On the first field trip of the season, the Freshwater Algae class at the University of Michigan Biological Station (UMBS) in Pellston climbs aboard one of the station’s pontoons, ready to sample the waters of Douglas Lake for phytoplankton, which are photosynthetic microorganisms that live suspended in the water column. The tradition goes back as long as professor Dr. Rex Lowe has been teaching at UMBS (which he’s done nearly every year since 1974).
The phytoplankton community of Douglas Lake was always “rich and diverse,” Lowe said, that is until zebra mussels were first found in the lake in 2001. The invasive mussels are known for their efficient filtering habits, cleaning the water column of suspended organisms (plankton) and clearing the water. Within about three years of the introduction of zebra mussels to Douglas Lake, he continued, the rich phytoplankton community had started changing as diversity decreased.
Phytoplankton, while small and often overlooked, are incredibly important as the base of aquatic food chains – and they contribute about half the oxygen to the atmosphere. When phytoplankton communities in lakes change in diversity, the base food supply changes, affecting organisms higher up the food chain like fish. Diatoms are one type of phytoplankton that are rich in omega-3 fatty acids, meaning they are a good source of food. In fact, fish are rich in omega-3 fatty acids (the main ingredient in fish oil) because they eat diatoms, Lowe explained.
Major shifts in the phytoplankton communities in Douglas Lake were concerning and disappointing, Lowe said, because of the negative implications on other organisms like fish. With the introduction of zebra mussels, the phytoplankton community was no longer a rich combination of diatoms, other algae, and blue-green algae, but rather became dominated by a non-nutritious alga, Microcystis. And because zebra mussels can also spit out the food they don’t want to eat, Lowe said, their filter-feeding habits fostered a bloom of Microcystis in recent years.
“The diatoms were gone,” he said.
Fortunately, however, Lowe, his co-professor Pat Kociolek, and their students noticed something encouraging last summer: the diatoms had returned. And, excitingly, the trend has continued this year.
“This year,” Lowe said, “there’s a really rich and abundant flora,” in the water column.
So, what happened? Why have the diatoms returned to Douglas Lake?
In addition to a rise in diatoms, all types of plankton have started to recover. Lowe theorizes this is likely due to a decline in the zebra mussel population in Douglas Lake. And while this improving phytoplankton diversity is encouraging, he likes to be cautiously optimistic until longer trend data is seen in subsequent summers. Algal communities do shift and fluctuate over time, after all, he said.
“At least for the past two years,” he said, “the phytoplankton are recovering. They are looking better.”
Douglas Lake is about 12,000 years-old, and for the last 5,000 years, the lake has had a rich phytoplankton community, said Lowe. Scientists can date algal communities by extracting cores of the lakebed and analyzing the sediments, where the cellular walls of diatoms can be identified. But when the zebra mussels were introduced to the lake, there is a dramatic shift in the variety of phytoplankton found in lakebed cores. Hopefully now, he said, that trend is reversing, and diversity is once again on the rise.
“As zebra mussels subside,” he said, “there’s probably more room for the phytoplankton.”
It’s difficult to say what might be causing the decline in zebra mussels, Lowe said, but he noted that invasive species often experience a boom in population, but then settle into a kind of equilibrium in their introduced ecosystem. While zebra mussels are still found in the lake, he said, anecdotally he’s heard there aren’t as many zebra mussels as there once was.
But whatever the reason for the recovery, Lowe said, “I’m just happy they’re reappearing,” because it means the aquatic food chain of Douglas Lake will once again be nutritious.
After a day of collecting “slime” in the field, hunched over a table, the seasoned phycologist peers into a microscope to see what beautiful things he’ll discover.
“It’s like seeing old friends again,” he described. “Looking through the microscope, it’s like looking at another world. It’s comforting when it’s a world you know.”
Hopefully, this comforting microscopic world will continue to be populated by old friends in future summers, too.