The Maui Nui Marine Resource Council is encouraging the public to contribute observations of green algae (Limu) along Maui’s coastlines. Sightings can be shared through the iNaturalist app and are automatically added to Coastal Groundwater Indicators in Hawai’i
The Marine Resource Council is sampling green algae to understand where Nitrogen-based nutrients (Nitrate, Nitrite, Ammonia) are coming from. In particular, lab analysis allows the detection of areas with nutrient pollution (too much nitrogen) and can distinguish between ‘natural’ nitrogen sources and those that are associated with sewage systems.
Green Algae (Limu)
Green algae is found in sources of freshwater and is commonly seen here on Maui’s shorelines. Tropical water usually has very low nutrients, but ground water brings nutrients from land to sea. Green limu can grow in less salty water than other marinen algae and can quickly grow when nutrients are available.
Ground Water
When it rains, some water is absorbed by plants and soil or runs off of the surface into lakes, streams, and the ocean. Some of the water makes it down deeper into the ground and slowly makes its way to the sea.
Nitrogen Levels
Because they grow where nutrients are scarce, many tropical algae can soak up and store nutrients, like nitrogen, when they are available. When excess nitrogen from land runs off and accumulates in the ocean it can create algae blooms.
Nitrogen Isotopes
Nitrogen naturally occurs in different isotope forms (Nitrogen-14 (N14) and (Nitrogen-15 (N15) which differ in weight because of different numbers of neutrons. Bacteria (common in sewage) prefer N14 and this means that in areas affected by sewage pollution, N15 will become more common in algal tissue. Tests are able to be done to show the levels of N14 compared to N15 that are found in limu. The results of these tests help determine the source of nitrogen, typically sewage or fertilizers.
How to Help
The Maui Nui Marine Resource Council welcomes help with identifying areas with groundwater. There is an iNaturalist project – Coastal Groundwater Indicators in Hawai’i – that anyone can contribute to. You can use your phone to upload a picture of limu to the iNaturalist app and all green limu observations get automatically added.