In the Seaweed Nursery: A Practical Overview


In this section, we will introduce the general techniques used in seaweed nurseries by providing an overview of growing sugar kelp (Saccharina latissima) on seed spools. The following outline of nursery processes is not a comprehensive nursery cultivation manual and is not transferable to all seaweeds. Detailed manuals and informational guides for cultivating sugar kelp and other seaweed species can be found at the end of this section.

We would also like to give a shoutout to one manual in particular from Ocean Approved (now Atlantic Sea Farms). This manual (which was published in 2013) is referenced significantly thoughout this section. The full manual can be found on the Maine Aquaculture Association website at 

A video (10:45) about seaweed reproduction in hatcheries and how researchers are working to develop advanced methods for aquaculture of seaweeds. Video credit: Maine Aquaculture Innovation Center. Available on YouTube:

Components of a Seaweed Nursery

The primary purpose of a seaweed nursery is (1) to create a highly controlled environment where it will be possible to (2) manipulate and respond to the complex life cycle of a given seaweed species in order to (3) raise viable juvenile sporophytes. With these goals in mind, the first necessary task in setting up a nursery is to make sure you have the appropriate equipment. This will include laboratory instruments with the capacity to control and manipulate environmental factors that are crucial to the growth and reproductive phases of seaweed. Factors that must be controlled include:

1. Sterile seawater

Wild seaweed grows in marine environments, so replicating the ocean in the nursery using seawater is necessary. Seawater can be natural, meaning it comes from the ocean, or artificial, meaning it is made from freshwater with salts added to it to mimic a marine environment. Natural seawater is more successful than artificial, but the latter can be used when need be.

Image credit: BBC

Regardless of which type of seawater is used in the nursery, it is key to make sure the water is clean and has no existing organisms living in it that may cause the culture to fail. When using natural seawater this means implementing a two step process of (1) filtering and (2) sterilizing the water before it is used in any system with seaweed. In the case of artificial seawater, it is easier to make clean water by using a pre-sterile freshwater input.

The amount of sterile seawater necessary for a seaweed nursery during the entire growth process will vary depending on the amount of seaweed being grown and the size of the tanks. That said, seawater must be replaced weekly (sometimes more frequently) for the duration of gametophyte settlement and juvenile sporophyte grow-out, which can be four to six weeks depending on species.

Image credit: Ocean's Balance

2. Nutrients

Regardless of which type of seawater is used in the nursery, it is key to make sure the water is clean and has no existing organisms living in it that may cause the culture to fail. When using natural seawater this means implementing a two step process of (1) filtering and (2) sterilizing the water before it is used in any system with seaweed. In the case of artificial seawater, it is easier to make clean water by using a pre-sterile freshwater input.

Nutrient Solutions

It is possible to prepare your own nutrient solution or purchase a pre-mixed nutrient solution.

Keep in mind that recommended nurtients may vary for different algal species. 

Nutrient Solution Options

3. Pest management

As with any cultivated organisms, there is a potential for pests to impede the growth of seaweed in a nursery. Filtration and sterilization of seawater is a critical first step for pest management that removes the majority of unwanted organisms in the system. However, some pests may still be present in the nursery system after water processing. For example, diatoms, which are silica-walled microalgae, are small enough to bypass filtration and can be introduced into the culture system via natural seawater or addition of wild seaweed. Diatom contamination is common in seaweed nurseries and their rapid reproduction can smother seed spools or juvenile seaweed.

To control diatom contamination, a saturated solution of germanium dioxide (GeO2) can be added to the seawater before it is put in tanks with the seaweed. GeO2 serves as an inhibitor for diatoms that prevents them from taking up silica, which they require for their cell wall. Without silica, diatoms are unable to grow and reproduce prolifically, which protects seedlings from potential contamination.

Image credit: Lab Roots

It is important to note that GeO2 can also be harmful to macroalgae when used in high concentrations; following instructions for GeO2 solutions provided in manuals for seaweed cultivation is key to successful diatom management and seaweed health. Use of GeO2 also disqualifies seaweed for Maine Organic Farmers and Growers Association (MOFGA) certification.

4. Temperature

Every species of seaweed has an optimal temperature range for growth, and a successful nursery system will have the ability to regulate temperature and maintain the optimal growth conditions. If the required temperature is lower than the room temperature, regulation in a nursery can be achieved using a refrigerator, cold room, or a seawater chiller.

Seawater chillers are used to keep water at a specific temperature even when the ambient air temperature is higher. They are often cheaper than refrigerated rooms or a cold rooms.

