Consequences of kelp loss: using restoration as a tool to inform ecology
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Consequences of kelp loss: using restoration as a tool to inform ecology

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Abstract

Canopy-forming kelp forests are found all over the world and operate as marine foundation species, creating underwater forests that provide structural habitat, shelter, and food for numerous other taxa (North 1971; Dayton 1972; Miller et al. 2018). In California’s coastal waters, these forests tend to be dominated by one of two species: Macrocystis pyrifera, the giant kelp, or Nereocystis luetkeana, the bull kelp, occupying southern and northern California regions, respectively. Both species form forests that extend throughout the water column and form thick surface canopies (Springer et al. 2010; Schiel and Foster 2015). Supporting a myriad of other species for seaweed gatherers and indigenous harvesters, as well as for local commercial and recreational fisheries (e.g., rockfish, red urchin, and abalone), these forests hold cultural and economic significance to coastal communities as well as California more broadly (Carr 1989; Turner 2001). Kelp forests face a number of environmental stressors which can operate independently or synergistically to cause localized kelp declines and, in some cases, widespread deforestation. For example, in southern California, hotspots of kelp loss have historically been driven by poor water quality attributed to increased coastal development (e.g., sedimentation, sewage discharge, contamination), as well as increased grazing pressure by purple sea urchins (Strongylocentrotus purpuratus). However, extreme storm events such as those associated with El Niño conditions can cause deforestation across much larger spatial scales (Wilson and Togstad 1983; Tegner and Dayton 1987). For example, storm waves from the 1982-1983 El Niño facilitated a loss of over 90% of the giant kelp canopy along Palos Verdes Peninsula, one of the largest M. pyrifera stands in California (Wilson and Togstad 1983). Shifts in urchin abundances that preceded the storm, in addition to dislodgement and redistribution of those urchins into deeper waters during the storm contributed to further weakening of the kelp and inhibited recovery in some areas (Wilson and Togstad 1983). While improvements to water quality resulted in the recovery of kelp in some areas, urchin grazing pressures continue to inhibit kelp growth and recovery back to the region’s historic coverage (Schiel and Foster 2015). In recent years, northern California’s N. luetkeana forests were hit with what scientists have called the “perfect storm” of conditions, resulting in catastrophic loss of this canopy-forming species (Rogers-Bennet and Catton 2019). While not completely understood, the suite of conditions that aligned to facilitate a loss of 90% of California’s bull kelp forests are three-fold: 1) elevated seawater temperatures, which weaken bull kelp individuals 2) sea star wasting disease, which led to the decimation of the sunflower sea star (Pycnopodia helianthoides), an important predator in the kelp forest system, and 3) most relevant here, an explosion of purple urchins (Strongylocentrotus purpuratus), which notoriously overgraze kelp forests when in high numbers (Rogers-Bennett and Catton 2019). What was once extensive bull kelp forests has now become desolate seascapes of bare rock, caked with purple urchins and red urchins (Mesocentrotus franciscanus). Deforestation is a challenge faced by nearshore kelp communities around the globe, and though each system has a unique suite of triggering conditions, the consequences are the same – profound loss of biogenic habitat and dramatically altered community structure and functioning. Restoration has emerged as a mechanism by which to facilitate kelp recovery around the world (Eger et al. 2020; Ray et al. in review). In California, recent kelp restoration efforts have focused on reducing the urchin grazing pressure exerted on the remaining kelp adults and new recruits. In partnership with local commercial urchin divers, nongovernmental organizations The Bay Foundation and Reef Check are working to reduce purple urchin densities along the Palos Verdes Peninsula and Mendocino coastline, respectively. Subsequent recovery of kelp, in response to urchin culling (Williams et al. 2021; Reef Check unpublished data), provides a unique experimental framework by which to explore physical and biological consequences of kelp loss, recovery, and the role kelp forests play in modulating their physical environment. Loss and regrowth of kelp forests can each have profound impacts on their surrounding environment. Here I explore the consequences of Macrocystis pyrifera forests’ disappearance and regrowth on the local surface gravity waves and alongshore current velocities as well as the consequences of Nereocystis luetkeana forest disappearance on jaw-test allometry of two important species of sea urchin (Strongylocentrotus purpuratus and Mesocentrotus franciscanus). Chapter one leverages the before-after experimental framework of an ongoing M. pyrifera restoration project along the Palos Verdes Peninsula to quantitatively distinguish energy dissipation of surface gravity waves due to the presence of a kelp forest from that due to frictional processes at the seabed. I found that the kelp forest had a detectable but modest capacity to damp wave energy. Interactions with the seabed alone reduced wave energy flux, on average, by 12% over 180 meters of travel, with an additional 7% reduction arising when an established forest was present. Kelp-associated decreases in wave energy flux were slightly greater for waves of longer periods and smaller wave heights than waves with shorter periods and larger wave heights. These findings suggest that Macrocystis pyrifera forests have a limited but non-trivial capacity to enhance shoreline protection from nearshore waves. Chapter two builds on the same before-after experimental framework of the M. pyrifera restoration project and quantifies alongshore current velocities outside and within a temperate rocky reef environment that twice underwent a transition from a barren state to one in which a thick surface canopy was present. Findings suggest there is a threshold density during forest emergence at which much of the attenuation of alongshore depth-averaged velocity occurs – three stipes per square meter with a surface canopy present. Incremental increases in damping occur as the forest matures, highlighting that relatively young, thin forests can induce substantially reduced flows. Additionally, the presence of a young forest’s subsurface canopy and its subsequent increase in height create a seasonally changing profile of reduced velocities through the water column. These results indicate greater complexity in how canopy-forming kelp influence nearshore flow properties than has often been recognized. Importantly, emerging forests can alter the nearshore environment through modulation of current speeds shortly following initial recruitment, with consequences for transport of larvae, nutrients, and sediment throughout the forest and adjacent habitats. Chapter three explores relationships among gonad production, size (i.e., test diameter), and jaw morphology (i.e., length, width, shape, weight) between Strongylocentrotus purpuratus and Mesocentrotus franciscanus, two dominant urchin species of California’s temperate rocky reefs. Within this chapter, I also characterize the extent to which those allometric relationships change across differing habitat conditions, classified as bull kelp (Nereocystis luetkeana) forest, reef with understory algae-only (no surface kelp canopy), and urchin barren, to better understand the role of habitat context on species-specific gonad, test, and jaw allometry. Both species of urchin exhibited greater production of gonad material in the kelp and understory habitats than the barren habitat, highlighting the stark differences in food availability across the habitats. The relationship between jaw length and test diameter did not differ between habitat conditions, in contrast to what has been documented in other kelp-barren systems (e.g., M. pyrifera forests and barrens of Monterey Bay, CA) and with other urchin species around the world (e.g., Heliocidaris erythrogramma). Further, M. franciscanus exhibited relatively wider jaws than S. purpuratus in the kelp habitat, however, such species-specific differences disappeared in the barren habitat, challenging the use of jaw shape to distinguish species within fossil records in lieu of habitat context. However, because M. franciscanus had relatively heavier jaws than S. purpuratus across all habitats, the relationship between jaw weight and test diameter could be leveraged to parse out distinct species from urchin remains. These results indicate greater complexity in the allometric relationships of urchin tests and their jaws, specifically when comparing between species and across differing or unknown habitat conditions. Habitat context should be considered when building growth models using jaw-test relationships for fisheries management, specifically for S. purpuratus, and when inferring species from midden and fossil records for reconstruction of human harvesting patterns across space and through time.

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