Palos Verdes Reef

The purpose of the Palos Verdes Reef Restoration Project��is to restore rocky-reef habitats and associated marine species on the Palos Verdes Shelf��that were impacted by contamination in the sediments from the discharge of DDT and PCBs from the Joint Water Pollution Control Plant’s Whites Point Outfall, as well as to restore reefs that have been impacted by sedimentation and scour.

This restoration project will fulfill the objective of the to restore fish and the habitats upon which they depend within the Southern California Bight. This reef will provide essential fish habitat and substrate for kelp, other marine algae, and marine invertebrates to become attached to, creating a productive rocky-reef ecosystem in an area with limited hard substrate.

Background

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Turbidity plume from the Portuguese Bend landslide (left: circa 1980s; right: April 2016). White arrows indicate location of sediment source.

The amount of giant kelp and rocky reef habitat on the Palos Verdes Peninsula has declined dramatically over the last century for several reasons. In particular, pollution from the Whites Point sewage outfall and sedimentation from landslides resulted in the loss of almost all of the giant kelp on the peninsula by the 1960s. Beginning in the 1970s, kelp has made a comeback due to restoration and enhancement activities including dewatering the Portuguese Bend Landslide which dramatically slowed sediment deposition, extending the sewage outfalls into deeper water and treating the sewage itself, as well as active kelp out-planting. However, in the section of the peninsula between Portuguese Bend and Point Fermin, much of the historic low-lying reefs continue to be heavily impacted by chronic sedimentation due to reef burial and scour and associated turbidity. 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.

Field Surveys and Analyses

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Sediment-impacted reefs at the future location of the Palos Verdes Restoration Reef

Over the past decade, we examined the potential efficacy of fishery production enhancement reefs in this region by conducting an intensive biological and physical sampling program throughout the subtidal areas of Palos Verdes Peninsula. Notable among all survey locations is a relatively high relief (~5 m) area of reef within the sediment impacted area, KOU Rock, 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.

We use a standardized comprehensive community monitoring survey method, the CRANE Protocol, that quantifies fishes, invertebrates, algae, and habitat characteristics within multiple depth zones at each site. In early 2020, we surveyed each module footprint using the CRANE protocol and searched for any presence of Caulerpa spp. or eelgrass (Zostera spp.).

From these CRANE surveys we are able to quantify:

Size structure, density, and species composition of fish, macroalgae, and invertebrate assemblages
Biotic and abiotic benthic cover
Substrate type and relief
Size structure of select commercially and/or ecologically important invertebrates

In order to further our understanding of how reef shape, size, structure, and relief affect the fish, invertebrate, and macroalgal communities, an additional 25 isolated reefs along and adjacent to Palos Verdes Peninsula were surveyed using the CRANE protocol. These reefs included six natural reefs, 10 manmade reefs built with quarry rock, three shipwrecks, three sites with scattered debris, two sites with quarry rock resting on discharge pipes, and one site with engineered shelters. In addition to typical survey techniques, total reef height was also measured by recording depth at the bottom and top of the reef.

We also conducted supplementary scuba surveys at 9 locations over several years to determine the sediment depth over rocky reef throughout this area in an effort to better characterize soft bottom habitat areas in the study area and determine proposed locations for the restoration reef blocks. Sites that were found to be primarily exposed rocky reef were excluded from successive surveys.

Reef Design Concept

Our goal in the reef design plan was to utilize limited resources and engineering criteria to develop a restoration reef plan that maximizes the biological benefits. These benefits include insights drawn from reefs at Palos Verdes and throughout the Southern California Bight and include species richness, diversity and biomass. Our research indicates that multiple factors including reef size, spacing, relief, rock size, heterogeneity, depth, sediment depth, location relative to kelp bed perimeter and flux all influence reef performance. We developed a secondary production model that specifically analyzes the production of fish biomass to evaluate reef performance. In this project, these factors were juxtaposed with the economic, physical and engineering constraints to develop the restoration plan.

Maximize heterogeneity in reef characteristics (e.g., relief, interstitial space, overall angle of outer reef surface) to increase biodiversity by increasing the availability of distinct micro-habitats within each block while being consistent with the size of natural reefs along Palos Verdes Peninsula.

Interstitial void space was also considered in our construction criteria due to the role it plays in supporting the density and diversity of fishes in general, and because large void spaces create critical habitat for several fish and invertebrates including California spiny lobster.
Stagger high relief piles within blocks and pile heights across adjacent piles within blocks.
Place high relief piles at the ends of each block to buffer any potential sedimentation of the 1 m relief piles in the middle of each block.
Size blocks similar to current natural reefs along Palos Verdes.
Increase the amount of outer reef edge (the relationship between perimeter and area) by not making blocks too large.

With these design elements in mind, additional placement criteria were followed for positioning restoration reef blocks across the Bunker Point restoration area in the restoration reef design.

Blocks do not overlap with persistent kelp canopy.
Blocks are placed at 15-20 m seafloor depth.
Vary the orientation of each block and each module.
Mimic natural features (reef width and orientation to natural features).
Blocks placed in a maximum of 1m sediment to limit long-term burial/sinking.
Incorporate 10-20 m sand channels between modules within a block to permit space for sediments moving with longshore current and wave action to move around/through modules.
Maintain connectivity with existing natural reefs.
Maximize distance between blocks to increase independence of each block.

The overall approach is to balance scientific study design considerations with maximizing the potential for an effective restoration effort across the range of important species and overall kelp forest biodiversity. Major motivations included incorporating heterogeneity throughout the restoration reef design both within (e.g., varying module heights within blocks) and amongst (e.g., varying block orientation).

Final Design Plan

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The restoration reef is designed as set of six “blocks”. Each block contains three modules (A, B, C). Each module consists of a 3 x 2 set of six “piles”. Blocks will be in two forms, either with a 3-m maximum module height (relief) or a 4-m maximum module height, with three replicate blocks per set, and three replicate sets of each height. This will permit a comparison of the two reef heights impact on fish biomass and production.

There is a 10 to 20 m wide sand channel between modules and at least 50 m of space between blocks. By separating the blocks and modules by the appropriate distances we can restore a greater amount of reef perimeter sand-rock ecotone habitat and we can increase the independence of replicate reef blocks.