Nutrient status of Otsuchi Bay (northeastern Japan) following the 2011 off the Pacific coast of Tohoku Earthquake

Abstract

On 11 March 2011, the Sanriku coast of Japan was struck by a massive tsunami associated with the 2011 off the Pacific coast of Tohoku Earthquake. The tsunami caused large disturbances in bottom sediments and severe destruction of human-made structures, natural landscapes and infrastructures such as sewage treatment systems and aquaculture facilities. A serious and continuing concern is a fundamental alteration of the nutrient regime in the impacted regions, which may exert a profound influence on the processes of ecosystem recovery and fishery reconstruction. We investigated the nutrient status of the water column of Otsuchi Bay, a small embayment that was heavily damaged by the tsunami. Our data collected between May 2011 and July 2014 revealed some prominent features in nutrient parameters that were potentially linked to effects of the tsunami. These features include (1) high turbidity and high concentrations of nitrite and silicic acid until September 2011, (2) the spread of seawater characterized by high phosphate concentration and low total inorganic nitrogen (ammonium + nitrate + nitrite) to phosphate (TIN/P) ratio during the mixing period between November 2011 and January 2012, and (3) an increase in the TIN/P ratio during the mixing period of subsequent years to become higher than it was before the earthquake. These data not only provide evidence of the alteration and subsequent recovery of the nutrient status of Otsuchi Bay after the earthquake but also suggest a possible tsunami-induced alteration in nutrient stoichiometry which appeared to last until at least the beginning of 2014.