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Archive for biogeochemistry

Unexpected global diatom decline in response to ocean acidification

Posted by mmaheigan 
· Tuesday, December 13th, 2022 

Biological impacts of ocean acidification have been the subject of intense research for more than a decade. While it is known that more acidic seawater will create difficulties for calcifying organisms (e.g. corals or coccolithophores), diatoms have so far been considered to be resilient against, or even benefit from, ocean acidification. But an overlooked biogeochemical feedback mechanism has revealed that diatoms are also under threat from ocean acidification.

Figure 1: Slower solubility of diatom shells in acidified oceans leads to global diatom decline. Diatoms build silica shells and produce organic carbon at the ocean surface. Today, much of the silica dissolves relatively quickly as the particles consisting of dead diatoms sink (e.g. after blooms). The resulting dissolved silicon is returned to the surface by upwelling waters, where it supports the growth of more diatoms. Under ocean acidification, the silica in sinking particles will dissolve slower, thereby reducing the return flux of dissolved silicon to the ocean surface as much of the marine silicon budget will become trapped in deep water. The result is a substantial global decrease in diatom biomass. (Figure source: Nature, Vol. 605, No. 7911, 26 May 2022, DOI: 10.1038/d41586-022-01365-z and 10.1038/s41586-022-04687-0)

Diatoms are the most important primary producers in the ocean and play an important role in transferring carbon dioxide (CO2) from the atmosphere into the deep ocean. Their most conspicuous feature is a silica shell formed around their cells. A comprehensive study published in Nature dove deep into the impacts of ocean acidification on diatoms and biogeochemical cycling. Their analyses of data from experiments, field observations, and model simulations suggest that ocean acidification could drastically reduce diatom populations. As a result of lower seawater pH, the silica shells of diatoms dissolve more slowly. However, this is not an advantage—it causes diatom shells to sink into deeper water layers before chemically dissolving and being converted back into the inorganic nutrient silicic acid. This means this nutrient is more efficiently exported to the deep ocean and so becomes scarcer in the light-flooded surface layer where diatoms require it to form new shells. Ultimately, this loss of silica from the surface ocean causes a global decline in diatoms, reaching -10% by the year 2100 and -26% by 2200. Since diatoms are one of the most important plankton groups in the ocean, their decline could lead to a significant shift in the marine food web or even a change in the ocean carbon sink.

This finding is in sharp contrast to the previous consensus of ocean research, which sees calcifying organisms as losers, but diatoms being little affected, or even a winner of ocean acidification. This also highlights the uncertainties in predicting ecological impacts of climate change and how small-scale effects can lead to ocean-wide changes with unforeseen and far-reaching consequences for marine ecosystems and matter cycles.

 

Authors:
Jan Taucher (GEOMAR, Kiel, Germany)
Lennart T. Bach (University of Tasmania, Hobart, Australia)
Friederike Prowe (GEOMAR, Kiel, Germany)
Tim Boxhammer (GEOMAR, Kiel, Germany)
Karin Kvale (GNS Science, Lower Hutt, New Zealand)
Ulf Riebesell (GEOMAR, Kiel, Germany)

When GEOTRACES‐based synthesis efforts improve global iron-cycling understanding

Posted by mmaheigan 
· Friday, December 18th, 2020 

Authors of a recent paper published in Global Biogeochemical Cycles conducted a detailed study of the residence times of total and dissolved iron (Fe) in the upper layers (0-250m) of the global ocean. Using historical (1980-2007) and recent GEOTRACES data, they compiled an impressive data set comprising dissolved, filtered and trap-collected particulate Fe spanning different biogeochemical oceanographic provinces. They also used indirect isotopic approaches to calculate Fe export from the surface layers (e.g., based on thorium-234-uranium-238 disequilibrium). The study revealed that upper ocean residence times of total Fe consistently fell between 10 and 100 days, despite a broad range of total Fe inventories and ocean biogeochemical settings. Conversely, dissolved Fe residences times were longer and more variable, cycling on sub annual to annual time scales. In addition to these detailed insights on upper ocean Fe cycling, these new data sets will help constrain the rate constant for total Fe export, an important term for exploring links between ocean Fe cycling and the global carbon cycle in ocean biogeochemical models.

Figure Caption: In-situ iron concentration and export (Ftot) estimates from numerous GEOTRACES efforts were combined with prior study results to constrain the residence time of iron in the upper ocean (diagonal lines, lower panel). Broad patterns in iron residence times emerged when contrasting coastal and open regions (pink vs. white), as well as with high and low latitude zones (black vs. white). Despite clear regional differences, however, the majority of residence times for total iron fell into a small range between 10 and 100 days.

 

Authors:
E. E. Black (former WHOI, current Dalhousie University, Lamont Doherty Earth Observatory)
S. S. Kienast (Dalhousie University)
N. Lemaitre (Institute of Geochemistry and Petrology, Zürich, Switzerland)
P. J. Lam (University of California, Santa Cruz)
R. F. Anderson (Lamont Doherty Earth Observatory)
H. Planquette (University Brest)
F. Planchon (University Brest)
K. O. Buesseler (WHOI)

This is a joint highlight with GEOTRACES

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