A bottom-up approach to remote sensing

What has measuring light levels underwater got to do with identifying human impacts? When studying the effects of human disturbance to the environment it is always important to ensure that comparisons are between similar environments. In the sea this normally means comparing communities from similar depths, wave conditions and sea-bed types. However, all natural systems are variable both in time and space. The key to any study that sets out to detect changes to the environment caused by human activity is separating the signal of impacts from the noise of natural environmental variability. In the Antarctic, disturbance from bergy bits (small icebergs) and the characteristics of sea ice are additional factors that need to be considered.

Research by the Human Impacts Program on the effects of contaminants leaching from old waste disposal sites into the sea indicate that macroalgae (seaweeds) are uncommon in Brown Bay, which is adjacent to the Thala Valley tip (see ‘Research into the clean-up of tips at Casey and Wilkes’ on p 37), but they are found in dense beds at apparently similar sites nearby. This may be an impact of the tip but, as there are no records of the seabed communities in Brown Bay before the tip was established, it is not possible to say with certainty whether algae were previously present.

Algae are plants and need light to photosynthesise; it is therefore possible that the reason they are not present in Brown Bay is that they are not receiving enough light. In Antarctica the light reaching the seabed varies according to the characteristics of the sea ice. Locations that lose the sea ice cover early in the summer will receive large amounts of light. In contrast, sites that usually remain covered with sea ice until December or January, such as Brown Bay, will receive less light.

Duration of sea ice cover does not explain the whole story. Sparkes Bay, south of Casey, has a flourishing algal community dominated by the large kelp (Himantothallus grandifolius) although sea ice remains in the bay until the middle of summer. Snow lying on top of sea ice considerably reduces the amount of light penetrating to the sea-floor. Near the coast the pattern of snow on sea ice is very patchy because hills and large boulders cause snowdrifts to accumulate down wind. Land formations around enclosed bays will therefore influence the amount of light getting to the seabed.

Light meters have been placed on the seabed to record the amount of light at various locations around Casey Station throughout the year. Some locations have abundant seaweeds; at others they are virtually absent. The information from the light meters will help ensure that, as we try to understand the possible effects of contaminated sites on the marine environment, we are comparing like with like.

Martin Riddle
Human Impacts Research Program Leader,
Australian Antarctic Division

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