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. 2017 Apr;20(3):553-567.
doi: 10.1007/s10021-016-0040-9.

Watershed Land Use and Seasonal Variation Constrain the Influence of Riparian Canopy Cover on Stream Ecosystem Metabolism

Jeremy M Alberts  1 , Jake J Beaulieu  2 , Ishi Buffam  1
Affiliations

Watershed Land Use and Seasonal Variation Constrain the Influence of Riparian Canopy Cover on Stream Ecosystem Metabolism

Jeremy M Alberts et al. Ecosystems. 2017 Apr.

Abstract

Ecosystem metabolism is an important determinant of trophic structure, nutrient cycling, and other critical ecosystem processes in streams. Whereas watershed- and local-scale controls on stream metabolism have been independently investigated, little is known about how controls exerted at different scales interact to determine stream metabolic rates, particularly in urban streams and across seasons. To address this knowledge gap, we measured ecosystem metabolism in four urban and four reference streams in northern Kentucky, USA, with paired closed and open riparian canopies, during each of the four seasons. Gross primary production (GPP), ecosystem respiration, and net ecosystem production (NEP) were all best predicted by models with season as a main effect, but interactions between season, canopy, and watershed varied for each response. Urban streams exhibited higher GPP during most seasons, likely due to elevated nutrient loads. Open canopy reaches in both urban and forested streams, supported higher rates of GPP than the closed canopy which reaches during the summer and fall, when the overhead vegetation shaded the closed reaches. The effect of canopy cover on GPP was similar among urban and forested streams. The combination of watershed and local-scale controls resulted in urban streams that alternated between net heterotrophy (NEP <0) and net autotrophy (NEP >0) at the reach-scale during seasons with dense canopy cover. This finding has management relevance because net production can lead to accumulation of algal biomass and associated issues like nighttime hypoxia. Our study suggests that although watershed urbanization fundamentally alters ecosystem function, the preservation and restoration of canopied riparian zones can provide an important management tool at the local scale, with the strongest impacts on stream metabolism during summer.

Keywords: ecosystem respiration (ER); gross primary production (GPP); phenology; riparian land cover; stream metabolism; urban streams.

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Figures

Figure 1.
Figure 1.
Conceptual diagram of drivers of stream ecosystem metabolism by spatial scale. The regional template is expected to shape climate, vegetation, and topography. Watershed-scale factors are an important determinant of nutrient availability and the hydrologic regime, while terrestrial organic matter (OM) and light conditions are most strongly controlled by local-scale riparian canopy characteristics. Seasonality is expected to influence all of these factors, leading to potential shifts in the interaction between watershed and local scale controls on GPP and ER. Dashed line reflects that hydrology is expected to influence the retention of terrestrial organic matter within stream channels. “Temp” represents stream-water temperature. Arrow size roughly represents the magnitude of influence.
Figure 2.
Figure 2.
Rates of stream Gross Primary Productivity (GPP) by watershed, canopy, and season. Dots represent the rate for individual study reaches during the specific sampling season. Lines connect the mean value for each canopy treatment by watershed type.
Figure 3.
Figure 3.
Rates of stream Ecosystem Respiration (ER) by watershed, canopy, and season. Dots represent the rate for individual study reaches during the specific sampling season. Lines connect the mean value for each canopy treatment by watershed type.
Figure 4.
Figure 4.
Rates of stream Net Ecosystem Productivity (NEP) by watershed, canopy, and season. Dots represent the rate for individual study reaches during the specific sampling season. Lines connect the mean value for each canopy treatment by watershed type.
See this image and copyright information in PMC

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