Coral diversity 10 months after Cyclone Ita

Reefs on the northern and eastern exposures received the full brunt of the cyclone-induced waves, so our objective was to document the status of reef communities in those areas and compare the results to identical surveys undertaken in 2011.  See also fish diversity.

The first survey site was North Reef. Coral species diversity was recorded on three replicated 50m x 2m wide belt transects at two depths.  Benthic (i.e. sea bottom) cover was recorded on point-intercept transects.  Superficially, North Reef was devastated – the majority of corals had succumbed to the physical forces.

The 8-10m depth zone at North Reef. Photo credit: Zoe Richards © WA Museum

Upon closer inspection however, a high number of small and juvenile colonies persisted, particularly encrusting Montipora species and individuals inhabiting crevices.

A juvenile colony of Acropora millepora survives cyclone Ita in a sheltered crevice at North Reef. Photo credit: Zoe Richards © WA Museum

Other survivors were large colonies of Porites and Diploastrea and the soft coral Isis hippuris.

Two of the key benthic survivors at North Reef are Porites lobata and Isis hippuris which inject some much-needed colour into the reefscape. Photo credit: Zoe Richards © WA Museum

Despite the clear visual changes to North Reef, the percent cover of hard coral has only marginally changed from that recorded in 2011. In the 3 – 5m and 8 – 10m depth zones, coral cover declined by 2% (from 14% to 12%) and 5% (from 12% to 7%) respectively.

The next survey site – Granite Bridge on the NW side of the island – showed a higher drop in coral cover; from 13% to 4% in the 3 – 5m depth zone and from 12% to 7% in the 8 – 10m depth zone (2011 to 2015 respectively).  The pattern was similar at Resort Reef where again percentage cover declined in both depth zones.

At all disturbed sites a large number of juvenile colonies were observed, suggesting at least a portion of the 2013/2014 cohort of coral spat had survived the cyclone.

Lots of juvenile corals from a variety of genera either survived the cyclone or have colonised the newly available space including these three charismatic species. Photo credit: Zoe Richards © WA Museum

It was also heartening to see that many Tridacnid clams and anemones had survived.

After surveying Big Vicky’s Reef I was optimistic about the ability of the Lizard Island coral community to recover from Cyclone Ita because, unexpectedly, hard coral cover had increased by 6-8% from the levels recorded in 2011.  Additional coral highlights included a spectacular colony of Acropora echinata and a large colony of the very fragile Pavona cactus.

Acropora echinata at Big Vicky’s Reef. © Daniela Ceccarelli

Unfortunately, the situation was quite different at Coconut Reef.  In 2011 Coconut Reef was the jewel in Lizard Island’s coral crown with 178 species recorded and coral cover ranging from 27% at 3-5m to 35% at 8-10m.  However, in our 2015 survey we were unable to locate a quarter of the coral species we recorded in 2011 and the level of hard coral cover had dropped to 9% at 3-5m and 12% at 8-10m.  This is a decrease of 18% and 13% for these two depth zones respectively.

Upturned Porites colonies lay as a testament to the physical forces imposed upon the corals at Coconut Reef.  Nevertheless the surviving corals are demonstrating remarkable resilience, with many regrowing from tiny remnants and fragments, or from their new aspect.

Upturned Porites boulders at 6m depth at Coconut beach. © Daniela Ceccarelli

With more data to analyse, it is too early to draw overall conclusions of the impact of cyclone Ita on the Lizard coral community; however my initial impression is that at least half of the island’s fringing reefs have been impacted, either moderately or severely.  A substantial amount of coral cover and coral biomass has been lost, and some genera, especially the Acropora have been hit hard.

Preliminary results suggest the lagoon reefs and those in the vicinity of Palfrey and South Islands largely escaped the wrath of Ita and are likely sources of larvae to enhance recovery at the more disturbed locations.  In the absence of recurrent disturbances, I am hopeful that the coral community will recover and that within a decade the debris from cyclone Ita will be assimilated by a new coral reef community.

Zoe Richards