Ok Tedi: Fly River Floodplain Isotope Study

Ok Tedi Mining Limited (OTML) in Papua New Guinea have been monitoring the effects of mining on the downstream ecosystem of the Fly River since 1983.

As a result of river bed aggradation and overbank flooding along the Fly River, the potential exists for mine impacts to the Fly River floodplain and off-river water bodies (oxbows, lagoons etc). OTML contracted WRM to undertake a study of oxbow lakes from control and exposed areas to test for any possible mine-related impacts on aquatic foodwebs.

WRM sampled the primary producers and primary, secondary and higher consumers of eight floodplain sites (3 exposed forested oxbow lakes, 3 control forested oxbow lakes and 2 blocked valley lakes) in February 2005. Stable isotopes of nitrogen and carbon were measured in the laboratory to determine sources of carbon (energy) supporting aquatic food webs. Significant differences were found in the biological and physical characteristics between control and exposed oxbow lakes. Differences in both the ∆ 13C and ∆ 15N signatures suggested a shift in the food webs of the exposed oxbow lakes. Fauna of exposed oxbow lakes appear to derive more algal carbon from periphyton sources (unicellular algae growing on submerged surfaces) than from phytoplankton, with control oxbow lakes displaying the reverse.

It appears that the dieback of rain forest along the shores of exposed oxbow lakes due to increased flooding has reduced near-shore shading and promoted the growth of floating grasses and submerged macrophytes, thus providing complex habitat and abundant substrate for colonisation by periphytic algae, with this habitat absent from unimpacted forested oxbows .