Euro-limpacs Deliverables

ABSTRACT - DELIVERABLE 127

Report on temporal variability of contemporary phytoplankton and benthic macroinvertebrate assemblages of boreal forest lakes

As a result of concerted international efforts to protect and restore natural resources threatened by acidification, rates of acid deposition have decreased markedly and widespread increases in surface water pH have been attributed to the improved air quality. Surface water chemistry exerts a major control on aquatic biodiversity, thus it is anticipated that improvement in surface water quality (e.g. raised pH) should result in biological recovery, albeit with inherent time lag responses, but records of biologic recovery are scarce so far.

Recovery can be defined as the movement towards a pre−acidified or reference state as stress decreases. Accordingly, finding no difference in water chemistry /or biology between previously acidified and reference lakes might be interpreted as a recovery from acidification.

This report intends to evaluate the effects of selection of organism group/trophic level and habitat on detecting recovery of lake ecosystems from acidification and to investigate the recovery rates of different indicators (water chemistry, biota), trophic levels (phytoplankton producers, macroinvertebrate consumers), and habitats (pelagic, benthic).

Several of the metrics analysed here showed significant, positive trends during the 16−year study period. Phytoplankton taxon richness and diversity recovered quickly and were strongly correlated to positive changes in pH in both reference and acidified lakes.

Littoral macroinvertebrate taxon richness and diversity also increased in reference and acidified lakes.

In contrast, sublittoral and profundal macroinvertebrate communities of both lake types showed clear negative trends, indicating that they may be more influenced by climate−related variables acting on habitat quality (e.g. ambient oxygen and temperature) than by changes in lake acidity. Moreover, this indicates that choice of not only organism but also habitat may be important in detecting biological recovery. Significant differences between acidified and reference lakes were obvious at the end of the study period, indicating that the acidified lakes have not fully recovered to the expected ecological targets.

Our findings indicate that recovery is a complex process influenced by multiple spatial and temporal factors and suggest the use of multiple organism groups and trophic levels to better detect and understand the processes and changes within the lake ecosystem.

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