Impacts of past climate variability on marine ecosystems: Lessons from sediment records

Sediment records and historical accounts are the only source of information on marine ecosystem status and variability at higher trophic levels before scientific observations began. Most reconstructions of paleoenvironments and higher ecosystem levels are based on measurable proxies in these sedimentary archives, ideally those that are directly and robustly related to environmental or biological variables of interest. Depositional characteristics place some constraints on the types of environments where proxy-based investigations into the dynamics of higher levels of marine ecosystems can be done and many proxy methods have large error ranges. But although sedimentary proxy records are often plagued by dating uncertainties and although proxies for higher ecosystem levels are scarce, the few studies performed in lakes and coastal upwelling environments have already yielded some fascinating insights into spatial and temporal scales of ecosystem variability in the past. Most available records are low-pass filtered with a bias to longer-term changes, which limits the window of detection to (interannual)-decades to centuries or even longer. Available data suggest pervasive bottom-up control of marine ecosystems in response to relatively subtle changes in external forcing, which telescopes into very high variability on higher trophic levels. Many paleo-curves suggest cyclicities or thresholds, which point to either cyclic external drivers or non-linear reactions. In particular, response of higher trophic levels to changes at the base of ecosystems appears to be highly non-linear and shows variations of several orders of magnitude, as exemplified by fish stocks in several areas (Baltic Sea, North Sea, upwelling systems, lakes in Canada) from historical records, observations and sediment archives.