What causes red tide? Research examines human impact

Research has found multiple reasons why red tides start and survive, but are humans partly to blame?

TAMPA, Fla. — The red tide continues to wash away hundreds of thousands of pounds of dead marine life every day.

In the Tampa Bay area, it is usually caused by Karenia brevis, a single-celled organism called a dinoflagellate. Most dinoflagellates are harmless, but K. brevis is very different, producing neurotoxins that can cause respiratory problems in humans and attack the central nervous system of fish and other wildlife.

But what makes red tides start and stay? Scientists say there is no clear answer, with many factors, both natural and man-made, contributing to harmful algal blooms, also known as HABs.

To begin with, K. brevis is considered an extremely opportunistic form of algae. It can survive water temperatures ranging from around 50° F at 90° F, but it thrives in temperatures between 72° F and 82° F. Although quite hardy, K. brevis grow at a much slower rate than other phytoplankton species.

They are not picky eaters, using a wide range of nutrients to survive and multiply, but they mostly need nitrogen and phosphorus to thrive.

In an assessment of the Florida red tides by the Mote Marine Laboratory, scientists conclude that there are a wide variety of factors contributing to enhanced red tides and why they can last for months. There is no single source, including pollution, that can account for all of the nutrients needed to maintain HABs.


Offshore upwellings initiating blooms

Most scientists agree that red tide blooms often start offshore and then move inshore due to winds and ocean currents. Deep nutrient-enriched water along the continental shelf can help start these blooms.

Most large bodies of water like oceans and gulfs will have nutrient reservoirs where colder, saltier water mixes with warmer, cooler water.

After reviewing data from blooms that occurred in the 1950s and 1960s, scientists conclude that red tide blooms often begin about 10 to 50 miles offshore. Once coastal, they can develop rapidly around inlets.

Other algae can help K. brevis get started

Trichodesmium, known as blue-green algae, can be found in the oceans. Scientists have found that blue-green algal blooms often occur before or at the same time as K. brevis blooms. They can actually provide important nutrients, like nitrogen, that K. brevis can use.

Research also reveals that dust clouds, including the sands of Africa, can contribute to these blooms. These clouds can actually create iron deposits in the Gulf of Mexico, leading to blue-green algae and K. brevis blooms. Tampa Bay recently experienced one of these Sahara Desert dust storms in June.

Dead Sea Life Could Mean More Nutrients

As K. brevis spreads, it can create mass fish kills. Scientists say dead sea life will drift to the bottom of the ocean before floating upward, while expelling these nutrients throughout the water column. All of these nutrients help sustain K. brevis as it moves inshore.

Coastal runoff and pollution

Although there is no specific evidence showing that pollution can trigger a red tide, it can prolong its duration. There is no conclusive evidence linking any specific source of coastal pollution to red tides, but any form of pollution can have consequences.

A study shows that coastal lagoons, bays and other similar enclosed or semi-enclosed coastal areas with restricted water movement can see pollutants can accumulate to high concentrations. The same study found that industrial and municipal waste discharges containing significant amounts of organic matter can create favorable conditions for strong red tides.

In a case study of Charlotte Harbour, researchers found that current levels of nitrogen flowing into the estuary are three times higher than they were before the 1800s. Due to improved from the quality of water treatment facilities and land use practices, river nitrogen levels began to decline in the 1980s and 1990s. However, research has predicted higher amounts after 1998 as populations continue to grow.

Statewide, Mote Marine Laboratory has seen substantial improvements in point source pollution control over the past three decades. However, diffuse pollution seems to have increased over time.

Mote Marine Laboratory scientists conclude that land use activities could affect nutrient levels, making some coastal areas more vulnerable. Once a red tide arrives, these increased nutrient levels can sustain K. brevis. The research adds that it is doubtful that coastal pollution can trigger the formation of a red tide, but it can support K. brevis and even provide a thriving environment.

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