A pattern-oriented model for assessing effects of weather and freshwater discharge on black coral (Antipathes fiordensis) distribution in a fjord

• Black coral survival is modelled in response to salinity change in New Zealand Fjords.
• Model parameters were estimated using a pattern-oriented approach.
• Upper depth limit of black corals is limited by exposure to low-salinity layers.
• Weather is ultimately a key driver of black coral distribution in New Zealand fjords.

Doubtful Sound and other New Zealand fjords are unique in that they provide suitable habitat for a black coral species (Antipathes fiordensis) at depths as shallow as 4 m. Research suggests the upper depth limit at which black corals can survive in Doubtful Sound is controlled by gradients in salinity (as opposed to light levels) within buoyant low-salinity layers (LSL). The thickness of the LSL in Doubtful Sound varies along the fjord and fluctuates as a function of rain, wind and freshwater tailrace discharge from the Manapouri Hydroelectric Power Station. A pattern-oriented survivorship model was developed to predict mortality and upper-depth distribution of black corals in Doubtful Sound in response to changes in the LSL. Model parameters were estimated using observed patterns of black coral distribution and time-series salinity measurements over a range of depths. Predicted upper depth limits in Doubtful Sound and a reference fjord (Milford Sound) were validated against field observations of black coral depth distributions on the adjacent rock walls. The model accurately forecasted depths of corals in mid to outer regions of Doubtful Sound, but underestimated upper depth limits in the inner region of both fjords. The discrepancies in inner fjord locations suggest that other factors, such as sedimentation, are also limiting establishment of long-lived corals in some locations. Data from hydrodynamic simulations of the LSL in Doubtful Sound were fed into the survivorship model to investigate black coral distributions under varying rain, wind and tailrace discharge scenarios. Results indicate that weather, rather than tailrace discharge, drives long-term patterns in the vertical distribution of black corals in Doubtful Sound.