Heavy nutrient load into lake fueled algae bloom

NOAA satellite images from Sept. 9 indicate that blue-green algae continues to bloom in the water or on the surface of a large part of Lake Okeechobee.

As more information has become available, we have a clearer picture of why 2018 has been such an extreme bloom year in Florida’s largest lake. In the past, the amount of algae-fueling phosphorus loaded into the lake hovered around 500 metric tons. This year, that number exceeds 1,040 metric tons, the second-highest phosphorus load into the lake in 45 years, according to data recently released by the South Florida Water Management District. A higher-than-average amount of nitrogen has also made its way into the lake.

Most of the nutrient-laden water flowed into the lake during a six- to eight-week period after Hurricane Irma last fall, and after heavy rains early this spring. The water has brought with it nutrients left in the soils, wetlands and bottoms of creeks from past agricultural activities that predate contemporary best management practices. The heavy rain also has brought nutrients from contemporary sources, including agriculture, urban areas and perhaps leaking septic systems.

The hurricane stirred up nutrient-rich bottom sediments in Lake Istokpoga, located northwest of Lake Okeechobee, raising the level of phosphorus in the water. Phosphorus loads from Lake Istokpoga to Lake Okeechobee subsequently have risen to 118 metric tons, four times the long-term annual average. The Kissimmee Valley has also been heavily inundated, contributing another 305 metric tons of phosphorus to the lake — also four times the long-term annual average.

These nutrients have fueled a bloom in Lake Okeechobee that began with the hot, sunny weather in June. The bloom had been dominated by Microcystis, a kind of blue-green algae that requires dissolved nitrogen and phosphorus in the water to grow.

The bloom appeared to be going away in July, but then reappeared. Why? If blooms decline in summer, it’s often because they run out of nitrogen, unless they’re a species that can obtain nitrogen from the atmosphere. Samples obtained in early August (see Aug. 9 update) showed the resurgent bloom was dominated by Anabaena, a species that can obtain nitrogen from the atmosphere. Phosphorus concentrations can remain high during a bloom by recycling in the water.

It is unclear what the present composition of blue-green algae is in the lake, because water samples have not been taken recently. We will update if that information becomes available.

Interestingly, the Taylor Creek/Nubbin Slough, a subwatershed, had below-average nutrient loads into Lake Okeechobee. This subwatershed sits on the northeast edge of the lake. It’s important to note that this slough has experienced the most intensive agricultural nutrient control program by the state and the industry.

This year’s bloom illustrates the problems faced by the large lake ecosystem — the land that drains into it has a huge pent-up quantity of phosphorus and nitrogen. Flood control canals can rapidly carry those nutrients into the lake, causing blooms.

A wet hurricane or just a very wet spring — followed by a hot summer — can determine whether there will be a massive bloom on the lake. Nutrient control is critical, because we cannot control the weather. Climate change only will make the situation worse.

The positive results in the Taylor Creek/Nubbin Slough subwatershed are encouraging. State- and federally funded projects continue in the entire Lake Okeechobee watershed in an attempt to control nutrient and water inputs into the lake from various source regions.

Completion of projects in the Lake Okeechobee watershed and the decline in blue-green blooms in the lake is decades away. The mandated target load for phosphorus from the watershed to the lake is 105 metric tons. But the actual load of around 500 metric tons has not declined for decades.

The target phosphorus concentration for the lake water is 40 parts per billion. This concentration has risen markedly since the 1980s and is routinely more than 100 parts per billion. Nitrogen inputs are also an important factor for blue-green algae blooms in this large lake; they are not as explicitly considered as phosphorus inputs at this time.

Editor’s note: Karl Havens has 35 years’ experience in aquatic research, education and outreach, and has worked with Florida aquatic ecosystems and the use of objective science in their management for the past 23 years. His area of research specialty is the response of aquatic ecosystems to natural and human-caused stressors, including hurricanes, drought, climate change, eutrophication, invasive species and toxic materials. He is a recipient of the Edward Deevey Jr. Award from the Florida Lake Management Society for his research dealing with Florida lake ecosystems.

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