Codium fragile



misc brown macroalgae

kelp lying over eelgrass

Seaweeds are another important group of plants in Long Island waters.  Seaweeds (a.k.a. – macroalgae) are a diverse group of organisms represented by a wide range of growth forms.  In general, seaweeds are divided into 3 groups based on their color – green, brown or red – though even within groups, color ranges greatly.  While seaweeds share some characteristics of “higher” plants, they lack the complex reproductive structures (flowers) and specialized functional tissues (roots, stems, and leaves) of seagrasses and land plants.  Seaweeds reproduce asexually (vegetative growth) and sexually (release of spores). 

seagrass vs seaweed diagramSeaweeds also display a wide range of growth strategies.  Some species, especially green seaweeds grow rapidly in response to nutrients (nitrogen) and may “bloom” in nutrient rich waters, only to reproduce and die after all the nutrients have been depleted.  Other species are more perennial.  These perennial species either never seem to die back (almost like evergreens) or die back under adverse conditions (e.g., high water temperatures, low light, etc.) but regrow when stress is alleviated.

Seaweeds are found in all of our coastal waters, with over 100 species represented.  Most seaweeds require a hard surface to which they can anchor, like rocks and shells, however, there are a few species that can be found over mud or sandy bottoms in the quiet back bays and creeks.  Seaweeds, like eelgrass, have the potential to form extensive beds that provide structure and habitat for marine animals.  Bed-forming seaweeds on Long Island include the perennial, brown seaweeds Sargassum (gulfweed) and Laminaria (kelp). Long Island also supports seaweed species that are non-native and invasive.  The first documented seaweed invasion on Long Island was by the green seaweed Codium (aka oysterthief, Sputnik-weed, deadman’s fingers) in 1957 in Orient Harbor.  The most recent introduction of a non-native seaweed was the discovery of the red seaweed Grateloupia on Montauk Point in 2001.  Currently, Long Island supports seven species of seaweed that are thought to be non-native.

Within a seagrass meadow, seaweeds contribute to the overall primary production as well as to biodiversity, and are therefore an part of the seagrass community. Algal epiphytes (algae growing on eelgrass blades) alone can contribute 20-60% of seagrass community net primary production. A healthy seagrass meadow will often have a fair amount of epiphytic as well as drift and benthic seaweeds present. The abundance of these forms or seaweed are mediated in healthy seagrass meadows by grazers and nutrient levels. Because seaweeds require less light and can tolerate higher nutrient levels than seagrasses, they will begin to overwhelm seagrass communities under eutrophic conditions, eventually contribute to the decline of the seagrass meadow.




Larkum, A.W.D., Orth, R.J., Duarte. C.M. (Editors), 2006. Seagrasses: Biology, Ecology and Conservation. Springer.


Table 1. Seagrass vs. Seaweed: What's the Difference?

(Adapted from Florida Fish and Wildlife Conservation Commission Seagrass vs Seaweed Info page)



Macroalgae (Seaweed)

Number of Species Worldwide




Have separate sexes
Produce flowers, fruits, and seeds

Produce spores


Evolved from terrestrial plants and have tissues that are specialized for certain tasks
Possess roots, leaves, and underground stems called rhizomes that hold plants in place

Relatively simple and unspecialized
Holdfast anchors plant to a hard surface; does not possess roots extending below the surface

Transport/ Classification

Use roots and rhizomes to extract nutrients from the sediment; use leaves for extracting nutrients from the water
Are categorized as vascular, with a network of xylem and phloem that transport nutrients and dissolved gases throughout the plant

Use diffusion to extract nutrients from the water
Not plants or animals, but protists


Next: Eelgrass Wasting Disease


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