Abstract

One of the largest tropical tidal ranges in the world occurs in King Sound, a semi-enclosed embayment in the tropical Kimberley region of Western Australia. Incubations of phytoplankton within King Sound displayed reduced photosynthetic efficiency, elevated maximum photosynthetic rates, and no measurable photo-inhibition. A response typical of high light adapted phytoplankton despite decreased water clarity and low ambient nutrient concentrations in the estuary. This is in contrast with the adjacent shelf where phytoplankton, associated with a deep chlorophyll maximum, display high photosynthetic efficiency, and strong light inhibition typical of low light adaptation. Remote sensing and numerical modelling suggest that spatial and temporal variations in tidal mixing drive changes in light variability and in photo-acclimation. In King Sound phytoplankton experience the largest variations in light over short timescales where diatoms dominate since they can rapidly acclimate to water column light conditions by adjusting pigment within the cell. The photo-physiological response of the phytoplankton in the Sound, suggests that acclimation to alternate weak and strong mixing exposes them to cyclical changes in light intensity delaying the onset of photo-inhibition, allowing higher maximum photosynthetic rates to be attained. These findings highlight the importance of a multifaceted approach to understanding the links between physics and photo-acclimation strategies employed by phytoplankton to more accurately determine rates of depth-integrated productivity in complex coastal areas.