Seaweed tarnishes our crown’s allure

Saturday April 08, 2017 Written by Published in Environment
An example of smothered coral in the lagoon at Muri. Coral heads cannot regenerate while they are covered in seaweed 17041525 An example of smothered coral in the lagoon at Muri. Coral heads cannot regenerate while they are covered in seaweed 17041525

This weekly column is supplied by Te Ipukarea Society. It deals with conservation and environmental matters of interest to the Cook Islands.

 

If Muri is the jewel in the crown of the Cook Islands, then the overly-abundant seaweed that was such an issue last year has been making that jewel lacklustre and tarnishing that crown.

Seaweed, otherwise known as marine macroalgae, are plant-like organisms.

They do not have roots to absorb their nutrition, but filter it from the water column. In place of roots they have holdfasts which they use to attach to rocks or other hard surfaces and to sand.

If you go for a snorkel in Muri or Papa’aroa for example you will find very little live coral, as it is all smothered by seaweeds. These coral heads will not regenerate while they are covered in seaweed as the coral larvae need a hard surface to settle on.

Seaweed is nothing new in our lagoon, and we have had outbreaks in the past, mainly of Boodlea algae, a genus of  bright green spongy/candy floss seaweed that was floating on the water surface in great carpets.

In fact, seaweed is a very important and useful part of our marine ecosystem, feeding some of our marine life and absorbing nutrients that pollute the lagoon from the land.

These nutrients are food for the seaweed, fuelling their growth and spread. Halimeda seaweed actually helps build up our beaches as, when it dies, its calcium filled skeleton adds to our sand. The problem starts when the balance of nature is disrupted.

Over the past two years we have had a much larger amount of Caulerpa cuppressoides seaweed in the Muri and Titikaveka areas. It spreads by fast-growing stems that spread across the rock or sand bed with fronds that rise from the stem.

While everybody seems to think this is a bad thing (and for tourism it is), in fact, this algae has been helping to clean our lagoon from the excessive pollution that is flowing in from the land. If the seaweed was not there absorbing the nutrients, the water quality would be much worse, and probably no longer safe to swim in.

The lagoon ecosystem is out of balance, with too many nutrients entering, and not enough leaving. So it would appear that the simple answer to reducing the amount of seaweed in the lagoon is to “re-balance” the ecosystem. Simple to say perhaps, but not that simple to achieve.

One way to help achieve this re-balance is to reduce the amount of nutrients entering the system. The demand for economic development has been at odds with that solution for many years, bringing us to where we are today.

A quick drive along the main road from Avana through to Vaimaanga can tell you this is not going to change soon, judging by the increase in holiday accommodation units and houses recently built and continuing to be built at this very moment. The run-off from development such as this can only mean more nutrients entering our lagoons.

On the positive side, government has prioritised improving the way sewage is being handled in the Muri area, though any actual improvement will likely take several years. In the short term, hopefully there will be much stricter controls with any new accommodation being built.

Nature has another way to help maintain a balanced ecosystem, with marine animals that eat seaweed.

These include surgeon fish (manini, maito, ume, etc) and sea urchins (vana, kina, avake), which all love to eat seaweed.

Nature has another way to help maintain a balanced ecosystem, with marine animals that eat seaweed. These include surgeon fish (manini, maito, ume, etc) and sea urchins (vana, kina, avake), which all love to eat seaweed.

It could therefore be considered that a ra’ui on these species in the areas of concern, or a ban on gillnetting inside the lagoon in these areas, would also be beneficial.

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