Paleoecological data are unique historical archives that extend back far beyond the last several decades of ecological observations. However, the fossil record of continental shelves has been perceived as too coarse and incomplete to detect processes occurring at decadal scales relevant to ecology and conservation. Here we show that the youngest (Anthropocene) fossil record on a continental shelf of the Adriatic Sea provides decadal-scale temporal resolution that is adequate for documenting an abrupt ecological shift affecting benthic communities during the 20th century. The magnitude and the duration of the 20th century shift in body size of a dominant bivalve species (Corbula gibba) is unprecedented given that this species was consistently small throughout the Holocene in the whole northern Adriatic Sea. The size shift coincided with compositional change of the benthic community, with the median per-assemblage abundance of C. gibba increasing from ~25% to ~70% in the late 20th century, and occurred at sites that experienced at least one hypoxic event per decade in the 20th century. This regime shift, which coincided with mass mortality of competitors and predators associated with higher frequency of seasonal hypoxic events, may reflect ecological release. The observed body size shift is coupled with a decline in the depth and rate of bioturbational mixing. This decline in burrowing benthic organisms resulted in the improved stratigraphic resolution of fossil assemblages, making it possible to detect sub-centennial ecological changes in the stratigraphic record on continental shelves.

Body size (length) of bivalve Corbula gibba from sediment cores in the northern Adriatic in the ESM 2 and the compilation of genus-level molluscan abundance data in death and living assemblages on the basis of published data in the ESM 6. See the ESM 1 for details. ESM 3-5 are R language scripts where body-size data  (ESM 3) are analyzed and the effects of time averaging on the detection of regime shift is simulated (ESM 5). The function for analogue matching is in the ESM 4.

 

See the ESM 1 for details