The most practical thresholds for use by policy makers and
stakeholders are those based on measurable control variables that
have a straightforward link to an ecosystem response (Table 1). The
easier the climate control variable is to quantify, as well as its effect
on ecosystem services, the more practical it is to implement policy
and effective monitoring measures. However, in practice direct
links between climate change control variables and ecosystem
responses are difficult to establish. Anthropogenic climate change
is quantified by several parameters that could be used as control
variables for defining the thresholds of freshwater ecosystems, e.g.,
atmospheric and surface water temperature, the length and onset
of seasons, and precipitation (Table 1). However, in most cases,
multiple control variables acting together must be considered. For
example, riverine ecosystems are strongly and simultaneously
influenced by variables like streamflow, erosion rates, temperature, and concentrations of micro-pollutants; though how to
account for all the variables in a threshold type model is not always
straightforward (Groffman et al., 2006). Other control variables
that are indirectly influenced by climate change as well as other
anthropogenic processes include water volume, salinity, dissolved
oxygen (DO) and pH. All of these parameters, in isolation or in
combination, could influence ecosystem services. In Section 4, we
review the state-of-the-art understanding of how shifts towards
ecological thresholds for freshwater ecosystems can be influenced
directly or indirectly by climate change. A schematic overview of
this presentation is provided in Fig. 2.