Short- and long-term impacts of variable hypoxia exposures on kelp forest sea urchins

Abstract

important sea urchins in kelp forests. We find that reproductive investment is the process that appears to be most sensitive to hypoxia, as well as to its exposure regime, with potentially important population and ecosystem level consequences. A remaining challenge is to understand how the changing patterns of variability in multiple environmental stressors will interact to impact organisms and ecosystems, and whether predictions based on laboratory studies hold under field conditions. For example, upwelling-driven hypoxia tends to occur in phase with low temperature and low pH, which can each have direct impacts on sea urchin metabolism, grazing, calcification, and reproduction 55-58. Experimental studies that integrate variable exposure patterns across such different co-occurring stressors will be needed to understand their potentially interactive effects on species and ecosystems 59,60 , as we move towards a more realistic and societally useful understanding of climate change impacts 54,61 .

Coupled changes in pH, temperature, and dissolved oxygen impact the physiology and ecology of herbivorous kelp forest grazers

“…franciscanus . These results are consistent with past studies showing reduced grazing in sea urchins due to reduced oxygen (Low & Micheli, 2018, 2020; Ng & Micheli, 2020) as well as acidification (Brown et al, 2014; Donham et al, 2021); however, the impacts of pH, temperature, and DO on gastropods are more variable.…”

“…These responses are generally consistent with how organisms respond to temperature alone (i.e., across a range of non‐stressful temperatures, increasing temperature increases metabolism; Brown et al, 2004). Additionally, Low and Micheli (2020) found purple sea urchin, S . purpuratus , metabolism declined significantly during rapid pulses (3–6 h) of low DO similar to the levels in our study.…”

Variable coastal hypoxia exposure and drivers across the southern California Current

“…There is a very limited amount of hypoxia threshold data available for species in the California Current system. Those that exist for benthic invertebrates suggest that both adult and juvenile life stages may be tolerant of oxygen levels much lower than 2 mg/L, but that physiological, behavioral, and ecological processes may be impacted at much higher levels of dissolved oxygen (e.g., 5.5 mg/L in sea urchins) 18,50,51 . These vulnerabilities also vary among taxa.…”

A low‐cost modular control system for multistressor experiments

“…To showcase some of the possible types of experimental treatments using this control system, we report seawater manipulations from three different experiments that utilized the system's multiple‐stressor and dynamic exposure capabilities. The biological data and results of these experiments are reported elsewhere (Low and Micheli 2020; Ng and Micheli 2020). From the HMS implementation of this control system, we present data from experiments involving (1) simultaneous manipulations of DO and pH; and (2) fluctuating DO levels.…”

“…We maintained four treatments, with two replicate tanks per treatment and 12 organisms in each tank, over a 9‐week experiment: constant ambient (8.0 mg/L), constant low DO exposure (5.5 mg/L), 3‐h “intermittent exposure” in which DO levels fluctuated between ambient and low conditions, and 6‐h “intermittent exposure” in which DO levels fluctuated between ambient and low conditions. These treatments were designed to reflect aspects of DO variability in the field (Low and Micheli 2020). We implemented the intermittent exposure treatments by designing sequences of DO set points that alternated between ambient (8.5 mg/L) and low (5.5 mg/L) levels of DO.…”