Light requirements of the seagrass, Zostera muelleri , determined by observations at the maximum depth limit in a temperate estuary, New Zealand

The Kaipara Harbour in New Zealand is one of the largest estuarine systems in the world, containing significant areas of subtidal seagrass habitat ( Zostera muelleri ). Light availability at the maximum depth limit for Z. muelleri was measured at 2.10 (0.19 SEM) and 4.91 (0.53 SEM) mol photons m −2 d −1 during the winter and summer monitoring periods, respectively. The primary drivers of benthic light availability were found to be surface light availability, the timing of the low tide and water clarity. Core sampling analysis suggested that biomass of seagrass growing at the maximum depth limit was low, indicative of light limitation. The results of this study suggest that the subtidal distribution of seagrass in the Kaipara Harbour is light‐limited and that reductions in water clarity due to changes in land use are likely to result in significant reductions in the extent and productivity of subtidal seagrass habitat.