Image credit: Ferro Tec

5. Lighting

There is a component of light that is required for plants and algae (including seaweeds) to perform photosynthesis. This is called “photosynthetically active radiation” (PAR). Different parts of the PAR spectrum are utilized by different photosynthetic pigments (e.g. chlorophyll a, phycocyanin) to produce energy for a seaweed’s growth.

Image credit: Black Dog LED

Fluorescent light bulbs can provide the correct light energy for growing seaweeds in a nursery, however it is important to know the spectral composition of the bulbs and ensure they produce the appropriate light for the species you plan to culture. More recently, LED lights are being used that allow for a finer tuning of the light wavelengths necessary for seaweed growth. In addition, light requirements may change at different stages of seaweed growth so implementing a strategy for managing light intensity and exposure is key. Automatic timers to control photoperiod and mesh with a variety of shade capacity to control light intensity are commonly used.

Specific environmental conditions and procedures for spawning and growing seaweed in a nursery vary by species and are largely dependent on the life cycle and reproductive strategy of the seaweed in question.

Compenents of a seaweed nursery. 

Video credit: Connecticut Sea Grant

An overview of nursery techniques for sugar kelp

The process of farming kelp begins in a nursery where spores from reproductively active kelp are used to grow juvenile sporophytes on an appropriate substrate. This process lasts between four and six weeks for Saccharina latissima and concludes when juvenile kelp is “seeded” in the open water at a farm. 

Prior to seeding, young kelp is raised in indoor aquarium tanks on a substrate of nylon string. Aquarium tanks are used to monitor and control factors including water quality, nutrients, pests, temperature, and light.  To form a substrate that can easily be transferred to the ocean when the kelp is ready for grow-out, nylon string is wrapped around a PVC tube known as a seed spool. Seed string on a seed spool is the preferred substrate for growing kelp in the nursery because it is compact but can be wrapped around a more sturdy substrate (like 7/16” poly line) once on the farm. 

Seed spools and aquarium tanks are prepped for growing juvenile kelp before any kelp is actually collected.

Seed spools ↓

Seed spools provide the substrate for seedling kelp. Seed spools are pieces of PVC tubing with thin nylon twine wrapped around them. Each spool has two to three hundered feet of seed string on it and is prepped prior to starting the innoculation process. 

Image Credit: Ocean Approved Kelp Manual

Nursery prep →

Aquarium tanks, chillers, lights, and seed spools should be prepared before starting the innoculation process. Each nursery set-up will look a little different depending on the space, the equipment used, and the number of seed spools being produced.

Image Credits: Cascadia Seaweed, Ocean’s Balance, Juneau Empire, USDA

Preparing settling tubes for seed spool innoculation. 

Image credit: Jaclyn Robidoux, Maine Sea Grant

Image credit: Jaclyn Robidoux, Maine Sea Grant

Tissue collection and preparation

The story of every great sugar kelp nursery starts with finding top notch reproductive tissue. Thinking back to the reproductive cycle of sugar kelp, you’ll remember that the first reproductive phase of kelp (spore release) begins with sorus tissue on adult sporophytes. To generate kelp spores in the nursery you must begin by finding sorus tissue from mature, reproductively active kelp. 

Mature, healthy sorus tissue in sugar kelp is typically found in a strip near the center of the kelp blade and appears darker and slightly raised compared to other tissue. Some sori run the entire length of the kelp blade while others are present as broken up patches. Sorus tissue that has dark patches fading into lighter patches could be showing signs of prior spore release, which is a good indicator of maturity. Kelp blades with heavy amounts of mucilage (feeling slippery to the touch), a dark strip of sorus with a raised texture, and areas of sorus that have already released spores will provide the best chance of successful releases in the nursery.

Kelp sorus tissue pictured in the image on the left appears healthy and would be good to use for innoculating seed spools. Kelp sorus tissue in the image on the right is not optimal BECAUSE, and would not be ideal to use for producing seed spools. 

Finding high quality sorus tissue in wild kelp depends primarily on two factors: location and season. Sorus tissue will only be found where there are thriving adult kelp individuals. The best sites to look for healthy sorus tissue are rocky, sheltered locations with rapidly moving currents just below mean low water. The kelp in these areas will be attached to some form of substrate such as cobble, boulders, or bedrock.

Wild kelp bed in Downeast, Maine. 

Video credit: Maine Aquaculture Innovation Center

Seasonality is also a key in finding sorus tissue since there are only two times throughout the year when sori develop on wild Maine kelps. Peak growth occurs in the fall between October and late November and is the ideal time to collect sori for nursery purposes, though many commercial nurseries start collecting earlier in September. Secondary growth peaks during the spring in April and May which is less conducive to nursery purposes since the season for farming Maine kelp happens between December and May. 

