Abstract

Ocean Acidification (OA) in marginal seas and shelf environments is a multifaceted phenomenon, most notably due to watershed export in the form of riverine influx. In this study, we examined the effect of decadal changes in alkalinity driven by fresh water influx from the Nile River before and after its damming, as well as the effect of this change on OA of the South-East Mediterranean (SE-Med) shelf. Two independent data sets were used to calculate the change in pH and aragonite saturation (Ω_ar) from 1948 to 2002: observational salinity data from which total alkalinity (A_T) was calculated and a δ¹³C record of the endemic Dendropoma reefs from which dissolved inorganic carbon (DIC) was estimated. Our calculations indicate the development of a localized low pH/low Ω_ar zone along the SE-Med shelf, associated to the Nile plumes. Following the damming of the Nile, a distinct acidification trend is observed. We infer an acidification rate of -0.0022 ± 0.0002 yr^-1 in the SE-Med following the damming; this rate exceeds those of the open-oceans but is comparable to other marginal seas. This trend is significantly correlated with the increase in atmospheric CO₂ since the 1960s, supporting the notion of a more substantial atmospheric impact from that period onward. However, despite evident acidification, the modern level of high alkalinity helped to buffer the system and maintained high levels of pH and aragonite saturation all year long.