Publication Abstract

Temporal variation in light availability in coastal benthic habitats: Effects of clouds, turbidity, and tides

K.R.N. Anthony, P. V. Ridd, A. R. Orpin, P. Larcombe and J. Lough

Temporal patterns of productivity and photophysiological characteristics of aquatic primary producers are linked to fluctuations in surface irradiance and fluctuations in the optical properties of the water column. Here, we analyse the contributions of clouds, turbidity and tides to variations in irradiance and predicted benthic primary productivity on a coastal coral reef over a period of two years (2001-2002). At 1.5 m below lowest astronomical tide (LAT, 3.8 m tidal range), attenuation by suspended solids (turbidity) accounted for 74-79% of the total annual variation in irradiance, clouds for 14-17% and tides for 7-10%. With increasing depth, the contribution from turbidity to irradiance variation increased asymptotically towards 95%. Fourier (spectral) analyses indicated that the benthic irradiance regime followed strong 8-week periodicities and weaker 2-4-week periodicities. The 8-week cycle was driven primarily by turbidity and secondarily by clouds, and matches the periodicity of the intraseasonal Madden-Julian atmospheric Oscillation. A weaker 3-4-week irradiance cycle was driven by turbidity, and a 2-week cycle was driven by tides and, to a lesser extent, by clouds. Comparisons of the benthic irradiance pattern with predictions of physiologically optimal irradiance levels for the coral T. mesenterina (parameter Ek) suggested that corals at the site alternate between states of potential light limitation and light stress with a 2-8-week periodicity, caused mainly by variations in turbidity. The impact of external sources of light reduction, such as episodic run-off events, on the energetics of benthic primary producers is likely to vary critically with the timing of such events.

Reference:

K.R.N. Anthony, P. V. Ridd, A. R. Orpin, P. Larcombe, and J. Lough (2005). Temporal variation in light availability in coastal benthic habitats: Effects of clouds, turbidity, and tides. Limnology and Oceanography 49: (6) 2201-2211.