Humans have been disturbing Earth’s landscape ever since we began constructing simple dwelling structures in the Paleolithic (400,000–500,000 years ago). The pace accelerated with the invention of the wheel in the middle Holocene (5,000–7,000 years ago), which enabled humans to travel farther and faster.
But humans haven’t been changing land only since antiquity. With the construction of the first man-made canal prior to the Iron Age some 3,600 years ago, we started to control the flow of water for agricultural practices and began perturbing Earth’s hydrologic system [Bishop et al., 2017]. This “replumbing” of Earth’s surface, recently referred to as anthroturbation [Zalasiewicz et al., 2014], rapidly expanded during the Industrial Revolution (beginning in about 1800 CE) and the “Great Acceleration” (about 1950 CE). The pattern of hydrological landscape modification—or replumbing—continues to this day.
Humanity’s replumbing of Earth goes hand in hand with the recent concept of the Anthropocene [Crutzen and Stoermer, 2000]. In fact, we posit that new boundaries with steep gradients in aquatic systems, created in the Anthropocene, are providing corridors for rapid organismal and biogeochemical change that warrant examination with a new perspective.
Thus, further understanding the Anthropocene must involve answering some key questions: How has this manipulation of the surface of our planet affected nutrient levels, species diversity, and evolution itself in aquatic ecosystems, and how will these changes continue into the future as climate changes?
To help answer these questions, we propose three specific ways forward.
1. Formally Designate Aquatic Critical Zones as a Focus of Study
Much of the foundational thinking on changing landscapes in the Anthropocene is focused on terrestrial systems. This terrestrial focus is, in part, an outgrowth of one synoptic goal of the National Science Foundation’s critical zone observatories program, which was chartered “to develop terrestrial observatories that could document and form predictions of the multi-scale and less visible transport of energy and material, and evolution of the Earth’s critical zone” [Chorover et al., 2011].
A critical zone is a “heterogeneous, near-surface environment in which complex interactions involving rock, soil, water, air, and living organisms regulate the natural habitat and determine the availability of life-sustaining resources” [National Research Council, 2001]. Although the general concept of critical zones has included aquatic systems, we propose that these areas are dynamic and vulnerable corridors of change, worthy of their own designation as aquatic critical zones (ACZs).
What would need to be done to fully recognize the importance of ACZs and understand their response to change? As a starting point, we see two domains for work:
- Natural ACZs have always existed, but the anthropogenic replumbing of ACZs has resulted in alterations of their spatiotemporal patterns. Landscape responses to these alterations, linked with molecular and organismal mechanisms, need greater consideration in global biogeochemical models.
- The impacts of anthropogenic and climate effects on ecosystems, on scales from genes to landscapes, have been recognized recently [Bishop et al., 2017; Scheffers et al., 2016]. However, these scales must be integrated; genetic change must be considered in conjunction with biogeochemical cycles and ecosystem fluxes, particularly in systems as vulnerable as ACZs.
Although we focus here on the adaptive biogeochemical and organismal ramifications of global change on ACZs, it is important also to recognize potential hydrologic, geomorphic, and ecosystem services that could be linked to define ACZs as a critical research area.
2. Develop a Human Footprint Index for Aquatic Critical Zones
In recent decades, alterations in the transition zone between terrestrial ecosystems and the open ocean, termed the land-ocean continuum, have produced numerous effects in the ACZs. These include hypoxic zones, eutrophication, and altered sediment transport (Figure 1).
Climate change has already affected 81% of ecological processes in marine systems and 74% of these processes in aquatic systems [Scheffers et al., 2016]. In the future, these effects will likely have complex interactions with climate change, which could result in climate feedbacks, ecosystem regime shifts, and potentially catastrophic impacts on the human systems that depend on ACZs.
Engineering projects have addressed issues at the human-aquatic interface, although often with unintended effects. For example, about 87% of Earth’s land surface is connected to the ocean by rivers. The approximately 16 million dams now in existence [Lehner et al., 2011] affect these connections between rivers and oceans; dams have altered biogeochemical processes, enhanced greenhouse gas production, and reduced sediment delivery to vital deltaic systems [Bianchi, 2016; Giosan et al., 2014]. The increased dominance of artificial structures in aquatic systems has been recognized as “ocean sprawl” [Bishop et al., 2017].
