The erosion of kelp forest resilience in an increasingly turbid environment

Abstract

Kelp forest declines are being documented in many regions. To mitigate further losses, it is imperative to understand the threats to kelp forests and the mechanisms by which they operate. Understanding such large-scale processes requires that standardised methodologies be developed and that experiments be conducted across regional gradients. To this end, I conducted a review of macroalgal clearance experiments. This method was then used to examine the influence of an important stressor on kelp forests. High turbidity due to sediment runoff from the land is an important and overlooked stressor on kelp forest ecosystems. To elucidate the resilience of kelp forests and their associated communities under increasing turbidity, I established replicate clearance and control plots within Ecklonia radiata forests spanning a turbidity gradient in north-eastern New Zealand. The recovery of kelp and changes in community composition were monitored over two years. The importance of kelp-derived organic carbon for benthic suspension feeders was also investigated to understand the importance of kelp forests in fuelling secondary production. This involved a second regional experiment in which two species of filter feeders were transplanted throughout the Hauraki Gulf. Their growth and isotope ratios were monitored relative to contrasting gradients in kelp and phytoplankton availability. This study demonstrated that kelp forests and their associated community were largely resilient to pulse disturbance across the examined turbidity gradient. However, high turbidity reduced the rate and extent of recovery at the adult sporophyte stage and facilitated a shift in structure to mixed stands of kelp and Carpophyllum flexuosum. Original understorey species assemblages varied with turbidity and experienced divergent trajectories following canopy loss, with greater turfing responses and impacts on sciaphilic species at high-light sites. However, after two years, understories had largely recovered. Finally, the growth of the two species of filter feeders was unrelated to kelp availability. This suggests that temporary kelp loss is unlikely to influence selective filter feeders through a loss of direct food provision. Nevertheless, increases in the frequency or intensity of disturbance to kelp forests are expected to have greater impacts on the forests and their communities, particularly when combined with increases in stressors.