Suspended sediments limit coral sperm availability

We have known for more than a decade that suspended sediments from dredging and other sources could impact coral fertilisation, but we didn’t know why. A WAMSI Dredging Science project is working to find the answers.

Lead researcher Gerard Ricardo from The University of Western Australia and the Australian Institute of Marine Science said the project team first designed an experiment to discover whether sediment was impacting the sperm or the eggs.

“We found that while the eggs were capable of fertilising in the presence of high sediment concentrations, far more sperm were needed to achieve adequate fertilisation success,” Gerard said. “This indicated that the sediment was ‘taking-out’ the sperm and preventing them from contacting the egg.”

“When we added sperm to the suspended sediments and noticed small flocs (flakes) appearing on the bottom of the containers, indicating the sediments were sticking to the sperm and both were sinking away from the floating eggs. We confirmed this hypothesis using microscopy to examine the flocs, as well as a sperm counts at the water surface which revealed a decrease in sperm numbers.

“Our results suggest that sediments may shrink the fertilisation window – a brief 1-2 hour period when sperm and eggs can fertilise before wind and waves dissipate them.

“The next step will be to determine what properties of the sediments cause the flocking and sinking of the sperm and subsequently which sediments present the greatest risk if dredging occurs during coral spawning events,” Gerard said.

The findings have been published in Scientific Reports: G. Ricardo, R, Jones, P. Clode, A. Humanes, A. Negri (Dec 2015) Suspended sediments limit coral sperm availability  doi:10.1038/srep18084

Fact File:

• Sediments can reduce the amount of sperm available to fertilise the egg
• Sperm can become tangled up in sediment flocs
• Multiple lines of evidence were used including fertilisation assays, flow cytometry and optical and electron microscopy to determine the mechanism that is responsible for the decrease in fertilisation with elevated suspended sediments.
• Sediment may further shrink an already brief (1-2 hour) ephemeral fertilisation window.

The work was carried out at the National Sea Simulator (AIMS) and the Centre for Microscopy, Characterisation and Analysis (UWA).

Naitonal Sea Simulator (SeaSim)

The WAMSI Dredging Science Node is made possible through $9.5 million invested by Woodside, Chevron and BHP as environmental offsets. A further $9.5 million has been co-invested by the WAMSI Joint Venture partners, adding significantly more value to this initial industry investment. The node is also supported through critical data provided by Chevron, Woodside and Rio Tinto Iron Ore.

Dredging Science

Research from the WAMSI Dredging Science Node has provided new insight into how seagrasses in the Pilbara may recover from disturbance events such as dredging.

While some seagrasses can potentially recover from dredging related pressures over a 200 kilometre radius, others require another meadow within five kilometres to survive, or need to be regenerated from a seed bank.

The research, which is important to the ongoing management of the region’s coastal biodiversity, has produced the first real insight into the genetic variability of the area’s seagrass and the level of connectivity among different seagrass populations.

Pilbara seagrass meadows are an important source of food and habitat for the endangered dugong, sea turtles and prawns, the latter of which makes up part of the region’s multi-million dollar commercial fishing industry.

Lead researcher, Edith Cowan University’s Kathryn McMahon, described how several WA Marine Science Institution (WAMSI) projects are revealing more about the genetic diversity and connectivity of Pilbara seagrasses and how this is revealing the possible ways seagrasses may resist or recover from disturbance, insights which are critical to better management of seagrass meadows.

“In one WAMSI project we determined where the seagrass meadows are, how much there is and how it varies seasonally throughout the year” Dr McMahon said. “In a complementary genetics project we examined which species had the higher genetic diversity and therefore a better chance to resist change and recover from dredging pressures.”

Of the 15 species of seagrass found in the Pilbara, the WAMSI team, including researchers from CSIRO, the University of Adelaide and Department of Parks and Wildlife, looked specifically at three of the most common; Halophila ovalis (6 populations), Halodule uninervis (8 populations) and Thalassia hemprichii (3 populations).

“What we found is that Halophila ovalis and Thalassia hemprichii have high genetic diversity in most meadows that we sampled,” Dr McMahon said. “Thalassia showed a high connectivity over distance; the fruit from one meadow can potentially float to a meadow up to 200km away and successfully recruit into that meadow.

“Whereas with Halophila ovalis and Halodule uninervis gene flow is over a much smaller scale. So a meadow which is completely lost could only recover if there was another meadow within about 5kms.

“So from a dredging perspective, if we lose a seagrass meadow, we can give an indication of when or if they are likely to recover within five years based on the WA EPA’s Dredging Guidelines of irreversible loss,” Dr McMahon said.

Seagrasses tend to be found in the coastal zone and grow in soft sand and muddy sediment but some species can also grow on reefs. They are found in more shallow water to a depth of 10 metres, but can be found in clear water down to 50-60 metres. In the Pilbara, seagrass meadows are mostly found in shallower water.

Based on genetic differentiation, two to three management areas were identified. Exmouth Gulf was distinct from Thevenard Island, Rosemary Island and Balla Balla, and Balla Balla was distinct from the island sites.

“Long distance migration was detected but this is likely to be rare based on the genetic differentiation among sites,” Dr McMahon said. “This long distance dispersal was occurring both in a northward direction, against the direction of the dominant oceanographic current, and southwards, with the direction of the dominant current. There was no association of genotypes with habitat. Some meadows appear to be resilient from a genetic perspective, but others not.”

Halophila ovalis genetic connectivity among sites over a local scale in the Exmouth region. This network analysis is stylised on the site map to show the major migration pathways (relative migration rates >0.8). 1=Exmouth Gulf 1, 2=Exmouth Gulf 2, 3=Mangrove Bay, 4=Muiron Is. North, 5=Muiron Is. South.

“As a result, we feel that incorporating analysis of genetic diversity of seagrass meadows (clonal richness, allelic diversity, heterozygosity) into pre-dredging surveys would allow dredging proponents to identify sites, which are more resilient and hence better able to cope with or recover from dredging related pressures, or conversely, less resistant and in need of more stringent management measures,” Dr McMahon said.

A summary of the genetic resilience of seagrass meadows in the Pilbara based on clonal richness, allelic diversity and heterozygosity. The potential to adapt to pressures over generations was based on allelic richness, to recover from declines in a generation was based on heterozygosity and to recover from complete loss using seed banks was based on clonal richness.

The WAMSI research has also found that the diversity of Thalassia hemprichii is higher in the Pilbara than in the Kimberley, which was unexpected.

“The hotspot for seagrass in this region is in the Coral Triangle of Indonesia and since the Kimberley is closer to this hotspot, we expected to find higher diversity there than in the Pilbara.  However, it may be that the massive tides in the Kimberly result in it being more isolated from the rest of the coast.” Dr McMahon said.

FACT FILE:

Seagrasses are clonal, marine flowering plants that form critical habitat in coastal waters. They are found in coastal waters of all continents except Antarctica, where they provide significant ecosystem services including: primary productivity; a food source for critically endangered fauna; habitat for many marine flora and fauna; sediment stabilisation; and carbon storage.

Globally, seagrasses are threatened with 29 per cent of the known areal extent estimated to be lost.  Since 1990 the loss rate is reported to have increased from 0.9% per year to 7% per year, comparable to loss rates reported for mangroves, coral reefs and tropical rainforests.

The WAMSI Dredging Science Node is made possible through $9.5 million invested by Woodside, Chevron and BHP as environmental offsets. A further $9.5 million has been co-invested by the WAMSI Joint Venture partners, adding significantly more value to this initial industry investment. The node is also supported through critical data provided by Chevron, Woodside and Rio Tinto Iron Ore.

Dredging Science