Introduction

Seaweeds, the varied and colorful marine algae commonly referred to as limu in Hawai‘i, play vital roles in the reef community, provide a nutritious source of food, and are significant in Hawaiian cultural practices. As photosynthetic organisms, marine algae are essential elements in a healthy coral reef system along with corals, fish and invertebrates. The Hawaiian Islands have over 500 species of algae known to science, and each year this list grows as scientists discover new species.

With so many limu species in Hawaiian waters, it can be overwhelming to identify all the limu you may encounter on a day at the beach unless you are a phycologist (a scientist who studies algae). Instead, you can often begin to sort limu by placing algae into broadly defined groups such as the taxonomic lineages (divisions) of red, green and brown algae or ecologically functional categories, such as the stone-like (coralline algae); upright and fleshy (macroalgae); and small and fuzzy (turf algae). Algae in Hawai‘i can also be distinguished as indigenous species (natives) and invasive species (aliens). Those indigenous limu that occur only in Hawai‘i are known as endemic species and about 20% of our limu are endemic.

Limu provide food for herbivorous fish, invertebrates and people, they also form living space for small animals, and some can even secrete chemicals that cue the larvae of fish and invertebrates to settle into a suitable habitat. Just like green plants, algae photosynthesize and thus split water and release free oxygen into their surrounding environment thereby enhancing dissolved oxygen so fish and invertebrates can respire. One very important ecological role of limu is the reef building and sand producing abilities of the coralline and other calcareous limu.

In a healthy Hawaiian reef ecosystem there is a remarkable diversity of corals, coralline algae, fleshy macroalgae, turf algae, colorful fishes, and invertebrates that all exist in an intricate balance. When the balance is upset by over-fishing, or the addition of excessive nutrients from the land, or especially a combined disturbance when both occur at the same time, the environment becomes degraded and the stage is set for invasive seaweeds to become dominant. 

Unfortunately, the shift in dominance from healthy reef to one dominated by invasive seaweeds is occurring on a number of reefs around the main Hawaiian Islands. As the photographs on the web site so graphically depict, invasive seaweed species are changing the structure of the reef community, resulting in a loss of biodiversity and a diminishing potential to maintain Hawaiian cultural practices that depend on a healthy reef.

This unit is designed to raise students’ awareness about the value of limu from ecological, cultural and economic perspectives, to introduce students to the ways that human activities are altering the coral reef ecosystem, and to explore what is being done to address the serious threats posed by invasive seaweeds in the Islands using the scientific method. Each of the instructional activities in the module focuses on selected Hawai‘i DOE Content and Performance Standards in science. The activities are to be conducted in life science, marine biology classes, as part of the voyaging curriculum, and at field sites to collect data.  The culminating activity encourages students to design and implement projects focusing on human activities that affect the diversity of the reef in their community. Students are encouraged to share what they have learned with others, to celebrate the biodiversity of Hawaiian reefs, and to examine ways that we can stop the spread of invasive species that threaten that diversity. 

This web site includes color photographs of a representative sample of limu species in the Islands. Also included are downloadable guides and additional suggested resources for use in the classroom.

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