From the perspective of sustainability, certain seaweeds can be gathered in a manner that will not adversely affect their viability. The classic example is Kelp – seaweeds of the Laminaria family. If during the spring the plant is gathered by cutting just beyond the top of the stem (more specifically, above the mericarp) to leave a little of the frond and, of course, the hold fast intact, the seaweed will regenerate from that point. I assume this can only be done once – but maybe twice is possible?
It’s the same with some land plants, for example, wild rocket (Perennial Wall Rocket -Diplotaxis tenuifolia): I have gathered this by twice cutting it down to just above the root in the spring. It always grows back – this is not the case, though, after the third cutting.
Secondly, there are health and safety issues relating to toxicity and pollution. Most of the following information was very kindly provided by Dr Stefan Kraan at the Irish Seaweed Centre, Martin Ryan Institute National University of Ireland, Galway.
There are roughly between 20-40 good edible seaweeds native to UK coastal waters. The variation in numbers reflects the highly subjective nature of taste. The figure doesn’t include any seaweeds that are out right disgusting, but it does include some that are rather bland and require a degree of creative expertise in the Kitchen to make them work as food. In fact, genuine and inspired creativity in the kitchen can potentially bring the number of available seaweeds from 40 to at least 100 or more. There are, however, non-edible varieties..
Among these are:
Desmarestia ligulata and D. viridis, they produce sulfuric acid esters. This is the plants defence mechanism to stop grazing from marine mollusks.
Species of the tribe Bonnemaisonaceae (such as Asparagopsis, Bonnemaisonia) all produce volatile halogenated iodine and bromine compounds.
There are other species with funny compounds that won’t kill you but make the seaweed taste pretty bad.
Seaweeds are known to take up heavy metals, radionucleotides and various other pollutants. Therefore do not harvest from areas close to places like Sellafield and or areas of heavy industry.
Iodine in kelps and Wracks can be high up to 1-3% of the dry weight. Depending on your conditions (thyroid and iodine sensitivity), this may be a problem. Then again, perhaps it could be a cure? Here is a fraction of the relevant research in this regard:
Detection of technetium-99 in Ascophyllum nodosum
from around the Welsh coast
Opinion of the Scientific Committee on Food on
the Tolerable Upper Intake Level of Iodine
Total arsenic, inorganic arsenic, lead and cadmium contents in edible seaweed sold in Spain