We want to help people understand the ocean. This is the first installment of a series to inform people about red tide and harmful algal blooms currently affecting Southwest Florida.

Red Tide

The first thing. We really don’t really know enough about red tide and the conditions that cause it to occur in high densities.

The name red tide is unfortunate. The water color in a red tide event is not usually red, nor does it have anything to do with the tide.

Red tide is caused by a single-celled organism, Karenia brevisKarenia brevis is a naturally occurring species of phytoplankton classified as a dinoflagellate; it is not a plant but has the ability to carry out photosynthesis.

Karenia brevis has the ability to reproduce very rapidly in favorable conditions. Rapid reproduction is common among phytoplankton species; these episodes of rapid population expansion are called blooms.

Red tide is caused by a Karenia brevis bloom.

Karenia brevis naturally produce compounds classified as brevitoxins, a group of compounds that can be neurotoxic to animals with backbones. In humans, when ingested in high concentrations, they cause the illness neurolytic shellfish poisoning.

Brevitoxins are not actively released into the water by living Karenia brevis cells, but upon the death of the cell, these compounds are released into the ocean.

Red tide has been recorded since the 1840s in the Gulf of Mexico in SW Florida. Karenia brevis cells are usually present in the waters of SW Florida, but at very low concentrations.

The conditions favorable for Karenia brevis blooms are not well understood, but blooms are often associated with upwelling events. Upwellings are the movements of deep, nutrient-rich waters to the shallow, warm, sunlit upper levels of the ocean.

These upwelling events are associated with wind and climatic conditions that push Karenia brevis populations towards shore.

Phytoplankton population growth in the ocean is commonly limited by concentrations of nitrogen or phosphorus in the water. Human activities produce and release significant amounts of nitrogen and phosphorus – from agriculture, landscaping and sewage – these nutrients make their way into rivers, and then to the ocean.

There is no evidence that red tides are more frequent because of human-influenced discharges from lakes and rivers, but evidence indicates they last longer, and are more intense in areas of river discharges.

The estuary adjacent to Sanibel receives water from the Caloosahatchee, which is fed by Lake Okeechobee. Therefore, Sanibel is receiving water from a very large area of land. This water is nutrient-enriched by both agriculture and high density human communities which add landscaping and sewage runoff.

When Karenia brevis moves into our region, the nutrient-rich conditions from the Caloosahatchee are favorable for intense, long-lived, high concentration blooms to occur.

How does red tide affect humans?

Shellfish filter and feed on plankton from the water; in doing so, they accumulate brevitoxins. Eating shellfish that have high concentrations of brevitoxins causes a serious illness, neurotoxic shellfish poisoning (NSP). 

Commercial shellfish operations are monitored very closely in the State of Florida, and commercially harvested shellfish are safe to consume during a red tide outbreak. However, shellfish should not be harvested and/or consumed during or shortly after a red tide outbreak.

Brevitoxins can also be suspended in the air at the surf line. These molecules can irritate the back of the throat and eyes when in the air in high concentrations. Airborne molecules can also provide challenges for people with pre-existing pulmonary challenges such as asthma or COPD. The airborne effects of brevitoxins are usually in areas close to the shore and diminish quickly as one moves inland.

The amount of brevitoxin absorption through the skin is unclear, but doesn’t not seem to be a significant pathway of absorption – although more research is ongoing.

Make your voice heard. Contact your representatives about our water quality issues.