Abstract
Globally, kelp forests are threatened by multiple stressors, including increasing grazing by sea urchins. With coastal upwelling predicted to increase in intensity and duration in the future, understanding whether kelp forest and urchin barren urchins are differentially affected by upwelling-related stressors will give insight into how future conditions may affect the transition between kelp forests and barrens. We assessed how current and future-predicted changes in the duration and magnitude of upwelling-associated stressors (low pH, dissolved oxygen, and temperature) affected the performance of purple sea urchins (Strongylocentrotus purpuratus) sourced from rapidly-declining bull kelp (Nereocystis leutkeana) forests and nearby barrens and maintained on habitat-specific diets. Kelp forest urchins were of superior condition to barrens urchins, with ~ 6–9 times more gonad per body mass. Grazing and condition in kelp forest urchins were more negatively affected by distant-future and extreme upwelling conditions, whereas grazing and survival in urchins from barrens were sensitive to both current-day and all future-predicted upwelling, and to increases in acidity, hypoxia, and temperature regardless of upwelling. We conclude that urchin barren urchins are more susceptible to increases in the magnitude and duration of upwelling-related stressors than kelp forest urchins. These findings have important implications for urchin population dynamics and their interaction with kelp.
Highlights
Kelp forests are threatened by multiple stressors, including increasing grazing by sea urchins
In the California Current Large Marine Ecosystem (CCLME) a ‘perfect storm’ of multiple factors has led to historic increases in purple sea urchin (Strongylocentrotus purpuratus) abundance and dramatic reductions (> 90% loss in canopy cover) in bull kelp (Nereocystis leutkeana) forests along the northern California coast, many of which have been converted to urchin barrens[5]
Our results indicate that purple sea urchins (S. purpuratus) from different habitats and maintained on habitat-specific diets exhibit differential susceptibility to both current day upwelling, and future-predicted increases in the magnitude and duration of abiotic stressors associated with upwelling
Summary
Kelp forests are threatened by multiple stressors, including increasing grazing by sea urchins. We assessed how current and future-predicted changes in the duration and magnitude of upwellingassociated stressors (low pH, dissolved oxygen, and temperature) affected the performance of purple sea urchins (Strongylocentrotus purpuratus) sourced from rapidly-declining bull kelp (Nereocystis leutkeana) forests and nearby barrens and maintained on habitat-specific diets. Disruption to the interplay between sea urchin grazing and kelp forest productivity can tip the balance between stable states that alternate between diverse kelp forests and species-depauperate urchin barrens and the transition from one state to the other can be initiated by several factors, including the abundance of algal food, predators, storm intensities, and incidence of d isease[3, 6]. Upwelling-driven increases in nutrients can lead to increases in algal primary productivity, that can outpace consumption by g razers[16]
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