As described in our main story in this edition of The Current, the increasing levels of phosphorus and nitrogen in many water bodies allow cyanobacteria to thrive in the warmer months of the year. And while wastewater treatment plants, septic leach fields, and other sources contribute some phosphorus and nitrogen, the top contributor is agriculture.
According to a report from a CHAB science conference held in April 2015 at Bowling Green State University in Ohio, conservation tillage practices recommended to reduce erosion have reduced particulate phosphorus runoff but have increased the runoff of dissolved reactive phosphorus (DRP)—a form more advantageous to cyanobacteria than the particulate form. The EPA blames the increase in DRP for CHABs getting worse in western Lake Erie even though the total phosphorus entering the lake has been relatively stable for the past 15 years.
And some common farming trends are clearly making things worse, according to scientists at the Bowling Green conference. These include a marked decrease in the use of winter cover crops; an increase in broadcast application of fertilizers; and the rapid spread of tile drainage systems to remove excess water from farm fields. These systems now constitute the major pathways by which DRP moves from cropland to streams.
There are some success stories, where financial incentives combined with voluntary changes initiated by farmers have resulted in lower nutrient loads. In Florida’s Lake Okeechobee basin, sugarcane farms have drastically reduced their phosphorus discharges over the last 25 years, according to a Florida Sun Sentinel article republished on the science news website Phys.org.
Still, greater changes in farming practices and more research on the effectiveness of BMPs is needed. “Even after decades of working with the agricultural community to propose the adoption of BMPs to reduce sediment and nutrient runoff, there’s still a poor understanding of the effectiveness of various practices,” said Tom Johengen, associate director with the Cooperative Institute for Limnology and Ecosystems Research (CILER) at University of Michigan.
The report on the Bowling Green conference also advocates better targeting of USDA funding. The $20 to $25 million spent annually in Lake Erie’s Maumee and Sandusky watersheds on BMPs should be targeted at “known nutrient loading hotspots.” Current USDA ranking systems do not accomplish this, but a LUMINATE modeling system developed for the Mississippi watershed could provide guidance, according to the scientists.
In 2015, legislatures and governors in Ohio and Florida enacted laws to regulate how and when farmers can apply fertilizer and manure. Johengen considers the Ohio law “a good first step in taking a proactive approach at controlling nutrient inputs.” But he said it will take some time for farmers to adapt to it, and more time to determine whether the law results in overall reduction of nutrient loads or just changes in the timing and patterns of those loads.
In Florida, the new regulations are viewed by many conservation groups as inadequate and overly reliant on voluntary compliance. But officials with the Department of Environmental Protection say the policy will require landowners to demonstrate that they are either complying with BMPs or not discharging phosphorus, according to the Sun Sentinel story.
Steve Davis, chief ecologist for the Everglades Foundation, told The Current that while he thinks “more could be done in the area of best management practices [around Lake Okeechobee], it’s important to point out that reducing inputs is not the only solution to the problem because we also have a legacy pollution problem.”
Davis and others say that in Lake Okeechobee—where recurring CHABs flow downstream and coat the shores and marinas on Florida’s west and east coasts with guacamole-like scum—any progress from changing agricultural practices will be tempered by release of nutrients stored in sediment. “Even if you could eliminate all the nutrient inputs, there’s a legacy of sediments rich in nitrogen and phosphorus that will continue to leach out into the overlying water column,” said E & E Ecologist Sharon Ewe, Ph.D. Pilot dredging projects to remove nutrient-rich sediment have had little success, says Ewe.
The pollution legacy story is similar in Lake Erie. Although the rates at which nutrients stored in sediments get recycled are poorly understood, according to Johengen, “This legacy will delay how the lake responds to reductions in annual inputs.”
“Reducing nonpoint source pollution from farms is much more difficult than the implementation of point-source regulations in the 1970s,” said Johengen. “Then we saw significant reduction in nutrient inputs within three to five years because of improved technology for phosphorus removal in both wastewater and detergents. The reductions we’re expecting to see from agricultural BMPs will take decades.”
Interested in learning more?
Sharon Ewe – Chief Ecologist
The Ag Angle: Addressing