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Kroeker, KJ, Micheli F, Gambi MC, Martz TR.  2011.  Divergent ecosystem responses within a benthic marine community to ocean acidification. Proceedings of the National Academy of Sciences of the United States of America. 108:14515-14520.   10.1073/pnas.1107789108   AbstractWebsite

Ocean acidification is predicted to impact all areas of the oceans and affect a diversity of marine organisms. However, the diversity of responses among species prevents clear predictions about the impact of acidification at the ecosystem level. Here, we used shallow water CO(2) vents in the Mediterranean Sea as a model system to examine emergent ecosystem responses to ocean acidification in rocky reef communities. We assessed in situ benthic invertebrate communities in three distinct pH zones (ambient, low, and extreme low), which differed in both the mean and variability of seawater pH along a continuous gradient. We found fewer taxa, reduced taxonomic evenness, and lower biomass in the extreme low pH zones. However, the number of individuals did not differ among pH zones, suggesting that there is density compensation through population blooms of small acidification-tolerant taxa. Furthermore, the trophic structure of the invertebrate community shifted to fewer trophic groups and dominance by generalists in extreme low pH, suggesting that there may be a simplification of food webs with ocean acidification. Despite high variation in individual species' responses, our findings indicate that ocean acidification decreases the diversity, biomass, and trophic complexity of benthic marine communities. These results suggest that a loss of biodiversity and ecosystem function is expected under extreme acidification scenarios.

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Nam, S, Takeshita Y, Frieder CA, Martz T, Ballard J.  2015.  Seasonal advection of Pacific Equatorial Water alters oxygen and pH in the Southern California Bight. Journal of Geophysical Research-Oceans. 120:5387-5399.   10.1002/2015jc010859   AbstractWebsite

Chemical properties of the California Undercurrent (CU) have been changing over the past several decades, yet the mechanisms responsible for the trend are still not fully understood. We present a survey of temperature, salinity, O-2, pH, and currents at intermediate depths (defined here as 50-500 m) in the summer (30 June to 10 July) and winter (8-15 December) of 2012 in the southern region of the Southern California Bight. Observations of temperature, salinity, and currents reveal that local bathymetry and small gyres play an important role in the flow path of the California Undercurrent (CU). Using spiciness (p) as a tracer, we observe a 10% increase of Pacific Equatorial Water (PEW) in the core of the CU during the summer versus the winter. This is associated with an increase in p of 0.2, and a decrease in O-2 and pH of 30 mu mol kg(-1) and 0.022, respectively; the change in pH is driven by increased CO2, while total alkalinity remains unchanged. The high-p, low-O-2, and low-pH waters during the summer are not distributed uniformly in the study region. Moreover, mooring observations at the edge of the continental shelf reveal intermittent intrusions of PEW onto the shelf with concomitant decreases in O-2 and pH. We estimate that increased advection of PEW in the CU could account for approximately 50% of the observed decrease in O-2, and between 49 and 73% of the decrease in pH, over the past three decades.