Environmental Dredging to Remove Fine-Grained, Organic-Rich Sediments and Reduce Inputs of Nitrogen and Phosphorus to a Subtropical Estuary

Abstract

Environmental dredging of fine-grained, organic-rich sediments, locally referred to as Indian River Lagoon (IRL) muck, have been promoted as an integral part of restoring the IRL, Florida, to a healthy ecosystem. In Turkey Creek, a tributary to the IRL, ~300 metric tons of N and ~70 metric tons of P were removed with 160,000 m3 of wet muck and sand via environmental dredging during 2016 and 2017. Within the established dredged area, muck removal efficiency was ~63%; some areas were not dredged deep enough to remove all the muck. An additional 24,000 m3 of muck located outside the dredged area were not removed due to the presence of docks and seawalls. Prior to dredging, benthic fluxes of dissolved N (as ammonium) and P (as phosphate) from sediments to the overlying water, adjusted to 25°C, averaged 11 mg N/m2/h and 2.5 mg P/m2/h, respectively. Where IRL muck was removed to expose the underlying sand or mixed sand and muck, benthic fluxes of N and P were 20- to 30-fold lower after dredging. Subsequent disturbances, including Hurricane Matthew in October 2016, redistributed residual muck, leaving the dredged area 26% muck-free. Where muck was incompletely dredged or reintroduced by slumping, fluxes returned to predredging values within 6 months as equilibrium was reestablished between sediments and interstitial water. Dredging produced a 50% increase in water depth and basin volume with positive increases in salinity and the total inventory of dissolved oxygen. This deeper basin also serves as a sediment trap that will sequester future inputs of muck and mitigate future benthic fluxes of N and P by reducing the transport of muck into the IRL.

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

“…Such high DIN concentrations in the IRL were not likely supported by tributaries such as Crane Creek and South Prong because incoming freshwater accounted for only ∼40% of the water in the lagoon during late 2017 (Figure 6A). This observation supports previously discussed sources of DIN to the IRL, north of stations 18 and B9, that likely included groundwater, sewer overflows, leakage from septic systems and benthic fluxes of ammonium (Lapointe et al, 2015(Lapointe et al, , 2020Barile, 2018;Fox and Trefry, 2018;Tetra Tech Inc and Closewaters LLC, 2021).…”

“…This observation for Turkey Creek can be partly explained by significant water diversion to the west plus water retention areas in wetlands. Additional sources of DIN to the IRL, north of the tributaries that we studied, likely include groundwater, sewer overflows, reclaimed water, leakage from septic systems and benthic fluxes of ammonium from fine-grained, organic-rich sediments (Lapointe et al, 2015(Lapointe et al, , 2020Barile, 2018;Fox and Trefry, 2018;Tetra Tech Inc and Closewaters LLC, 2021). The relative importance of these potential sources of N is not well quantified for the IRL.…”

Field‐Validated Detection of Aureoumbra lagunensis Brown Tide Blooms in the Indian River Lagoon, Florida, Using Sentinel‐3A OLCI and Ground‐Based Hyperspectral Spectroradiometers

“…Field research has linked the potential sources of nutrient pollution causing brown tides to agricultural fertilizer runoff (Zhang et al, 2007), leeching of N from onsite sewage disposal systems (OSDSs, Barile, 2018;Lapointe et al, 2015Lapointe et al, , 2017, and the accumulation of legacy nutrients resuspended by natural and humaninduced actions (Dunne et al, 2011;Fox & Trefry, 2018;Reddy et al, 2011;Yang et al, 2013). Some research suggests that the limiting nutrient for brown tide is organic nitrogen derived from OSDS, which is ultimately reduced to ammonium or urea (Gobler & Sañudo-Wilhelmy, 2001;Gobler & Sunda, 2012;Kang et al, 2015;Lapointe et al, 2017).…”

Soft-Sediment Communities of the Northern Indian River Lagoon, FL, United States

“…This is especially apparent in sites 16 and 27, which had the largest proportion of this size sediment and the lowest abundances and species richness. Throughout the IRL, "muck, " a sediment composed of high organic content and fine sediment grains is widespread and characterized by a lack of biota (Fox and Trefry, 2018).…”