The influence of a petrochemical discharge on the bioturbation and erosion potential of an intertidal estuarine mudflat (Humber estuary, UK)
The influence of sedimentary variables on the structure and function of infaunal estuarine and marine communities is well studied but less is known of the influence of biota on sediment properties. Feeding and burrowing activity, locomotion, the production of faecal pellets and biological secretions (bioturbation) have important implications for sediment structure, chemistry, transport characteristics and the flux of nutrients and contaminants. Although spatial and temporal patterns in bioturbation have been studied to some extent, little attention has been given to the effects of pollution. The present study examines the effects of an intertidal petrochemical discharge into the Humber estuary (UK), from BP chemicals (Saltend) Ltd on the structure and function of the communities. Field and laboratory techniques were used to determine the effects of community change on bioturbation potential. In addition, a laboratory flume was constructed to measure sediment erosion potential with field measurements being taken using a Cohesive Strength Meter (CSM). The physico-chemical properties of the sediment, changes to the infaunal community structure, bioturbation potential and the interaction of these variables were used to explain differences between the erosion potential of sediments subject to varying levels of contamination. The main study was carried out on the Saltend mudflats near Hull, with sites at various distances from the outfall being used. A further set of control sites on the adjacent, and largely unaffected, mudflat at Paull were also used. In terms of the sediment properties, sites closest to the outfall showed the greatest degree of anoxia and the highest chlorophyll-a and carbohydrate concentrations, with all three parameters being seasonally influenced. No consistent spatial or temporal patterns were found for any of the other parameters (water and organic content, particle size). Whilst the infaunal communities were characteristic of estuarine areas, macrobenthic community response followed the Pearson & Rosenberg (1978) model for organic discharges with high abundance and low species diversity being associated with the more polluted sediments. Close to the discharge, there was an impoverished community consisting predominantly of highly abundant oligochate worms. With increasing distance from the outfall, species diversity and biomass increased with Hediste diversicolor becoming increasingly dominant and the appearance of Corophium volutator, Streblospio shrubsolii and Macoma balthica. Bioturbation potential was significantly reduced (in terms of depth and burrow volume and density) by increasing effiuent concentrations and with proximity to the discharge. The diversity of both feeding and sediment modification guilds was also reduced as a result of the discharge. Both field and laboratory studies indicated a stabilising effect of this type of pollution. Using the CSM, critical shear stress values were found to be significantly lower from unpolluted sites, indicating higher erosion potential, than those from sites close to the discharge. As a result of this, the total mass of sediment eroded from unpolluted sites was significantly higher than that from polluted areas. A similar trend was observed in the laboratory with sediments treated with an effluent concentration of 32% being considerably more stable than untreated sediments. Flume studies also indicated the stabilising effect of pollution with suspended particulate matter (SPM) concentrations and mass of sediment being transported as bedload being significantly higher for unpolluted sediments. These differences in erosion potential were attributed to the direct effects of the effiuent on the physico-chemical properties of the sediment, the effects of the effiuent and sediment type on macrofaunal community structure and function and the differences in bioturbation potential between sites. The implications of these findings in the wider context of coastal management are discussed.