Biodeposition from four bivalves species using natural estuary water in Australia

Brief description

The threatened status of shellfish reefs has been well established globally (e.g Beck et al 2011) however the ecological consequences of these losses is still largely unknown. In Australia, shellfish reefs are one of the most imperilled marine habitat types (Gillies et al 2018), due to historical overharvest and widespread eutrophication of coastal waters through the use of fertilizers, livestock and human waste.

Marine bivalves are important ecosystem engineers providing habitat, shelter and a food source for other species in benthic soft-sediment environments. In addition, filter-feeding bivalves link benthic and pelagic components of ecosystems through filtration and excretion. Through their filter feeding, they produce large amounts of faeces (digested seston) and pseudofaeces (rejected particles bound up in mucus) which are deposited on the benthos. This process brings energy and nutrients from the pelagic system to the benthic system (bentho-pelagic coupling).

The removal of large quantities of seston can serve an important ecosystem function by improving water quality and clarity. The filtration of water performed by bivalves has been demonstrated to reduce water turbidity, improving light penetration and thereby enhancing growing conditions for seagrasses (Wall et al 2008). In systems where healthy populations of bivalves remain, they can filter a volume equivalent or larger than the entire estuary volume within the residence time of the water (zu Ermgassen et al 2013). While such densities of oysters are rare today, this highlights the critical ecosystem services that are lost when oyster reefs decline. Furthermore, it demonstrates the potential functions that can be regained through oyster reef restoration.

Given the increasing awareness of the decline of these ecosystems, interest in restoration efforts to restore critical ecosystem functions has been growing. However, conservation and restoration decision making is underpinned by reliable quantification of relevant ecosystem services (zu Ermgassen et al 2016). For example, there are plans to restore some of the natural oyster reefs of Sydney Rock Oyster (Saccostrea glomerata) in Port Stephens, New South Wales. One of the main drivers motivating this restoration project is restoring lost ecosystem services.

The filtration rates of Australian oysters has been demonstrated in aquarium studies using filtered water augmented with algae, yet little is known about filtration and biodeposition rates of oysters using raw seawater. In this study, we provide the first evaluation of the filtration and biodeposition rate of four species of bivalves using raw seawater, providing a proxy for natural biodeposition rates. As such, this study provides a first indication of the filtration/nutrient cycling function that may be restored following oyster restoration efforts.

Lineage

Maintenance and Update Frequency: asNeeded

Statement: Study site
All experiments were conducted at the Port Stephens Fisheries Institute, New South Wales, Australia.

Oyster feeding and biodeposition
Four bivalve species were used for these experiments. Two bivalve species that are components of shellfish reefs in Port Stephens were collected by hand from natural shellfish reefs, Sydney rock oysters Saccostrea glomerata and hairy mussels Trichomya hirsuta. Pacific oysters Crassostrea gigas and flat oysters Ostrea angasi were collected from local aquaculture facilities.

Bivalves with similar mean (±SE) shell height were transported to Port Stephens Fisheries Institute where they were hung submerged off the wharf until needed. Raw seawater was pumped in each day from an intake located at the end of a jetty located at Port Stephens Fisheries Institute. The seawater was kept well mixed using bubbled air and an ambient temperature throughout the experiment.

The experimental design included 16 feeding trays, (30 cm x 22 cm x 10 cm), which each had a single inflow, mesh baffles to reduce turbulence, and two separate outflows. Raw seawater was distributed to the 16 feeding trays at the rate of 500 ml ± 50ml min-1. A single bivalve was placed, fully submerged in each tray and left undisturbed until it consistently produced biodeposits. If a bivalve did not produce biodeposits within 60 min it was replaced. Biodeposits were not suspended by water flow, but settled near the oyster. During 30-60 min timed trials, all faeces and pseudofaeces from each bivalve were collected by pipette without disturbing feeding activity, and placed in a separate container. Biodeposits were collected continuously throughout the trials. Continuous collection and tray design minimised the chance of biodeposit loss before collection. Two sequential collections of biodeposits were performed for each bivalve. At the conclusion of the trials, biodeposits were filtered onto separate (ashed and preweighed) glass fibre filters using a pump. Filters with biodeposits were washed to remove salt by gently pumping through deionised water, placed in 47 mm petri dishes, and stored at -20 oC until processed in the laboratory. The filters were dried for > 24 h at 70 oC, weighed, ashed for 4 h at 450 oC, and weighed again to measure organic content

Bivalve characterisation
Bivalves were measured for shell height (longest axis), width, and depth. Tissue was removed from the shell, and tissue and shell were dried separately at 70 oC for > 48 h to obtain dry weights

We used biodeposit measurements to calculate total biodeposit production (mg h-1), organic content, inorganic content, and C and N content.

Seston analysis
To characterise the seston, we collected triplicate water samples during the feeding trials and filtered through pre-weighed Whatman glass microfiber filters, Grade GF/F. Seston filters were processed as described for biodeposits.

Water quality analysis
Three water samples were taken for each water trial and send to a commercial water quality laboratory for analyses of total phosphorous, total nitrogen, and total organic carbon.

Biodeposit composition
Two samples were collected for each species and send to a commercial water quality laboratory for analyses of total carbon, total nitrogen and carbon/nitrogen ratio.

Notes

Credit
National Environmental Science Program (NESP) Marine Biodiversity Hub

Credit
Department of the Environment, Australian Government