Fig 1.
Distribution of brittle stars and oxythermal conditions over the past decade on the Caribbean coast of Panama.
(Left column) Pie charts show proportion of individuals from two species of brittle star, Ophioderma cinereum (orange) and Ophiomastix wendtii (blue), found at each survey site. Animals were collected for laboratory experiments from 3 m at the inner bay (red asterisk) and outer bay (blue triangle) reef sites. (Center column) Daytime temperature (top) and pO2 (bottom) surveyed on September 25th, 2017, at 3 m across 83 sites. (Right column) Time series of temperature and pO2 over the mean seasonal cycle (upper panels) from a decade of weekly measurements 2010–2020 and the mean diurnal cycle (lower panels) from 37 d of hourly data collected between Oct.–Nov. 2019 (light blue shaded box). Solid colored lines in time series show average values and shading denotes the standard deviation. Base map from: https://stridata-si.opendata.arcgis.com/datasets/SI::bathymetry-of-the-republic-of-panama.
Fig 2.
Physiological tolerances, behavior, and survival of O. cinereum (orange) and O. wendtii (blue).
(a) Low pO2 limits (Pcrit) under ambient (29°C) and high temperatures (32°C). (b) Non-lethal upper thermal limits (CTmax) and (c) Factorial Aerobic Scope (FAS) under normoxia or low oxygen/hypoxia, 19.5 or 6.5 kPa, respectively. (d) Time to reach physical exhaustion in either normoxic conditions (light colors) or hypoxic conditions (dark colors) for each species. (e) Percentage of individuals that ‘escaped’ from experimental hypoxic conditions (< 6.5 kPa) by climbing out of a low oxygen layer. (f) Percentage of living individuals after experiencing low pO2 (1.0 kPa) while resting, with all O. wendtti individuals dying after a 3.1 h duration. This time period is equivalent to one extreme low pO2 event on the reef. Significantly different mean values (p < 0.05) in different treatments for the same species are indicated by capital letters, while significantly different mean values in the same treatment among different species are indicated by lowercase letters. Pairwise comparisons were conducted using the Estimated Marginal Means test with Least Significant Difference test correction. Error bars indicate ± SE.
Fig 3.
Distribution and time series of Φ throughout the reef for O. cinereum and O. wendtii.
The Metabolic Index is computed from spatial and temporal observations of temperature and pO2 (see Fig 1) and the temperature-dependent hypoxia traits for each species: O. cinereum (c, d), O. wendtii (e, f). The solid lines show average values and shading denotes the standard deviation around the mean. The Factorial Aerobic Scope (FAS) of each species (dashed lines) denotes the value of Φ below which maximum metabolic rate at the observed temperature would be limited by local oxygen availability. Base map from: https://stridata-si.opendata.arcgis.com/datasets/SI::bathymetry-of-the-republic-of-panama.
Fig 4.
Bivariate frequency distribution of temperature and oxygen, from one decade of weekly daytime sampling.
Orange coloration represents the environmental space of the inner bay, with panel (a) showing all measurements made from each temperature and oxygen combination during the 10-y sampling period. This is statistically modelled in panels b-d (Eq 2, S4 Fig). Light yellow colors indicate low occurrences or rare conditions and red colors indicate common, frequent (i.e., average) conditions. The ecophysiological exclusion thresholds (Φ = Φcrit) are drawn over the environmental space for both O. cinereum (cyan lines) and O. wendtii (blue lines). Exposure to conditions that limit activity due to hypoxia are represented by the fraction of time that conditions fall below the species-specific thresholds (Φ < Φcrit). (c, d) Projected future bivariate frequency distributions assuming temperatures increase by 1 or 3°C, respectively. The frequency of stressful conditions (i.e., hypoxia exposure, Φ < Φcrit) below the cyan and blue lines becomes more frequent as temperature rises.
Fig 5.
Impact of future warming on hypoxia exposure across species traits.
(a, b) Exposure to hypoxia for both brittle star species, computed as a fraction of time with Φ < FAS in the current climate (dots) and under three different climate warming scenarios (curves). Observed relationship between temperature and pO2 extrapolated to warmer conditions (Fig 4) yields a large increase in hypoxia exposure (red line), that disappears when increased pO2 variance is neglected (green and blue lines). (c) Hypoxia exposure in the inner bay for a projected warming of 3°C across the range of active hypoxia tolerance and temperature sensitivity traits (Eo and Ao) for known marine species globally (black dots) [7], and newly measured O. cinereum (orange circle) and O. wendtii (blue circle). Exposure on the reef would currently exclude most known species at a 5% exclusion threshold (black line). If future pO2 variance is constant, exclusion of additional trait space from exposure is minimal (dashed red curve), whereas the observed trend toward increased pO2 variance erodes habitat for a broad swath of currently viable ecotypes (solid red curve), including O. cinereum.