Long-term ecosystem change: how environmental pressures combine

Posted on July 2, 2014

Understanding complex interactions between the many human-induced pressures that affect freshwater ecosystems is a major challenge, not least because in many cases the combined impact of several stressors can’t be predicted by simply adding the single-stressor effects. In many lake environments, human activities have led to significant declines in levels of calcium, magnesium, phosphorus and metals, and increases in temperature, sodium, chloride, dissolved organic carbon, ammonia and the prevalence of invasive species. The causes of such trends include climate change, nutrient inputs and lakeshore development. In a novel study, Canadian biologists used a replicated large-scale field survey to disentangle the single and interacting effects of long-term environmental changes on lake biota. They focused on zooplankton because these animals are important links in the food web and are useful sentinels of aquatic response. In 2004-2005 the research group resampled 34 lakes (0.02 – 6.8 km2 in area) that had originally been sampled in the 1980s, and compared the two sets of data, using stepwise linear regression to quantify the effects of different factors on the abundance and diversity of zooplankton taxa. They found that over the twenty-year period total zooplankton abundance declined, while zooplankton diversity increased. The relative abundance of large water fleas increased while the contribution of cyclopoid crustaceans dropped. These changes were associated with falling acidity and a 20% decline in total phosphorus. Increasing chloride concentrations were also implicated, having a stronger impact in this multiple-stressor situation than would otherwise be expected. By interacting with water quality impacts, climate change, in the form of increasing temperatures, behaved as a threat multiplier. And the establishment of an invasive species, the spiny water flea Bythotrephes longimanus, interacted with total phosphous to reduce the relative abundance of small cladocerans. In summary, it was clear that interactions between human-induced stressors were widespread and had a strong, large-scale influence on aquatic communities.

Reference: Palmer, M.E. & Yan, N.D. 2013. Decadal-scale regional changes in Canadian freshwater zooplankton: the likely consequence of complex interactions among multiple anthropogenic stressors. Freshwater Biology 58, 1366–1378. http://onlinelibrary.wiley.com/doi/10.1111/fwb.12133/pdf