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.