Exploring lake ecosystems: hierarchy responses to long‐term change?

Abstract

Shifts in climate regime have provoked substantial trophic-and species-dependent changes within ecosystems. With growing concerns of present global warming, we examined potential lake ecosystem responses, natural hierarchy responses (i.e. immediate responses at lower system levels as opposed to delayed responses at higher system levels), and possible shifts among abiotic (physics, nutrients) and biotic (phytoplankton, zooplankton) system components. Specifically, we analyzed decadal data collected from Mü ggelsee, a lake in Berlin, Germany, for climate-induced abiotic and biotic changes, their timing and type, and classified them as abrupt permanent, gradual permanent, abrupt temporary, or monotonic. We further categorized variable changes as a function of system hierarchy, including lake physics (ice, temperature, stratification), nutrients (phosphorus, nitrogen, silicate), plankton, and levels of integration (i.e. species, taxonomic groups, and total plankton). Contrary to current theory, data suggest abrupt responses did not occur in a hierarchy-dependent manner, nor was a clear pattern observed among functional system-based categories. Abrupt permanent changes were the most prominent response pattern observed, suggesting they may be driven by largescale climatic oscillations and by surpassed thresholds, as noted in previous case studies. Gradual changes coincided with affected abiotic parameters spanning an expansive time range; for example, climatic effects in spring preceded changes in nutrient limitation. Variables displaying no long-term changes pointed to compensation processes caused by, e.g., simultaneously acting forces of warming trends and climate-independent changes in trophic state. Nevertheless, the complexity of response patterns at the single system level manifested clear chronological regime shifts in abiotic and biotic parameters in spring and, to a lesser extent, in summer. With regard to projected global warming, the majority of currently unaffected system levels may face impending thermal thresholds, achievement of which would result in an accelerated shift in ecosystem state.

The response of temperate aquatic ecosystems to global warming: novel insights from a multidisciplinary project

“…Similarly, interannual variability of lake summer crustacean plankton responded more strongly to temperatures around the clear-water phase than to summer temperatures (Huber et al 2010). However, such critical time windows often cannot be defined by certain calendar months or weeks but by the hydrographic cycle or important cardinal events such as the timing of ice-off or the timing of the clear-water phase (Wagner and Adrian 2009a). To account for such dependencies and to provide a framework for comparison across ecosystems, Wagner et al (2012a) propose a classification scheme which combines hydrographically defined periods of the year with a cold-warm classification based on long-term temperature records.…”

Environmental Impacts—Lake Ecosystems

“…Stratification periods of more than three weeks caused a switch from a dominance of non-N-fixing cyanobacteria to a dominance of N-fixing cyanobacteria species, thus affecting not only biomass of cyanobacteria but also ecosystem functioning (Wagner and Adrian 2009a). In addition to warming-related changes in species composition ), habitat shifts northward toward temperate-zone lakes over the last few decades have been observed for Cylindrospermopsis raciborskii, an invasive freshwater cyanobacterium, originating in the tropics (Padisak 1997).…”

“…The crossing of critical thresholds may result in abrupt changes in particular elements of an ecosystem Scharfenberger et al 2013) or entire ecosystems-the famous example being the alternative stable states of clear versus turbid lakes (Scheffer and Carpenter 2003). Abrupt changes within ecosystems are already known under warming trends experienced in the recent past for a number of variables spanning abiotic and biotic components, such as nutrients or algal blooms (Wagner and Adrian 2009a; for review see Adrian et al 2009Adrian et al , 2012. The underlying forces are often unclear, but may involve competition for common resources and the crossing of critical thresholds in the abundance of conspecifics (Scharfenberger et al 2013) or multiple overlapping environmental forces (Huber et al 2008).…”

“…Exceeding direct and indirect temperature thresholds (length of thermal stratification) has been shown to trigger processes such as the onset and magnitude of cyanobacteria blooms (Wagner and Adrian 2009a;Huber et al 2012). Stratification periods of more than three weeks caused a switch from a dominance of non-N-fixing cyanobacteria to a dominance of N-fixing cyanobacteria species, thus affecting not only biomass of cyanobacteria but also ecosystem functioning (Wagner and Adrian 2009a).…”

“…Thus, in terms of their duration, seasons should be defined by cardinal events within the lake itself, rather than by fixed calendar dates. Important markers successfully used to define phenology-adjusted seasons in lakes include temperature thresholds, ice-off dates, the timing of the clear-water phase, and periods of stable thermal stratification (Rolinski et al 2007;Wagner and Adrian 2009a;Huber et al 2010). Wagner et al (2012) proposed a seasonal classification scheme tuned to specific hydrographic-sensitive phases for dimictic lakes across a latitudinal gradient: inverse stratification (winter), spring overturn, early stratification and the summer stagnation period.…”