From recurring harmful algal blooms – including brown tides – to catastrophic seagrass losses, fish kills and unusual marine mammal deaths – including the threatened Florida manatee – the Indian River Lagoon is environmentally distressed. For decades, water managers, policy makers and environmental activists have implicated fertilizer use as the primary contributing source responsible for about 71 percent of these impairments in the lagoon.

Consequently, fertilizer restrictions have been implemented in counties and municipalities along the 156-mile-long Indian River Lagoon on Florida’s Atlantic coast to reduce nutrient inputs from urban and agricultural land uses to achieve total maximum daily loads for the lagoon. Excess nutrient inputs, particularly nitrogen, often result in increased harmful algal blooms, seagrass die-offs and fish kills. The hope was that water quality would improve by reducing the nitrogen load. 

While these restrictions were well-intended, a new study by Florida Atlantic University’s Harbor Branch Oceanographic Institute reveals fertilizer use is not the root cause of these environmental problems in the Indian River Lagoon. It’s sewage.

Findings of the study, published in the journal Marine Pollution Bulletin    in July show recent estimates for residential fertilizer contributions to the Indian River Lagoon are much lower than the originally defined contribution of 71 percent. In fact, current nitrogen loading estimates represent a 21 percent contribution from residential fertilizers compared to 79 percent from septic systems. These loading estimates are similar to those reported in other septic system-impacted urbanized estuaries.

Following five years of mandatory wet season fertilizer blackouts along the lagoon, researchers discovered water quality and harmful algal blooms have worsened in the northern Indian River Lagoon and Banana River, leading to unprecedented seagrass die-offs and starvation of manatees.

To assess the effectiveness of these fertilizer bans, researchers collected seawater and macroalgal samples at 20 sites “pre” and about five-years “post” bans. They tested by comparing dissolved seawater nutrient concentrations and tissue nutrient and isotope data of brown tides and macroalgae. Gathering evidence from stable nitrogen isotope values enabled researchers to discriminate between sewage, rainfall and fertilizer, providing a unique “fingerprint” of the samples they collected.

“Our comparative pre- versus post-ban nutrient data indicate that the wet season fertilizer blackouts were not as effective as hoped,” said Brian Lapointe, Ph.D., senior author and a research professor at FAU Harbor Branch. “Our findings also suggest that the increasing concentrations of dissolved inorganic nitrogen and phosphorus observed in some segments of the lagoon following five years of fertilizer bans would support the worsening trend of algal blooms.”