Myth Busting: Seaweed & Fertilizers

Seaweed is definitely riding a wave of interest and investment. Some of this attention is justified as seaweed has tremendous potential to transform aquaculture and offers numerous benefits.

One such benefit is that seaweed only needs water and light to grow—no fertilizers required.

Well, obviously this is not true. Seaweed, like all plants, need nutrients to grow. Nitrogen and phosphorous are two of the most important. The lack of nutrients has a tremendous impact on growth and yields on seaweed farms. Where water is “too clean” seaweed just doesn’t grow or won’t grow well. In Japan and China seaweed farms are supported by the addition of nutrients, i.e., fertilized.

Thus, farms are best located where there is access to nutrients. This is not so simple though. Ideally, seaweed farms should be sited where there is a natural upwelling of clean ocean nutrients. Alternatively, farms can be sited where man-made nutrients are in the water, such as agricultural run-off. The rub here is that where there are man-made nutrients there are often other pollutants and toxins—which the seaweed will absorb thus making it less suitable for human consumption if not outright dangerous.

An irony of the nutrient dilemma is that seaweed is also being heralded as a carbon remediator. Large open water seaweed farms are being planned and built that are sited where there are no nutrients to support the seaweed growth. The solution? Pump nutrient rich water up from below the ocean surface level. This nutrient rich water is actually a significant carbon sink and sequesters carbon for hundreds if not thousands of years. Thus, these farms are using sequestered carbon to grow seaweed—virtually no different than using fossil fuel-based fertilizers!

The take away here is that like all aquaculture, seaweed aquaculture is not quite as simple as it seems. Successful and responsible aquaculture is knowledge based and often not intuitive. Simplistic catch phrases may draw attention to seaweed aquaculture, however if these teasers are misleading, in the long run they may harm the industry by creating unrealistic expectations or unfulfilled promises.

The Maine Seaweed Exchange is committed to developing standards for seaweed farm siting that takes into account nutrient sources as well as the impact of seaweed farms and how siting effects carbon remediation and food safety.