Spatially variable effects of copper on sessile invertebrates across a marina

Heavy metals often occur at elevated concentrations in bays and estuaries surrounded by urbanised areas and can cause substantial ecological changes to marine communities. The toxic effect of heavy metals can be modified by a several physico-chemical processes in the marine environment. These processes can vary greatly over small scales, so it is likely that the ecological effects of heavy metals on marine biota also vary substantially over similar spatial scales. We used a manipulative field experiment to assess the spatial variation in the effect of copper on sessile invertebrates within a marina and tested whether this variation was influenced by variation in localised water flow. Effects of copper were not consistent over relatively small spatial scales (i.e. metres) for all sessile invertebrate species. Three of the twenty sessile invertebrates from our experimental assemblages showed significant small scale variation in the effect of copper at the scale of metres. For these taxa, the effect of copper varied in magnitude and direction. We had predicted that the effect of copper would diminish with increased water flow, but this did not occur for any taxa. We suggest that variations in the presence of organic or inorganic compounds or in pH within the marina may provide alternative explanations for the spatially variable effects detected in this study. Our study's findings highlight the need for future field assessments of the complex physical, chemical and biological influences on the ecological effects of heavy metals, particularly at small spatial scales. Such studies are the key to improving our prediction about the effects of heavy metals on marine organisms and aid in better characterisation of the risk of heavy metals in the marine environment.