Bivalve Mass Mortality Caused by Toxic Dinoflagellate Blooms in a Berriasian-Valanginian Lagoon, Bornholm, Denmark

Abstract

Thin lagoon-margin clay beds of the Berriasian-Valanginian Jydegdrd Formation contain monospecific assemblages of the bivalve Neomiodon angulata and the gastropod Viviparus cariniferus and polyspecific assemblages of Viviparus and juvenile bivalves. The three assemblage types were all formed by mass mortality. The N. angulata assemblage resulted from seasonal toxic dinoflagellate blooms, the V. cariniferus assemblage from desiccation of shallow ponds, and the polyspecific assemblage from storm washover. Paleoecologic, sedimentologic, and stable-isotope data show that the shallow bay or lagoonal waters fluctuated between almost fresh and brackish. Rapid changes in temperature and especially salinity can therefore be invoked to explain the mass mortality events. There is no direct evidence for hypersaline conditions. The beds with the Neomiodon angulata assemblages contain a highly unusual dinoflagellate flora consisting of only two species, one of which, Sentusidinium pelionense, constitutes over 99% of the assemblage. This suggests the occurrence of repeated virtually monospecific blooms of S. pelionense in the shallow, brackish water. Toxins formed during the bloom probably caused seasonal mass mortality of the opportunistic, salinity-tolerant Neomiodon faunas. The Viviparus cariniferus assemblages were probably killed during progressive desiccation of shallow lagoon-margin ponds. The polyspecific gastropod-bivalve assemblages were formed by washover during storms. All three types of assemblages were thus caused by mass mortality. The first two monospecific assemblages are non-transported census populations, while the third was formed by transport.