Stress in estuarine communities: Lessons from the highly-impacted Bilbao estuary (Spain)

The highly-impacted Bilbao estuary was studied toquantify the response of estuarine communities tostress. Total heterotrophic biomass, a measure ofsediment infauna, declined sharply from the mouth tothe upper reaches of the estuary. Differentstatistical methods selected the depletion ofdissolved oxygen (DO) at the bottom of the watercolumn as the main correlate with the observedpattern. Biomass size-spectra (BSS) of estuarinecommunities suggested that `unimpacted' sites at themouth of the estuary had a high proportion of biomassin the larger size classes under regular andpredictable environmental conditions. In contrast, inthe impacted middle and inner parts of the estuarybiomass was concentrated in the smaller size classes,indicating a clear effect of stressful environmentalconditions on larger bodied organisms.A progressive increase of DO values has beenforecasted by the local Water Authority after thecompletion of a sewerage plan for the whole estuary,and predictions of water quality criteria were used toestimate community biomass figures following thismanagement. In order to predict the recovery at thelevel of individual species, several bioassays withthe native species Nereis diversicolor andScrobicularia plana were performed. Results obtainedwith the latter, more-sensitive species, suggest thatchemical contamination in sediments from the Bilbaoestuary has the potential to cause deleterious effectson indigenous benthic organisms, which could delayfaunal recovery in some parts of the estuary oncewater-oxic conditions improve.