Lapointe, Brian E., et al. “Evidence of sewage-driven eutrophication and harmful algal blooms in Florida’s Indian River Lagoon.” Harmful Algae 43 (2015): 82-102. http://dx.doi.org/10.1016/j.hal.2015.01.004
Imagine walking from your car to the beach on a sunny, hot day in southeast Florida. The smile on your face begins to fade when you see unusually green water and a sign that reads, “No Swimming Allowed; Harmful Algae Bloom Present”. This sequence of events is not uncommon for residents situated along the expansive Indian River Lagoon (IRL). The IRL is a shallow (~ 0.8 m deep) and narrow (~ 3 km wide) lagoon that extends 251 km along Florida’s east-central coast (Fig. 1). The IRL watershed (area of land that drains into the lagoon) is massive and populated by nearly 1.7 million people! Groundwater containing contributions from septic tank sewage flows from urban areas on the watershed and ends up in the lagoon. Once this water reaches the lagoon, excess nutrients cause eutrophication (nutrient enrichment), harmful algal blooms (HABs), fish kills, and loss of seagrass and coral reef ecosystems.
In the IRL, benthic macroalgae and phytoplankton depend on nutrients and sunlight for survival. It is true that nutrients are essential for growth, but too much of a good thing can sometimes be a VERY bad thing. Some species of macroalgae and phytoplankton in the IRL are adapted to thrive under excessive nutrient conditions. These species quickly “take off” and form giant blooms that turn the water green or brown (Fig. 2). These blooms are harmful and create “dead zones” (areas of little to no oxygen) in the lagoon. Without sufficient oxygen, other organisms in the IRL (seagrass, fish, and aquatic mammals) will suffer. In a two-year study, researchers set out to assess changes in nutrients throughout the IRL and to better understand patterns of HABs.
Seawater samples were collected at 20 fixed sites in the IRL; samples were also taken from reference sites (REF) on reefs outside the lagoon (Fig. 1). Each site in the IRL was sampled at three different times in the two-year period. The three sampling events were characterized by “wet vs. dry” seasonality (Dry 2011, Wet 2011, and Wet 2012). Due to the enormous length of the IRL, the 20 sites were grouped and averaged by region. The five regions being Mosquito Lagoon (ML), Northern IRL (NIRL), Banana River (BR), Central IRL (CIRL) and Southern IRL (SIRL). Seawater samples at each site were measured for chlorophyll a and nutrients (nitrogen and phosphorous). Seagrass abundance was also estimated.
Nitrogen was found to be the most dominate nutrient in the IRL. The concentration of total dissolved nitrogen (TDN) did not significantly vary with seasonality. However, TDN did vary amongst the different regions in the IRL. TDN was highest in BR with a mean value of 82.30 ± 3.12 µM. Average TDN in ML and NIRL was also high, indicating a larger nitrogen pool in the northern IRL (Fig. 3). The reference site had a much lower average TDN concentration (13.21 ± 0.62 µM) when compared with IRL sample regions. So, thus far we know that the upper (northern) regions of the IRL had higher concentrations of nitrogen!
Average chlorophyll a varied in the IRL with highest concentrations occurring in the ML (49.94 ± 11.58 µg/L), BR (39.52 ± 8.06 µg/L) and NIRL (30.06 ± 5.16 µg/L) regions (Fig. 4). There was considerable variation between sampling events, especially in the northern IRL. Average chlorophyll was insanely high (>100 µg/L) in ML in the Wet 2011 sampling season and in BR in the Dry 2011 season. Mean chlorophyll a concentrations at the reference site were much lower than northern IRL regions (Fig. 4). Percent seagrass cover also varied with sampling region; the lowest mean value occurred in the BR (32.70 ± 6.78 %), which was one of the sites with enormously high chlorophyll. So, the northern IRL had higher amounts of nitrogen and chlorophyll coupled with low cover of seagrass!
Conclusion and Significance:
Nitrogen was found throughout the IRL and was particularly abundant in northern regions. High nitrogen concentrations in these areas were associated with massive phytoplankton blooms. Enormous “superbloom” events with chlorophyll a values greater than 100 µg/L (basically plankton soup!) occurred in specific seasons in both the ML (Wet 2011) and BR (Dry 2011). These “superblooms” reduced regional seagrass cover. In addition to nutrient runoff, the northern IRL is known for having a very long residence time (flushing of the lagoon water). The northern IRL is more “closed off” from the ocean in comparison to other regions. This causes things to move very slowly. Thus a combination of slow moving water, high nitrogen input, and adequate sunlight created harmful blooms in the northern IRL.
As the name implies, harmful algae blooms are harmful and can pose a serious threat to the health of the IRL. Runoff from sewage septic tanks on the IRL watershed is the the main source of nutrient input into the lagoon and drives harmful algae blooms. The economic value of the IRL has been estimated at $3.7 billion (mainly used for recreational and commercial fishing). High fish kills associated with “dead zones” will impact the economy of the IRL region. Loss of seagrass in the lagoon will negatively impact manatees who rely upon it as a food source and fish who use it for protection. The overall quality and health of the IRL depends on future efforts to better understand and manage nitrogen groundwater pollution. Improved sewage treatment facilities and more efficient nutrient removal techniques must be employed, especially in the northern IRL.