Ashley N. Meara, Sarah C. Gray, Environmental and Ocean Sciences, University of San Diego
The concentration of dissolved oxygen (DO) in the oceans has decreased by about 2% over the past 50 years due to rising global temperatures and models predict further declines this century. However, little research has been done to examine how benthic foraminifera communities vary across this changing DO gradient offshore of southern California. Foraminifera serve as important bioindicators to assess environmental health, reconstruct past climates and predict future climate change, as their tests are composed of calcium carbonate with morphology and porosity unique to their individual rate of gas-exchange. Given these factors, foram shells provide the most valuable geochemical archive of ocean conditions and climate spanning the last 100 million years, and play a vital role in the ocean carbon cycle. These shells are preserved in marine sediments as calcite, the stable polymorph of calcium carbonate.
The southern California continental margin is characterized by variable bathymetry, isolated low oxygen basins and high productivity, serving as an ideal location to investigate the impacts of variance in dissolved oxygen and climate on benthic foraminifera. The objectives of this research were to 1) characterize how foraminifera communities vary spatially and with depth and DO and 2) to examine if there have been changes in these communities over the past 20 years. In 2018-19, sediment samples from the sediment-water interface were collected using a multicorer at five sites of variable depth (200-900 m) from 10 to 170 kilometers offshore of southern California. Two of these sites had been sampled annually from 2001-2012. Though the communities at all sites included most taxa, the relative percent and abundance of each taxa varied between sites. Sites with reduced DO were dominated by Uvigerina and Bolivina, with a large decrease in Cassidulina. Generally, the same morphotypes were seen across two decades of observation at two sites without a clear pattern of secular variability. Further studies should be done to examine how hypoxic-associated species vary across the full depth range of the oxygen minimum layer and whether these taxa are developing morphological adaptations to cope with the changing environment.
