Re-defining sediment transport models over coral reefs and seagrass meadows

Novel research within WAMSI’s Dredging Science Node will redefine how current dredged sediment transport models predict key pressure parameters such as sediment deposition rates within ecologically significant marine habitats.

Sediment deposition and subsequent smothering of marine habitats such as corals and seagrasses is one of the mechanisms by which dredging can impact on the environment.

However, according to Professor Ryan Lowe from The University of Western Australia, current sediment transport models are severely lacking in their ability to predict rates of sediment deposition and re-suspension over coral reefs and seagrass meadows with any degree of confidence.

Canopies formed by seagrass meadows impose drag forces that can trap sediment. This is not accounted for in sediment transport models.

“The first step in the Environmental Impact Assessment (EIA) process for proponents of new developments is to make predictions on the likely extent, severity and duration of their impacts on the environment,” Professor Lowe said. “To do this for projects involving dredging, proponents use sediment transport models that make predictions of where dredge plumes will go and what impacts they will have when they get there.”

“Current sediment transport models assume that the seafloor is essentially flat and that nothing is growing on it. However, in reality the large roughness, or canopies, formed by coral reefs, seagrass meadows and sponge gardens impose substantial drag forces that will alter turbulent flow structure over very small spatial scales and can trap sediment. As a consequence, current sediment transport models can grossly underestimate the rates of sediment deposition that occur in and around these important habitats.”

Sawhorse instrument frame deployed at Ningaloo Reef with hydrodynamic and sediment transport instrumentation. Photo: Andrew Pomeroy

Professor Lowe and UWA collaborator Dr. Marco Ghisalberti are leading a research program combining field and laboratory techniques to address this problem.

“In the field we are measuring turbulent flow structure and sediment concentrations above and within the coral reef and seagrass meadow canopies,” Professor Lowe said. “These direct measurements are compared with various conventional sediment transport models and highlight the major deficiencies.

“We are also conducting parallel and complementary laboratory experiments. The advantage of laboratory experiments is that we can examine in detail the mechanisms and processes in a controlled setting. We can control the densities and heights of canopies, and factors like whether they are completely submerged or not. In this way we can precisely measure transport rates, near bed turbulence, sheer stress and look at the effect of canopies on transport rates and subsequent deposition,” he explained.

Laboratory experiments of sediment transport through artificial canopies

The ultimate goal of this research is to develop new and improved transport formulations and algorithms that can more accurately predict rates of sediment deposition and the subsequent impacts to seabed communities.

“If we can achieve this, then both the Environmental Protection Authority and project proponents will have greater levels of confidence in the prediction of impacts during the EIA process,” Professor Lowe said. “And this is what the Dredging Science Node is all about.”

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

While sea sponges may not be as charismatic as corals, an industry funded WAMSI project is finding their strength may lie in their resilience to change.

One of the most important roles of the sponge is providing habitat for vertebrate and invertebrate species, but one of the most impressive is its ability to filter huge volumes of water – a one kilogram sponge can filter more than 20,000 litres of sea water every day. It makes them a critical link between what’s happening in the benthic environment (the ecological region at the very bottom of the sea) and what happens in the pelagic zone (mid to surface region).

Because they have a big influence on what’s happening around them, a team of researchers at the Australian Institute for Marine Science (AIMS) led by Dr Nicole Webster is testing their sensitivity to reductions in water quality.

Mari Carmen Pineda monitoring sponges in the AIMS SeaSim lab

“Field surveys of the filter feeding communities in the Pilbara region led by Drs Jane Fromont and Christine Schoenberg really highlighted the amazing diversity and abundance of sponges,” Dr Webster said. “It has confirmed that sponges are the dominant filter feeders.”

“This project is looking at sponges of different species, different morphologies and different nutritional strategies to see how they respond to the various dredging related pressures,” she said.

Little is understood about the effects of dredging pressures on sponges. However, some of the known effects include bleaching or the loss of photosynthetic microbes from low light conditions, clogging of their aquiferous systems from high levels of suspended solids or complete burial or smothering of the sponges due to high rates of sedimentation.

Sponge filtration capability

The sponge’s aquiferous system is made up of channels and small chambers lined with specialised cells (choanocytes) that create currents of water and retain nutritive particles.