Recent work in terrestrial systems quantified anthropogenic effects on mammalian movement with a human footprint index (HFI) that included as key components predator-prey interactions, nutrient cycling, and disease transmission [Tucker et al., 2018]. The development of an equivalent index for aquatic systems would be valuable to quantifying and assessing anthropogenic impacts to ACZs.
Such an HFI could include changes in nutrients, suspended particulates, and water residence times as key components. Analogues to the terrestrial HFI described above, including predator-prey interactions and pollution and disease transmission, could also be included.
Greater cross-disciplinary research that links specific anthropogenic disturbances in ACZs with organismal and biogeochemical changes (Figure 2) would facilitate greater communication between evolutionary geneticists, environmental microbiologists, geochemists, and ecosystem scientists.
3. Study the Genetic Effects of Manipulating ACZs
Vicariance biogeography is a field that looks at the ways in which organism populations become separated and species diverge. At its inception, studies in this field described how large-scale geographic barriers (e.g., mountain ranges and rivers) affected the geographic spread of genetic characteristics [Wiley, 1988].
Anthropogenic manipulation of ACZs results in the formation of new barriers and corridors [Bishop et al., 2017] in Earth’s plumbing. At times, these modifications can reach the scale of natural geomorphic features associated with the vicariance biogeography of multicelled animals.
Physical barriers or corridors from anthropogenic disturbance directly affect the movements of living organisms, but indirect effects, such as alterations in nutrient availability, have also been observed. For example, recent research suggests that large-scale disturbance from such artificial structures as levees and dams in ACZs may have the capacity to affect the drivers of evolutionary change in multicellular organisms, as in the case of altered migration patterns of marine mammals [Bishop et al., 2017]. Drivers of such change include mutation, migration, genetic drift, and selective pressures. What’s more, these drivers could produce dynamic and complex interactions with a changing climate.
In addition to altering the evolutionary trajectories of higher organisms, anthropogenic disturbance in ACZs is also likely to influence microbial community structure and function. Microorganisms, as the engines of Earth’s biogeochemical cycles, clearly are at the front lines of response to changing biogeochemical gradients in ACZs. However, microbes will likely respond to anthropogenic change differently than multicellular creatures because of complexities associated with microbial biogeography. Plus, humanity’s replumbing of Earth’s surface is likely to result in changes in community composition and function.
Environmental factors can influence microbial communities on a landscape scale, but evaluating their response to global change is challenging. New frameworks are emerging that consider microbial response (e.g., community composition) versus effect (e.g., functional traits) in the context of global change. Linking alterations in community composition and function with ecosystem-scale processes may improve our ability to predict ACZs’ response to global change, although some microorganisms may exhibit response traits that may not translate to changes in ecosystem-scale processes.
Earlier studies called for further integration of the geosciences and microbial ecology [Moran et al., 2013], an exhortation we support and now extend to include an emphasis on ACZs in a changing climate.
We propose that the integration of microbial ecology with chemical biomarker studies will allow for a comprehensive evaluation of biogeochemical cycles and microbially driven processes, providing data and hypotheses necessary for critical insights into the complex interacting processes underlying the global impacts of anthropogenic change in ACZs.
Critical Needs in Aquatic Critical Zones
Numerous studies have evaluated ecosystem responses to climate-driven changes, including alterations in temperature, acidity, salinity, and hydrology. One consistent theme is the susceptibility of ACZs to altered regimes and their potential nonlinear response to climate change.
Recent studies all point to how ACZs are highly susceptible to change and that their response to change not only may alter the composition and function of their resident organisms but also will have disproportionate impacts on neighboring human populations that depend on them. These studies, however, only scratch the surface.
We urgently need more work to investigate how human influences are affecting ACZs to get a full picture of the extent of the Anthropocene and better predict how these systems may respond to change.