ARTICLE | doi:10.20944/preprints202303.0519.v1
Subject: Environmental And Earth Sciences, Water Science And Technology Keywords: dissolved sulfide; iron; hypoxia; buffering capacity; environmental restoration; coastal waters; Mikawa Bay; dead zone
Online: 30 March 2023 (02:58:33 CEST)
This study examined the spatial-temporal distribution of sulfur and iron compounds (dissolved sulfide, iron sulfide, and ionized iron) in sediments from April 2015 to March 2016 at four stations in Mikawa Bay, Japan. Seasonal changes were observed in dissolved sulfide, iron sulfide, and ionized iron in the upper part of the sediment (0–4 cm depth) at all stations. The maximum concentration in the upper part of the central bay was 2.8 mmol L-1. The maximum values of dissolved sulfide (ranging from 1.4 to 8.1 mmol L-1) at stations located in a water way varied among stations. The iron sulfide concentration in the upper part of the sediment at a station where dissolved sulfide concentration in the waterway was relatively low exceeded that at other stations in the waterway during spring to summer. Ionized iron concentration was highest at the station where the dissolved sulfide concentration was low. The study results suggest that iron plays an important role in determining the magnitude of dissolved sulfide accumulation in sediments by binding with dissolved sulfide. The results imply the possibility of mitigating the accumulation of free sulfides, which causes extreme hypoxia, by artificially adding sufficient iron to the seabed.
ARTICLE | doi:10.20944/preprints202303.0412.v1
Subject: Environmental And Earth Sciences, Environmental Science Keywords: asari clams; upwelling of hypoxic water masses; mass mortality; hydrogen sulfide; chain of biological die-off
Online: 23 March 2023 (09:49:49 CET)
To investigate the mass mortality of asari clams triggered by upwelling-driven hypoxia, we conducted biological and oceanographic observations of the Rokujo tidal flat in Mikawa Bay, central Japan. In addition, a simple laboratory experiment was conducted using sediments on a tidal flat containing macrobenthos to examine the possibility of hydrogen sulfide formation in tidal flats. The results of field observations showed that the number and biomass of asari clams decreased from September to October in the tidal flat when hypoxia was intermittent. In particular, hypoxia persisted for approximately one week from September 21, which was associated with the calm weather and stagnation of tidal currents owing to the neap tide. The hydrogen sulfide concentration in the water directly above the sediment exceeded 30 mg L-1 after 3 days of incubation. Our results suggest that the mass mortality of asari clams may be caused by the high concentration of hydrogen sulfide produced by sulfate reduction, which is attributed to the chain of biological die-off caused by the upwelling of hypoxic water mass accompanied by meteorological events, and calm conditions such as the neap tide period.