Assessing shallow temperate reef ecosystem health and stability in the Hauraki Gulf

Abstract

Kelp forests are vital ecosystems dominating shallow temperate rocky reefs worldwide and are generally considered their “natural” healthy state. However, kelp forests are declining in some regions due to anthropogenic disturbance, reflecting a decline in ecosystem health. Conserving and restoring reef ecosystem health is critical. However, reef ecosystem health is poorly defined, and adequate transferable quantitative approaches are lacking. This thesis develops and applies approaches to assessing reef ecosystem health and tests these using benthic reef community monitoring data from northeastern New Zealand. Temporal variation in ecosystem health and the effect of marine protection on ecosystem stability were tested at specific locations. Spatial variation in ecosystem health and its variance in response to overfishing and environmental context was also tested across the study region. Reef ecosystem health generally declined at Long Bay over a 13-year-period, characterised by the collapse of the gastropod community, shortening of macroalgal canopies and an increase in non-indigenous species richness, and correlating with increased sediment cover on the reefs. However, the causal link remains undetermined. Long-term no-take protection in the Leigh Marine Reserve was found to enhance kelp forest ecosystem stability and health by facilitating macroalgal recovery and resisting shifts to denuded alternate states. Across the wider Hauraki Gulf, the impacts of fishing on reef ecosystem health were evident as shifts from kelp forests to urchin barrens, but these impacts varied among locations and depths. The Hauraki Gulf’s shallow reef ecosystems are predominantly macroalgal dominated and relatively healthy compared to other regions with extensive anthropogenically-caused deforestation. Where overfishing has driven ecosystem shifts from kelp forests to urchin barrens, marine protection can restore ecosystem health, but this can take decades. Therefore, prompt protection of larger areas is advised. The approaches developed here are valuable for assessing spatial and temporal variation in reef ecosystem health and could be applied to other regions or adapted for different ecosystems. The specific approach and indicators used will depend on the questions and scale of impacts in a given region. The approaches developed produce output that is informative, easy to understand and communicate, and will facilitate effective ecosystem monitoring and management.