Living (Stained) Deep-Sea Foraminifera Off Hachinohe (Ne Japan, Western Pacific): Environmental Interplay in Oxygen-Depleted Ecosystems

Abstract

Live (Rose-Bengal stained) deep-sea foraminiferal faunas have been studied at five stations between 500-2000-m depth along the NE Japanese margin (western Pacific) to understand how complex environmental conditions (e.g., oxygen depletion, organic matter) control their structure (i.e., diversity, standing stocks, and microhabitats). All stations are characterized by silty sediments with no evidence of recent physical disturbances. The three stations located between 760-1250 m are bathed by dysoxic bottom waters (,45 mmol/L). Although high organic-carbon contents are recorded at all stations (.2.2% DW), only the oxygendepleted sites are characterized by higher concentrations of sugars, lipids, and enzymatically hydrolysable amino acids (EHAA). Sedimentary contents in chlorophyllic pigments decrease with water depth without any major change in their freshness (i.e., [Chl a/(Chl a + Pheo a)] ratios). Both Uvigerina akitaensis and Bolivina spissa are restricted to the stations bathed by dysoxic waters, proving their oxygendepletion tolerance. In such conditions, both phytophagous taxa are obviously able to take advantage of labile organic compounds (e.g., lipids and EHAA) contained in phytodetritus. Nonionella stella and Rutherfordoides cornuta survive in oxygen-depleted environments probably via alternative metabolic pathways (e.g., denitrification ability) and a large flexibility in trophic requirements. At stations where oxygen availability is higher (i.e., .70 mmol/L in bottom water) and where bioavailable organic compounds are slightly less abundant, diversity indices remain low, and more competitive species (e.g.

Long-term monitoring of bottom environments of the continental slope off Otsuchi Bay, northeastern Japan

“…Minima were recorded on 24 June (178 μM) and on 19 August (152 μM). Because the DO shows a decreasing trend from the sea surface to 1000 m depth in the area off Sanriku (Ogura 1970;Fontanier et al 2014;Nomaki et al 2015), we inferred that the DO decreases that we observed were caused by intermittent upwelling of deeper water. During the periods when DO was low, σ t was increased.…”

“…The upper slope site is in an active fishery area, and trawling is frequently conducted around its water depth, except between July and August. The bathyal site is within the O 2 -depleted zone of the North Pacific (DO < 45 μM; found at water depths from 700 to 1300 m; Ogura 1970;Fontanier et al 2014). Although there is little fishery activity at the bathyal depth, some important fishery resources such as Sebastolobus macrochir is distributed at that depth (e.g., Hattori et al 2007;Sakaguchi-Orui et al 2014), and knowledge of benthic habitats is essential information for understanding the ecology of such fish.…”

Impact of the 2011 off the Pacific coast of Tohoku earthquake on a deep-sea benthic ecosystem: evidence from living and dead benthic foraminifera on the landward slope of the Japan Trench

“…Bolivina spissa, nearly only found from cluster Ib, has been reported in the oxygen minimum zone of the Pacific Ocean (Ingle et al, 1980;Quinterno and Gardner, 1987;Fontanier et al, 2014). Fontanier et al (2014) studied living benthic foraminiferal assemblages between 500-2000 m depth off Hachinohe (northeast Japan). They reported that the B. spissa distribution is restricted to stations bathed by dysoxic waters between 760-15 1250 m depth.…”

“…The tsunami disturbances also affected benthic communities on the continental shelf and upper continental slope (Toyofuku et al, 2014). Five months after the tsunami, the shelf ecosystems off Shimokita (northeast Japan) were characterized by high 10 species diversity and did not show a reduction in density, but an opportunistic fauna dominated the deeper site (211 m depth).…”

“…134 Cs 30 was released into the environment after the Fukushima Daiichi Nuclear Power Plant (FNPP1) accident, and the first major releases occurred on March 15 (Mathieu et al, 2018). These radionuclides, particularly short-lived 134 Cs (2.06 years), have been used to distinguish post-sediment from pre-sediment of the 2011 Tohoku-oki Earthquake (Oguri et al, 2013;Toyofuku et al, 2014;Ikehara et al, 2014Ikehara et al, , 2016.…”