The underwater light climate of billabongs of the Alligator Rivers Region, NT
Open File Record 20
Walker TD, Kirk JTO & Tyler PA
Supervising Scientist Division
The distribution of photosynthetically-active radiation (PAR) underwater has been investigated for a number of Alligator Rivers billabongs, together with the physical, chemical and biological conditions of the waters which determine this underwater light climate. Of the three parameters most likely to affect light conditions, namely dissolved organic colour (gilvin), turbidity and chlorophyll, turbidity was shown to be the dominant influence. Gilvin concentrations were always low and only seldom did chlorophyll modify the spectral properties of the underwater PAR. The presence of finely-divided sediments, turbidity, so characteristic of many of the billabongs, specifies a blue and green-deficient spectrum and a shallow euphotic zone. Laboratory studies indicate that turbidity exerts its influence through absorption rather than scattering. The greater the turbidity, the greater the limitation of PAR to a narrow band of red wavelengths. The resulting spectrum resembles closely that of non-turbid but highly humic waters in other parts of the world. In the clearest billabongs there is a transmission peak in the yellow, as in oligotrophic lakes and in the ocean.
In most billabongs, turbidity increases markedly through the Dry with consequent contraction of both the spectral bandpass of PAR and the euphotic depth. Other billabongs show capricious fluctuations in optical conditions. Nourlangie is unique in that its clarity improves as the Dry progresses. For most of the billabongs the Wet is a time of fluctuating water clarity, with the euphotic depth reaching an annual maximum at the Dry/Wet interchange.
There was high correlation between Secchi depth and the euphotic depth determined from PAR measurements. There was no re1ationship between Secchi depth and chlorophyll concentrations and the Secchi depth cannot be used as an indicator of trophic conditions as it often is elsewhere in the world. Nonetheless, with its close correlation with euphotic depth and with its simplicity and ease of operation, it becomes an excellent routine monitoring tool if used with care.