ANTHROPOGENIC CHANGE VS NATURAL VARIABILITY: LESSONS LEARNED FROM THE CONTINUOUS PLANKTON RECORDER

• Main Author: Rivero-Calle, Sara
• Other Authors: Del Castillo, Carlos, Gnanadesikan, Anand
• Format: Thesis
• Language: English
• Published: Johns Hopkins University 2016
• Subjects: plankton oceanography ecology biogeochemistry; North Atlantic
• Online Access: http://jhir.library.jhu.edu/handle/1774.2/60651

The Continuous Plankton Recorder (CPR) surveys have sampled plankton in the North Atlantic using ships-of-opportunity since 1958. The resulting unique, methodologically-consistent, multi-decadal datasets can distinguish between anthropogenic and natural variability. I show an example of a plankton group that may be driven by anthropogenic effects, another one that is likely due to natural variability and discuss changes in the North Atlantic plankton community structure in the context of multiple stressors. As anthropogenic CO2 emissions acidify the oceans, calcifiers such as coccolithophores are expected to be detrimentally affected. Our first study shows that CPR coccolithophore occurrence increased basin-wide from ~2 to over 20% from 1965-2010. Using Random Forest models to examine >20 possible environmental drivers, I found that CO2 and the Atlantic Multidecadal Oscillation (AMO) were the best predictors. Since coccolithophore photosynthesis is strongly carbon-limited, increasing CO2 and temperature may be accelerating its growth rate. It is generally assumed that the distribution of the marine nitrogen fixer Trichodesmium, is restricted to warm, (sub)tropical oligotrophic waters. However, the second study demonstrates that Trichodesmium are widely distributed in “cold” North Atlantic waters. Trichodesmium presence near the British Isles increased approximately five-fold during the 1980s-1990s. Using NCEP reanalysis wind and pressure anomalies, and the Sahel precipitation anomaly, I propose that this can be explained by an increase in the Saharan dust source, coupled with wind and pressure anomalies that opened a pathway for iron-rich dust transport. As Trichodesmium can grow in temperatures below 20o C, the N2 fixation capability of Trichodesmium strains from extra-tropical regions must be reevaluated. This has important implications for the global Nitrogen budget. The third study expands the first study to examine nine plankton groups, testing the hypothesis of a plankton phase shift in the North ...