Assessing the response of microphytobenthic biofilms to multiple stressors

Abstract

Ecological systems are exposed to a range of drivers, often referred to as stressors if detrimental or resulting from anthropogenic activities, which affect their functioning. To understand how these systems respond to stressors requires testing their effects on scaled ecosystems or biological components therein, or making predictions based on a mechanistic understanding of their modes of action. However, few stressors occur in isolation, and many – perhaps most – ecological responses are affected by non-additive interactions; net effects which are greater, or less than, the sum of the individual effects of multiple stressors.

Cumulative stressor effects on estuarine, intertidal microphytobenthic (MPB) biofilms were examined, as knowledge concerning responses to such effects in soft-sediment habitats is presently limited. The MPB perform key roles in estuaries, such as primary production and biogenic sediment stabilisation. Unexpected, combined stressor effects may propagate to higher trophic levels, and inhibit ecosystem functions provided by the MPB. Mesocosm experiments examined the effects of the herbicide glyphosate and nanoparticles of titanium dioxide, revealing variable stressor responses which appeared to be affected by contextual influences, such as abiotic environmental factors and biofilm condition. A meta-analysis of the experimental data suggested that the nanoparticles mitigated some of the detrimental effects of the herbicide on the biofilms.

Experimental findings, with MPB-driver relationship data, were used to predict the probability of detecting stressor effects in natural systems, as well as to estimate statistical power and inform experimental design. Field experiments demonstrated that whilst exposure to glyphosate was likely to detrimentally affect MPB assemblages in accordance with findings from mesocosm experiments, there was little evidence to suggest that antagonistic interactions occurred in a natural system. The ramifications of these results for ecosystem management are considered, and gaps in our understanding of cumulative stressor effects in estuarine systems and more broadly are discussed.