Abstract
Ecological communities are increasingly exposed to multiple chemical and physical stressors, but distinguishing anthropogenic impacts from other environmental drivers remains challenging. Rarely are multiple stressors investigated in replicated studies over large spatial scales (>1000 kms) or supported with manipulations that are necessary to interpret ecological patterns. We measured the composition of sediment infaunal communities in relation to anthropogenic and natural stressors at multiple sites within seven estuaries. We observed increases in the richness and abundance of polychaete worms in heavily modified estuaries with severe metal contamination, but no changes in the diversity or abundance of other taxa. Estuaries in which toxic contaminants were elevated also showed evidence of organic enrichment. We hypothesised that the observed response of polychaetes was not a ‘positive’ response to toxic contamination or a reduction in biotic competition, but due to high levels of nutrients in heavily modified estuaries driving productivity in the water column and enriching the sediment over large spatial scales. We deployed defaunated field-collected sediments from the surveyed estuaries in a small scale experiment, but observed no effects of sediment characteristics (toxic or enriching). Furthermore, invertebrate recruitment instead reflected the low diversity and abundance observed during field surveys of this relatively ‘pristine’ estuary. This suggests that differences observed in the survey are not a direct consequence of sediment characteristics (even severe metal contamination) but are related to parameters that covary with estuary modification such as enhanced productivity from nutrient inputs and the diversity of the local species pool. This has implications for the interpretation of diversity measures in large-scale monitoring studies in which the observed patterns may be strongly influenced by many factors that covary with anthropogenic modification.
Highlights
The intense and extensive development by humans across the planet has subjected much of the world’s biological diversity to multiple severe anthropogenic stressors [1,2,3] including frequent chemical and physical disturbances, which are often concentrated in urban and industrial areas [4]
We investigated changes in taxa richness and abundance between modification categories because these measures are closely linked to contaminants [31,43], and are robust to taxonomic changes when sampling across large spatial scales e.g. estuaries [44]
Sediment chl-a, porewater ammonia and total organic carbon (TOC) were elevated in heavily modified estuaries suggesting significant inorganic and organic enrichment of these sediments (Table 1, Figure S2)
Summary
The intense and extensive development by humans across the planet has subjected much of the world’s biological diversity to multiple severe anthropogenic stressors [1,2,3] including frequent chemical and physical disturbances, which are often concentrated in urban and industrial areas [4]. Recent efforts to monitor ecological impacts have focused on integrating information collected from chemical and ecological monitoring into a more holistic understanding of ecosystem condition [18,19,20] How this information is collected remains fiercely debated and ranges from laboratorybased experiments, field-based manipulative experiments (generally small-scale), and small- and large-scale observational studies (either snapshot or examining change through time). These each have advantages and disadvantages [21,22], but to distinguish the effects of anthropogenic stressors from other environmental drivers [23] and inform management-oriented biomonitoring requires multiple lines of evidence and experimental studies in order to reliably interpret patterns of community composition and diversity [22,24,25]
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