Archive for Gulf of Mexico

Timing matters: Correcting float-based measurements of diurnal oxygen variability

Posted by mmaheigan 
· Friday, November 6th, 2020 

Despite its fundamental importance to the global carbon cycle, climate, and marine ecosystems, oceanic primary production is grossly under-sampled. Autonomous platforms represent an important frontier for expanding measurements of marine primary productivity in time and space, but this requires the establishment of robust, standardized methods to obtain reliable data from these platforms. Using data from profiling floats deployed in the northern Gulf of Mexico, authors of a recent study published in Biogeosciences demonstrated, for the first time, that daily cycles of dissolved oxygen can be observed with Argo-type profiling floats. The floats were instructed to profile continuously, resulting in about one profile every three hours. The floats recorded data both on the ascent (upcast) and the descent (downcast). Adjacent casts showed hysteresis in gradient areas, i.e. a lag in the concentration measurement, due to the slow response time of oxygen sensors.

Figure 1: Example of raw oxygen measurements from a downcast (dark purple line) and an upcast (dark green line) and corrected profiles (lighter purple and green lines) in (a) density and (b) pressure coordinates. (c) Upcasts and downcasts (top 150 m) plotted against each other with raw data (purple) and data corrected according to the new method (red). (d) The root-mean-square difference (RMSD) between the upcast and downcast after correcting casts for a range of time constants (τ), showing an optimal τ value in this case of 76 s (red dot).

To correct for these measurement errors, the authors developed a method to determine sensor response time in situ, using an established process for correcting sensor response time errors. This method requires a timestamp associated with each observation. The response time parameter (τ) was determined by correcting consecutive profiles taken in opposite directions using a range of possible values and finding the minimum root-mean-square-difference between them (Figure 1). In light of these findings, future oxygen measurements from Argo floats should be transmitted with time stamps for a calibration period during which up- and downcasts are recorded to facilitate response time correction. The method developed here will contribute to more accurate measurement of dissolved oxygen, thus improving the quality of derived quantities such as primary productivity.

 

Authors
Christopher Gordon (Dalhousie University)
Katja Fennel (Dalhousie University)
Clark Richards (Fisheries and Oceans Canada)
Nick Shay (University of Miami)
Jodi Brewster (University of Miami)

Gulf of Mexico: A blue carbon hotspot of mangroves, seagrass and marshes

Posted by mmaheigan 
· Wednesday, February 20th, 2019 

The Gulf of Mexico (GoM) is an important global hotspot that comprises over 2.1615 million hectares of blue carbon habitats, including mangroves, seagrasses, and salt marshes, which collectively store 480.5 Tg of organic carbon (Corg) just in the upper 1 meter of sediment. Some of these important areas of carbon sequestration are protected or conserved, but much of the area is vulnerable, as 69 million people (US and Mexico) live within 50 miles of these blue carbon habitats, so the potential for development and subsequent habitat loss is high. In a recent study published in Science of the Total Environment, the estuaries around the GoM were delineated to determine areal extent and associated carbon stocks for all three habitats.

Figure 1: Map of blue carbon extent and stock for six sub-regions in the Gulf of Mexico estuaries and the Florida Shelf. The areal extent in hectares (ha) and associated organic carbon (Corg) stock in Tg is listed for each blue carbon system (MN = mangroves, SG = seagrass, SM = saltmarsh) in each sub-region. The underlying blue carbon map shows the distribution of mangroves (red), saltmarsh (yellow), and seagrass (blue) (used with permission from Chmura and Short, 2015).

 

Of the GoM blue carbon systems studied, mangroves sequester the most carbon, storing nearly 200 Tg Corg over 650,482 ha (Figure 1). Seagrass is ubiquitous throughout the GoM basin, spanning over 1 million ha and storing 184 Tg Corg, Salt marshes, which are predominantly found in the northwestern quadrant of the GoM account for just under 100 Tg Corg. In addition to presenting these updated blue carbon stock estimates for the GoM, this study estimates anthropogenic impacts on GoM blue carbon storage and compares GoM vs. Atlantic shoreline blue carbon habitat stocks and extents.

 

Authors:
Anitra L. Thorhaug (Yale University)
Helen M. Poulos (Wesleyan University)
Jorge López-Portillo (Instituto de Ecología Mexico)
Jordan Barr (Elder Research)
Ana Laura Lara-Domínguez (Instituto de Ecología Mexico)
Tim C. Ku (Wesleyan University)
Graeme P.Berlyn (Yale University

Seagrass carbon dynamics: Gulf of Mexico

Posted by mmaheigan 
· Thursday, March 1st, 2018 

Seagrasses have died-off in great numbers, resulting in the release of stored carbon. Seagrasses represent a substantive and relatively unconstrained North American and Caribbean Sea blue carbon sink in the tropical Western Hemisphere. Fine-scale estimates of regional seagrass carbon stocks, as well as carbon fluxes from anthropogenic disturbances and natural processes and gains in sedimentary carbon from seagrass restoration are currently lacking for the bulk of tropical Western Hemisphere seagrass systems.

To address this knowledge gap, in the subtropics and tropics, a recent study yielded estimates of organic carbon (Corg) stocks, losses, and restoration gains from several seagrass beds around the Gulf of Mexico (GoM). GoM-wide seagrass natural Corg stocks were estimated to be ~37.2–37.5Tg Corg. A unique method involving quadruplicate sampling in naturally-occurring, restored, continually-historically barren, and previously-disturbed-now-barren sites provided the first available Corg loss measurements for subtropical-tropical seagrasses. GoM Corg losses were slow, occurring over multiple years, and differed between sites, depending on disturbance type. Mean restored seagrass bed Corg stocks exceeded those of natural seagrass beds, underscoring the importance of seagrass restoration as a viable carbon sequestration strategy. For restored seagrass areas, the older the restoration site, the greater the Corg stock.

Organic carbon stocks for Gulf of Mexico sediments for the top 20 cm of sediment in always barren, impacted barren, natural seagrass, and restored seagrass sites. Natural and restored seagrass beds had significantly higher organic carbon stocks than impacted barren or always barren sediments.

Seagrass restoration appears to be an important tool for climate-change mitigation. In the USA and throughout the tropics and subtropics, restoration could reduce sedimentary carbon leakage and bolster total blue carbon stores, while facilitating increased fisheries and shoreline stability. Although well-planned and executed restoration of seagrass is more difficult than mangroves or marshes, there are >1 million hectares of degraded seagrass habitats that could be restored, which would greatly increase blue carbon sinks and support diverse marine species that rely on seagrass for habitat and food.

 

Authors:
Anitra Thorhaug (Yale School of Forestry)
Helen M. Poulos (Earth Sci., Wesleyan Univ.)
Jorge López-Portillo (Inecol, Mexico)
Timothy C.W. Ku (Earth Sci., Wesleyan Univ.)
Graeme P. Berlyn (Yale School of Forestry)

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