Date of Award
Doctor of Philosophy in Oceanography
Kelton W McMahon
The export of organic matter from the surface to deep ocean has major implications for global biogeochemical cycles, the transfer of energy across food webs, and the sequestration of carbon through the biological pump. Therefore, understanding the physical and biological conditions that control these processes is important to understanding pelagic-benthic coupling and resulting controls on ocean productivity, fisheries production, and Earth’s climate regulation. Equally important is consideration of and collaboration with diverse audiences that need to understand this information to make scientifically informed decisions about climate and environment policies that come from this research. Despite the importance of this knowledge, access to long-term data sets on the controlling mechanisms of export production are scarce and urgently needed to test assumptions about 1) the sources and transformations of organic matter through different food web pathways and 2) the variability of these processes across climatic, oceanographic, and ecological changes through time. This thesis applies recent advances in molecular isotope geochemistry approaches to biological archives of food web processes in the surface ocean (long term pelagic copepod archives) and deep ocean (benthic deep-sea coral archives) from the Gulf of Maine to shed new light on how changing ocean circulation, mixing, and stratification alter biogeochemical cycling, primary production, and metazoan and microbial heterotrophic processes leading to the formation of exported organic matter. Through strategic generalized additive modeling (GAM) approaches and transdisciplinary collaborations across science, sculpture, video, educator, public media, and communication experts, this study was able to identify pronounced regimes changes in pelagic-benthic coupling and its underlying drivers over a multi-decadal time series. These regime changes were linked to a shift in proximate drivers from changes in water mass, mixing, and stratification to unprecedented warming in the recent decade, which related to changes in copepod abundance and pelagic food web dynamics. Surprisingly, there was a strong and persistent microbial loop geochemical signal recorded in C. finmarchicus, including evidence of multiple microbial trophic transfers in the food webs supporting these large copepods that were invisible to traditional geochemical food web metrics. Geochemical records of these microbial food web processes in pelagic system were directly exported to the benthic deep-sea coral record, which had a striking resemblance, in both magnitude and trend, to the geochemical record in the large-bodied copepod Calanus finmarchicus. Tight pelagic-benthic coupling, driven by the large, fast sinking fecal pellets of C. finmarchicus, provided a direct mechanism to export microbial loop production to the benthic system. We observed a long-term trend towards increasing reliance on microbially reprocessed organic matter that mirrored regional warming trends in both the pelagic and benthic food webs of the Gulf of Maine. Pelagic-benthic coupling in the Gulf of Maine was strongly influenced by variations in water mass nutrient delivery and mixed layer depth, which in the early rate of change periods drove physical-nutrient-production dynamics, though as the average mixed layer depth deepened in the most recent two decades, the closer proximity of the two systems facilitated the continued pelagic-benthic coupling despite the recent decreases in C. finmarchicus abundance. These results provide a new critical framework for understanding the central role that copepods play in pelagic food webs and deep ocean export as well as how they may change in a warming future ocean. By transforming these complex physical, chemical, and biological ecosystem-level relationships into transdisciplinary data visualizations, we increased the collective reach and associated impact of this research through a more holistic and inclusive approach to presenting science.
Nowakowski, Catherine, "PELAGIC-BENTHIC COUPLING IN THE GULF OF MAINE: MULTIDECADAL MOLECULAR ISOTOPE RECORDS OF ZOOPLANKTON AND DEEP SEA CORALS BIOARCHIVES" (2023). Open Access Dissertations. Paper 1526.