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