River guardians warn of toxic blooms

The Swan River Trust has warned that the waterway is at increasing risk of toxic algal blooms because Perth is getting more of its rainfall later in the year.

Less than a month after Perth had one of its heaviest December downpours, the trust said such events were becoming increasingly common in spring and early summer.

Jeff Cosgrove, the agency's acting principal scientist, said that under the changing climatic conditions, nutrients were likely to be flushed into the Swan and Canning rivers much later than normal.

Dr Cosgrove said this posed a problem for the system because the nutrients would be exposed to hot, dry weather over summer, which could spark a bloom of toxic algae.

Such an event would be devastating for the rivers and lead to mass fish deaths and bans on swimming as happened after the last major algal bloom in 2003.

The trust said it had responded to more than 100 complaints about damage to trees along foreshores since 2008, with vandalism suspected in 29 of those cases.

Though there was a drop in suspicious damage last year, the trust said it would continue to pursue cases and hand out warnings or infringements.

In extreme cases, it would also prosecute offenders and install signs where trees had been removed.

The City of South Perth, which manages several parks next to the Swan and Canning rivers, said it had probed 12 cases where trees had been poisoned, ring-barked or cut down.