Global warming benefits the small in aquatic ecosystems

Abstract

Understanding the ecological impacts of climate change is a crucial challenge of the twenty-first century. There is a clear lack of general rules regarding the impacts of global warming on biota. Here, we present a metaanalysis of the effect of climate change on body size of ectothermic aquatic organisms (bacteria, phyto- and zooplankton, and fish) from the community to the individual level. Using long-term surveys, experimental data and published results, we show a significant increase in the proportion of small-sized species and young age classes and a decrease in size-at-age. These results are in accordance with the ecological rules dealing with the temperature-size relationships (i.e., Bergmann's rule, James' rule and Temperature-Size Rule). Our study provides evidence that reduced body size is the third universal ecological response to global warming in aquatic systems besides the shift of species ranges toward higher altitudes and latitudes and the seasonal shifts in life cycle events.

Fig. 1.

The tested hypotheses regarding the impact of warming on body size at different biological scales.

Fig. 2.

Location of the study areas. 1–4: Long-term survey of freshwater fish communities in large rivers. 5–7: Long-term survey of brown trout populations. 8: Long term survey of North Sea fish community. 9–10: Long term survey of herring and sprat populations in the Baltic Sea. 11–13: Sampling of bacteria and phytoplankton communities and of Pseudocalanus sp. (zooplankton) in temperature-controlled mesocosms. Numbers in brackets refer to published climate–size relationships reviewed in this article (16, 17).

Fig. 3.

Mean effect sizes (i.e., mean weighted temporal trend statistic S; ±95% confidence intervals). Negative or positive trend values indicate temporal decrease or increase, respectively. Mean temporal trends are significant if their 95% confidence intervals did not contain 0. Community body size shift and species shift hypotheses were tested by using 4 freshwater fish communities. To test the species shift hypothesis, small species were defined as species with a maximum size below the first quartile of the maximum size of all of the species in the community. Proportions of small species are calculated in terms of species richness (SR) and abundances (Ab.). Population body size shift and population age-structure shift hypotheses were tested by using 28 and 18 fish populations, respectively. Size-at-age shift hypothesis was tested by using 28 age classes. Significantly different means for marine (M) vs. freshwater (F) populations are represented. To increase readability some effect sizes are divided by a factor x (indicated in the figure as /x).

Fig. 4.

Change in size structures under warming. (A) Cell size of bacteria subjected to different level of warming (+0, +2, +4, and +6 °C) compare to a reference thermal regime (dT) [means (open and closed circles), standard errors (gray lines), and raw data (closed rectangles) in the different replicates are represented]. (B) Mean cell size of phytoplankton at different level of warming (dT) and different light conditions [percentage of the natural light intensity above cloud cover (Io); 16% Io: hanging triangles; 32% Io: circles; 64% Io: standing triangles] (after figure 3c of ref. 16). (C) Effect of maximum length on distribution trends (expansion or shrinkage) of fish species in the North Sea during the past 20 y (after figure 4b of ref. 17). (D) Size of female adult Pseudocalanus sp. at different level of warming (dT) (symbols as for A).