A new review has found that turbidity and sedimentation, two of the most widely recognised threats to coral reefs, can have an effect on coral reproduction before, during and after spawning.

Elevated turbidity regularly occurs in shallow, tropical marine environments driven primarily by wind-driven waves but this can be exacerbated by anthropogenic activities such as dredging. The effects on adult corals and the sensitivity of their early life-history stages has been well documented, but the review, published in Marine Pollution Bulletin highlights new potential mechanisms that suspended sediments can have on the reproductive cycle including gametogenesis, spawning synchrony and on gametes in the water column.

The WAMSI study, conducted by researchers from the Australian Institute of Marine Science (AIMS) and The University of Western Australia (UWA), is working to help environmental managers predict how corals will react to changing pressures on their environment.

Co-author from UWA’s Centre for Microscopy, Characterisation and Analysis, Gerard Ricardo, said the review highlighted the need to be able to quantify the extent of the changes in the environment in order to accurately predict how coral spawning would be affected.

WAMSI researcher Gerard Ricardo (UWA) collecting freshly spawned eggs for laboratory experimentation. (AIMS)

“This review was partly motivated by the recent resources boom in tropical Australia, the need for dredging for coastal infrastructure and shipping channels to export mineral and petroleum products, and current environmental regulations around protecting coral spawning events during dredging campaigns,” Gerard explained. “However, the findings are equally as applicable to natural events in turbid-zone communities driven by wind-wave induced resuspension.”

Lead author AIMS’ Dr Ross Jones said that while sediments can act as an energy source for adult corals, there are overwhelmingly more than 30 possible causal pathways whereby turbidity-generating activities can negatively affect reproduction and early life-cycle stages.

“We know that it is only very subtle changes in light quantity and quality that trigger spawning,” Dr Jones explained. “A loss in natural synchronicity could ultimately affect the arrival of the gametes at the surface, which could affect the separation for each species, a process which prevents or reduces hybridization between closely-related species by spawning at slightly different times.

Reproductive cycle of the broadcast spawning Acropora species with indicative timings based on the studies of Hayashibara et al. (1997), Okubo and Motokawa (2007), Okubo et al. (2008) and Ball et al. (2002).

In recognition of the sensitivity of the early life-cycle stages of corals, and their importance to the marine community, policy makers have attempted to protect coral spawning periods from sediments generated by dredging-related activities. Since 1993, dredging projects in Western Australia that are close to reefs are required to temporarily stop when corals are spawning five days before spawning and up to seven days afterwards.

This management approach has also been adopted in some dredging projects on the Great Barrier Reef and the possibility of introducing this practice to other locations such as Singapore has been suggested but it’s a contentious issue internationally because it can significantly inflate costs.

One of the most contested issues is the length of the window and also whether dredging-related turbidity-generating activities need to cease entirely or whether dredging can continue but must adhere to more conservative water quality guidelines.

Sperm tangled in sediment particles. WAMSI scientists are examining whether this impacts on the reproductive success of corals (Image: AIMS)

“The successful use of a coral spawning environmental window as a management tool depends on a shutdown period which encompasses the entire period that turbidity-generating activities could have an effect on spawning and ultimately the successful recruitment of juveniles into the next generation,” Dr Ross Jones said. “The window needs to contain sensitive stages such as settlement and early post-settlement survival.”

In Western Australia there is a well-known main autumn spawning period, but more recently a significant spring spawning period has been identified.

“The presently applied 12 days coral spawning shutdown period is too short to fully encompass the full settlement period, especially settlement and early post-settlement survival,” Dr Jones said.

“Extending the window before and after the predicted spawning date seems an obvious next step to also accommodate effects on gametogenic and spawning synchrony and to fully cover the settlement period. This may however significantly limit time that turbidity-generating activities could occur near coral reefs in any given year.

“Although the approach seems logical, the question is whether this approach is reasonably practicable and whether the resulting intermittent and protracted dredging operation would result in a better net environmental benefit than a shorter campaign.

“What we do know is that conducting dredging activities at a time that avoids coral spawning periods and settlement periods constitutes a best management practice,” Dr Jones said

[R. Jones^,,G.F. Ricardo^,,A.P. Negri^, (2015) Effects of sediments on the reproductive cycle of corals. Marine Pollution Bulletin doi:10.1016/j.marpolbul.2015.08.021]

This project was funded by the Western Australian Marine Science Institution as part of the WAMSI Dredging Science Node, and made possible through investment from Chevron Australia, Woodside Energy Limited, BHP Billiton as environmental offsets, and by co-investment from the WAMSI Joint Venture partners. This research was also enabled by data and information provided by Chevron Australia. The commercial investors and data providers had no role in the data analysis, data interpretation, the decision to publish or in the preparation of the manuscript.

Dredging Science