Taxon-specific prey response to the invasion of a pelagic invertebrate predator, revealed by comparison of pre- and post-invasion time series

Quantification and attribution of the food web changes associated with the invasion of non-indigenous species in the marine realm often remain a challenge. One of the pelagic non-indigenous species of concern in the recent history of aquatic bioinvasions is the predatory cladoceran Cercopagis pengoi, which invaded the Baltic Sea in the early 1990s. While several studies have reported immediate declines in abundances of its potential prey, the long-term effects of C. pengoi on the food webs remain to be examined. Based on the long-term time series (1968–2018) in the Gulf of Riga (Baltic Sea), we found significant declines in abundance of the cladoceran Pleopis spp. and copepod Eurytemora affinis by 90 and 80%, respectively, are associated with the invasion of C. pengoi as well as significant alterations in seasonal abundance patterns of Pleopis spp., E. affinis and cladoceran Bosmina spp. The invasion of the non-indigenous predator has led to the changed prey abundance–temperature relationships. Special caution was taken in data preprocessing, to minimize the likelihood that observed changes in the zooplankton prey could be associated with factors other than the invasion of C. pengoi, such as temperature and storminess.