Assessing drivers of variation in the distribution of macroalgal forests across a complex marine environment in Central New Zealand

Understanding the relative importance of different stressors in driving habitat loss is key to developing management strategies. Long-term declines in macroalgal forests on rocky reefs in central New Zealand have been suggested to be due to several anthropogenic stressors including sea urchin grazing resulting from overfishing of predators, elevated turbidity associated with land-based sediment inputs and warming waters. To better understand key drivers influencing macroalgal forests, we examined how the distribution of macroalgal forests varies across large environmental gradients on rocky reefs in Tōtaranui-Queen Charlotte Sound and Kura Te Au-Tory Channel, central New Zealand. Drop-camera surveys carried out at 102 reef sites were used to examine how the depth distribution and prevalence of macroalgal forests and urchin barrens relate to seawater temperature, turbidity, tidal flow, wave exposure and sea urchin density. Urchin barrens were the most prevalent habitat recorded, covering 46 % of the reefs examined. The extent of urchin barrens was reduced at sites with higher wave action, lower water temperatures, and within a marine protected area. Macroalgal forests in Tōtaranui were largely restricted to <7 m depth, with their lower depth limit being most strongly related to urchin density and wave exposure. These results suggest that sea urchins are a major factor limiting macroalgal forests within Tōtaranui, but this effect varies in a complex manner with environmental factors. This correlative study suggests management actions focusing on controlling sea urchin populations within Tōtaranui may promote macroalgal forest recovery and increase their resilience to other environmental stressors.