Nitrates are hurting our waters and our fishing

By now, we can all just stipulate that nitrate levels in Iowa rivers, streams and groundwater are unhealthily high, that most nitrate pollution drains or leaks from farm fields and that little will be done about it.

This chronic condition has made headlines this year — mainly because 600,000 central Iowans have to ration water because treatment plants can’t keep up with elevated levels in the Des Moines and Raccoon rivers.

The unwatered lawns and gardens call attention to the more ominous specter of increasing threats to human health posed by chronically high nitrate levels in the source water for human consumption.

In addition to the long-established connection between elevated nitrates in drinking water and Blue Baby Syndrome, studies are finding potential links to several kinds of cancer (in a state with the second-highest rate of new cancer diagnoses) as well as to birth defects and thyroid ailments.

But this is not about that. This is an outdoor column. This is about the effects of elevated nitrate on fish that have to live in nitrate-tainted water.

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Just as excessive applications of nitrogen fertilizer fuel bumper crops of corn, excessive concentrations of nitrate in water fuel growth of algae and aquatic plants. As these plants die and decompose, they consume dissolved oxygen, limiting oxygen available to aquatic animals and in some cases suffocating them.

That’s what happens every year in the Gulf of Mexico dead zone — a condition that prompted formulation of the Iowa Nutrient Reduction Strategy, a catalog of voluntary conservation practices ostensibly intended to cut nutrient pollution by 45 percent when it’s “scaled up” at some indefinite time long after people younger than me are dead.

While algae-related fish kills are seldom reported in Iowa rivers and streams, the Iowa Department of Natural Resources last week advised against swimming at 14 state park beaches because of elevated levels of bacteria or algae. Last year the DNR issued 15 beach advisories for microcystin, a toxin produced by blue-green algae.

Fisheries biologists say eggs and fry are much more susceptible to the ill effects of high nitrate levels than adult fish.

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Interior rivers research biologist Greg Gelwicks said the DNR has not studied the impacts of nitrates on fish. In Iowa’s interior rivers, “sediment and phosphorus have a bigger negative impact on fish,” he said.

Sediment degrades the gravel and rock substrate on which game fish prefer to spawn, Gelwicks said. Unlike nitrate, which is invisible in water, sediment turns it murky, impairing the ability of sight-feeding predators like bass, walleyes and northerns to find their prey, he said.

Another major threat to the welfare of interior river fish is the timing of spring floods, said Mike Siepker, the DNR’s northeast Iowa fisheries bureau chief.

Fisheries personnel have little discretion in the timing of stocking hatchery-raised walleye fingerlings — the backbone of Iowa’s popular walleye fishery, he said. They must be stocked when they reach the 2-inch stage or they will begin to eat each other, Siepker said.

Smallmouth bass and northern pike, which maintain their populations through natural reproduction, spawn in a narrow window dictated by day length and water temperature.

The survival and recruitment rates of all three species decline if stocking or spawning occurs when rivers are high, swift and muddy, as was often the case from 2016 to 2020, Siepker said. In the years since, spring river conditions have been mostly favorable, yielding strong year classes of game fish, he said.

I always welcome dry springs not just because they aid game fish production and my personal access to the places they live; but also because the plants in my garden prefer the nitrate-laden water from my well to that which falls from the sky.