Phytoplankton adapt to changing ocean environments

Abstract

Model projections indicate that climate change may dramatically restructure phytoplankton communities, with cascading consequences for marine food webs. It is currently not known whether evolutionary change is likely to be able to keep pace with the rate of climate change. For simplicity, and in the absence of evidence to the contrary, most model projections assume species have fixed environmental preferences and will not adapt to changing environmental conditions on the century scale. Using 15 y of observations from Station CARIACO (Carbon Retention in a Colored Ocean), we show that most of the dominant species from a marine phytoplankton community were able to adapt their realized niches to track average increases in water temperature and irradiance, but the majority of species exhibited a fixed niche for nitrate. We do not know the extent of this adaptive capacity, so we cannot conclude that phytoplankton will be able to adapt to the changes anticipated over the next century, but community ecosystem models can no longer assume that phytoplankton cannot adapt.

Keywords: climate change; evolution; phytoplankton; realized niches.

Fig. 1.

Monthly environmental conditions averaged over the upper mixed layer (1, 7, 15, and 25 m depth) from the CARIACO Ocean Time-Series Program: temperature (°C), irradiance (mol⋅m^–2⋅d^–1), and nitrate concentration (µmol⋅L^–1). The vertical dotted line is drawn at the boundary (January 1, 2004) between the cool and warm periods. The straight lines are linear regressions: temperature = (24.6 ± 0.3) + (0.09 ± 0.03) t, R² = 0.05, P < 0.005; irradiance = (18.1 ± 0.9) + (0.05 ± 0.11) t, R²= 0.001, P = 0.65; nitrate = (1.06 ± 0.14) – (0.045 ± 0.017) t, R² = 0.04, P = 0.03, where t is time in years since January 1, 1996, errors are one SE, and the shaded region is the 95% confidence interval on the line. The R² is very low because of the tremendous interannual variation relative to the trend.

Fig. 2.

The change in the distribution of mean niches in response to warming for species before and after January 1, 2004, in the CARIACO Ocean Time-Series in pairs of panels: temperature, irradiance, and nitrate concentration. (Top) Mean niche before January 1, 2004, with species only observed in this early, cool period shown in dark blue. (Bottom) Mean niche after January 1, 2004, with species only observed in this later, warm period shown in dark red.

Fig. 3.

Change in mean niche for the 49 species observed in both the warmer and cooler periods as a function of the mean niche in the early, cooler period for temperature, irradiance, and nitrate concentration. There is an approximate linear relationship for temperature and irradiance indicated by the linear regressions for temperature [ΔT = (0.43 ± 0.06) – (0.38 ± 0.11) (T_early – 24.74); R² = 0.19; P < 0.002] and for irradiance [ΔE = (0.56 ± 0.16) – (0.55 ± 0.12)(E_early –15.80); R² = 0.30; P < 0.001, errors are one SE]. T_early is the temperature niche from the early, cooler period with mean over species of 24.74 °C, and E_early is the irradiance niche from the early, cooler period with mean over species of 15.87 mol⋅m^–2⋅d^–1 (see Table 1). The symbol color indicates the functional group of each species: diatom (green, open circles), dinoflagellate (dark green, filled circles), cyanobacteria (cyan), coccolithophorid (black), and silicoflagellate (gray). There are no significant differences in responses to changing conditions across the taxonomic groups.