Driving questions: (1) What are the primary constraints on metabolism in headwater streams, and (2) how are these constraints changing with climate change?
Climate change at Hubbard Brook = warmer and wetter winters
Warmer and wetter winters dominate the climate record at Hubbard Brook Experimental Forest (New Hampshire). The mirror lake ice record documents reduced ice cover by about 4.4 days every decade and increased variability in the trend (Data: ) Similarly, the snow free period has increased by about 20 days since 1959 (Groffman et al., 2012).
Though these effects on productivity (less ice and more snow free days) have been evaluated in terrestrial and lake systems, I am interested in how warmer and wetter winters impact stream metabolism.
What does warmer winters mean for streams?
We hypothesize that temperature, light, and nutrient availability are the primary constraints on productivity at Hubbard Brook. Further, headwater streams have a seasonal reliance on each of these constraints.
Stream's seasonal dependence on light
To evaluate if streams experience variable light regimes year to year, we modeled predicted photosynthetically active radiation (PAR) entering the surface of the stream using StreamLight (Savoy et al. In Review). StreamLight uses MODIS satellite imagery, stream width, and approximate tree height to model incident light entering the stream.
We found that: a) “peak stream light” (blue dots) occurs between January and May and b) the cumulative amount of light that enters a stream annually is variable. For example, if we just look at stream light as a constraint for productivity, 2010 seems like a bad year for productivity.
Spring is the "lightest" time of the year
When we plot the total variability of stream light (gray bars), you can see that it variable annually since 2001. However, if we remove the portion of light that enters the stream in the winter, we are left with “biological” stream light (light blue bars). Of that portion, 53% of the light enters the stream in the spring.
All this to say, if you are an autotroph, then spring should be an important time for you.
Climate change is shifting the light regimes for streams
To translate this light availability hypothesis into a conceptual model, you can imagine the times of year where a stream is truly “active”: when the stream is warm enough, has available nutrients (snowmelt), and well-lit. These “windows of metabolic opportunity” are bookended by snowmelt and canopy closure in the spring, and litterfall and persistent snowpack in the fall.
Streams that exhibit window-like metabolism
To see if some streams do exhibit "window-like" metabolism, I looked through the StreamPULSE dataset (Data: Appling et al. 2018 data.streampulse.org). Of the "small streams" (<10 m) in the dataset, 27% had >50% gross primary productivity (GPP) in the spring, while only 9% had >50% ecosystem respiration (ER) in the fall.
These windows of opportunity are variable year to year
At Hubbard Brook, the spring window ranged from 28 to 85 days
The spring window was calculated as the last day of snow to 75% canopy closure. Data: ___
While, the fall window ranged from 37 to 105 days
The fall window was calculated as 75% litterfall to first day of persistent snowpack. Data: ___
More frequent storms could interrupt the spring window
Zooming back into the stream, if most of the productivity in the stream is from algae, algae may be susceptible to disturbance. Therefore, disturbance events may interrupt productivity.
While intermittent snowpack may open small windows of opportunity
Using camera traps, we can understand more about the ice phenology of the stream and compare this to measurements of productivity.