Strong structures at the bases of kelp plants called holdfasts hold the kelp firmly attached to rocks, so that even when currents are at their strongest, kelp is in no danger of being swept away. Unlike land plants seaweeds do not gain their water or nutrients such as nitrogen and potassium from the ground but instead absorb both directly from the water through surface of the plant.

Algae use a variety of strategies to remain attached to the rocks, even under the enormous forces of waves crashing down on them and strong currents. These include the use of powerful adhesives, which may have potential for human use, and physical structures that take advantage of small imperfections in the rock surface.

Another challenges for larger plants living in water are the properties of water that result in light being selectively absorbed with depth. As light penetrates water certain colours are absorbed. The blades of kelp plants are like leaves on trees and contain the pigments that allow them to absorb sunlight and convert it into sugars through photosynthesis. The different colours of algae are due to the presence of different pigments that allow these plants to absorb different parts of the light spectrum that penetrates through the water.

In addition to the holdfast and blades, kelps also have stem like structures known as stipes for both support the blades as well as make food. In some species gas filled vesicles, or bladders, provide uplift for the blades and stipes and keep the plants up in the water columns allowing them to gain greater access to light.

Kelps are representatives of one three main groups of larger algae, each group being distinguished by their colour. The three main groups of larger marine algae are

• Red algae (Rhodophyta) - often found growing under kelp forests or in low light conditions
• Green algae (Chlorophyta) - often found in shallow waters such as the tops of reefs or rockpools
• Brown algae (Phaeophyta) - the group that includes kelps as well as a large number of smaller species found on rocky shores.

Plants from each of these groups may be found almost everywhere along the coast where rocky outcrops or reefs occur, where there is a suitable substrate on which the seaweeds are able to grow.

Marine algae have a complex life cycle that involves often two quite distinct phases, one of which is microscopic. Algae do not have flowers and reproduce by releasing eggs and sperm into the water where fertilisation occurs. In some species these fertilised eggs attach to a rocky substrate and begin to grow, whereas in others they form a separate plant that in turn produces spores that colonise bare rocks.