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Climate Change

Do increasing air temperatures mean increasing stream temperatures? Spoiler, yes!

Article: Rice, Karen C., and John D. Jastram. “Rising air and stream-water temperatures in Chesapeake Bay region, USA.” Climatic Change 128, no. 1-2 (2015): 127-138. DOI:10.1007/s10584-014-1295-9

 

Are air temperature and stream water temperature connected?

 

A stream in the Chesapeake Bay watershed. Credit: http://www.chesapeakebay.net/images/issues/Forest_Buffers_page_image.jpg

A stream in the Chesapeake Bay watershed.                      Credit: The Chesapeake Bay Program

While the consequences of climate change are numerous and complex, perhaps the most pressing and well-known effect is a gradual warming of the global air temperature. This warming also means that the surface water temperature of the ocean is gradually increasing. The Intergovernmental Panel on Climate Change estimated the global ocean is undergoing a temperature increase of 0.12°C per decade . This temperature increase may seem small, but it can have huge effects on aquatic ecosystems.

While most scientific literature and models agree that the increase in air temperature is largely controlling the increase in ocean surface water temperatures, there is a big debate as to whether air temperature has an effect on stream temperatures. Mathematical predictive models estimate air temperature should modulate stream temperatures while literature based on previous studies suggest that there is not much of a connection.

Temperature increases in streams which flow into estuaries can have huge impacts on both the biology and the physical make-up of these regions. For one, changes in stream temperature can cause shifts in the phytoplankton and aquatic vegetation populations. Additionally, warmer stream inputs can cause more stratification (when water layers have different densities and do not mix as well) and less dissolved oxygen, potentially leading to enhanced hypoxic (low oxygen) events.

This study by Rice and Jastram (2015) expands on previous work in the Chesapeake Bay watershed suggesting stream temperatures are increasing.

The approach: Data and Statistics

 

Figure 1: Maps showing Chesapeake Bay watershed, locations of sites, and temporal trends for 1960–2010 for a) monthly mean air temperature, and b) instantaneous stream-water temperature. Red symbols indicate increasing trends and blue symbols indicate decreasing trends. Solid and open symbols show trends that are statistically significant and not significant, respectively (from Rice and Jastram, 2015)

Figure 1: The Chesapeake Bay watershed, study sites, and temporal trends for 1960–2010 for a) monthly mean air temperature, and b) instantaneous stream-water temperature. Red symbols indicate increasing trends and blue symbols indicate decreasing trends. Solid and open symbols show trends that are statistically significant and not significant, respectively (from Rice and Jastram, 2015)

Temperature data from 129 stream stations and 85 meteorological stations were compiled for this study. These sites were selected since all has at least 90% of data coverage from 1961-2010, the time period used for this study. Monthly mean air temperatures were used, but the stream water temperatures were taken more sporadically (between 0-27 times per year). Anomalies, or differences from the mean, where calculated using a base period from 1971-2000.

Changes in stream water temperature were determined using a simple linear model. Thus, a positive slope would indicate that stream water temperatures were increasing. Additionally, probability density functions from the first half of the time series (1961-1985) were compared to the second half (1986-2010). This analysis provides insight into changes in the mean and standard deviation of the stream temperatures over this period.

The stream water temperatures where compared to the closest air temperature station to assess if air temperature is statistically related to stream water temperature.

What did they find?

Climate change is a complex process and does not manifest the same way in all locations. In this region, 61 of the 85 air temperature stations were determined to have an increasing trend over the 51 year time series. The median temperature increase for these stations together was 0.023°C per year. Similarly, 49 of the 129 streams investigated also had an increasing water temperature trend, with 8 stations showing a decrease in stream water temperature and the remaining having no trend. The median stream water temperature increase for all the streams with positive slopes was 0.012°C per year.

Figures 2: Probability density functions of anomalies for 1961–1985 and 1986–2010 relative to 1971–2000 for a) air temperature, and b) stream-water temperature (from Rice and Jastram, 2015).

Figures 2: Probability density functions of anomalies for 1961–1985 and 1986–2010 relative to 1971–2000 for a) air temperature, and b) stream-water temperature (from Rice and Jastram, 2015).

The probability density functions (Figure 2) also demonstrated that mean air and stream water temperatures were greater in 1986-2010 compared to 1961-1985. This again demonstrates that the stream temperatures were warmer in the near-present (1986-2010) than the past (1961-1985).

Another interesting factor was the geospatial trend. The increase in stream water temperatures was greater in the northern Chesapeake Bay watershed compared to the southern region. This shows that the stream water warming in the northern watershed may be occurring at a faster rate.

The researchers also applied a principal component analysis to predict how land-use in the Chesapeake Bay watershed may be influencing the trends in stream water temperature. They found that dams were one of the most influential factors; in other words, the more dams, the faster the increase in stream water temperature. Similarly, the land-use difference between open agriculture and a forest were also important. It was hypothesized that shading provided by a forest caused the stream water temperature increases to occur slower compared to a cropland.

Significance

 Both air and stream water temperatures were found to be significantly increasing in the Chesapeake Bay watershed. The increase in stream water temperatures will likely put pressure on the native flora and fauna in this region. For example, a shift to warmer temperatures will likely cause cool-water fish to find these streams less suitable while invasive species and pathogens may find these streams more appealing. Additionally, soluble reactive phosphorus becomes more soluble in water temperatures, creating an increased potential for eutrophication in Chesapeake Bay. Lastly, 85% of the variance in stream water temperatures could be explained by the air temperature, suggesting that air temperature is a primary modulator of the stream water temperatures.

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