It’s quite well known that sponges can reduce their pumping activity in response to high levels of particulates in the sea water, but how long they can maintain that reduction in pumping activity before their energy reserves, which they need for growth and reproduction, become entirely depleted is not yet known.

Dr Mari-Carmen Pineda and PhD student Brian Strethlow are testing the sponges under controlled conditions in the AIMS National Sea Simulator (SeaSim) to define cause/effect relationships to the dredging related pressures of light attenuation, sedimentation (smothering) and elevated suspended sediments (clogging).

Carteriospongia foliascens with  mucus and sediment layer sloughing

Cliona orientalis with brittle star inside its osculum cleaning the sediments

“To determine the sponge stress response we measure a whole suite of health parameters in our experimental animals including: changes to respiration rates, bleaching or loss of symbiotic microbes, changes to sponge pumping activity and cellular necrosis,” Dr Pineda said.

“Overall we are finding that sponges seem largely tolerant of short term dredging-related pressures and that light and suspended sediments on their own do not cause severe stress in the short time (days),” she said. “Sponges also demonstrate an array of different mechanisms for coping with sedimentation, such as the development of new oscula (exhalent pores), sediment sloughing and removal of sediment from the aquiferous canals by infauna

Coscinoderma matthewsi with open osculla through the sediment layer

such as brittle stars. “

“On the other hand, long-term exposure (>2 weeks) to dark conditions and high levels of suspended sediments (>30 mg/L), seems to have an impact on growth rates and symbiotic microbes, although most sponges can recover once conditions return to normal.

“Overall, cup shaped sponge morphologies and phototrophic species (ie those that rely on photosynthetic microbial symbionts for nutrition) are the most sensitive to dredging related impacts, and some of them do not possess the ability to recover,” Dr Pineda said.

The SeaSim experiments will add to data gathered from pre-dredging surveys by divers and towed video. The field surveys are also due to be repeated in July post-dredging off Onslow to determine what changes have occurred and how this can be applied to management of dredging operations in the future.

This research was funded by Woodside, Chevron, BHP and WAMSI partners

 

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

In the second part of our report into measuring the effects of light reduction and sediment burial to determine the capacity for northwest seagrasses to withstand change, we move from the field to the lab for some surprising results.

The WAMSI Dredging Science Node project has brought together researchers from Edith Cowan University (ECU), The University of Western Australia (UWA) and the Commonwealth Scientific and Industrial Research Organisation (CSIRO) to test tolerance levels that have

UWA’s seagrass growth facility (Crawley campus)

previously never been determined for seagrass species in the far northwest.

Dr John Statton manages the research’s experimental tank system at UWA’s seagrass growth facility (Crawley campus) where the effects of light stress and sediment burial are being measured under controlled conditions on three commonly co-occurring northwest Australian

tropical seagrass species (Halodule uninervis, Halophila ovalis and Cymodocea serrulata).

“We’ve been keeping plants under different light intensities for a number of weeks now. It was quite clear that within the first three weeks the plants at low light intensities (4 and 11% surface irradiance) had lower photosynthetic rates and growth rates than unshaded plants.

“Some of the responses were as expected but what was interesting is that it took a long time for all except one of the species, Halophila ovalis, to die,” Dr Statton said.

Halophila ovalis

Halophila ovalis has the smallest leaf and storage reserves of the three seagrasses under observation and is the only variety of the three that can also be found off the southwest coast.

In the second stage of the research, the plants were subjected to fine sediment (Rocla Quarries WA sponsored UWA with high-grade fine sediments) burial at a depth of between 4mm to 70mm but the effects of light were found to have a far greater impact.

“Only the deepest burial treatments resulted in in some adverse effects,” Dr Statton said. “The seagrasses adapted and grew vertically to new sediment heights then, much like suburban lawns, they put out runners spreading across the new sediment surface. So their initial response to being buried in sediment was to increase the length of their leaves then grow vertically.”

The next step to the lab work will be to install the combined sediment burial and light interaction experiment to look at answering the questions as to what synergies there are when the stresses are combined.

 

The first part of this story was published in the February WAMSI Bulletin:

WAMSI research finds northwest seagrass in a world of its own (Part 1 – in the field)

Related Links:

Tropical Seagrass examined for light pressures (article by Science Network WA)

 

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