Treatment of the underwater light field in ocean biogeochemical models has been attracting increasing interest, with some models moving towards more complex parameterisations. A simple test of the sensitivity of a typical, highly simplified parameterisation, to adjustment of the phytoplankton light attenuation parameter using both steady-state and future projections reveals a range of values to which the model primary production is relatively insensitive in steady-state but to which it becomes increasingly sensitive under climate forcing. Parameter value choice can determine the magnitude and sign of global net primary production trends in a high CO<sub>2</sub> forcing scenario. Ocean oxygen is particularly sensitive to parameter choice. With climate forcing, two simulations establish a strong biogeochemical feedback between the Southern Ocean and low latitude Pacific that highlights the potential for regional teleconnection and serves as a reminder that shifts in fundamental properties (e.g., light attenuation by phytoplankton) over deep time have the potential to alter biogeochemical climate.