Study links human waste to harmful algal blooms

OKEECHOBEE — A research paper published by Elsevier gives more evidence to the link between faulty septic tanks and the harmful algal blooms that plagued the St. Lucie Estuary in 2016. The study recommends conversion from septic tanks to sewer systems near the waterways to reduce nutrient loading, as well as more storage north of Lake Okeechobee to reduce the need for excessive freshwater releases from the lake.

While researchers found the local basin runoff contributed 70 percent of the nutrients that fed the algal blooms, they also note that excessive releases of freshwater from Lake Okeechobee disrupt the salinity levels in the estuaries and are harmful to that ecosystem even if there is not an algal bloom.

The paper, “Septic systems contribute to nutrient pollution and harmful algal blooms in the St. Lucie Estuary, Southeast Florida, USA,” was written by Brian E. Lapointe, Laura W. Herren and Armelle L. Paule, of the Harbor Brand Oceanographic Institute at Florida Atlantic University, Marine Ecosystem Health Program.

Elsevier is one of the world’s major providers of scientific, technical and medical information.

“Nutrient enrichment is a significant global-scale driver of change in coastal waters, contributing to an array of problems in coastal ecosystems,” state the authors. “The St. Lucie Estuary in southeast Florida has received national attention as a result of its poor water quality (elevated nutrient concentrations and fecal bacteria counts), recurring toxic microcystis aeruginosa blooms, and its proximity to the northern boundary of tropical coral species in the United States. The St. Lucie Estuary has an artificially large watershed comprised of a network of drainage canals, one of which (C-44) is used to lower the water level in Lake Okeechobee. Public attention has primarily been directed at nutrient inputs originating from the lake, but recent concern over the importance of local watershed impacts prompted a one-year watershed study designed to investigate the interactions between on-site sewage treatment and disposal systems (OSTDS or septic systems), groundwaters, and surface waters in the St. Lucie Estuary and nearshore reefs.”

Study results found multiple lines of evidence that indicated the St. Lucie Estuary is contaminated by septic systems. The researchers found high levels of dissolved nutrients concentrated adjacent to two, pre-1978 residential communities. They found the artificial sweetener sucralose present in groundwater near residential sites. They also found that dissolved nutrients documented at residential groundwater monitoring sites were higher than those recorded at nonresidential sites. They noted high fecal counts which result in periodic public heath warnings for Treasure Coast waterways.

“Consequently, there is a need to reduce nitrogen and phosphorus loading, as well as fecal loading, from the St. Lucy Estuary watershed via septic-to-sewer conversion projects and to minimize the frequency and intensity of the releases from Lake Okeechobee to the St. Lucie Estuary via additional water storage north of the lake. These enhancements would improve water quality in both the St. Lucie Estuary and Lake Okeechobee, reduce the occurrence of toxic harmful algal blooms in the linked systems, and improve overall ecosystem health in the St. Lucie Estuary and downstream reefs,” the study states.

In addition to the periodic problems with algal blooms, the St. Lucie Estuary has experienced health advisories for high fecal bacteria counts, the study notes.

The U.S. Environmental Protection Agency and the Florida Department of Environmental Projection have identified the St. Lucie Estuary as an impaired waterbody.

The St. Lucie Estuary “receives freshwater inputs from an artificially large watershed as the result of a network of canals constructed in the early-to-mid 1900s to alleviate flooding and increase development potential,” the researchers state.

In addition to local basin runoff, water from Lake Okeechobee is sometimes released into the C-44 canal at Port Mayaca. These releases are based on the Lake Okeechobee Regulation Schedule, through which the U.S. Army Corps of Engineers tries to keep the lake level above 12.5 ft. and lower than 15.5 ft. (If the lake is below 12.5 ft., water from the C-44 canal may flow into the lake, as happened during the dry season in 2016.)

In the C-44 canal, the lake water mixes with direct basin runoff, which is higher in nutrient loading than the lake water. For example, per the South Florida Water Management District data, the lake water averages around 120 parts per billion (ppb) phosphorus while the C-44 runoff averages around 300 pbb.

Massive freshwater releases into the C-44 canal are currently necessary when the lake level rises too rapidly from heavy rainfall runoff north of the lake in the Kissimmee River valley. These freshwater releases lower the salinity levels in the coastal estuaries, and makes these waterways more vulnerable to freshwater algal blooms.

While the lake water is believed to have carried the seeds of the 2013 and 2016 algal blooms, the nutrient loads in the estuaries were sufficient for the blooms to grow rapidly and become toxic, according to the study, “indicating wastewater nitrogen as a primary nitrogen source fueling the blooms in the St. Lucie Estuary.”

The nitrogen levels in the microcystis aeruginosa samples collected mid-estuary during the 2013 and 2016 blooms were highly enriched compared to samples from the C-44 canal, the researchers state.

“With such high nutrient availability, microcystis has the potential to double its biomass in approximately two days. This combination of a toxic gradient and nutrient availability provides additional evidence that significant bloom development has been occurring in the St. Lucie Estuary itself rather than the upstream seed sources,” the study states.

Microcystis aeruginosa can produce toxins, but does not always do so. Certain conditions — such as rapid growth — can cause the algae to release toxins.

In 2016, toxins were found in the bloom in the C-44 canal, and toxin levels increased as the bloom moved from the C-44 to the estuaries, the researchers found.

In other words, in 2016, the algae was present in the lake — as it always is as part of that natural ecosystem — but grew thicker when it entered the C-44 and fed on the increased levels of phosphorus and nitrogen there, and even thicker and more toxic when it entered the estuaries which were even richer in nutrients than was the C-44 canal.

“Recent water and nutrient budgets for the St. Lucie Estuary indicate that between Water Year 1997 and 2015, about 30 percent of the nitrogen came from Lake Okeechobee, compared to 70 percent from the St. Lucie River watershed and tidal basin. Although more attention has been given to the contribution of freshwater releases from Lake Okeechobee to this system, it is also important to understand the significance of nutrients (e.g., atmosphere, fertilizers, wastewater) from local watersheds to achieve nutrient mitigation for the St. Lucie Estuary and the downstream ecosystems,” the authors explain.

The microcystis aeruginosa blooms are just part of the problem facing the coastal waterways.

“In addition to phytoplankton-based harmful algal blooms, degradation of nearshore reefs by macroalgal harmful algal blooms likewise result from inputs of land-based sources of nitrogen and phosphorus,” the study found.

“Land-based discharges to these nearshore reefs likely contribute to biological indicators of stress,” the paper states.

The results of the one-year study provide evidence of wastewater contamination impacting not only the St. Lucie Estuary but also the nearshore reefs. In addition, studies indicate that climate change will increase the nitrogen loading into the world’s rivers by about 20 percent. “They estimate that offsetting this increase will require a 33 percent reduction in nitrogen inputs, representing a major challenge to planners and resource managers. In the St. Lucie Estuary study area, septic-to-sewer programs, which include nutrient (nitrogen and phosphorus) removal from advanced wastewater treatment, could help meet this challenge and mitigate future harmful algal bloom events,” the researchers state. “These improvements within the local watershed combined with increased storage and nutrient management north of Lake Okeechobee would collectively improve the conditions in both the St. Lucie Estuary, adjacent portions of the Indian River Lagoon and the nearshore reefs downstream of the St. Lucie Inlet while better protecting human health.”

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