Diatoms in the food chain

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Flood of ‘08 served up feast for fish
May 28, 2009

By Jennifer Yauck
One year ago this month, a series of severe storms moving across the Midwest dropped nearly a foot of rain on the Milwaukee area in just 10 days. The deluge caused widespread flooding that damaged homes and businesses, washed out roads, closed the airport, and belched plumes of sediment, debris, and sewage into Lake Michigan.
But the flood may also have delivered food to some of Lake Michigan’s fish when they needed it most.
Among the many things the area’s swollen rivers carried out to Lake Michigan during and after the storms was a mix of the rivers’ microscopic algae (phytoplankton), including diatoms, said Carmen Aguilar, a scientist at the Great Lakes WATER Institute. Diatoms are one of the largest forms of phytoplankton and the only form having glass-like cell walls made of silica. Diatoms also are packed with lipids, making them a nutritious meal for newly hatched fish and the tiny aquatic animals that newly hatched fish also eat.
“Compared to other phytoplankton, diatoms are like juicy steaks,” said Aguilar, who has been studying Lake Michigan’s diatoms and other phytoplankton for over a decade.
According to Aguilar, diatoms historically accounted for most of the phytoplankton found in the sun-fueled spring blooms in the Milwaukee-area waters of Lake Michigan. Those blooms typically begin in shallow, nearshore waters in late April and May, and spread to offshore waters in May and June, where they eventually sink. In the past, the diatom-rich blooms nourished the tiny animals that in turn nourished the babies of spring-spawning fish such as perch, which hatch around mid-June.
This satellite image of Lake Michigan was taken July 10, 2008, one month after the storms. The colors reflect the concentration of chlorophyll, a pigment found in phytoplankton, in the lake’s surface waters. Higher concentrations of chlorophyll (red) indicate a higher abundance of phytoplankton (most of which were diatoms in this case), while lower concentrations (blue) indicate lower abundance. The highest abundances of phytoplankton are seen at the mouths of major rivers around the lake (1-Milwaukee River drainage; 2-Two Rivers; 3-Sturgeon Canal; 4-Northern Green Bay/Death’s Door; 5-Muskegon Lake drainage; 6-Portage Lake/Frankfort drainage area). ~courtesy William Balch, Bigelow Laboratory for Ocean Science
But the makeup of the blooms began changing around 2003, according to Aguilar. That’s when the invasive quagga mussel-a voracious filter feeder with a fondness for diatoms-became well established in Lake Michigan. As a result of the quaggas’ feeding habits, the diatom population has declined significantly in recent years in the areas that Aguilar studies-within about 40 miles of Milwaukee-if not beyond.
The diatom decline, in turn, has left each year’s batch of newly hatched perch and other baby fish struggling to survive on a less nutritious diet, said Aguilar. As a result, these fish have a smaller chance of growing into adults and producing new young to sustain their populations. For an already struggling fish like perch, whose population in the Wisconsin waters of Lake Michigan dropped from a high of 23.6 million to a low of 264,000 since the early 1990s, that’s not good news.
But last year’s baby fish may have caught a break in the form of the June flood, which Aguilar said forced massive amounts of water-and diatoms-through the area’s rivers and out into Lake Michigan in a giant plume. She and her colleagues investigated the plume during a research cruise aboard the WATER Institute’s Neeskay June 10, 2008, just two days after the heaviest rains. What they found surprised them. “Outside the plume it was the same old, same old, but inside the plume there were diatoms everywhere,” said Aguilar. “Everybody was like, ‘Wow!’”
What’s more, the diatoms flourished unusually late into the summer-mid-July-and drifted unusually far-to the middle of the lake-as the plume expanded. Based on analysis of water samples for silica, the material found in diatom cell walls, and chlorophyll, a green pigment found in diatoms and other phytoplankton, Aguilar estimates some areas of the lake had two to three times more diatoms in 2008 than in 2007.
Most importantly for perch, all those diatoms entered the food web right around hatching time. Aguilar and fellow scientist Russell Cuhel suggest that such coincidences can dramatically improve baby perch survival, and may explain why some years have a “baby boom” of perch larvae. Such booms have occurred in the past during so-called El Niño years, when storms increased the lake’s turbulence and stimulated diatom growth in May and June, they said. And anecdotal reports suggest that the alewife, another Lake Michigan prey fish that also would have benefited from the 2008 influx of diatoms, reproduced in much higher numbers last year compared to other recent years.
So far, 2008 hasn’t stood out as a boom year, based on initial assessments by the Wisconsin Department of Natural Resources. But booms often do not become evident until several years later, when fish are bigger and more easily captured during assessments. Such was the case for perch born in 1998, 2002, and 2003.
Should 2008 eventually prove to be a strong year, it would provide a small boost to the diminished perch population, said Aguilar-but it would still only be a small step toward significant recovery.
Jennifer Yauck is a science writer at the Great Lakes WATER Institute. GLWI (glwi.uwm.edu) is the largest academic freshwater research facility on the Great Lakes.
AMAZING FLOOD

The Milwaukee River drains an area of almost 700 square miles before converging with the Menomonee and Kinnickinnic Rivers in downtown Milwaukee. After a series of severe rainstorms between June 5 and 14, 2008, water flowed through the Milwaukee River at a rate as high as 60,000 gallons per second, according to U.S. Geological Survey readings taken at Estabrook Park. That’s 32 times the normal June flow rate of 1,870 gallons per second!