The episodic dynamics of salt transport in the Waitematā Estuary, a well-mixed estuary in New Zealand

Abstract

This study investigates the response of salt transport to freshwater discharge events and tides in the Waitematā Estuary. The salt transport is assessed by both observations and numerical simulations using a gridded salt balance and isohaline salt transport. The Waitematā Estuary represents a unique type of estuary in parameter space: it varies between well-mixed and strain-induced periodic stratification during the spring-neap tidal cycle. Spring tides often lead to a net inflow of salt when the freshwater discharge is below average. Higher freshwater discharge (>10 m³s^-1) results in net downstream salt transport regardless of the amplitude of the tides. The estimates of estuarine parameters show that the whole estuary becomes partially-mixed when the freshwater discharge increases by another order of magnitude (100 m³s^-1). Based on future regional climate projections of less average rainfall with more intense events and rising temperatures, the estuary is expected to experience less frequent but stronger salt outflow. This study contributes observations from a previously undocumented part of estuarine parameter space.