Seaweed farming is an emerging industry in the U.S. with an annual increase of 132% from 2017 to 2018. While most of this growth has been in the East of the U.S. and Alaska, California’s strict regulations have limited the growth of this industry. This changed in 2020, when Humboldt State University, in partnership with Greenwave , installed the first and only commercially licensed seaweed farm to be operational in California state marine waters. The objectives of this project were to: 1) evaluate the growth of Pacific dulse Devaleraea mollis , previously known as Palmaria mollis, cultivated at a range of depths and during different seasons, 2) estimate nutrients removed by D. mollis from the water , and 3) analyze for heavy metals and pesticides to determine if the red seaweed would be considered safe for human consumption.
The study site was located in a pre-permitted lease area of Humboldt Bay in northern California. Two long-lines were used, each consisting of a 107m surface line attached to a mooring buoy at both ends and supported by roughly 25 floats. The mooring buoys were anchored with 100-pound Danforth anchors. Each long-line had 50 vertical weighted grow lines (droppers) separated every two meters. Each dropper was seeded with a 15g starter bundle of D. mollis at 0m, 1m, 2m, and 3m deep. Lines were monitored weekly for bundle presence/absence and sampled once a month to measure growth rate over two 4-month farm seasons (September-December 2020, April-July 2021). Water parameters were also tracked and measured. Tissue samples were collected before deployment and at harvest and sent for analysis of nutrient composition, heavy metals and pesticides.
A nonparametric Friedman test was used to test a null hypothesis that seaweed growth for all depths is equal with an alternative hypothesis that at least one depth’s growth is different. It rendered a Q value of 6652.800 and p <0.000 for Trial 1, and a Q value of 8209.296 and p < 0.000 for Trial 2 which is significant. Therefore, we reject the null hypothesis at the alpha 0.05 level, and conclude that at least one depth is different and that the best depth to grow Pacific dulse is 0 and 1 m. It is estimated that a total of 453.5g of carbon, 108.1g of nitrogen, and 9.3g of phosphorus were removed from the water. All pesticides were found to be undetectable, and heavy metals were either undetectable or below FDA action levels.