Extreme Runoff of Chemical Species of Nitrogen and Phosphorus Threatens a Florida Barrier Island Lagoon

Abstract

Extreme runoff of stormwater to poorly flushed barrier island lagoons often adds excess nitrogen (N) and phosphorus (P) that can promote subsequent, sometimes intense, harmful algal blooms (HABs). Successful management of such estuaries requires special appreciation of when and how to control concentrations and fluxes of chemical species of N and P during high flow. Toward that end, monthly surveys and episodic rain-event sampling were carried out from December 2015 to March 2018 for two contrasting tributaries of the Indian River Lagoon (IRL), a barrier island lagoon in Florida. One tributary, South Prong Saint Sebastian River, flows through predominantly agricultural, forested and open land, whereas the second tributary, Crane Creek, traverses mainly residential-commercial land. Concentrations of some N and P species in these tributaries increased with increased flow and could be described with statistically significant equations for concentration versus flow rate, thereby supporting flow-rate-dependent flux determinations. Drainage basin yields (fluxes per square km) varied with land cover/use. Calculated annual yields of dissolved organic N (DON) and dissolved inorganic P (DIP) averaged ∼70% greater for South Prong Saint Sebastian River from high flow through thicker, more organic- and P-rich soils. In contrast, yields of nitrate + nitrite were 100% higher for Crane Creek from widespread application of N-fertilizer to thin layers of turfgrass overlying sand, plus runoff of N-rich reclaimed water. Two major weather events highlighted our study and foreshadow impacts from climate change. Seven months of drought from November 2016 to May 2017 were followed in September-October 2017 by excess rain, runoff and flooding from Hurricane Irma. Consequently, >50% of freshwater fluxes and ∼60% of N and P fluxes from South Prong Saint Sebastian River, Crane Creek and other IRL tributaries occurred during 2 months in 2017. Lagoon-wide inputs provided enough bioavailable N and P to help support a nanoeukaryotic bloom for >5 months, with chlorophyll a values >50 μg L–1. The bloom was co-dominated by the brown tide alga, Aureoumbra lagunensis, and an unidentified nanoeukaryotic green alga. Decreased salinity, low concentrations of dissolved inorganic N and P, and decreasing dissolved organic P (DOP), combined with biological factors, diminished the IRL bloom by mid-2018.

High-Resolution Estimation of Suspended Solids and Particulate Phosphorus Using Acoustic Devices in a Hydrologically Managed Canal in South Florida, USA

“…The low values of TSS and TPP estimates from this study are not uncommon in the region of Central and South Florida [ 51 , 52 ]. For example, studies conducted in Lake Worth Lagoon (South Florida) and in the tributaries of the Indian River Lagoon (East Central Florida) generally had TSS concentration averages less than 12 mg L −1 [ 52 , 53 ] and an average TPP concentration in the water column of less than 70 μg L −1 [ 52 ]. Similarly, in a study performed along the L-40 Canal, north of the L-29 Canal (our study), the average TSS concentration did not exceed 18 mg L −1 , and the average TPP concentration was 71 μg L −1 [ 51 ].…”

“…Similarly, in a study performed along the L-40 Canal, north of the L-29 Canal (our study), the average TSS concentration did not exceed 18 mg L −1 , and the average TPP concentration was 71 μg L −1 [ 51 ]. Such low TSS concentrations have been found in <5% of rivers globally [ 52 ]. However, the average TPP in TSS from our L-29 study, which was 2.05 μg mg −1 , is higher than the range of global averages for rivers (1–2.01 μg mg −1 ) [ 54 , 55 ].…”

“…Instantaneous discharges have been compared to storm events in their capacity to mobilize sediments and increase their concentrations and export [ 28 ]. In rivers, the first flush occurs at the start of a rain or storm event, where higher TSS can be recorded during the onset of very high flows that decrease over the duration of the event [ 52 ].…”

Nutrient fluxes from recent deposits of fine-grained, organic-rich sediments in a Florida estuary

“…The global challenge to reduce eutrophication is daunting. Controlling external sources of nutrients has been a mainstay for mitigation of estuarine eutrophication for many years (Kemp et al, 2009;Trefry and Fox, 2021). Point sources, such as wastewater treatment plants, are usually a starting point for reducing external nutrient sources (Diaz et al, 2008).…”