The algae Karenia brevis, found along Florida’s west cost, causes red tides. When present in the high concentrations of a bloom, it produces neurotoxins that can cause massive damage to marine life. Normal concentrations of the algae in Gulf waters are less than 1000 cells per liter. Bloom concentrations can be millions of cells per liter.

Karenia brevis is a dinoflagellate, called that because of its two whiplike flagella. It uses them to swim up or down, taking advantage of both sunlight and food availability. Its size is about one-half the diameter of a human hair.

Why red tides are a puzzle

Red tides in the Gulf are a natural occurrence. It is not clear whether human activities have changed their frequency or severity. It is also not clear where the nutrients necessary to support a bloom come from. Nitrogen and phosphorus are both needed for a red tide. This part of the Gulf is rich in phosphorus, so nitrogen availability probably controls blooms. But where does the nitrogen for a bloom come from? One attractive theory is that it comes from another marine organism, called Trichodesmium, which can capture nitrogen from the air. It needs iron to make this happen. Iron comes from Saharan dust that blows into the Gulf during the summer. So, red tides should happen when Saharan dust is most plentiful. But they don’t – a really nice theory shot down by data. There is little current research on the root cause of red tides.

Many people believe that red tides are fueled by nearshore nitrogen from human activities. But they start in offshore waters unaffected by human activities. Red tide algae reproduce very slowly compared to algae found nearshore. These nearshore algae are absent offshore, so it doesn’t matter there. But when the red tide algae move to the nearshore, the nearshore algae should out-compete them. In fact, the opposite happens.

A severe red tide happened in 2018, causing great damage to marine life in Sarasota Bay. As a result, Tidy Island became an ideal laboratory for observing its affects. There will be a lot more said about this.