Do nitrogen cuts benefit the Baltic Sea?
Scientists disagree over proposals to cut nitrogen inputs to curb “dead zones” and toxic algae.
Swedish cottagers were greeted by a foot-thick scum of rotting algae on coastal island shores last summer as the Baltic Sea heaved up one of its worst cyanobacteria blooms ever. Sweden’s inability to rein in the algal blooms, despite a world-class nutrient-control program, has elicited criticism of the government’s tough policy of cutting both nitrogen and phosphorus. Now, a panel of five eminent North American scientists has released a report that supports Sweden’s stand on phosphorus, but they remain deadlocked over how to handle nitrogen.
Eutrophication of Swedish Seas [1.4MB PDF] calls for aggressive reductions in phosphorus inputs to alleviate algal blooms and oxygen-starved “dead zones” in the Baltic Sea and inlets along Sweden’s east coast. The report, which was released on March 20, explains that algae thrive on both nitrogen and phosphorus. Moreover, when the ratio of nitrogen to phosphorus is below a unique threshold, growth is limited by nitrogen, but when the ratio is high, phosphorus controls the growth rate. The experts, who were called in by the Swedish EPA, agree that Baltic algae are being fed too much phosphorus, but they differ on whether nitrogen has a special role to play.
Worldwide significance
The debate holds lessons for resource managers worldwide who are struggling with the growing problem of nutrient over-enrichment of brackish coastal waters, says Fred Wulff, a marine ecologist at Stockholm University.
“The case for phosphorus reductions has been well demonstrated,” says Robert Hecky, an aquatic ecologist at the University of Waterloo (Canada) and a coauthor of the report. Sweden cut phosphorus inputs to the Stockholm archipelago by 90% from 1970 to 1980, slashing algal production in these waters by 50% by the mid-1980s, according to the report.
In 2004, Sweden contributed roughly 13% of the excess phosphorus dumped in the Baltic each year from sewage treatment plans and farm runoff, according to the Helsinki Commission, a coalition of 9 Baltic countries. Additionally, the 9 member countries of the Helsinki Commission have a nonbinding agreement to reduce phosphorus emissions by 50% from 1989 levels by 2005, but not all have reached the target, says Roger Sedin, director of the zero eutrophication program at the Swedish EPA.
However, the main reason that Sweden’s aggressive phosphorus removal program hasn’t had much impact on the Baltic is because of inaction by other countries, especially former members of the Soviet Union.
Great opportunity for improvement
Scientists have seen no substantial reduction in overall phosphorus loading over the past 25 years, Hecky says. Great opportunity exists for improvement, beginning with removing phosphorus from detergents and from point sources, the report concludes.
Sweden began to invest in nitrogen reduction from farms and sewage treatment plants in the mid-1990s as a response to mandatory goals set by the EU’s Water Framework Directive as well as research suggesting that nitrogen cuts could reduce algal blooms, Sedin says. The Helsinki Commission member countries also have a non-binding agreement to reduce nitrogen emissions by 50% from 1989 levels by 2005, but none have achieved this goal, he says.
In the waters of the Stockholm archipelago, nitrogen loads dropped by roughly 50% after 1998 compared with the 1980s, and algal production declined by about 15–40% in the summer, according to the report. Whereas the spring algal bloom is still limited by phosphorus, by summertime the algae are short of nitrogen and this is when the declines in production are seen, demonstrating the efficacy of nitrogen removal, says Ragnar Elmgren, a marine ecologist at Stockholm University.
Nitrogen removal controversies
“There are two ways in which nitrogen removal has been controversial: Has it actually worked in coastal areas, and can it do anything for the open Baltic?” Elmgren asks.
“We’re seeing different things,” Hecky responds. The nitrogen-removal technology extracted ammonia and organic matter, both of which remove oxygen from the water as they decompose, he explains. By reducing the biological oxygen demands associated with the excess ammonia and organic matter, the technology boosted oxygen concentrations in the bottom waters. Because phosphorus that has accumulated in the sediments over many years can be released to the water under low-oxygen conditions, the nitrogen removal technology—by elevating oxygen levels—may have also reduced phosphorus inputs from the sediment, thereby driving down algal production, Hecky says.
Experiments show that algal growth in the waters of the Stockholm archipelago is limited by both nitrogen and phosphorus, similar to estuaries on the U.S. east coast, says Don Boesch, a biological oceanographer at the University of Maryland and a coauthor of the report. Because algal growth in the archipelago is limited by nitrogen in the summer, the nitrogen cuts are decreasing summertime algal production; this reduces organic matter inputs to the bottom and elevates oxygen levels, helping to keep phosphorus locked in sediments, he says.
Effects of nitrogen cuts not clear
However, the effect of the nitrogen cuts on the open Baltic has been far less clear. The debate has focused on data showing that cyanobacteria, blue-green algae, “fix”, or absorb, 300,000–600,000 tons (t) of nitrogen from the air every year, a level that makes it the single largest source of the nutrient to the Baltic. The findings led critics in the Swedish EPA and the Swedish scientific community to charge that controlling nitrogen is fruitless because the cuts from sewage and farming will instead favor the growth of cyanobacteria.
When phosphorus concentrations are high, algal growth is limited by nitrogen, which gives cyanobacteria a competitive edge over other algae in midsummer because they can supply themselves with nitrogen from the air, Hecky explains. In the Baltic, algal production is still tuned to phosphorus, and if nitrogen is removed too fast, it could create favorable conditions for cyanobacteria, he cautions. These toxic blooms then defeat any cuts by Sweden or the EU by fixing nitrogen from the air, Sedin says.
However, the springtime bloom is dominated by other algal species, such as diatoms, which thrive on the nitrogen-rich water that is brought up from the bottom of the Baltic in spring, Elmgren says. When the spring bloom dies and sinks, it ultimately robs the bottom water of oxygen, triggering release of phosphorus in the sediment that feeds the cyanobacteria bloom in midsummer, he says.
Should both nitrogen and phosphorus be controlled?
“Both nitrogen and phosphorus should be controlled,” says Daniel Conley, a nutrient biogeochemist at the National Environmental Research Institute (Denmark). Studies show that hypoxia in the Baltic is responsible for releasing 50,000–80,000 t of phosphorus from the sediment every year, far more than the 15,000 t from external sources, he says. Phosphorus release from sediments and the accompanying cyanobacteria blooms will continue until the low-oxygen zones are eliminated, and that will require cuts in both phosphorus and nitrogen, he says.
This analysis appears to be borne out by a model developed by Wulff. It shows that if all the Baltic countries removed 95% of the phosphorus and 70% of the nitrogen from sewage treatment plants, then the oxygen-poor dead zone would decline by about 25%, and cyanobacteria blooms would dramatically decrease.
The Swedish EPA is evaluating the report and is likely to put more emphasis on phosphorus removal while maintaining current nitrogen-reduction measures, Sedin says.
