Thresholds in Aquatic Communities in Response to Ex-Urbanization and Environmental Change

Principal Investigator:

Matthew Baker

Start/End Year:

2011 - 2014


University of Maryland, Baltimore County

Co-Principal Investigator:

Joseph Sexton, University of Maryland, College Park

Strategic focus area:

Viable coastal communities and economies


OBJECTIVES: The overall goal is to better understand specific consequences of watershed urbanization, and assess biological community responses to resulting changes in stream water quality. Our objectives are to (1) build a database of impervious surface expansion for the past 25 years from freely available, but as yet uncorrected satellite images, (2) track chemical changes in stream monitoring data associated with ex-urbanization, (3) link changes in watershed land cover and stream chemistry to biodiversity and species abundance, and (4) assess biotic responses for evidence of community thresholds. Our objectives will result in an extremely valuable geographic data set, inform water quality management and regulatory criteria, and provide a necessary exploratory analysis of spatial and chemical factors influencing aquatic communities through time and space.

METHODOLOGY: We will use well-established technology to create a spatio-temporal database of impervious surface area (ISA) for tributary watersheds of the Chesapeake Bay throughout central Maryland from 1985-2010 using filtered and corrected Landsat satellite images and reference ISA maps gathered from municipalities within the study area. Landsat images will be processed through the Landsat Ecosystem Disturbance Adaptive Processing System (LEDAPS) and fit to municipal ISA data by regression tree models that convert the surface reflectance into novel, spatio-temporally comprehensive ISA estimates. A subset of municipal data and high-resolution images will be used for validation. Watershed summaries of ISA will be used to assess changes in MBSS sentinel and MDNR core/trend sites through non-parametric change point analysis (nCPA) of water quality time series, and Threshold Indicator Taxa ANalysis (TITAN) will be used to detect biological responses associated with temporal changes in both ISA and water quality through time.

RATIONALE: Recent publications describing Threshold Indicator Taxa ANalysis (TITAN) reveal that existing methods for detecting biological responses to environmental change fail to detect thresholds in aquatic community structure. Using taxon-specific change-point analysis to decompose community signals, TITAN demonstrates that subtle increments in watershed urbanization or water quality can result in significant biodiversity losses. Because many gradient analyses assume spatial and temporal equivalence, they are biased by environmental covariates in space and insensitive to temporal dynamics. To fully understand biotic responses to urbanization it remains necessary to test spatial-gradient studies with time-series analysis. We propose to utilize novel techniques for tracking incremental patterns of ex-urbanization and both water quality and biological information available through the MBSS to assess bioassessment time series for signs of degradation and recovery. Our work has the potential to profoundly impact management and policy geared towards protecting water bodies from environmental degradation and detecting impacts of restoration.


This section describes how this project has advanced scientific knowledge and/or made a difference in the lives of coastal residents, communities, and environments. Maryland Sea Grant has reported these details to the National Oceanic and Atmospheric Administration (NOAA), one of our funding sponsors.

RECAP: Scientists developed a new analytical approach that improves detection and measurement of changes in biological communities in Chesapeake Bay tributaries over time and at different locations. This line of research may help researchers and local and state officials to plan better to prevent and manage the effects of development and urbanization on that plants and animals in Maryland’s streams.

RELEVANCE: The project’s overall goal is to better understand responses of biological communities to changes in stream water quality caused by urbanization and development in watersheds. Existing methods for detecting these responses have methodological limitations that limit their precision for representing/detecting changes over time, such as the population decline of a species, genus, or family. The new approach developed in this project complements and informs interpretation of those existing techniques.

RESPONSE: The principal investigators -- Matthew Baker of the University of Maryland, Baltimore County, and Joseph Sexton, University of Maryland, College Park – created an annual database of impervious surface maps from satellite images covering a 25-year period for six Maryland counties and Washington, D.C. The researchers linked these maps to existing data sets on stream chemistry and macroinvertebrate abundance in the Chesapeake Bay watershed. The scientists looked for evidence of thresholds, where incremental changes in urbanization and water quality were associated with disproportionately large changes in the composition of biological communities, as reflected in the occurrence and abundance of individual species or other taxonomic groups.

RESULTS: This study provided a proof of principle for a new analytical approach for accurately mapping urbanization and land development and tracking associated changes in biological communities. Of 70 sampling sites where macroinvertebrate species were measured, 16 sites showed evidence of abrupt changes consistent with a threshold response.

The researchers identified several specific cases where threshold responses indicated urbanization was a likely cause, yet other cases showed incremental responses to increasing development. The scientists are working to expand the model’s predictive power by incorporating a larger data set from more Bay tributaries in Maryland.

Knowledge of how ecosystems respond to land-use changes, and how rapidly, has profound implications for mitigation efforts and properly managing those ecosystems. The researchers have begun to disseminate their new analytical approach to local officials and natural-resource managers. It may prove useful for analyzing long-term trends in and effects of development, construction, population growth, and changes in regulatory policy.

The method has spawned research proposals and efforts in other major metropolitan regions of the United States.

Related Publications:

Baker, ME; Schley, ML; Sexton, JO. 2019. Impacts of Expanding Impervious Surface on Specific Conductance in Urbanizing Streams Water Resources Research55(8):6482 -6498. doi:10.1029/2019WR025014. UM-SG-RS-2019-09.

Song, XP; Sexton, JO; Huang, CQ; Channan, S; Townshend, JR. 2016. Characterizing the magnitude, timing and duration of urban growth from time series of Landsat-based estimates of impervious cover Remote Sensing of Environment175:1 -13. doi:10.1016/j.rse.2015.12.027. UM-SG-RS-2016-31.

Sexton, JO; Song, XP; Huang, CQ; Channan, S; Baker, ME; Townshend, JR. 2013. Urban growth of the Washington, DC-Baltimore, MD metropolitan region from 1984 to 2010 by annual, Landsat-based estimates of impervious cover. Remote Sensing of Environment129:42 -53. doi:10.1016/j.rse.2012.10.025. UM-SG-RS-2013-03.

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