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IRL Report Card: South Indian River Lagoon

The South Indian River Lagoon extends from the St. Lucie County line south to northern Palm Beach County. It includes three regions that have the same regulatory targets: South IRL North, South IRL Central, and South IRL South.


The South Indian River Lagoon has three major tributaries entering it (Taylor Creek and the St. Lucie River enter the South IRL – North) and the Loxahatchee River enters the South IRL-South.

South Indian River Lagoon – North

The South Indian River Lagoon (IRL) – North includes the area of the lagoon in St. Lucie County and northern Martin County. This region includes two tributaries, Taylor Creek and the St. Lucie River. The St. Lucie River is the main discharge for Lake Okeechobee waters to be released to Florida’s east coast. Water released episodically from Lake Okeechobee drastically reduces salinity, increases turbidity, and often transfer harmful algae to the South Indian River Lagoon – North region and the Atlantic Ocean.

Because regulatory targets are measured as an annual average, unless the Lake Okeechobee discharges occur for many days, the change in water quality is barely evident in the annual average. You can see from the graph that water quality in the South IRL – North is pretty good and has frequently exceeded the regulatory target over the past 14 years. When Lake Okeechobee discharges occur for a long period of time (like in the year 2016) a decline in water quality occurred in this region that was followed by a rapid decline in seagrass.

Although water quality in this regions appears to be good, habitat quality (seagrass) has always been poor and in fact, rarely approached a passing grade.

South Indian River Lagoon – North
Water and Habitat Quality



South Indian River Lagoon – North Tributaries

There are two tributaries that drain into this region. Taylor Creek is a small tributary that enters the lagoon just north of the City of St. Lucie. The St. Lucie River enters the lagoon in Martin County, just south of Stuart, and exits the lagoon into the Atlantic Ocean through the Jupiter Inlet. Tributary targets were calculated based on the TMDL established in each tributary. See the Health Assessment Methodology Report linked here for more information on targets.

The water quality graph of the South IRL – North and its tributaries shows that the St. Lucie River water quality has historically not had a major impact on the South IRL overall, but this pattern changes in 2016, where a decline in both is evident, likely a result of the extensive Lake Okeechobee discharges that happened that year. However, even the water quality in Taylor Creek was poor that year, a tributary not effected by Lake Okeechobee.

South Indian River Lagoon – North Tributaries Water Quality



South Indian River Lagoon – Central

South IRL – Central includes the area of the lagoon within Martin County that extends from just South of the St. Lucie River to the Palm Beach County line. There are no major tributaries entering this region of the lagoon, although it is heavily impacted by the St. Lucie River located just north of this region’s northern boundary. A glimpse at the graph shows that the 2016 Lake Okeechobee continuous discharge had a devastating effect on seagrasses in this region of the lagoon. As with the South IRL – North, the water quality trend over 23 years is positive, indicating improvements over time, but the habitat quality is rapidly declining.

South Indian River Lagoon – Central
Water and Habitat Quality



South Indian River Lagoon – South

The South IRL – South includes the southern portion of Martin County south of Hobe Sound and the very top of Palm Beach County where the Loxahatchee enters the lagoon, the region’s only tributary. This region has some of the best water quality in the lagoon, consistently well above the regulatory target. Perhaps this is why the state stopped monitoring water quality in 2012. From the graph you can see that regardless of the good water quality, seagrass (habitat quality) declined after 2013 and took a major turn for the worse in 2017.

South Indian River Lagoon – South
Water and Habitat Quality



South Indian River Lagoon – South Tributary (Loxahatchee River)

The Loxahatchee River that enters the South IRL South is the only region in the entire Indian River Lagoon that consistently meets water quality targets. Notice that every part of the Loxahatchee is above average, but that the data doesn’t start until 2007.

South Indian River Lagoon – South Tributary (Loxahatchee River) Water Quality