Seaweed Beds

Seaweed beds are nature’s fast food outlets, for the marine animal that can’t be bothered chasing down a hard to find meal in the open ocean. Macroalgae (big seaweeds) can produce 2kg of plant material per m2 of bottom per year, about the same as a paddock or a seagrass meadow. That is about 5 times more productive than inshore plankton.

While seagrass is tough, seaweed is pretty easy for many animals to eat whole. It also breaks down more quickly for those animals at the small end of the food chain with little mouths. It’s obviously pretty fattening because there may be up to 50,000 of these “mezograzers” on every m2 of seaweed bed. There can be up to 50 separate species of these little critters on just one plant. These are nature’s chubby ‘fast food’ eaters. While little shrimp-like things like amphipods can tackle seaweed, not many fish can eat whole seaweed. Those that can (like Luderick or Herring Cale) are so successful that they can make up half of the total weight of fish on reefs in Southern Australia. Luderick have a bacteria filled second gut, just like a cow, to help it chow down on the seaweed.

One of the most common southern Australian reef fish are Purple Wrasse and Blue-throated Wrasse. They mostly get their energy from seaweed indirectly, by eating the small grazing shrimp-like animals that are eating the kelp. It is common at certain times of the year, to see large clouds of mysid shrimps hovering over the kelp. In fact, almost everything bastard trumpeter, leatherjackets, handfish and sea dragons will have a go at these little, but very tasty, amphipods, isopods and mysids.

OK so some fish and shrimp-like animals swim up and eat seaweeds. You can even see little holes forming in fronds as they get nibbled away. What you might not know is that seaweeds shed much more food into the water that you can’t see. Large amounts of organic material leach from seaweeds and are known as mucilage (basically seaweed snot). Big seaweeds like Giant Kelp also shed the tips of their fronds as tiny particles, as a mechanism to get rid of fouling plants and animals. Up to 25% of the plant is lost as mucilage or as dissolved organic matter. A lot more of the seaweed is lost this way, than by grazing from other animals like fish or urchins. Although you can’t see this with the naked eye, seaweed beds are literally oozing great masses of food into the surrounding water column.

Bacteria goes after it all first and 6 grams of bacteria can polish off 100 grams of dissolved weed particles. Other small critters (ciliates, flagellates and amoebas) chew on the bacteria. Sifting through the scraps These microscopic organisms are food for other filter feeding animals, like the colourful tunicates and sponges that inhabit the reef. This flood of ‘easy’ food might partly explain why tunicates are often very common on seaweed beds, while other invertebrate animals usually find it too hard to compete for space with the bigger algal plants.

The tunicates provide structure for green algae which in turn provides habitat for breeding fish and other small animals. Pyura sea squirts are better adapted than most to survive out on the looser sand. Out there they can feed without too much competition from other filter feeders. They also don’t get ‘beaten up’ by the lashing of big seaweed fronds. As this particular species is a vegetarian, they have most likely found a good spot where the phytoplankton oozing out of the seaweed bed collect thanks to local current eddies.

The productivity of seaweed beds can make them hotspots for biodiversity. So when you hear someone say they are an “open water diver” they are actually a ‘temperate seaweed bed’ diver, or ‘sub-tidal reef’ diver. The exciting stuff to look at is mostly found there and divers rarely venture away from this very narrow, and relatively shallow strip of rock hugging the edge of our vast (and relatively barren) continental shelf. These seaweed beds are like a smorgasboard of quick fattening meals in a desert of fairly nutrient-poor surroundings