Consequences of human-mediated marine intrusions on the zooplankton community of a temperate coastal lagoon

Abstract

Barrier bars separating lagoons from oceans are frequently breached as a management tool to prevent flooding of terrestrial ecosystems. The effects of such human-mediated openings on zooplankton have been investigated only in one tropical system. We investigated the temperate Waituna Lagoon, New Zealand, over a 2-year period when the barrier bar was ‘artificially’ breached on three occasions. Increases in salinity associated with opening of the barrier bars greatly influenced zooplankton community composition, and recovery of communities was dependent on the rate at which salinity returned to pre-disturbance conditions. As such, resilience of zooplankton in coastal lagoons is a function of the lagoon conditions returning to those experienced prior to barrier breach, rather than being a result of the zooplankton community simply recovering from a single defined disturbance event. In contrast to the tropical lagoon studies, temperature in Waituna Lagoon was inferred to explain a significant proportion of the variability in zooplankton community composition, independent of salinity. Appropriate timing for the opening of barrier bars by management authorities in temperate lagoons, which would allow the greatest opportunity for freshwater zooplankton communities to recover rapidly, will rely on determining the best time for rapid barrier bar reformation and high freshwater inflow rates (i.e. the recovery of zooplankton relies on return to initial conditions). However, such an approach is in direct conflict with the opening of barrier bars for management of water levels.