Archive for Ross Sea

A half century perspective: Seasonal productivity and particulates in the Ross Sea

Posted by mmaheigan 
· Tuesday, April 2nd, 2019 

Studies of cruise observations in the Ross Sea are typically biased to a single or a few year(s), and long-term trends have predominantly come from satellites. Consequently, the in situ climatological patterns of nutrients and particulate matter have remained vague and unclear. What are the typical patterns of nutrients and particulate matter concentrations in the Ross Sea in spring and summer? How do these concentrations affect annual productivity estimates?

Patterns of nutrient and particulate matter in the Ross Sea can play a wide-ranging role in a productive region like the Ross Sea. Smith and Kaufman (2018) recently synthesized austral spring and summer (November to February) observations from 42 Ross Sea research cruises (1967-2016) to analyze broad biogeochemical patterns. The resulting climatologies revealed interesting seasonal patterns of nutrient uptake and particulate organic carbon (POC) to chlorophyll (chl) ratios (POC:chl). Temporal patterns in the nitrate and phosphate climatologies confirm the role of early spring haptophyte (Phaeocystis antarctica) growth, followed by limited nitrogen and phosphorus removal in summer. However, a notable increase in POC occurred later in summer that was largely independent of chlorophyll changes, resulting in a dramatic increase in POC:chl. A gradual decline in silicic acid concentrations throughout the summer, along with an increased occurrence of biogenic silica during this time suggest that diatoms may be responsible for this later POC spike. Revised estimates of primary productivity based on these observed climatological POC:chl ratios suggests that summer blooms may be a significant contributor to seasonal productivity, and that estimates of productivity based on satellite pigments underestimate annual production by at least 70% (Figure 1).

Figure 1. Bio-optical estimates of mean productivity using a constant POC:chl ratio (black dots and lines) and modified estimates of productivity using the monthly climatological POC:chl ratios (red dots and lines), in a) the Ross Sea polynya region and b) the western Ross Sea region.

 

By clarifying typical seasonal patterns of nutrient uptake and POC:chl, these climatologies underscore the biogeochemical importance of both spring haptophyte growth and previously underestimated summer diatom growth in the Ross Sea. Further investigation of the causes and consequences of elevated summer ratios is needed, as assessments of regional food webs and biogeochemical cycles depend on more accurate understanding of primary productivity patterns. Likewise, these results highlight the need for continued efforts to constrain satellite productivity estimates in the Ross Sea using in situ constituent ratios.

For other relevant work on seasonal biogeochemical patterns in the Ross Sea, please see https://doi.org/10.1016/j.dsr2.2003.07.010. And for intra-seasonal estimates of particulate organic carbon to chlorophyll using gliders, please see: https://doi.org/10.1016/j.dsr.2014.06.011.

 

Authors:
Walker O. Smith Jr. (VIMS, College of William and Mary)
Daniel E. Kaufman (VIMS, College of William and Mary; now at Chesapeake Research Consortium)

 

 

 

The Ross Sea deep microbial community’s role in sequestering CO2

Posted by mmaheigan 
· Thursday, November 9th, 2017 

Antarctic shelf systems generate the densest waters in the world. These shelf waters are the building blocks of Antarctic Bottom Water, the ocean’s abyssal water mass. These bottom waters have the potential to sequester carbon out of the atmosphere for millennia. One such form of marine carbon is dissolved organic carbon (DOC). DOC is produced in the surface ocean via primary production and is the global ocean’s largest standing stock of reduced carbon.

In a recent study, Bercovici et al (2017) used hydrographic and biogeochemical measurements to assess the mechanism that brings DOC into the shelf waters of the Ross Sea, the shelf system in the Pacific sector of Antarctica. These mechanisms include sinking particles, brine rejection caused by katabatic winds in the Terra Nova Bay polynya, and vertical mixing. This study revealed that DOC is primarily introduced into the deeper shelf waters via convective overturning and deep vertical mixing upon the onset of austral winter. Substantial DOC enrichment of shelf waters suggests that this carbon is exported off the shelf into Antarctic Bottom Water. However, this study finds much of the excess Ross Sea shelf DOC is actually consumed and remineralized to CO2 by deep microbial communities at the slope of the Ross Sea shelf, ultimately sequestering this carbon into the ocean’s interior.

