Award: OCE-2023664

Changing ocean conditions, such as marine heatwaves, are precipitating ecosystem collapse. Once productive kelp forests are being reduced to sea urchin dominated barren grounds. Undersea forests in northern California have declined by 95% in just one year during an unprecedented marine heatwave, losing biodiversity and important fisheries. Yet, we do not understand the mechanisms that tipped the system to sea urchins overgrazing. This NSF project focuses on - predicting ocean settlement of larval sea urchins, the impact of ocean warming and acidification on larvae, juvenile recruitment and adult sea urchin energetics, urchin persistence in barrens, and carbon production to help explain the responses we have seen and establish effective restoration solutions. Larval Transport Modeling The project constructed (PI Simons lead scientist) biophysical models that include both the physical circulation processes as well as biological processes in 3 dimensions. These models showed that when the biology of larvae are incorporated into the model, such as larval vertical migration, then more model larvae stayed close to the adult sea urchin reefs. Further, the primary driver of large sea urchin settlement pulses was the temperature experienced by adults during gonad maturation. This suggests an early warning system could be established to predict where and when large sea urchin recruitment events might occur. Temperature and Early Larval Biology We examined the impact of temperature and stressful ocean acidification conditions across the generations. If the parents experienced stressful conditions would this harden the larvae, or weaken them? We found there was some early advantage for larvae whose parents had seen warm temperatures but this advantage disappeared in the later stages. The temperature experienced by the larvae themselves was the primary factor impacting their success and survival. In experiments designed to examine the effects of temperature in conjunction with food availability with Ms. Munstermann we looked at whether larvae with abundant food resources can overcome the stress of warmer temperatures? We found larval purple sea urchins with abundant food survived warm stressful temperatures but, poor food led to a decrease in optimal temperatures. This highlights how temperature and food concentrations are acting synergistically to drive larval survival rates and how southern and northern California sea urchins may respond differently to ocean warming. Temperature and Adult Sea Urchins Adult sea urchins responded to increased temperatures in lab experiments by reviving up their metabolism and eating more. We found with Dr. Spindel that warm water resulted in 100% increases in metabolism and 200% increases in grazing when combined with modern day acidification levels. However, these increased grazing rates did little to benefit the sea urchin since both food conversion and test growth were reduced by approximately 50%. Therefore, sea urchins are predicted to have more destructive grazing behaviors in the warm, acidic waters of the future while gaining little advantage from super charged grazing. Metabolomics is an important new tool which can assess how organisms are responding to stressful environments by looking at the physiological snapshot of a living cell. We characterize metabolite products in sea urchin cells in response to either kelp forest or barrens habitats. We identified metabolomic markers of barrens sea urchins to determine the starvation response pathways and how they are able to withstand these stressors to persist in barrens. Sea Urchin Ecosystem and Carbon Cycling Monitoring results in collaboration with Dept Fish and Wildlife show that purple sea urchins are at extremely high densities 20/m2 (Figs. 1-4) while the sunflower sea star, an urchin predator, was locally extinct. We found that both newly settled and one year old purple sea urchins were at their highest densities in 2 decades (Rogers-Bennett et al. 2024). Further, there was a significant relationship between the numbers of larvae settling and the numbers of one year olds found 1.5 years later in the recruitment modules (Fig.5). This suggests that much of the increase in sea urchin numbers was the arrival of newly recruited small individuals from exceptionally successful reproduction. Understanding blue carbon, in the form of bound carbon and carbon released is essential for mitigating excess carbon. Here we estimate the carbon bound in sea urchin tests in northern California a region that recently switched from kelp forest to a sea urchin dominated system. We quantify sea urchin desnity using using Scuba surveys and rocky habitat area that in 2022-23 to estimate there are approximately 4.0 billion purple and an additional 318 million red sea urchins equating to 7.4 M kg of carbon in sea urchins. The rapid creation of these billions of sea urchins and their shells via calcification generated carbon emissions. We estimate the formation of these sea urchin barrens generated 17 M kg of CO2 released (4.6 M kg of Carbon) into the ocean (Fig.6) highlighting the need to better understand carbon cycling in the ocean. Last Modified: 01/27/2025 Submitted by: LauraRogers-Bennett