Coastal Services Center

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Managing a Nuisance Aquatic Plant

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Ulva lactua, commonly known as sea lettuce, is a naturally occurring species of macroalgae in Delaware's Inland Bays that becomes a nuisance to recreation and environmental health during the summer months. Managers are using a single beam acoustic seafloor classifier to identify where sea lettuce is growing to effectively target state-sponsored harvesting efforts.

The Project:
Mapping a Nuisance Algae for Effective Harvesting in Delaware's Inland Bays

Increased nutrient loading has caused naturally occurring sea lettuce populations in the Inland Bays to grow large and dense. As the summer temperatures rise, the algae dies and decays, depleting the water's oxygen supply and suffocating important recreational fish and shellfish. In addition, the rotting algae wash up on shore where nauseating odors discourage beach-goers and other tourists.

Decaying sea lettuce is easy to locate by its pungent odor and the black oily film it leaves on the water's surface. In the past, state-sponsored sea lettuce harvesters focused their removal efforts on areas with decaying algae – removing the algae after it became a nuisance. While this helped reduce some of the negative impacts, coastal managers wanted a proactive approach that would enable harvesters to remove dense populations of living sea lettuce before it began posing problems.

During the spring of 2000, the Delaware Department of Natural Resources and Environmental Control (DNREC) Delaware Coastal Programs (DCP) began an annual effort to map and monitor sea lettuce populations in Delaware's Inland Bays. The project has two ongoing goals:

1. determine where dense populations of sea lettuce are growing
2. monitor the distribution of algae over several years to determine how harvesting is affecting the population size and distribution

A harvester used for removing sea lettuce operating in Rehoboth Bay, Delaware.

Mapping Algae with a Single Beam Acoustic Sensor

The naturally turbid waters of Delaware's Inland Bays make it impossible to map sea lettuce for the entire area using a method like aerial photography. In addition, the water is too shallow for mapping with side-scan or multibeam sonars. Working with the NOAA Coastal Services Center, researchers with DCP decided to use a single beam acoustic seafloor classifier (RoxAnn™) to map the sea lettuce. This type of acoustic sensor uses a single sound pulse that is capable of penetrating the turbid water and operates in the extremely shallow waters of Delaware's Inland Bays.

A single beam acoustic sensor is ideal for the DCP because it works in a wide range of water depths (one to thousands of meters deep), is portable and fits on almost any boat, and can be shared with other agencies for other uses, such as bathymetry surveys or other seafloor substrate mapping.

Using a towed video for field calibration, the researchers can survey a single bay (approximately 5-km x 7-km area) with transects spaced approximately 300 meters apart over the course of one week. The data can be quickly processed to derive coarse sea lettuce distribution maps which can be provided to sea lettuce harvesters within a week of data collection.

What appear to be lines on this map are actually acoustic data points collected along intertwining transects in Rehoboth Bay, Delaware. The points are classified as algae (green) or no algae (light blue). The points were processed and used to derive a coarse map of the algae distribution (dark green squares).

The Result

Rapidly produced algae maps enable harvesters to target their efforts on the densest and largest populations of sea lettuce. The speed with which these maps are produced is critical for effective harvesting, because within a few days, a productive population of sea lettuce can die off and begin decaying. By knowing exactly where the sea lettuce is located, harvesters can target the densest algae populations without accidentally harvesting areas that do not need it. Today, everyone – boaters, locals, tourists, and coastal managers – are breathing easier because of the sea lettuce maps.

For More Information

• Delaware Department of Natural Resources and Environmental Control (DNREC) Delaware Coastal Programs (DCP)
• NOAA Coastal Services Center's Benthic Mapping Project

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