Date: Thursday, February 06, 2025
Time: 01:00 pm
Location
CRH 3101

Skylar Gerace Dissertation Defense

Thursday, February 06, 2025 | 01:00 pm | CRH 3101
Skylar Gerace
Graduate Student
Event Details

Title: Nutrient and respiration stoichiometry in marine ecosystems

Abstract: Earth System Models predict that increasing ocean stratification is reducing the transport of nutrients to the upper ocean and dissolved oxygen to the deep ocean. However, the anticipated decline of upper ocean nutrients has not yet been observed due to challenges in detecting low concentrations at the ocean surface. Additionally, while the decline in deep ocean oxygen has already been observed, most of this rate exceeds model predictions. Here, I quantified the elemental stoichiometry of upper ocean nutrients and organic matter respiration to identify mechanisms linking stratification to these predicted biogeochemical changes.

In the first chapter, I showed that nutricline depths, where nutrient concentrations are well-detected, proxy upper ocean nutrient availability and reveal its changes at a global scale. First, I used an Earth System Model to demonstrate that nutricline depths strongly correlated with the vertical fluxes of nitrogen (N) and phosphorus (P). Then, I analyzed cruise observations spanning the past five decades to quantify the N-nutricline and P-nutricline over time. Their trends revealed that phosphorus availability is decreasing worldwide, especially throughout the southern hemisphere, while nitrogen availability remains stable. Model simulations supported the hypothesis that reduced iron stress and increased nitrogen fixation can buffer the availability of nitrogen, but not phosphorus, against increasing stratification. This mechanism may be intensifying P-limitation throughout the ocean, which is expected to continue until nitrogen fixation is impeded by low phosphorus availability.

In the second and third chapters, I showed that respiration stoichiometry varies with depth and under sea ice due to changes in plankton community composition. The molar ratio of oxygen consumed per organic carbon respired decreased with depth throughout the euphotic zone, but increased throughout the twilight zone. At the surface of the Arctic Ocean, this ratio is positively related with sea ice formation and the bacterial proportion of the plankton community. These findings suggest that the ratio is primarily influenced by plankton size structure, consistent with previous studies in other regions. As stratification increases at low latitudes and likely selects for smaller plankton cells, this mechanism may contribute to oxygen loss in areas where deoxygenation remains largely unexplained.

The Department of Earth System Science acknowledges our presence on the ancestral and unceded territory of the Acjachemen and Tongva peoples, who still hold strong cultural, spiritual and physical ties to this region.