Physical and biological processes have the potential to introduce carbon into the dense shelf waters (blue) in the Ross Sea. Incoming waters (yellow) are modified from the Southern Ocean’s circumpolar waters. At the onset of winter, cooler temperatures and katabatic winds cause brine rejection. The rejection of brine, sinking particles and vertical mixing are all potential mechanisms for bringing DOC to the dense shelf waters. At the shelf slope, outflowing shelf waters ultimately contribute to Antarctic Bottom Water formation. This research furthers our understanding of global carbon cycling through demonstrating that Antarctic shelf systems have the potential to sequester organic carbon into the abyssal ocean.

Authors:
Sarah K. Bercovici (Rosenstiel School of Marine and Atmospheric Science, University of Miami)
Bruce A. Huber (Lamont Doherty Earth Observatory, Columbia University)
Hans B. Dejong (Stanford University)
Robert B. Dunbar (Stanford University)
Dennis A. Hansell (Rosenstiel School of Marine and Atmospheric Science, University of Miami)

Phytoplankton increase projected for the Ross Sea in response to climate change

Posted by mmaheigan 
· Thursday, October 26th, 2017 

How will phytoplankton respond to climate changes over the next century in the Ross Sea, the most productive coastal waters of Antarctica? Model projections of physical conditions suggest substantial environmental changes in this region, but associated impacts on Ross Sea biology, specifically phytoplankton, remain unclear.

In a recent study, Kaufman et al (2017) generated and analyzed model scenarios for the mid- and late-21st century using a combination of a biogeochemical model, hydrodynamic simulations forced by a global climate projection, and new data from autonomous gliders. These scenarios indicate increases in the production of phytoplankton in the Ross Sea and increases in the downward flux of carbon in response to environmental changes over the next century. Modeled responses of the different phytoplankton groups to shoaling mixed layer depths shift the biomass composition more towards diatoms by the mid 21st century. While diatom biomass remains relatively constant in the second half of the 21st century, the haptophyte Phaeocystis antarctica biomass increases as a result of earlier seasonal sea ice melt, allowing earlier availability of low light, in which P. antarctica thrive.

 

Modeled climate scenarios for the 21st century project phytoplankton composition changes and increases in primary production and carbon export flux, primarily as a result of shoaling mixed layer depths and earlier available low light.

The projected responses of phytoplankton composition, production, and carbon export to climate-related changes can have broad impacts on food web functioning and nutrient cycling, with wide-ranging potential effects as local deep waters are transported out from the Ross Sea continental shelf. Future changes to this ecosystem have taken on a new relevance as the Ross Sea became home this year to the world’s largest Marine Protected Area, designed to protect critical habitat for highly valued species that rely on the Ross Sea food web. Continued coordination between modeling and autonomous observing efforts is needed to provide vital data for global ocean assessments and to improve our understanding of ecosystem dynamics and climate change impacts in this sensitive and important region.

 

For other relevant work on observing phytoplankton characteristics in the Ross Sea using gliders, please see: https://doi.org/10.1016/j.dsr.2014.06.011.

And for assimilation of bio-optical glider data in the Ross Sea please see: https://doi.org/10.5194/bg-2017-258.

 

Authors:
Daniel E. Kaufman (VIMS, College of William and Mary)
Marjorie A. M. Friedrichs (VIMS, College of William and Mary)
Walker O. Smith Jr. (VIMS, College of William and Mary)
Eileen E. Hofmann (CCPO, Old Dominion University)
Michael S. Dinniman (CCPO, Old Dominion University)
John C. P. Hemmings (Wessex Environmental Associates; now at the UK Met Office)

 

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