Prepared by E. Allen Stewart III P.E. October 2021
The Indian River Lagoon System (IRLS) includes about 156 miles of open estuarine
water which runs from north to south along Florida’s east coast. The IRLS developed
over the past 5,000-7,000 years as a low nutrient, (oligotrophic) estuarine system with a surface water area of about 353 square miles and a limited watershed area of only 893 square miles. The lagoon itself was, and remains, restricted from significant tidal influence by a series of barrier islands. These barriers were interrupted by four inlets (now five) along the length of the lagoon.
The pre-development IRLS relied largely upon direct atmospheric input (rainfall and dry deposition) as the major nutrient source. These nutrients which found their way into the lagoon waters through groundwater seepage and small surface creeks were captured by expansive seagrass lawns and associated epiphytes, both serving as the primary producers. Because of the low nutrients, the IRLS generated little net ecosystem production (NEP), i.e., excess production. Consequently, accretion of organic residual was low, which facilitated the establishment of a stable quasi-steady state dynamic.
The ecology of the IRLS changed dramatically with extensive development of a
technological society during the twentieth century. By the year 2000 as a result of
canalization; watershed expansion; and urban, residential, industrial and agricultural
development, the IRLS had become a eutrophic (highly productive) system which was trending towards a shift from seagrass dominated primary production to phytoplankton (suspended algae) dominated primary production. With this shift, and the subsequent decline in seagrasses, combined with disruption in hydrologic and nutrient dynamics, ecological degradation began to impose upon the local economy and the quality of life.
In 2009 a TMDL for nutrient loading was established by the Florida Department of Environmental Protection (FDEP) for the three impaired segments of the IRLS—these
being the North IRL, Central IRL and Banana River Lagoon.
As part of the TMDLdevelopment, a direct relationship was established between seagrass coverage, as determined through average maximum depth of seagrass growth, and nutrient mass loading to the lagoon segments. This relationship was used to determine nutrient loading limits (TMDL). The original model used in projecting stormwater loading was later replaced with an improved model, the Spatial Watershed Iterative Loading or SWIL, and the TMDL was adjusted accordingly. The present TMDL as noted in the 2021 Basin Management Action Plans (BMAP) is 1,975,106 pounds per year total nitrogen and 224,319 pounds per year total phosphorus for the three segments.
In 2011 the IRLS experienced an extensive algae bloom, and unprecedented blooms
followed in subsequent years, often accompanied with fish kills and excessive loss of
seagrass—about 50% loss of seagrass coverage occurred from 2011 to 2018. As a
consequence of reduced seagrass coverage, per the Florida Fish and Wildlife Conservation Commission (FWC) in 2021 the IRLS experienced “unprecedented
manatee mortality due to starvation”.