Algae harvesting

A number of studies carried out in Oyster and Princess Royal harbours have highlighted the decline of seagrass cover and density.

The decline of seagrass in the harbours was first formally noticed in 1984. Surveys of seagrass communities done in 1962 were compared with surveys done in 1984 and showed that by 1984 45% and 66% of seagrass cover in Oyster Harbour and Princess Royal Harbour respectively had been lost.

The Albany Harbours Environmental Study expanded on this and showed that by 1988 almost 90% of seagrasses in Princess Royal Harbour and 80% in Oyster Harbour had been lost since 1962 (when the seagrass meadows were considered to be in pristine condition). Accumulations of macroalgae reaching densities that exceeded 1000 g dry weight per square metre were also evident in shallow areas of both harbours.

The cause of the seagrass decline was worked out through evidence and experience from other bays and estuaries in Western Australia. Fast-growing, highly productive macroalgae and epiphytes smothered the seagrass and reduced their light supply. The over-supply of nutrients to the harbours and their high availability in the water and sediments provided an excellent environment for the growth of these algae. Seagrass, however, is less able to make use of increased nutrients.

Recent surveys

A survey of both harbours in 1996 brought back good news about the abundance of seagrass and algae.

In Princess Royal Harbour substantial seagrass regrowth was happening in the deeper basin and shallows around South Spit. A dramatic improvement in seagrass distribution was noticed in Oyster Harbour, with some evidence to show that there was a recolonisation happening. The eastern side of the harbour showed the most improvement since 1988. There were the first signs of a seagrass recovery, though the survey confirmed that only a small fraction of the original, pristine seagrass meadows remained.

Furthermore, the spread and amount of macroalgae had declined since 1988 in both harbours, with the algae being concentrated in smaller, although at times denser, patches.

Algae harvesting

The original recommendation to harvest the algae to try and reduce its harmful effects was based on the belief that removing the algae would increase the likelihood of seagrass recovery by reducing the direct smothering of the seagrass by algae and increasing the amount of light. The removal of the algae would also reduce the amount of nutrients available in the harbours.

The scale of the operation meant that it was impossible to remove all the algae. The operation was costly and seemed to reduce the amount of algae by only a small amount.

In 1997 the work was reviewed by scientists whose conclusions led to the recommendation to stop harvesting. The reduction in nutrients getting into the harbour, as industry met the EPA's standards for nutrient discharge, meant that the amount of algae growth had been reduced. The review recommended that funds be spent to reduce even further the amount of nutrients getting into the harbour. The harvesting operation stopped in April 1998.

All told, the operation removed about 30 000 t of algae over seven years, containing about 3 t of phosphorus and 50 t of nitrogen. The algae was deposited at the Hanrahan Road tip. Residents collected some of it and used it as garden mulch.