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A workshop was held in Wageningen, The Netherlands, in September 2017 to collate data and literature on three aquatic ecosystem types (agricultural drainage ditches, urban floodplains, and urban estuaries), and develop a general framework for the assessment of multiple stressors on the structure and functioning of these systems. An assessment framework considering multiple stressors is crucial for our understanding of ecosystem responses within a multiply stressed environment, and to inform appropriate environmental management strategies. The framework consists of two components: (i) problem identification and (ii) impact assessment. Both assessments together proceed through the following steps: 1) ecosystem selection; 2) identification of stressors and quantification of their intensity; 3) identification of receptors or sensitive groups for each stressor; 4) identification of stressor-response relationships and their potential interactions; 5) construction of an ecological model that includes relevant functional groups and endpoints; 6) prediction of impacts of multiple stressors, 7) confirmation of these predictions with experimental and monitoring data, and 8) potential adjustment of the ecological model. Steps 7 and 8 allow the assessment to be adaptive and can be repeated until a satisfactory match between model predictions and experimental and monitoring data has been obtained. This paper is the preface of the MAEGA (Making Aquatic Ecosystems Great Again) special section that includes three associated papers which are also published in this volume, which present applications of the framework for each of the three aquatic systems.

Effective management of introduced species requires an understanding of their effects on native species and the processes that structure the habitat. The introduced European polychaete Sabella spallanzanii dominates epifaunal assemblages in south-eastern Australia, yet little is known about how it affects the structure of the surrounding assemblages. The present study investigated the differences between infaunal assemblages in the presence and absence of S. spallanzanii using clumps of real and mimic polychaetes. Both the real and mimic clumps had the same effect on an existing assemblage with fewer numbers of small crustaceans in the sediment under the clumps. The effects of S. spallanzanii on infaunal colonisation and larval abundances above and below the S. spallanzanii canopy were also investigated. Larval effects varied among taxa, depending on position (above and below the canopy) for bivalve larvae and presence/absence of S. spallanzanii for gastropod larvae. There was no effect of the S. spallanzanii clumps on infaunal colonisation. These results suggest that the effects of S. spallanzanii on larval abundances and colonisation may not be as significant as the effects on post-colonisation processes that structure macrofaunal assemblages in soft sediment habitats.

Understanding biological responses to nutrient enrichment under different environmental conditions is integral for the effective management of eutrophication in coastal environments. However, current conceptual models of nutrient enrichment are limited as they are based on studies that only consider a single source of nutrients, when in reality it is more likely that enrichment is a result of multiple sources. Here, we test the hypothesis that biological responses to nutrient enrichment in intertidal mudflat assemblages depend on the source by comparing enrichment from a controlled release fertilizer with that from decomposing macroalgae. Sediment at two sites in Port Phillip Bay, Victoria, were dosed with the different nutrient sources and monitored through time. After six weeks, the macroalgae-enriched plots had significantly higher abundances and biomass of some taxa of deposit-feeding polychaetes. In the fertilizer-enriched plots, the porewater nutrients increased but there was no detectable change in abundances or biomass of infauna, suggesting that the nutrients did not assimilate into the foodweb. The rate of assimilation of anthropogenic nutrients potentially occurs over longer time scales compared with the rapid assimilation and biological responses to decomposing macroalgae. Responses to the different nutrient sources provide new insights into the complexity of nutrient enrichment models that are applied to the management of aquatic ecosystems worldwide.

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