Archive for bottom water

Water clarity impacts temperature and biogeochemistry in Chesapeake Bay

Posted by mmaheigan 
· Thursday, December 3rd, 2020 

Estuarine water clarity is determined by suspended materials in the water, including colored dissolved organic matter, phytoplankton, sediment, and detritus. These constituents directly affect temperature because when water is opaque, sunlight heats only the shallowest layers near the surface, but when water is clear, sunlight can penetrate deeper, warming the waters below the surface. Despite the importance of accurately predicting temperature variability, many numerical modeling studies do not adequately parameterize this fundamental relationship between water clarity and temperature.

In a recent study published in Estuaries and Coasts, the authors quantified the impact of a more realistic representation of water clarity in a hydrodynamic-biogeochemical model of the Chesapeake Bay by comparing two simulations: (1) water clarity is constant in space and time for the calculation of solar heating vs. (2) water clarity varies with modeled concentrations of light-attenuating materials. In the variable water clarity simulation (2), the water is more opaque, particularly in the northern region of the Bay. During the spring and summer months, the lower water clarity in the northern Bay is associated with warmer surface temperatures and colder bottom temperatures. Warmer surface temperatures encourage phytoplankton growth and nutrient uptake near the head of the Bay, thus fewer nutrients are transported downstream. These conditions are exacerbated during high-river flow years, when differences in temperature, nutrients, phytoplankton, and zooplankton extend further seaward.

Figure 1: Top row: Difference in the light attenuation coefficient for shortwave heating, kh[m-1] (variable minus constant light attenuation simulation). June, July, and August average for (A) 2001, (B) average of 2001-2005, and (C) 2003; difference in bottom temperatures [oC] (variable minus constant). Bottom row: Difference in June, July, and August average bottom temperature for (D) 2001, (E) average of 2001-2005, and (F) 2003. Data for 2001 are representative of low river discharge, and 2003 are representative high river discharge years.

This work demonstrates that a constant light attenuation scheme for heating calculations in coupled hydrodynamic-biogeochemical models underestimates temperature variability, both temporally and spatially. This is an important finding for researchers who use models to predict future temperature variability and associated impacts on biogeochemistry and species habitability.

 

Authors:
Grace E. Kim (NASA, Goddard Space Flight Center)
Pierre St-Laurent (VIMS, William & Mary)
Marjorie A.M. Friedrichs (VIMS, William & Mary)
Antonio Mannino (NASA, Goddard Space Flight Center)

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)

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