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- Title
Drivers of Biogeochemical Variability in a Central California Kelp Forest: Implications for Local Amelioration of Ocean Acidification.
- Authors
Hirsh, Heidi K.; Nickols, Kerry J.; Takeshita, Yuichiro; Traiger, Sarah B.; Mucciarone, David A.; Monismith, Stephen; Dunbar, Robert B.
- Abstract
Kelp forests are among the world's most productive marine ecosystems, and they have the potential to locally ameliorate ocean acidification (OA). In order to understand the contribution of kelp metabolism to local biogeochemistry, we must first quantify the natural variability and the relative contributions of physical and biological drivers to biogeochemical changes in space and time. We deployed an extensive instrument array in Monterey Bay, CA, inside and outside of a kelp forest to assess the degree to which giant kelp (Macrocystis pyrifera) locally ameliorates present‐day acidic conditions which we expect to be exacerbated by OA. Temperature, pH, and O2 variability occurred at semidiurnal, diurnal (tidal and diel), and longer upwelling event periods. Mean conditions were driven by offshore wind forcing and the delivery of upwelled water via nearshore internal bores. While near‐surface pH and O2 were similar inside and outside the kelp forest, surface pH was elevated inside the kelp compared to outside, suggesting that the kelp canopy locally increased surface pH. We observed the greatest acidification stress deeper in the water column where pCO2 reached levels as high as 1,300 μatm and aragonite undersaturation (ΩAr < 1) occurred on several occasions. At this site, kelp canopy modification of seawater properties, and thus any ameliorating effect against acidification, is greatest in a narrow band of surface water. The spatial disconnect between stress exposure at depth and reduction of acidification stress at the surface warrants further assessment of utilizing kelp forests as provisioners of local OA mitigation. Plain Language Summary: Anthropogenic emissions increase atmospheric carbon dioxide (CO2), leading to increased dissolved CO2 in the ocean. Elevated CO2 concentrations increase the ocean's acidity (ocean acidification), threatening marine ecosystems. When kelp photosynthesizes, CO2 is removed from seawater (reducing acidity), and oxygen is produced. We do not know if kelp photosynthesis is enough to reduce acidity and protect the local ecosystem from acidification. To understand how kelp impacts its local environment over time, depth, and space, we deployed a novel array of monitoring instruments in Monterey Bay, CA. We observed patterns in temperature, pH (acidity), and oxygen over days to weeks related to the strength of offshore winds and the movement of deep, cold, acidic, low‐oxygen water into the shallow kelp environment. Below the surface, pH and oxygen were similar inside and outside of the kelp forest. Interestingly, surface pH was slightly higher (less acidic) inside the kelp relative to outside, suggesting that the kelp canopy reduced acidity. However, we observed the highest acidity in deep water, indicating that the impact of the kelp canopy (reducing surface acidity) does not extend to where we see the greatest acidification. Therefore, the ability for kelp to lessen acidification stress may be greater at the surface. Key Points: Offshore processes drove event‐scale variability and mean pH, O2, and temperature inside and outside a central California kelp forestThe influence of the kelp canopy on local biogeochemistry was greatest within the upper several meters of the surfaceThere was a spatial disconnect between maximum exposure to acidification stress at the bottom and the potential refuge at the surface
- Subjects
BIOGEOCHEMICAL cycles; OCEAN acidification; OCEAN dynamics; MARINE geophysics; PHYTOPLANKTON
- Publication
Journal of Geophysical Research. Oceans, 2020, Vol 125, Issue 11, p1
- ISSN
2169-9275
- Publication type
Article
- DOI
10.1029/2020JC016320