Silicon biogeochemistry in the open-ocean surface waters : insights from the\ud Sargasso Sea and equatorial Pacific

Graduation date: 2008Diatoms are a ubiquitous group of plankton responsible for 20-40% of oceanic\ud primary production, and a higher fraction of organic matter export to the ocean\ud interior. Diatoms actively transport dissolved inorganic silicon into their cells, and\ud through the process of silicification (i.e. biogenic silica production) they build tough\ud and intricate shells, known as frustules. With a global distribution and the ability to\ud persist in high numerical abundances, diatoms dominate the biological cycling of Si in\ud the oceans. The biogeochemical cycling of Si has been well studied in the coastal\ud ocean, specifically in upwelling regions where diatoms are generally the dominant\ud phytoplankton group. However, much less is known about the role diatoms play in the\ud open ocean; which comprises the vast majority of the oceanic surface area. Outside of\ud the Southern Ocean, only 11 studies (prior to 2003) directly examined surface-water Si\ud biogeochemistry in the open ocean. Current knowledge about Si biogeochemistry in\ud the open ocean suffers from what can only be described as gross under-sampling. This\ud dissertation reports on the surface-water Si biogeochemistry in two open-ocean\ud regions: the northwestern Sargasso Sea and the eastern equatorial Pacific. The three\ud research chapters are linked by the examination of spatial or temporal variability in\ud surface-water Si biogeochemistry. Chapters 2 and 4 examine scales of temporal\ud variability in Si biogeochemistry in the Sargasso Sea. The results demonstrate that\ud biogenic silica concentrations, and presumably Si biogeochemical processes, vary on daily, seasonal, multi-year (e.g. ~3-4 years), and decadal time scales. In Chapter 3\ud data were gathered over a ~2.6x106 km2 area (i.e. larger than the Bering Sea) in the\ud equatorial Pacific. Within that area biogenic silica production showed little spatial or\ud temporal variability. Additionally, the estimated contribution to new production\ud (productivity supporting the export of organic matter to the ocean interior) by diatoms\ud in this region was 4-10 times higher than the diatom contribution to total autotrophic\ud biomass