Enteromorpha (Enteromorpha spp.)

Synonyms: Ulva flexuosa, Ulva intestinalis, Ulva prolifera

Common Names: Green alga, gutweed

Description:

Enteromorpha is a group of hollow, tube-like green algae that has recently turned up in Michigan waters. It's not a separate genus from a modern taxonomic standpoint: genetic studies published in the early 2000s showed that species long called "Enteromorpha" actually belong within the genus Ulva, the same group as common sea lettuce, so the name lives on mostly as an older, still-widely-used label [1]. MISIN tracks it because one of these forms, Ulva (formerly Enteromorpha) flexuosa, reached bloom proportions in a Muskegon-area coastal lake on Lake Michigan in 2003 ? a naturally marine and brackish-water alga showing up and thriving in an inland Great Lakes setting [2].

Habit:

Enteromorpha forms long, hollow, tube-shaped strands, usually light green and often twisted or crumpled, giving it an "intestine-like" look that inspired both its old scientific name and its wrinkled appearance [3]. The tube wall is just one cell thick, and strands are typically 0.04 to 0.3 inch (1 to 7 mm) wide, growing up to about 8 inches (20 cm) long, though most strands stay smaller [3]. It's easiest to spot from late summer into fall, when warm water and nutrient runoff favor rapid growth, and blooms can wash ashore in large mats or foul boat motors and other equipment sitting in the water [2].

At-a-glance cues:

1. Long, hollow, tube-shaped strands with a wall just one cell thick [3]
2. Light green color, often twisted, wrinkled, or "intestine-like" in appearance [3]
3. Grows in dense mats on rocks, docks, or other submerged surfaces, and can break free to float as loose clumps [3][4]
4. Can turn up unexpectedly in inland lakes and streams, not just typical coastal salt- or brackish water [2]

Stems:

1. Habit: cylindrical, tube-like strands throughout, sometimes flattened or expanded and blade-like near the tip [3]
2. Wall: a single cell thick [3]
3. Size: 0.04 to 0.3 inch (1 to 7 mm) wide, up to about 8 inches (20 cm) long [3]
4. Color: light green [3]

Habitat:

Enteromorpha (now classified within Ulva) is typically found in saltwater and brackish coastal habitats ? estuaries, salt marshes, and areas where fresh water mixes with salt water ? and can tolerate a wide range of salinity, including quite low-salinity conditions [4]. In calm water, mats can detach and drift as free-floating clumps [4]. Its documented appearance in Michigan is a notable exception to that typical coastal-brackish pattern: it established in Muskegon Lake, a freshwater coastal lake on Lake Michigan, where past industrial activity had left the water somewhat saltier than a typical inland lake, and nutrient enrichment combined with a lack of grazing pressure likely helped it take hold [2].

Reproduction:

Enteromorpha most likely arrives in a new water body attached to a boat hull, motor, dock equipment, or other gear that has been in infested water, since even small fragments can regrow into new mats. Once established, it grows and reproduces very quickly [3].

Its life cycle alternates between two stages: a gametophyte stage that releases huge numbers of swimming sex cells (gametes) that pair up and fuse, and a sporophyte stage that releases swimming spores, which grow into the next gametophyte generation, and so on [3]. In its native coastal habitats this release is timed to the tides, letting the alga synchronize reproduction with the rhythm of the water it lives in [3]. In an inland lake like Muskegon Lake, where there's no tide to cue this cycle, populations can still spread and bloom quickly whenever warm water and nutrient-rich conditions are present [2][3].

Impact and Damage:

Enteromorpha isn't known to be directly toxic or hazardous to people or pets, but a bloom can be a real nuisance and hazard on a lake or shoreline: it can foul boat motors and equipment, wash up in large, unpleasant mats along beaches, and disrupt swimming and other recreation, as happened at Muskegon Lake [2].

For natural areas, the ecological picture is more serious. Dense mats ? sometimes called "green tides" when they occur at large scale ? can deplete oxygen in the water and increase hydrogen sulfide production in the sediment as the algae decompose, conditions that have been linked to population declines in other aquatic life and, in severe outbreaks, to fish and shellfish die-offs [1]. Thick mats can also shade out native seagrass beds and submerged vegetation, harming the fish and invertebrate communities that depend on them, and can disrupt feeding by wading birds that rely on clear, open shallow water [1]. In inland settings like Michigan's Great Lakes coast, an unexpected bloom is a sign that nutrient enrichment and reduced grazing pressure have created conditions favorable to a normally coastal species establishing far from its typical range [2].

Similar species:

Enteromorpha isn't simply confused with the green algae genus Ulva ? DNA studies in the early 2000s showed that the tube-shaped algae long called "Enteromorpha" are genetically part of Ulva itself, the same genus that includes common sea lettuce [1]. Species once known as Enteromorpha flexuosa, Enteromorpha intestinalis, and Enteromorpha prolifera are now more properly called Ulva flexuosa, Ulva intestinalis, and Ulva prolifera ? the "Enteromorpha" name persists mainly out of habit in older records and casual use, not as a distinct group. Anyone trying to identify a bloom is really distinguishing among different forms within Ulva rather than telling Enteromorpha apart from a separate, similar genus.

Monitoring and rapid response:

Signs to watch for:

1. Long, hollow, tube-like green strands forming mats on docks, rocks, or other submerged surfaces, especially in late summer and fall [3]
2. Large floating clumps washing ashore, or algae fouling boat motors and equipment [2]
3. An unexpected bloom in a normally low-algae lake or stream ? a sign nutrient levels or salinity may have shifted [2]

What you can do:

1. Reduce fertilizer use and nutrient runoff from your property into nearby lakes and streams, since excess nutrients are a key driver of blooms [4]
2. Rinse and clean boats, motors, and gear thoroughly after use in water with a known bloom, before moving to a different lake or stream, to avoid carrying fragments to a new location
3. For a small, localized mat on a private dock or shoreline, physically removing and disposing of the algae (not composting near the water) can offer short-term relief
4. Natural-area and lakefront associations: coordinate broader nutrient-reduction efforts across a watershed, since a single property's runoff reduction has limited effect on a lake-wide bloom

When to call a professional:

1. Large or recurring blooms across a lake or waterway, which call for a coordinated water-quality assessment rather than individual removal efforts
2. Suspected new occurrences in a water body without a documented history of this alga, since confirming identification matters for tracking its spread
3. For chemical control options, contact a licensed applicator or your local cooperative extension office.

Report sightings to MISIN. Help track this species' spread by submitting observations through the MISIN web site or the MISIN mobile app (Android, iOS, Windows, and macOS). Photos at multiple angles paired with a precise GPS location are the most valuable data for confirming new occurrences and triggering rapid-response review.

Credits:

1. Zhang Y, He P, Li H, Li G, Liu J, et al. (2019). "Ulva prolifera green-tide outbreaks and their environmental impact in the Yellow Sea, China." National Science Review 6(4):825?838. semanticscholar.org
2. Lougheed VL, Stevenson RJ (2004). "Exotic Marine Macroalga (Enteromorpha flexuosa) Reaches Bloom Proportions in a Coastal Lake of Lake Michigan." Journal of Great Lakes Research 30(4):538?544. semanticscholar.org
3. USGS Nonindigenous Aquatic Species Database, Great Lakes Program. Ulva (Enteromorpha) flexuosa species profile. nas.er.usgs.gov
4. Existing MISIN record's original habitat description (estuaries, salt marshes, salinity tolerance, free-floating clumps), independently consistent with USGS NAS [3].

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