From frequent harmful algal blooms – including brown tides – to catastrophic losses of seagrass, fish mortality and extraordinary marine mammal mortality – including the threatened Florida manatee – the Indian River Lagoon suffers from ecological distress. For decades, water managers, policymakers, and environmental activists have considered fertilizer use the major contributing source responsible for about 71 percent of this lake damage.
As a result, county and municipal fertilizer restrictions have been implemented along the 156-mile Indian River Lake on Florida’s Atlantic coast to reduce nutrient inputs from urban and agricultural land uses to achieve maximum total daily lake loads. Excess nutrient inputs, especially nitrogen, often increase harmful algal blooms, seagrass mortality, and fish mortality. The hope was that the water quality would be improved by reducing the nitrogen load.
While these restrictions were well-intended, a new study conducted by the Harbor Branch Oceanographic Institute at Florida Atlantic University reveals that fertilizer use is not the root cause of these environmental problems in the Indian River Lagoon. It’s sewage.
Study results published in the journal Marine Pollution BulletinAnd Recent estimates show the Indian River Lagoon’s residential fertilizer contributions are much lower than the originally specified contribution of 71 percent. In fact, current nitrogen loading estimates are 21 percent for residential fertilizers compared to 79 percent for septic systems. These loading estimates are similar to those reported for urban estuaries affected by the sewage system.
After five years of mandatory rainy-season fertilizer interruptions along the lagoon, researchers have discovered that water quality and harmful algal blooms have worsened in the North Indian Lagoon and Banana River, leading to unprecedented seaweed mortality and starvation of manatees.
To assess the effectiveness of these fertilizer bans, the researchers collected samples of seawater and macroalgae at 20 sites “before” and about five years “after”. They were tested by comparing the concentrations of seawater dissolved nutrients, tissue nutrients, and isotope data for brown tides and macroalgae. Gathering evidence from stable nitrogen isotope values enabled the researchers to distinguish between wastewater, rain and fertilizer, providing a unique “fingerprint” for the samples they collected.
“Our comparative nutritional data before and after the ban suggests that blackouts on fertilizer during the rainy season were not as effective as hoped,” said Brian Lapointe, PhD, senior author and research professor at FAU Harbor Branch. “Our findings also indicate that the increased concentrations of dissolved inorganic nitrogen and phosphorus observed in some parts of the lake after five years of fertilizer bans will support the declining trend of algal blooms.”
The researchers analyzed a total of 450 samples of macroalgae, including 211 collected before the ban and 239 collected after the ban. During the rainy season, 217 samples of macroalgae were collected, while 233 samples were collected during the dry season. They examined whether there was an associated decrease in dissolved ambient nutrients or a change in tissue nutrients and/or stable isotope values for phytoplankton or macroalgae that might indicate a shift in available nutrients and measured the stoichiometry feeding eutrophication in the lake.
“The deteriorating conditions in the Indian River Lagoon show the urgent need for more comprehensive mitigation measures because fertilizer regulations are unlikely to be a standalone solution,” said Rachel Broughton, corresponding author and research scientist at FAU Harbor Branch. “Our data indicate the essential role of the impact of human waste in the lake, indicating that current management measures have not been sufficient in mitigating environmental pollution.”
The significantly higher carbon to nitrogen ratio in the brown tide in 2012 compared to 2016 indicates increased bans on post-fertilizer nitrogen fertilization. The highest stable nitrogen isotope values occurred in the Banana River during the 2016 brown tide and closely matched values for partially treated wastewater, which would be expected in this highly urban area with old wastewater collection systems and secondary treatment without denitrification.
The researchers observed a similar rise in nitrogen-to-phosphorus ratios in the Banana River in the rainy season, showing how small-celled brown tides can maintain blooms by scavenging nutrients at low concentrations and skewing nitrogen into phosphorus. These results confirm the conclusions that phosphorus reduction plays a major role in brown tidal dynamics, particularly in relation to reproductive decline.
“The initial overestimation of nitrogen contributions from residential fertilizer applications led to broad public support and the passage of several fertilizer laws along India’s River Lagoon during our study period,” said Lapointe. “Now, it would be prudent to prioritize reducing nutrient inputs from human waste into the lake, before mitigating the effects of inland nutrient sources, when possible.”
Study co-authors are Lynn E. Wilking, a harmful algal bloom marine biologist with Consolidated Safety Services, Inc. , under contract with NOAA. and Laura Herren, a biologist at FAU Harbor Branch.
This work was supported by the Save Our Seas Specialty License Plate Fund administered through the Harbor Branch Oceanographic Institute Foundation (HBOIF) and the Florida Center for Coastal and Human Health, which was developed by HBOIF and receives ongoing support from HBOIF.