Forecasting the combined effects of urbanization and climate change on
stream ecosystems: from impacts to management options
Kären C. Nelson1*, Margaret A. Palmer1,2, James E. Pizzuto3, Glenn E. Moglen4, Paul L. Angermeier5,6,
Robert H. Hilderbrand7, Michael Dettinger8,and Katharine Hayhoe9
*Corresponding Author – kanelson@umd.edu
1 Department
of Entomology, University of Maryland, College Park MD, 20742; 2 Chesapeake Biological
Laboratory, University of Maryland Center for Environmental Science, Solomons, MD 20688; 3Department
of Geology, Penny Hall, University of Delaware, Newark DE 19716; 4 Department of Civil and
Environmental Engineering and National Center for Smart Growth Research and Education, University of
Maryland, College Park MD 20742; 5 U.S. Geological Survey, Virginia Cooperative Fish & Wildlife Research
Unit, Virginia Tech, Blacksburg VA 24061; 6 The Unit is jointly sponsored by the U.S. Geological Survey,
Virginia Tech, Virginia Department of Game and Inland Fisheries, and Wildlife Management Institute; 7
University of Maryland Center for Environmental Science, Appalachian Laboratory, Frostburg MD 21532 ; 8
US Geological Survey, Branch of Western Regional Research, Scripps Institution of Oceanography, La Jolla
CA 92093; 9 Department of Geosciences, Texas Tech University, Lubbock TX 79409
Running Title: Urbanization, Climate Change and Stream Ecosystems
Word Count: 9115
Summary
1. Streams collect runoff, heat, and sediment from their watersheds, making them highly vulnerable to
anthropogenic disturbances, such as urbanization and climate change. Forecasting the effects of these
disturbances using process-based models is critical to identifying the form and magnitude of impacts likely to
be experienced by stream ecosystems. Here, we present a new model that integrates five submodels
(downscaled climate projections, stream hydrology, geomorphology, water temperature, and biotic responses)
to predict responses of stream fish growth and reproduction as a function of climate and urbanization
stressors. We illustrate its utility and application to management using Piedmont headwater streams in the
Chesapeake Bay watershed of the U.S.
2. For the biotic submodel, we employ a unique hybrid of population dynamics and habitat suitability
modelling to model entire fish assemblages based on readily available biotic information (including
preferences for habitat, temperature, and food, and characteristics of spawning) and day-to-day variability in
stream conditions. The model takes into account such disparate processes as stream warming due to
impervious surface, removal of riparian vegetation, and/or increased air temperatures; increased productivity
and decomposition; loss of cooler water for spawning; flashier flow regimes; and higher siltation rates
affecting both feeding and spawning.
3. We illustrate the use of our model by comparing predictions for two scenarios, a baseline scenario (low
level of urbanization with 10% impervious surface, 20% forest cover, remainder split between low-density
residential and agricultural; no on-going construction; and present-day climate); four climate scenarios
(Hadley under a medium-high (A2) emissions scenario, Hadley under a medium-low (B2) emissions scenario,
PCM A2 and PCM B2); and the same four climate change scenarios plus urbanization (30% impervious
surface, 2% forest cover, significant construction activity).
4. For our study sites, all four downscaled climate models (Hadley A2, B2; PCM A2, B2) predict a significant
warming of stream temperatures which would be exacerbated by urbanization. Short-term spikes in
temperature in urban stream could be as high as 7ºC immediately following rain storms. Three of the four
climate models did predict stormier precipitation events that move more water – up to 27% more under
climate change alone and in combination with urbanization, up to 45% more water during high flows.
Sediment movement on the streambed is expected to increase under all scenarios with the greatest increase
occurring in the scenario that included both climate change and increasing urbanization.
5. In terms of fish response, the urbanization scenario resulted in stress on 8 of 39 species while climate
change affected the majority of species (22 to 29, depending on the climate scenario used). The scenario
combining urbanization and climate change increased the number of stressed species two to threefold,
suggesting that considerable change in community composition and loss of diversity could occur under the
generally accepted future scenarios of more urbanization and warmer climates in the mid-Atlantic. Almost
every recreationally important species (trout, bass, yellow perch, and bluegill) had declines in the growth and
reproduction indices ranging from 40% to 90%. Among the 10 currently most common species, six showed
decreases of at least 10% in at least one score.
6. The interaction of climate change and population growth may entail high costs for headwater streams,
including a loss of ecosystem structure and services. Our model suggests that the combination of
urbanization with climate change will be more dangerous than climate change alone. On a local scale,
stakeholders cannot control global climate drivers, but they can mitigate land use impacts. Therefore we
recommend a number of proactive measures to provide ecological insurance against species loss or
population declines, including re-growth of riparian vegetation, improved or retrofitted stormwater
management, controls on sediment, and (where possible) land preservation programs. These proactive
measures will need to be implemented at large spatial scales to work effectively, and delays will inevitably
exacerbate the impacts of both climate change and urbanization on these headwater systems.
Keywords
Climate change; land use change; urbanization; fish assemblage; headwater stream; siltation; flow regime;
temperature regime