Sediment contributing invasive dreissenid species in a calcareous shallow lake-possible implications for shortening life span of lakes by filling.
Although the ecosystem transforming impact of the invasive dreissenid mussels has been widely reported in short-to-mid time scale studies, little is known about the contribution of the spent shells to sediments accumulating on the lake bottom. The question whether the shell production significantly reduces the lifespan of the lake by increasing sedimentation rate is particularly interesting in those shallow lakes where the calcium supply is sufficient to maintain the high mussel biomass production permanently, and where the alkaline water does not favor shell dissolution. Lake Balaton, a large calcareous, shallow lake in Central Europe invaded by dreissenids (Dreissena polymorpha, Dreissena rostriformis bugensis), provides an ideal testing ground for this scenario. Therefore, we made calculations based on recent population abundance datasets (2000-2018), estimated the whole habitable, hard surface coastline and the muddy bottom of the pelagic area which is also gradually becoming inhabited by D.r. bugensis, using high resolution aerial photographs and analyzing seismic sections. We created four scenarios: (1) if no dreissenids are present (applying basic sedimentation rate); (2) if D.r. bugensis had not been introduced to the lake (only D. polymorpha); (3) if D.r. bugensis occupies the hard surfaces of the coastline (the current dominant situation); (4) if D.r. bugensis colonizes the entire lake bottom (a probable future model). Different sedimentation rates obtained from the literature were used to model the filling of Lake Balaton. The shell production of the new invader, D.r. bugensis can shorten the lake's lifespan by one to two-thirds, depending on the model, and whether the mussel density currently observed at the shoreline is extended to the whole lake bottom. Attention is called to shallow calcareous lakes with low pre-invasion sedimentation rates in which the shell contribution of invasive mollusks has the potential to shorten lifespan.