Benthic Opportunistic Polychaete/Amphipod Ratio: An Indicator of Pollution or Modification of the Environment by Macroinvertebrates?

The development of sensitive indicators reflecting the state of the environment is an important issue for the monitoring of marine ecosystems. The spionid polychaete Marenzelleria arctia and pontoporeiid amphipod Monoporeia affinis are common macrobenthic species in the brackish Gulf of Finland (the easternmost Baltic Sea). This paper aims to apply the Benthic Opportunistic Polychaetes Amphipods (BOPA) and Benthic Opportunistic Annelid Amphipods (BO2A) indices based on the polychaete/amphipod ratio as indicators of the environmental state in this region. We analyzed the relationships between environmental variables and benthic indices based on samples from two benthic surveys in 2019 (10 sites) and 2020 (9 sites). The coastal sites were characterized by worse water quality (i.e., the higher concentration of hydrocarbons, total phosphorus and chlorophyll-a), but cleaner sediments (i.e., total phosphorus, organic carbon, polyaromatic hydrocarbons and trace metals) than offshore sites. The BOPA and BO2A correlated positively with the level of water pollution and negatively with sediment pollution. The activity of the benthic organisms seems to strongly influence the concentration of contaminants in sediments, so this factor hinders the use of BOPA and BO2A indices for the assessment of bottom sediments quality in the eastern Baltic Sea. At the same time, this study shows that BOPA and BO2A indices can be used in assessing water quality.