It’s been stormy for three days and today huge swathes of the beach are covered with seaweed that has been washed up. The giant chunks of seaweed, lime green and brown in color, look like giant spinach thrown all over the beach. The birds ply through the seaweed and the bounty of shells brought up by the storm. Oystercatchers work in packs, while the sea gulls do their sea gull thing, squawking and flapping around, taking off and landing, splashing in the water, picking around through the seaweed, while other groups of gulls stand in the sand staring forlornly out to sea as if waiting for someone or something to arrive.
What is seaweed and what are these seaweeds that now cover the beach?
Seaweed is type of algae. Algae are a diverse group of generally photosynthetic organisms most of which live in water. They range from single celled organisms to large seaweeds growing many meters in size.
One common way to classify seaweed is simply based on color: green seaweeds, brown seaweeds, and red seaweeds. Most of the seaweed washed up on the beach is a species of green seaweed intermixed with lesser amounts of a brown seaweed.
The bright green seaweed is appropriately called sea lettuce (Ulva lactuca). It looks like lettuce—lactuca, the species name is Latin for lettuce. And it is actually edible. It can be used in salads and soups and contains a fair amount of protein.
Sea lettuce is found throughout the world, but this particular species is native to New Zealand. Sea lettuce forms sheets up to 30 centimeters in diameter. The blades of the leaves are bright green, cellophane-like and almost translucent. It feels slimy to touch and particularly at this time of year is found floating around in the water when you go swimming.
Tauranga harbor and the surrounding area where we live are notorious for having large blooms of sea lettuce when the conditions are right in the summer months. Multiple factors including water temperature, weather patterns and nutrient run-off affect sea lettuce blooms, that is, how much sea lettuce grows in a given year and often consequently washes up on shore. When it begins to rot, sea lettuce gives off a sulfurous smell. In 2009 in France, both a horse and later a cleanup crew driver are felt to have succumbed and died from the fumes of rotting sea lettuce on the beach.
The other seaweed that has been washed up by the storm is a brown seaweed called common kelp (Ecklonia radiata).
Various species of kelp are found in oceans throughout the world. The plants, tethered to underwater rocks, grow near the shoreline in large underwater groupings. Charles Darwin is said to have been one of the first to characterize these as underwater forests. Individual plants of certain species of kelp can grow up to 80 meters long with some species growing up to one-half meter each day.
Common kelp, the type of kelp that has washed up onto our beach, is a dominant seaweed in the northern waters of New Zealand and grows in underwater thickets and forests on the rocks and small islands near Mount Maunganui where we live. This seaweed grows to about one meter in length and has a number of fronds arising from a central supporting stem or stipe. The leaves, more accurately called blades, vary slightly in appearance; some are smooth while others are corrugated. What is called a “holdfast”, a root-like structure, tenaciously holds the kelp plant to rocks underwater.
A third common type of seaweed that is seen in New Zealand is common bull kelp (Durvillaea antarctica). We saw a great deal of it along the shore on our trip to the South Island. The Maori name for it is rimurapa. It is larger and more robust than the two seaweeds previously described. It grows in areas where there is a tidal surge, that is, where the effects of the tide sweeping in and out are felt by the shoreline. The stipe of common bull kelp is particularly strong and flexible allowing it to withstand wave action. You can bend it and virtually tie it in knots without being able to break the stipe. From the stipe the kelp divides into multiple broad blades or straps. The blades are green to brown in color.
Most tidal types of seaweed have actual air bladders to allow the blades or straps to float up near the surface of the water. This is important because it allows the blades to float free and away from the rocks so the plant doesn’t get abraded or torn apart on the rocks themselves. The second reason is that this allows the blades to float up on or near the surface of the water, allowing them to be able to capture sunlight (photosynthesis) with every surge of the waves. The bull kelp along the New Zealand coasts is somewhat different; rather than having air bladders, the blades have an internal honeycombing structure that traps air and allow the blades to float.
Poke a hole through one of the blades and you can make shoes, flippers, and even a hat! The Maori would also use the kelp blades to make bags called poha for storing food. Apparently kelp blades can also be roasted and eaten, or used to make a jelly.
When storms come and large storm tidal surges come, it tears kelp and other seaweeds from the rocks and the seaweed washes up on the beaches or floats around in the water. Since the bull kelp blades are able to float, bull kelp is able to colonize other areas, termed “rafting”, after it is broken off by storms.
Over the past week, the seaweed that has washed up has begun to rot and dry out. I can smell the sulfurous smell from the sea lettuce. It turns out that this is a particularly large amount of seaweed that has washed up on the beach here this week. How much? “Heaps” as they might say here. One lifeguard, quoted in the local paper, says this is the most seaweed to be washed up in the last 18 years. A contracting company has been hired to haul at least some of the offending seaweed off the beach.
Seaweed—all the different varieties—are all part of the ocean’s ecosystems and provide an environment for a host of other plants and animals. For me, while living here next to the ocean, I have grown to appreciate in some small measure how vast the ocean truly is, and simply how much goes on in the oceans of which we generally remain oblivious. Seaweed is part of that story.
All photos by the author except as noted.