Once healthy sorus tissue has been collected, it needs to be prepped for spore release in the nursery. The purposes of preparing sorus tissue are: 

  1. To isolate viable reproductive tissue from vegetative tissue
  2. To remove as many potential biofouling organisms as possible
  3. To  put the reproductive tissue in a state where spore release can be artificially induced

The process of preparing the tissue involves excising it from the thallus, cleaning it, and partially drying it– all while keeping it cool. Both cutting and cleaning the tissue at a cool temperature are key for isolating viable sori. Meanwhile, drying the tissue prepares it for spore release upon re-emergence in cold salt water. Sorus tissue should be prepped as soon as possible after collection.

Sorus tissue preparation

Diagram from the Ocean Approved Kelp Manual.

A video (10:20) about preparing sorus tissue for innoculating seed string, featuring Adam St. Gelais. Video credit: University of New England. Available on Vimeo:

Zoospore release

Once prepped, sori need to sit in a cool, dark place for 14 to 24 hours before spore release begins. This waiting time is a good opportunity to double check all of the aquarium equipment to make sure it is working properly. Lights, water chillers, and air bubblers should be ready to rumble as soon as the spore release is initiated. Settling tubes, where the inoculation of seed spools takes place, must also be clean and ready to go.

When it is time to release the kelp spores, the chilled and moderately dry sorus tissue must be re-emerged in a solution containing essential nutrients, cold seawater, and germanium dioxide (optional). Spore release can occur anywhere between a minute and an hour after the kelp tissue is re-emerged in water and will give the water a murky/cloudy appearance.

If spore release is not successful within an hour, it is possible to try re-drying the tissue and putting it back in the refrigerator for 14 to 24 hours before attempting a second release. However, if release is not successful after the second attempt, the sorus tissue is not viable and should be discarded.

When spore release is successful, the liquid in the beaker will be murky. Spores left on a paper towel during the sorus preparation process is a good indicator that spore release could be successful. 

Image credit: Ocean Approved Kelp Manual

Releasing zoospores

Diagram from the Ocean Approved Kelp Manual.

Bio Blurb: Spores vs. Zoospores

You may have noticed that the title of this subsection about spore release refers to ‘zoospores’ rather than spores. So, what is the difference between these two terms? ‘Zoospore’ is used to indicate that the haploid reproductive cells released by an organism are motile using a flagella. In other words, zoospores have tiny tail-like structures that help them swim around in the water. ‘Spore’ is a more general term that refers to a particular reproductive stage but does not specify if the cells in this stage are motile or not.

Zoospores under the microscope.

Video credit: Jaclyn Robidoux, Maine Sea Grant

Counting the zoospores

When spores are successfully released from the sorus tissue, they are present at a very high concentration in the seawater solution. To enable the optimal settlement of spores on seed string, the solution containing the spores needs to be thinned by mixing it with additional seawater and nutrient mixture. Diluting the spores to a concentration of ~7,500 spores/mL is best for inoculating the seed string evenly without causing suffocation due to an overabundance of individuals.

In the nursery, a microscope and a special tool called a hemocytometer are used to calculate the concentration of spores in the initial spore release solution. This concentration along with the recommended stocking density for inoculation (7,500 spores/mL) and the volume of water used in the settling tubes are then used to calculate the amount of spore release solution that should be used for inoculation with the following formula:

Hemocytometer schematic. 

Image credit: Chincilla Life Sciences

Calculating stocking density

Diagram from the Ocean Approved Kelp Manual.

Using a hemocytometer

A hemocytometer is a device used in labs to estimate the number of cells in a volume of liquid (the concentration). The hemocytometer is a specialized microscope slide that holds a specified quantity of liquid and has a cover slide with calibrated grid marks on it. Using the hemocytometer with a microscope, you are able to count cells in parts of the grid and estimate the overall concentration of cells in your liquid. 

Click to enlarge the image. 

Image credit: Ocean Approved Kelp Manual

There are multiple ways to count cells using a hemocytometer. People do not count all of the cells in the grid as that would take too long. Instead, a subsection of the cells are counted and used to estimate the concentration of cells per mL of liquid. In the image above, the number of zoospores in each of the four corner squares is counted, added together, and divided by four. This average is then multiplied by 10,000 to obtain a value for the number of spore per mL. 

Example 1: Try calculating the number of spores yourself! Then click here for the answer.

Count the number of spores in the four corner squares, divide by four and multiply by 10,000.

