Seasonal variations of water column nutrients in the inner area of Ariake Bay, Japan: The role of muddy sediments

To investigate seasonal variations of nutrient distribution in the mudflat-shallow water system, we conducted field surveys once a month from August 2007 to July 2008 in the inner area of Ariake Bay (IAB), Japan. The NH₄ ⁺-N concentration of the water column increased in autumn because of the high NH₄ ⁺ release from the sediments, ranging from 850 to 3,001 μmol m^-2 day^-1. The NO₃ ^--N concentration was maximal in January, which was thought to be caused by NO₃ ^- release from the oxic sediments and by NO₃ ^- regeneration due to water column nitrification. The PO₄ ^3--P concentration of the water column was high in summer-autumn due to the high PO₄ ^3- release from the reduced sediments, ranging from 22 to 164 μmol m ^-2 day^-1. We estimated the total amounts of DIN and PO₄ ^3--P release (R _DIN and {R-{{\mathrm{P}{{\mathrm{O}}-4}}}} , respectively) from the muddy sediment area of the IAB. In summer-autumn, R _DIN and {R-{{\mathrm{P}{{\mathrm{O}}- 4}}}} corresponded to about 47.7 % of DIN input and about 116.6 % of PO ₄ ^3--P input from the river, respectively. Thus, we concluded that the muddy sediments were an important source of nutrients for the water column of the IAB during summer-autumn. In addition, we found that phosphorus necessary for the growth of Porphyra (Porphyra yezoensis, Rhodophyceae) would be insufficient in the water column when phosphorus during the Porphyra aquaculture period is supplied only from the river. Therefore, the phosphorus release from the muddy sediments was thought to play an important role in the sustainable production of Porphyra in Ariake Bay.