier about these little things that matter,” he said.

The impacts of spiny waterfleas are spreading beyond Lake Superior.

“We’re finding that spiny waterfleas are moving inland, particularly to lakes along the Gunflint Trail,” said Branstrator. “Lake Superior is a likely source for these inland invasions but we don’t know how these small creatures are getting across the land. Recreational equipment and aquatic birds are our prime suspects, but our evidence is only inferred.”

Picoplankton Communities in Lake Unique

Picoplankton are so small that 500,000 of the fat ones can fit on the head of a pin. Most of these extra-small plankton are bacteria (bacterioplankton), packaged in simple cells without nuclei, organelles, or chlorophyll. (For enquiring minds, the rest are composed of prokaryotes and microeukaryotes.) Despite their paltry size, these cells can divide at rates comparable to or exceeding phytoplankton in some aquatic systems. This is important because bacterioplankton depend on phytoplankton for energy and their relationship has implications for the aquatic food chain.

Randall Hicks, professor with UMD’s Biology Department, notes that although bacterioplankton can divide faster than phytoplankton they are not as productive. He and his colleagues estimate carbon production of bacterioplankton is six to 12 percent that of phytoplankton. They also found that bacterioplankton account for a minimum of 91 percent of Lake Superior’s picoplankton community as things heat up towards August.

One of the intriguing aspects of picoplankton’s role in aquatic ecosystems is the way they muscle in on the center of the food web (see diagram). The most energy-efficient route in the food web occurs when zooplankton eat phytoplankton. However, like “taking the scenic way,” an alternative route become available thanks to bacertioplankton. In the microbial loop, as this path is called, bacterioplankton consume the dissolved organic compounds that leak from phytoplankton as well as dead phytoplankton cells. The bacterioplankton are then eaten by exceedingly small grazers like flagellated microplankton (tiny plankton with whip-like tails). These microplankton are in turn wolfed down by ciliate protozoans, which are then gulped by zooplankton.

diagram dipicting the microbial loop. Tradtional energy flow is Sunlight > Phytoplankton > Zooplankton > Fish. Microbial Loop is Sunlight > Phytoplankton > Dissolved Organic Matter > Bacterioplankton > Flagellated Microplankton > Ciliate Protozoans > Zooplankton.

“This detour in the food web is not nearly as energy efficient as the straight ‘zooplankton-eats-phytoplankton’ road,” said Hicks, “but it’s well-traveled in many aquatic systems.”

Bacterioplankton abundance and productivity and possibly size are dictated by water temperature and the availability of dying phytoplankton. When the Great Lakes thermally stratify throughout the summer, the researchers found that different picoplankton communities usually form in the upper and lower layers. “The genetics of picoplankton communities near the surface (at 5 meters depth) were very different from the communities we found about 40 to 65 meters deeper, below the thermocline,” said Hicks.

However, Hicks and his team found that this genetic diversity by temperature layer isn’t as pronounced in Lake Superior as it is in the other Great Lakes. The DNA of picoplankton they collected from the surface was 80 percent similar to the DNA of deeper-dwelling picoplankton. (In the other lakes, the communities were 27 to 71 percent similar.)

The researchers also found that Lake Superior’s surface picoplankton is unique, sharing less than 50 percent of its DNA with communities from the other Great Lakes (where they were more than 85 percent similar). They hypothesize that the picoplankton communities floating in the waters of Lake Superior differ because the resources available to them are from phytoplankton typical of cold waters with low productivity.

So why should we care about these microorganisms? Hicks says that even though the microbial loop food web pathway is not very efficient, “Bacterioplankton are extremely abundant and can tap into the large pool of dissolved organic materials in lakes better than phytoplankton. This places aquatic microbes in the important role of recycling carbon and many other nutrients back into the grazing food web,” he said.

If you would like to read any of the Journal of Great Lakes Research articles forming the basis of this article, they are available for free from Minnesota Sea Grant. Look for the following journal reprints on the journal reprints order form: JR 492, JR 493 and JR 502.

By Sharon Moen
October 2005

Return to October 2005 Seiche

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