Abstract
Human activities can alter aquatic ecosystems through the input of nutrients and carbon, but there is increasing evidence that these pressures induce nonlinear ecological responses. Nonlinear relationships can contain breakpoints where there is an unexpected change in an ecological response to an environmental driver, which may result in ecological regime shifts. We investigated the occurrence of nonlinearity and breakpoints in relationships between total dissolved nitrogen (TDN), total dissolved phosphorus (TDP), and total dissolved carbon (DOC) concentrations and ecological responses in streams with varying land uses. We calculated breakpoints using piecewise regression, two dimensional Kolmogorov-Smirnov (2DKS), and significant zero crossings (SiZer) methods. We found nonlinearity was common, occurring in half of all analyses, with some evidence of multiple breakpoints. Linearity, by contrast, occurred in less than 14% of cases, on average. Breakpoints were related to land use gradients, with 34–43% agricultural cover associated with DOC and TDN breakpoints, and 15% wetland and 9.5% urban land associated with DOC and nutrient breakpoints, respectively. While these breakpoints are likely specific to our study area, our study contributes to the growing literature of the prevalence and location of ecological breakpoints in streams, providing watershed managers potential criteria for catchment land use thresholds.
Highlights
In ecology, we often measure the effects of a driver on a response to better understand ecological structure and function
We focused on relationships between three solutes: dissolved organic carbon (DOC), total dissolved nitrogen (TDN), and total dissolved phosphorus (TDP) and ecological responses of various stream communities and water quality parameters
We found that nonlinearity was prevalent, being detected, on average, in more than half of all tested solute-ecological responses by piecewise regression
Summary
We often measure the effects of a driver on a response to better understand ecological structure and function. There is a pressing need to better understand how human activities propel ecosystems past breakpoints that lead to ecological regime shifts Prior to assessing this need, it is important to first quantify breakpoints in the relationships between common variables, such as dissolved nutrients, carbon, other water quality parameters, and ecological responses. We focused on relationships between three solutes: DOC, total dissolved nitrogen (TDN), and total dissolved phosphorus (TDP) and ecological responses of various stream communities and water quality parameters Using these data, we had two primary objectives which differed from the original studies; 1) to quantify dissolved solute (nutrient and carbon) breakpoints across a variety of stream community and water quality responses using multiple breakpoint detection methods, and 2) to determine the level of anthropogenic land use associated with above- and below-breakpoint solute concentrations. We expected that the percentages of anthropogenic land use would differ depending on whether measured solute concentrations were above- or below-breakpoint, which would suggest a land use effect on ecological regime shifts
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