Status and Trend

Status describes the condition of a particular indicator at one moment in time. Trend describes change in an indicator over time. The status of an indicator often depends on its quantity or size defined as increase minus decrease over a previous time interval. Trend is a statistically meaningful departure from a previous condition measured over an interval of time between two or more previously documented conditions. Trend analyses describing changes in Chesapeake Bay watershed stream health span time intervals of five or more years.

Water Flow

Chesapeake Bay health is greatly affected by freshwater flows from streams draining its watershed. Variations in the amount of freshwater flow entering the Bay can change water temperature and salinity levels, affect the amounts of nutrients, sediment, and contaminants delivered to non-tidal and tidal waters, and change the amount of instream habitat available to freshwater flora and fauna. Knowledge of daily, monthly, and annual amounts of freshwater flow in streams, interpreted from continuous analyses of USGS stream flow data, is essential to understand ecosystem conditions and evaluate status and trends in stream health within the Bay watershed (U.S. Geological Survey, 2022; fig. 1).

Water Quality

The health of aquatic communities in the Chesapeake Bay and its tributaries is linked to water quality. Some key indicators linked to water quality are nitrogen, phosphorus, and suspended-sediment loads (status) and changes in loads (trends). Data continuously compiled since 1985 from a non-tidal network of 123 USGS monitoring sites across 5 States provide nitrogen, phosphorus, and suspended-sediment load observations for approximately 78 percent of the 64,000 square-mile Chesapeake Bay watershed (Mason and others, 2021b). Long-term trends in water quality calculated for sites with at least 30 years of data, and short-term water-quality trends are calculated for sites with at least 10 years of data. Short-term water-quality trends provide reliable annual and monthly load estimates if at least five years of data are available.

Water Temperature

Stream productivity and the metabolic rate of aquatic organisms are dependent on temperature (Cushing and Allan, 2001). Stream temperature can fluctuate naturally due to atmospheric conditions, topography, stream flow and streambed heat transfer, or as the result of human influences, such as land use, thermal pollution, water storage impoundments, and climate change (Caisse, 2006; Webb and others, 2008). Water temperature data are derived from a multi-agency dataset of available stream temperatures (Clune and others, 2023). Data selected and analyzed for status and trends include historical stream temperature time -series that span the varying latitude, elevation, and temperature ranges of the Bay watershed (fig. 2). Data selection is dependent upon the overall data quality and representative daily observations available for aggregation (daily mean, minimum, and maximum values).

Hydromorphology

Instream physical habitat is the product of a complex interplay between flow, sediment, geomorphology, and vegetation. This interaction, referred to as hydromorphology (Orr and others, 2008), supports a range of processes at multiple spatial and temporal scales that can create a diverse patchwork of habitats, water depths, flow types and velocities, substrate, and channel forms. These ecologically relevant habitats can support overall stream health and a diverse ecological community. Status and trends in hydromorphology are evaluated using two main data sources: (1) rapid-habitat assessment data from multiple sources (Chesapeake Bay Program, 2021), and (2) analyses of USGS streamgages for changes to channel geometry and local hydraulics (U.S. Geological Survey, 2023; fig. 3).

Salinity

Freshwater salinization, or the increase in ionic concentrations in freshwater ecosystems, is a key water-quality issue in the Chesapeake Bay watershed. Higher-than-normal salinity levels affect freshwater ecosystems in many ways, one of them is the disruption of osmotic-pressure regulation in aquatic organisms (Griffith, 2014). Although salinity is typically low in freshwater streams in the mid-Atlantic region, many sources of salinity are found in the watershed, including applications of agricultural limestone, road deicer material (fig. 4), mining leachate, weathering of human-made surfaces, household water softeners, and other point-source discharges. Specific conductance data (an indicator of salinity) are compiled from a multi-agency dataset (Fanelli and others, 2023). This dataset includes data collected at various frequencies (for example, annually, monthly, 15-minute intervals), which is used to quantify a set of ecologically relevant metrics to compute status and trend analyses.

Toxic Contaminants

More than three-quarters of tidal waters in the Chesapeake Bay are partially or fully impaired by toxic contaminants (Chesapeake Bay Program, 2022b, Chesapeake Bay Program, 2022c); an example being compounds resulting in toxicity effects to fish which could lead to human exposure. Because there are no Bay-wide Total Maximum Daily Loads (TMDLs) for individual toxic contaminants or contaminant classes, these data vary more than data for other domains with regards to sample collection and analysis methodology, frequency, and location. These data have often been collected with a different purpose than assessment of status and trends, and data may be geographically focused. Further complicating the understanding of toxic contaminants in the Bay watershed is the variability in the fate and transport of the individual compounds in contaminant classes, the prevalence of the contaminants in different landscapes (agricultural and urban), the inclusion of legacy pesticides (polychlorinated biphenyls [PCBs] and organochlorides), current use pesticides (mercury, polycyclic aromatic hydrocarbons [PAHs]), and emerging contaminants such as per- and polyfluoroalkyl substances (PFAS). Select toxic contaminants, such as mercury and PCBs, drive most fish-consumption advisories in the watershed. This results in state datasets for contaminants in fish tissue over time (Banks and others, 2022). Data for other constituents and in other environments are less common, and are typically associated with specific, localized impairments or contaminated sites.

Although methods exist to assess status and trends of individual toxic contaminants at management-relevant scales, the inconsistencies across collection agencies and time coupled with the limited sample frequency in some media, make calculations of trends infeasible. In limited geographically focused areas (for example, in reaches with fish-consumption advisories for specific toxic contaminants) within the Chesapeake Bay watershed, the quantity of contaminants residing within the specified media (for example, within fish tissue) and rates-of-change in quantity over time, are identified. Even in these more data-dense media and locations, input data limits the statistical confidence in calculations.

Biological Aquatic Communities

Two freshwater assemblages: (1) benthic macroinvertebrates, and (2) fish, are key indicators of stream health and habitat condition. These two assemblages can also be used to measure progress in improving stream health and fish habitats. Two Chesapeake Bay watershed-wide compilation efforts provide data to address this need: (1) benthic macroinvertebrate data compiled during the generation of the Chesapeake basin-wide index of biotic integrity for stream macroinvertebrates (Chessie BIBI; Smith and others, 2017); and (2) fish sampling data compiled for development of an assessment of fish habitat (Krause and Maloney, 2021). These two datasets create numerous approaches for which site-specific measures of benthic macroinvertebrates and fishes may be characterized, including metrics describing assemblage composition (life histories and tolerance classifications), taxonomic composition, multi-metric composite indices, and presence or abundance of specific species (fig. 5).