FORT PIERCE, Fla. — Sewage and not excess fertilizers are the main culprit that is polluting the Indian River Lagoon, according to a new study released Tuesday by Florida Atlantic University.
Officials including water managers, policymakers and environmentalists have long implicated fertilizer as the main cause of algal blooms, the death of marine life and the loss of seagrass.
This has prompted fertilizer restrictions in Florida counties and municipalities along the Indian River Lagoon to reduce nutrient runoff into the waterway. Excess nutrient inputs, particularly nitrogen, often result in the increase of toxic algae and other negative impacts on the lagoon.
While well-intended, Florida Atlantic University's Harbor Branch Oceanographic Institute found that fertilizer is not the main cause of these environmental problems but instead is sewage.
The findings of the study, published in the journal Marine Pollution Bulletin, show recent estimates for residential fertilizer contributions to the Indian River Lagoon are much lower than the originally defined contribution of 71%.
The study found that current nitrogen estimates represent a 21% contribution from fertilizers compared to 79% from septic systems.
Scientists monitor overlooked algae toxin in Indian River Lagoon
After five years of fertilizer blackouts along the lagoon during the wet season, researchers found that water quality and harmful algal blooms actually worsened in the northern Indian River Lagoon and Banana River. This led to historic seagrass loss and the starvation of manatees.
"Our comparative pre- versus post-ban nutrient data indicate that the wet season fertilizer blackouts were not as effective as hoped," 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."
Researchers analyzed a total of 450 macroalgae samples, including 211 that were collected pre-ban and 239 collected post-ban for the study.
"The deteriorating conditions in the Indian River Lagoon demonstrate the urgent need for more comprehensive mitigation actions as fertilizer ordinances are not likely to be a standalone solution," Rachel Brewton, corresponding author and a research scientist at FAU Harbor Branch, said. "Our data indicate a primary role of human waste influence in the lagoon, which suggests that current management actions have been insufficient at mitigating environmental pollution."
The study found a significantly higher carbon-to-nitrogen ratio of the brown tide in 2012 compared to 2016, indicating greater nitrogen enrichment post-fertilizer bans.
"The initial overestimation of nitrogen contributions from residential fertilizer applications led to broad public support and the passage of numerous fertilizer ordinances along the Indian River Lagoon during our study period," Lapointe said. "Now, it would be prudent to prioritize reducing human waste nutrient inputs into the lagoon, prior to mitigating the impacts of internal nutrient sources, when possible."