Restoring a Nearshore Rocky Reef Ecosystem in the Challenge of an Urban Setting
Daniel J. Pondella II, Jonathan P. Williams, Chelsea M. Williams, Jeremy T. Claisse, and David Witting
Abstract.—The subtidal rocky reefs that surround the Palos Verdes Peninsula in Los Angeles County, California, USA are subject to multiple anthropogenic impacts, including many (at least partially) human-induced landslides over the past half century, which have resulted in chronic sedimentation (e.g., reef burial and scour) and associated turbidity effects along a major stretch of this coastline. The amount of rocky reef habitat has significantly decreased; as such, determining the optimum technique for restoring this lost habitat is the focus of this study. Over the past decade, we mapped and intensively surveyed the nearshore physical and biological characteristics both inside this impacted area and in surrounding reference areas that contain extensive rocky reef habitat with established kelp forests. Notable among all survey locations is a relatively high-relief (~5 m) area of reef within the sediment impacted area that consistently has the highest fish biomass density among anywhere on the peninsula. The high structural relief prevents sediment accumulation, scour, and subsequent reef burial, and this reef ultimately served as the example for the design of sets of quarry rock reef “blocks” that together form the proposed restoration reef. Our primary objective was to use the 63,500 metric tons of quarry rock the budget would allow us to create the most productive habitat by restoring the natural reef environment while balancing scientific study design considerations (i.e., replicated reef components at multiple spatial scales) with maximizing the potential for an effective restoration effort across the range of important species and overall kelp forest biodiversity. To meet this objective, we considered multiple criteria that incorporated engineering specifications and biological performance and were informed by the scientific literature and results of natural and artificial reef surveys in this region. Ultimately, the design incorporated heterogeneity at multiple spatial scales while attempting to maximize high relief components, surface area to volume ratio, perimeter, ecotones, and small-scale current flow features and nutrient flux and while being consistent with the size of natural reefs along the Palos Verdes Peninsula. Further, placement and spacing of individual reef blocks (i.e., 2 × 48 m heterogeneous quarry rock reefs) included space for sand channels between blocks to permit sediment transport and create sand/rock ecotone habitats while remaining close enough to each other and existing natural reefs to maintain biological connectivity. Reef blocks were also located at the depth (15–20 m) where the most productive reef habitat in the region was observed. Finally, we discuss a proposed pre- and post-construction monitoring program and additional studies that could be performed that would leverage the replicated elements in the restoration reef design to inform future reef restoration programs.