Global assessment of species-specific habitats of planktonic foraminifera : an ecosystem modeling approach

Abstract

Over the last few million years, the Eartha s climate system has changed continuously on decadal to millennial time scales. Past climate conditions have been reconstructed based on fossil evidence of marine microorganisms, such as planktonic foraminifera. Planktonic foraminifera exhibit species-specific seasonal production patterns and different preferred depth habitats. To precisely reconstruct past climate conditions these spatial and temporal variations within the individual species distribution have to be considered. In this regard, an ecosystem modeling approach can help to gain a better knowledge about species-specific habitat shifts under climate change. In this study, a planktonic foraminifera model is used to predict monthly concentrations of the colder-water species Neogloboquadrina pachyderma, Neogloboquadrina incompta, and Globigerina bulloides, and of the warm-water species Globigerinoides ruber (white) and Trilobatus sacculifer throughout the world ocean. In particular, the seasonal distribution of the polar species N. pachyderma in the surface mixed layer of the North Atlantic Ocean during the last glacial period was investigated. In response to changes in the sea ice cover and food supply, the peak timing of N. pachyderma is shifted from the last glacial period to modern conditions by several months. However, for a more realistic simulation of species-specific habitats, the planktonic foraminifera model PLAFOM was adapted to allow for resolving the vertical dimension. This new model version estimates the foraminiferal biomass of the colder- and warm-water species as a function of temperature, nutrition, competition, and in particular light. To predict the species concentration over different water depths the model code of the improved version of the planktonic foraminifera model was added to the code trunk of the ocean component of a global earth system model. This model produces seasonally and vertically coherent distribution patterns that are in good agreement with available observations without any explicit parameterization in the vertical dimension regarding their ontogeny. The colder-water species exhibit a seasonal cycle in their depth habitat in the polar and subpolar regions: during the warm season they occur at mid-depth, while during the cold season they ascend through the water column and are found in the near-surface layer. The warm-water species show a less variable depth habitat and occur almost consistently close to the sea surface throughout the year in the tropics and subtropics. This emergence of species-specific depth habitats in the model that are consistent with available observations indicates that the population dynamics of planktonic foraminifera species may be driven by the same factors. Here the impact of global warming on the speciesa spatial and seasonal distribution patterns has been investigated. In response to changes in the temperature and food supply, the habitat range as well as the peak timing of both the colder-water and warm-water species will likely shift. In general, planktonic foraminifera do not respond uniformly to climate change due to their different ecological preferences. Their habitat is altered in time and space, and depending on the ambient conditions either warm-water or colder-water species benefit strongly from these changes. Knowing how individual planktonic foraminifera species adapt to changing environmental conditions can help to obtain more precise estimates of the geological past and can provide implications for future climate change.