Has Dreissena polymorpha, an older immigrant in a coastal Baltic lagoon, responded to appearance of a new non-indigenous dreissenid?

Although a non-indigenous species, Dreissena polymorpha (zebra mussel) has constituted a constant and abundant component of the Szczecin Lagoon biota. A congeneric dreissenid, D. rostriformis bugensis (quagga), was first recorded in the north of the Lagoon in 2014, already in abundance. In summer 2015, the quagga dominated over the zebra mussel in abundance and biomass. Quagga individuals there were much larger than those of D. polymorpha, while elsewhere in the Lagoon the zebra mussel still dominated the dreissenid assemblages. In summer 2016, the population structure of D. r. bugensis was substantially modified and its abundance was greatly reduced. D. polymorpha became the dominant again. In terms of biomass, after the initial domination, D. r. bugensis biomass declined substantially. The zebra mussel, although represented mostly by small individuals, has become a biomass dominant too on account of its high abundance. As of mid-2016, there has been no clear peak (evidencing the appearance of juveniles) in the quagga size distribution. The recent dreissenid samples show a large share of empty quagga shells, an effect not observed earlier. Therefore, it is plausible that the older immigrant, so far at least, has outcompeted the newcomer.