In the fall of 2017, we established a sugar kelp nursery at the University of Rhode Island to produce “seed spools” of kelp to be planted at six different oyster aquaculture sites in Rhode Island. We followed methods for nursery cultivation of sugar kelp that were previously published in the “New England Seaweed Cultivation Manual: Nursery Systems” by Connecticut Sea Grant. This is the tale of the kelp as it progressed from spore to string to sea during the fall of 2017 and spring of 2018.
Wild Kelp Collection and Preparation
On a late fall day, wild reproductive kelp was collected in Rhode Island by SCUBA diving. Reproductive kelps are easily recognized because they have a dark brown stripe down the middle— this is where the spores are. Kelp are reproductive throughout the year, but generally have a peak in reproduction in the spring and the fall. Since kelp farming occurs during the winter (in Rhode Island between November 1 and May 1), we collect wild kelp in the fall to raise in the nursery. After collection, tissue is cleaned in the lab, wrapped in damp paper towels, and placed in the fridge over night.
Spore Release and Seeding Spools
The next day, the tissue is taken out of the fridge and put into cool seawater. Over the course of the next hour of so, the seawater turns from crystal clear to milky brown as it fills with millions of microscopic, swimming kelp spores. After counting the spores with a microscope and diluting the seawater to the appropriate spore concentration, the spore solution is added to settling tubes. Settling tubes are large, closed PVC tubes that contain seawater and nutrients. Inside of the settling tubes are the seed spools. Seed spools are PVC pipes with nylon or cotton string carefully wrapped around them—this string is what the kelp spores will attach to and grow on. In order to encourage the spores to settle on the string, we place the settling tube with spore solution in a fridge, adjusted to 50F, in the dark for 24 hours.
The next day, the seed spools are ready to go to the nursery, where they will be for the next 6-8 weeks before returning to the sea. Over the course of the next three or so weeks, we will monitor the seed spools in the nursery, maintaining them at 50F. Kelps are temperate seaweeds and prefer cool temperatures. Every week, the spools will be moved to a new aquarium with clean seawater and nutrients. At the end of each week, we will increase the amount of light shining on to the spools. The gradual increase in light level serves as a natural signal to the kelp and encourages growth. When kelp spores initially settle on the seed string, they grow into microscopic (less than 10 cell!) male and female gametophytes. Over the course of several weeks, the light will encourage the gametophytes to develop and signal them to reproduce. Female gametophytes use hormones to attract sperm released from male gametophytes. After fertilization, a new golden-brown kelp sporophyte (think of the large kelp blades you see washed up on the beach) will grow out of the female gametophyte. When this process happens in the nursery, the bright white string on the seed spools begins to turn amber brown—a truly beautiful sight!
Planting and Monitoring Kelp Lines
After roughly three more weeks, the kelp sporophytes will reach 1-5 mm in size and will be ready to plant on the farm. We wait until the water temperatures are below 60F, generally at the beginning of November, to plant the kelp lines. In 2017, we planted our first kelp lines on November 1 at all of our farm sites. Planting kelp lines is a fairly quick process because of the design of the seed spool. Once we are on the farm, we take a thick line, called the longline, that will support the kelp as they grow, and put it through the middle of the PVC seed spool. We then take the end of the thin kelp line and splice it through the longline, securing the end with several knots. At this point, we simply back the boat up at a slow pace and watch as the kelp line wraps around the longline. When we reach the other end, we secure the kelp line in place with some additional knots.
After the line is seeded, it’s secured about 3 feet below the water surface. Putting the line below the surface protects it from any ice that may form during the winter months and from high-energy waves on the surface. Once the line is secure, the hard work is done. Now, over the process of 5-6 months, we will monitor the kelp growth. Every few weeks, we visit the farms to check the lines. Samples of kelp are collected to measure how much carbon and nitrogen they have in their tissues—a measurement of how suitable the site is for kelp farming and a measurement of how much carbon the kelp is sequestering. We also determine the growth rate of kelp blades using a hole punch. Kelp grown from the bottom of the blade, so we can use a hole punch to mark a spot and measure how far the hole moves over time!
Late December in Rhode Island was very cold, but by mid January we had at least 12” of growth on most of our lines, with some already reaching 3 feet in length. As the day-length increases and water temperatures rise, the kelp will continue to increase its growth rate. In the late spring, our kelp will begin to double in size every couple of weeks. By harvest time, which happens at the end of April, we expect kelp on most of the farms was between 6 and 9 feet long!
Written by: Lindsay Green-Gavrielidis