Kelp Changes the Water and the Shore Above It

A brown band visible offshore at low water is usually kelp, anchored to rock and standing several metres tall. Its presence changes wave conditions, water clarity and what ends up on the beach.

Kelp needs rock, light and cold

Kelp attaches to hard substrate with a holdfast rather than roots, so it cannot grow on sand. Its distribution therefore maps the rocky parts of the seabed exactly.

It also needs light, which limits it to shallow water, and it favours cool, nutrient-rich conditions. Warm, clear, sandy coasts have little or none.

This is why the same latitude can offer dense kelp on one shore and none on another. The controlling factors are the seabed and the current, not the region.

The forest slows the water

A stand of kelp absorbs a meaningful amount of wave energy before the water reaches the shore. Beaches behind established beds tend to be calmer than exposed neighbours.

Swimmers notice a distinct drag and a change of rhythm on entering a bed. Movement through it is slower and the surface chop flattens.

Small boats notice it differently, since fronds foul propellers and rudders. Local knowledge about where the beds sit is largely knowledge about where not to motor.

It is a nursery before it is anything else

The structure of a kelp bed provides cover, so juvenile fish and invertebrates concentrate in it. Predators follow, which is why fishing effort clusters at the edges.

Snorkellers see more life over kelp than over open sand for the same reason. The complexity of the habitat supports more species in a small area.

The beds also hold carbon and produce a continuous supply of detached material that feeds the wider system. Very little of what grows there is eaten in place.

Why the beach smells after a gale

Storms tear kelp free and pile it along the strandline in heavy, glossy heaps. Decomposition begins immediately and produces a strong sulphurous smell for a few days.

That wrack is not pollution. It supports flies, sandhoppers and the birds that eat them, and it traps sand that would otherwise blow away.

Beaches that are mechanically cleaned lose that function. The sand becomes more mobile and the shoreline food chain loses its base, which is why some resorts now clear selectively.

Kelp has long been harvested

Coastal communities have cut and gathered kelp for centuries, using it as fertiliser, as fuel and as a raw material for processing into other products.

The practice left visible traces in some places, including stone-walled drying areas and access tracks down to otherwise pointless stretches of rocky shore.

Modern harvesting is regulated because removing the canopy exposes the shore behind it. The argument is now about how much can be taken before the shelter effect is lost.