Seabed surveys for sewage solutions

A muddy site on the Davis sea floor, with many sea pens and several giant isopods visible on the surface, and many bivalve siphons visible at the sediment interface
A muddy Antarctic sea floor, with many sea pens and several giant isopods visible on the surface, and many bivalve siphons visible at the sediment interface (Photo: Glenn Johnstone)
A sandy site on the Davis sea floor, with some green macroalgae and many bivalve siphons evident.Davis sea floor habitat dominated by macroalgae.Davis sea floor habitat dominated by invertebrates - sea cucumbers, anemones and ascidians.Diver ascending with a bag of samplesA portion of a polychaete reef in Long Fjord, Davis, with a range of other invertebrates living in and on it, including sponges, nudibranchs, starfish and worms.

The environmental impact of sewage discharged from Davis station into the surrounding bay was the focus of an intensive scientific diving program in Antarctica last summer (2009–10). The secondary wastewater treatment system at Davis failed several years ago and sewage has since been macerated and discharged to the ocean. While this practice meets the minimum requirements of the Protocol on Environmental Protection to the Antarctic Treaty, understanding the impact of the sewage outfall on the environment will inform future management of the site and the potential installation of a new treatment plant.

As reported in a previous issue of this magazine (17: 20-21, 2009) the three main research questions for our team (six divers and 11 associated support and research personnel) were:

  • How well does the sewage disperse in the vicinity of the outfall and are there better alternative locations for the outfall?
  • What are the nature and extent of the impacts of the present sewage outfall?
  • How toxic is the sewage effluent to local species?

To answer these questions we aimed to survey and characterise the habitats, chemistry and biological communities at sites near the sewage outfall and compare these to ‘control’, or reference sites, far away from the possible influence of the sewage. A rapid survey technique was developed especially for this purpose, to enable comprehensive sampling at as many sites as possible in a five-week window of opportunity for diving. A site was defined as an area of approximately 50 m radius around the work boat at anchor, as this was the area that divers could easily and safely work. In each site a number of different tasks were performed:

  • Habitat surveys were conducted to determine the types of physical and biological habitats and their variation from site to site. To do these, four 25 m tape measures were laid on the seabed. Along these the divers recorded the proportion of different substrata types (rock, sand, mud), and types of biological cover (seaweed and invertebrates such as sponges). This information is very important when comparing many different sites, as the types of habitat can have a very strong influence on the types of biological communities present.
  • Photoquadrat surveys were performed to collect detailed information on the seabed biological communities. A special frame with a camera mounted on it was used to take a photograph of an area of the seabed of a fixed size. These photos were taken in plots along each of the habitat survey tapes. There were two plots on each tape, 5 m long x 2 m wide, and 8–10 photos were taken in each plot for analysis back in Australia.
  • Sediment cores were collected by divers for a range of purposes. Large cores (10 cm diameter x 10 cm deep) were collected to examine the biological communities living within the sediment.  Typically, hundreds of small invertebrates (worms, crustaceans, snails and bivalves) were observed in each core. The types of species present and their abundance will provide a measure of ecosystem health as certain types of communities can be characteristic of polluted or disturbed sites. The divers also took smaller cores (5 cm diameter x 10 cm deep) for analysis of physical (grain size) and chemical characteristics. A range of chemical analyses are now being conducted on these cores, including for metals, sewage biomarkers, chemicals, detergents and oils. Smaller cores (taken using 60 ml syringes with the ends cut off) were also extracted for analysis of microbial and micro-invertebrate communities. Microbial communities in particular can provide an excellent indication of the presence of sewage contamination. Replicates of all three core types were taken in two plots, approximately two metres in diameter and at least 10 m apart.
  • Invertebrates and fish were collected for several different studies.
The rapid survey technique was designed so that these tasks could be completed at a site in just two dives (with two pairs of divers), but this was dependant on water depth, as dive times are shorter in deeper water.

For much of the season we had to navigate underwater in very poor visibility, due to the plankton bloom that occurs over summer, reducing water clarity to only a few metres of green soup. Later in the season the visibility improved and we encountered some amazing wildlife, particularly in the mouth of Long Fjord where we found very diverse communities of invertebrates. Here there were extensive reefs of serpulid polychaetes (tubeworms), which in turn provided habitat for an amazing array of invertebrates living in and on them, including colourful sponges, ascidians, molluscs and echinoderms. These areas could be biodiversity hotspots and certainly warrant future investigation. Much of the Davis coastline was dominated by macroalgae – red, green and brown seaweeds occupying extensive areas of seabed and contributing to local primary productivity.

Davis can be a very unforgiving place to dive from a boat. There is very little shelter from winds as the coastline is low lying and open. We had many days of marginal weather, where the winds were not too strong to prevent us from operating on the water, but strong enough to make it very cold and unpleasant. However, our work boat, Pagodroma, had a nice little cabin, and once we got the heater working we could all crowd in and warm up between dives and munch on frozen sandwiches.

The strong winds caused the boat to swing widely on its anchor, making it difficult for both the divers underwater and the tenders on the surface manning the umbilicals. Unfortunately we were not blessed by the weather of the so-called Riviera of the South, with many more windy days than calm ones. Given the demanding and difficult nature of the field work, it was important to have a team of people we could rotate in and out of the boat over the course of a week, giving divers and tenders a chance to get out of the weather for a day and spend some time in the laboratory, processing the many samples we were collecting.

Early results from the analysis of our samples, surveys and photoquadrats indicate that the discharge from the outfall is being carried south along the Davis shoreline towards Marchant’s Landing. However, we will not know the extent or potential impact of the sewage until all the analyses are complete.

In all we surveyed 30 sites, conducted approximately 2.8 km of habitat surveys, took about 2000 photoquadrats, collected more than 400 sediment cores and did 176 dives. Not bad for four and a half weeks!

JONNY STARK

Benthic ecologist, Australian Antarctic Division