  • Square 1 = 11 spores
  • Square 2 = 10 spores
  • Square 3 = 8 spores
  • Square 4 = 12 spores

Total Counted = 41 spores

Spores per mL = [(41/4) x


Spores per mL = 102,500

Another way to estimate the concentration of cells is to count the number of zoospores in the large center square of the hemocytometer and multiply that by 10,000. Try this method using the example image below. 

Click to enlarge the image. 

Image credit: Ocean Approved Kelp Manual

Example 2: Try calculating the number of spores yourself! Then click here for the answer.

Count the number of spores in the large
center square and multiply by 10,000.

Square 5 = 62 spores
Total Counted = 62 spores

Spores per mL = [62 x
Spores per mL = 620,000

Calculating the volume of spore release water to add to settling tubes

The next step after calculating the number of spores per mL in the release water is to use this value along with the target stocking density and the volume of seawater in container being innoculated to determine how much release water to add. The formula below is used for this calculation.

The recommended stocking density in for kelp zoospores is 5,000 to 10,000 (average 7,500) spores per mL. The volume of seawater in settling tubes depends on the set-up you are using, but for this example, let’s use 2,300 mL. 

Try calculating the volume of release water to use based on the number of spores per mL from example 1. Then click here for the answer!

Using the formula and values provided, 168 mL from the release beaker would be added to each settling tube.

Try calculating the volume of release water to use based on the number of spores per mL from example 2. Then click here for the answer!

Using the formula and values provided, 37.09 mL from the release beaker would be added to each settling tube.

Gametophyte settlement

Once kelp zoospores are released into the water column, they are motile (remember they have swimming flagella) and can swim around for up to 24 hours before settling and beginning their second growth phase as gametophytes. During this stage as single-celled individuals, the kelp spores are most vulnerable to rapid die-off, contamination, or failed settlement. 

Settling tubes are used to create a highly controlled environment in which the zoospores can settle on the seed spools and begin their lives as gametophytes. Settling tubes are essentially mini, temperature-controlled, nutrient-rich, zoospore-baths for the seed spools. The seed spools are kept in this environment for 24 hours to allow the zoospores to attach to the nylon string substrate before being transferred to their longer-term home in aquarium tanks. At the point of transfer, the spores that have settled on the seed string will have developed into gametophytes.

Innoculating seed spools

Diagram from the Ocean Approved Kelp Manual.

Transfer to aquaria and monitoring

Moving the seed spools to the production aquaria from the settling tubes is a relatively quick process but should be done carefully to make sure the delicate gametophytes survive and remain attached to their substrate. The production aquaria should be prepared ahead of time with cold, filtered seawater; nutrient solution; a lightsource set to a 12 hours on/12 hours off photoperiod; and a system to provide aeration to the tanks. The number of seed spools in an aquarium will depend on the size of the tank, but spools should not be in danger of touching each other or the tank walls when placed upright.

Transfering seed spools to aquaria

Diagram from the Ocean Approved Kelp Manual.

Once the inoculated seed spools are comfortably moved into their new aquarium home, the primary objective of nursery management becomes monitoring and maintenance. The appropriate growth conditions need to be maintained for the kelp gametophytes to develop, produce their gametes, become sporophytes via fertilization, and mature to a point where they can be transferred to the farm. 

The process of growing kelp from the gamete phase to a juvenile sporophyte large enough to transfer to the farm takes between four and six weeks in the nursery. During this time, environmental conditions in the tank such as light intensity, water temperature, nutrient level, pH, and contamination are monitored and adjusted as necessary. A complete water change in the tank is done once a week and requires a second tank for transferring the seed spools to while the first tank is cleaned.

Keeping tabs on how nursery equipment is working and the status of kelp growth in your aquarium is super important to successful seed production, and you should be prepared to check on the kelp nursery daily. Maintenance issues or contamination should be addressed immediately to keep the young kelp thriving.

Progression of sugar kelp growth during the nursery stage

Image credits: Ocean Approved Kelp Manual

Where to buy kelp seed in Maine

These are a few of the options for purchasing kelp seed in the state of Maine. Additional options may be available.

Atlantic Sea Farms

Atlantic Sea Farms provides all kelp seeds to partner farmers for free, but also makes organic kelp seed spools availabe for purchase to those outside their farmer network.


Atlantic Sea Farms kelp seed

Ocean's Balance

Ocean’s Balance offers seaweed farmers kelp seed spools grown in their state-of-the-art nursery.


Ocean’s Balance kelp seed

Springtide Seaweed

Springtide seaweed takes orders for organic seeded seaweed spools.


Springtide Seaweed kelp seed