Shelf Nitrate Response to Upwelling
2024-01-01
Nitrate is an essential nutrient for phytoplankton growth, which forms the base of the marine food web. This project explores the response of Oregon shelf nitrate concentrations to upwelling events using newly available observational data from the Ocean Observatories Initiative.
Nitrate is an essential nutrient for phytoplankton growth, which forms the base of the marine food web. In the Northern California Current System, which spans from Northern California to British Columbia, upwelling events driven by southward along-shelf winds bring nutrient-rich waters to the surface, fueling primary productivity. Despite the importance of nitrate to ocean ecosystems, long-term, in situ measurements of nitrate concentrations have been limited.
In order to address this gap in observational data, the Ocean Observatories Initiative (OOI) Coastal Endurance ArrayExternal link has deployed a profiler equipped with an optical nitrate sensor (the SUNA V2External link ) along the Newport Hydrographic Line (NHL)External link , a long-term monitoring site off the coast of Oregon. This profiler collects relatively high-frequency, long-term nitrate data throughout the water column during the summer upwelling season, providing a unique opportunity to study the response of shelf nitrate on weekly to monthly time scales.
Coastal upwelling is a process where deep, nutrient-rich waters are brought to the surface, driven by southward along-shelf winds. These deep waters are particularly rich in nitrate due to the accumulation of decomposed organic matter as the water travels throughout the ocean. When upwelling occurs, these nutrients are transported to the euphotic zone, where they can be utilized by phytoplankton, leading to increased primary productivity.
The inner-shelf, a region where water depths are shallow (less than about 50 m), has sunlight penetrating all the way to the seafloor in addition to strong vertical mixing, making it a particularly productive area for phytoplankton growth. Observing the response of nitrate concentrations to upwelling events in this region is crucial for understanding the dynamics of marine ecosystems and the role of upwelling in supporting shelf ecosystems.
The OOI Coastal Endurance Array profiler data is available from 2016 to the present, providing a long-term record of nitrate concentration in the inner-shelf region. This data is shown in the figure below: the left plot is a time series of the nitrate over one summer upwelling season, with lighter colors representing shallower depths and darker colors representing deeper depths; the right plot is a depth profile of nitrate that you can select. Feel free to play around with it and see if you can notice any of the weekly or monthly variability!
Nitrate Concentration
The weekly timescale variability should be immediately obvious; the nitrate concentration often goes from greater than 30 mg/L to near 0 mg/L in a matter of days. This is due to the variable winds that drive the upwelling process and strengthen and weaken on roughly 5-10 day timescales.
If you looked at some of the depth profiles, you may have noticed that sometimes there is a big difference between the nitrate concentration at the surface and the nitrate concentration at depth, while other times there is little difference. This is a reflection of the stratification of the water column, which is a measure of how well-mixed the water is vertically. When the water column is well-mixed, the nitrate concentration is similar throughout the water column; when it is stratified, there is a bigger difference between surface and bottom nitrate concentrations.
The monthly variability in nitrate concentration is a bit more difficult to pick out from the time series plot, but is still very important. In order to examine the monthly variability, we can look at the monthly mean nitrate concentrations from the inner shelf, which are shown in the figure below.
Monthly Mean Nitrate Concentration
Now, the seasonal cycle of nitrate concentration is much more clear: the mean nitrate concentration increases in the spring, before reaching a maximum in July and then decreasing for the rest of the summer and early fall. Interestingly, this seasonal cycle in nitrate aligns with seasonal cycles also observed in physical water propertiesExternal link , such as velocity, temperature, and salinity, which, like nitrate, are also controlled in part by the along-shelf winds through the upwelling process. This demonstrates the importance of coastal upwelling and therefore winds in delivering nutrients to the inner shelf.
As we have seen, nitrate is highly variable on weekly, upwelling event timescales and has a similar seasonal evolution to other physical shelf variables. This suggests that nitrate is closely linked to the upwelling process and the winds that drive it. Nitrate is therefore an important link between the physical environment and the biological response of the shelf ecosystem. However, when upwelling circulation is strong, phytoplankton are quickly swept offshore and don't have time to develop into large blooms. This means that the relationship between nitrate and chlorophyll, an imperfect proxy of phytoplankton biomass, is not as straightforward as it would initially seem.
Indeed, the competing influence of increased nitrate along with lower retention times in the inner shelf result in an optimal upwelling strengthExternal link , strong enough to supply sufficient nitrate to maintain blooms but not too strong as to transport all the phytoplankton offshore, away from the inner shelf. This can be seen in the figure below, which shows nitrate and chlorophyll concentrations as a function of a weighted average of the recent along-shelf winds (here, negative values correspond to the southward winds that drive upwelling). We can see that, as predicted, the maximum chlorophyll concentration indeed occurs at a moderate wind strength and nutrient concentration.
The OOI Coastal Endurance Array's profiler data has provided a unique opportunity to study the response of shelf nitrate concentrations to upwelling events. The data shows that nitrate is highly variable on weekly timescales and has a seasonal cycle that aligns with other physical shelf variables. The relationship between nitrate and chlorophyll is not straightforward, but rather has a maximum at moderate wind strengths, suggesting an optimal upwelling strength for phytoplankton growth. Together, these findings advance our understanding of the role of upwelling in supporting shelf ecosystems and the dynamics of marine food webs.
Like all of my research, all of the data and analysis code for this project is available on my GitHubExternal link . The nitrate data can be downloaded via my OOI Nitrate RetrieverExternal link command line interface, which utilizes uv for managing script dependencies . If you found this project interesting and want to get in touch, I look forward to hearing from you! You can find my contact information on below.