Figure 1.

Environmental fluctuations occur across length scales and timescales in diverse habitats. (A) Kilometer-scale algal bloom on Western Lake Erie. Courtesy of NOAA (https://oceanservice.noaa.gov/facts/hab-solutions.html). (B) A microscale nutrient hotspot is created as a marine diatom secretes metabolites into the local environment. Chemotactic bacteria accumulate around the hotspot, as shown by the time projection (Smriga et al. 2016). (C) A temporal map of European river flooding events, which bring water on kilometer scales to local soils (Blöschl et al. 2017). Colors indicate time of year and arrows indicate how yearly events change over time as the climate changes. (D) Microscale image of chemicals seeping into a soil-like porous medium. The void spaces between soil grains (gray) are filled with either air (black) or water (blue): the water saturation and pore size affect how fluid and chemicals are transported through the medium. Chemical heterogeneities (colorbar) arise due to the formation of preferential paths (Jiménez‐Martínez et al. 2017). White arrow indicates flow direction, as chemical seeps into soil. (E) Microscale image of the gut epithelium and its 100–500-µm-sized crypts (black arrows) (Shamsuddin, Phelps and Trump 1982). Each crypt is lined by mucus-secreting cells and this mucus contributes to maintain strong gradients in oxygen, antimicrobials and flow, accessible to resident bacteria (blue asterisks).