Influence of seasonally deposited phytodetritus on benthic foraminiferal populations in the bathyal northeast Atlantic. the species response

Abstract

Cores were obtained with a multiple corer at a bathyal site (1320 to 1360m depth) in the Porcupine Seabight during April and July 1982. In July (but not April) the sediment surface was overlain by a layer of phytodetritus, material rapidly sedlmented from the euphotik zone following the spring bloom. The phytodetrital fraction of samples (0 to 1 cm layer of subcores; 3.46 cm2 surface area) removed from the July cores harboured dense, low-diversity populations of benthic foraminifers which resembled the phytodetritus-dwelling assemblages already described from the much deeper (4550 m) BIOTRANS site in the northeast Atlantic. Our new observations consolidate the view that phytodetritus is a microhabitat for some deep-sea benthic foraminiferal species. The bathyal populations were dominated by Alabaminella weddellensis (75 '70 of total) and also included Episton~inella exigua and Tinogullmia sp. nov. These 3 species occurred also in the BIOTRANS phytodetrital assemblages. The April san~ples and the total July samples (phytodetritus plus sediment fractions) yielded diverse foraminiferal populations of similar density and species richness. However, there were some important taxonomic dfferences. In particular, the 8 species consistently present in the phytodetritus were significantly more abundant in the July san~ples, while the most common species in the April sanlples (01.dmrnina sp. nov. A ) was entirely absent during July. We argue that the infhtence of phytodetritus, rather than spatial variability (patchiness), was responsible for some of the differences in species abundances. Other species, however, maintain more stable populat~on densities. Our results suggest that deep-sea benthic foraminifers, like those living in shallow water, probably display a variety of life-history strategies and populdtion dynamics. INTRODUCTIONThe delivery of organic material to the food-limited deep-sea benthic ecosystem is a central topic in biological oceanography (Angel 1984, Fowler & Knauer 1986, Bruland et al. 1989) One potentially important pathway is provided by rapidly sedimented phytoplankton blooms (Takahashi 1986) which accumulate during the spring and early summer on the sea-floor as a layer of 'phytodetritus'. This material was first reported in sediment cores and bottom photographs taken at depths of 1000 to 4500m in the Porcupine Seabight (an area centred around 51°30'N; 13'00' W) and on the adjacent abyssal plain (Billett et al. 1983, Lampitt 1985, Rice et al. 1986). Elsewhere in the northeast Atlantic it has been observed in the Rockall Trough (Barnett et al. 1982), the northern Bay of Biscay (Sibuet 1984, p. (Gardner et al. 1984). Phytodetritus is known to be ingested by deposit-feeding echinoderms (Billett et al. 1988) and other inegabenthic animals (Thiel et al. in press). Evidence from the BIOTRANS site (4550m depth) indicates that it is also degraded rapidly by deep-sea bacterial populations (Lochte & Turley 1988) and colonised and eaten by small benthic foraminifers (Gooday 1988a). The presen...

Paleoceanography of the southern Agulhas Plateau during the last 150 ka: Inferences from benthic foraminiferal assemblages and multispecies epifaunal carbon isotopes

“…: AGULHAS 150 Ka BENTHIC FORAMINIFERA d 13 C phytodetritus [Gooday, 1988[Gooday, , 1993[Gooday, , 1996Gooday and Lambshead, 1989;Turley et al, 1993;Smart and Gooday, 1997;Cornelius and Gooday, 2004]. This could explain why higher BFARs are strongly dominated by peaks in only one or two small sized species as occurs during MIS 2 (A. weddellensis, N. iridea) and between MIS 5b and MIS 4 (E. vitrea) while lower BFARs correspond to moderate abundance of both small and large size species, such as the combination of N. iridea, A. weddellensis, Uvigerina spp.…”

“…Its presence in the benthic assemblage therefore is a good indication of conditions favorable for enhanced carbonate preservation. Nonionella iridea has been related to high-sediment C org content [Mackensen et al, 1990], phytodetrital inputs (but it does not occur within the phytodetrital aggregates and probably benefits from bacterial densities beneath phytodetritus, [Gooday and Lambshead, 1989;Gooday and Hughes, 2002]), and episodic pulses of labile C org flux to the seafloor [Duchemin et al, 2005[Duchemin et al, , 2007. The highest concentration of N. iridea is coincident with high C org concentrations during early MIS 2.…”

“…>63mm per gram of dry weight sediment) may not be related in a predictable manner to the C org flux reaching the seafloor. Indeed, the benthic foraminiferal fauna in core MD02-2589 is dominated by small species: A. weddellensis, N. iridea, E. exigua, and E. vitrea which are typically present in the <125 mm size fraction [Gooday and Lambshead, 1989;Gooday, 1986Gooday, , 2003Duchemin et al, 2007]. Epistominella exigua and A. weddellensis are also known to feed on, and reproduce in phase with seasonal pulses of fresh [Mackensen et al, 1993a]).…”

“…In such cases the influence of productivity-induced low-d 13 C overprint on epibenthic d 13 C values needs to be taken into consideration when reconstructing deep ocean nutrient inventories and water mass circulation [Bickert and Mackensen, 2004;Rutberg and Peacock, 2006]. Such influence may be constrained by d 13 C analysis of an epifaunal foraminiferal species that is physically associated with the presence of phytodetritus as a food source such as, for instance, Epistominella exigua (Brady) that is known to grow and reproduce rapidly when phytodetritus is available [Gooday, 1988[Gooday, , 1993[Gooday, , 1996Gooday and Lambshead, 1989;Gooday and Malzone, 2003;Cornelius and Gooday, 2004]. However, little is known about the stable isotope fractionation of E. exigua during calcite precipitation as only few studies have focused on the isotopic composition of this species in recent sediments [Woodruff et al, 1980;Corliss et al, 2006] and its use in paleoceanography [Mackensen et al, 1989[Mackensen et al, , 1994Labeyrie et al, 1996].…”

“…Therefore studies of the benthic foraminiferal community structure can provide useful environmental information because changes in the abundance and distribution of deep sea benthic foraminifera are mainly controlled by the oxygenation levels in the bottom and interstitial waters [e.g., Bernhard and Sen Gupta, 1999], the quantity, quality [Loubere and Fariduddin, 1999a] and seasonality of the C org arriving at the seafloor [Loubere, 1998;Fariduddin, 1999a, 1999b;Sun et al, 2006]. Once it is defined which benthic foraminiferal species are physically associated with the deposition of phytodetritus [e.g., Gooday, 1988Gooday, , 1993Gooday and Lambshead, 1989], abundance variations of these species in the fossil record can be used to reconstruct seasonality in the geological past [Thomas et al, 1995;Thomas and Gooday, 1996;Nees, 1997;King et al, 1998;Bornmalm et al, 1999;Ohkushi et al, 2000;Kawagata, 2001;Rasmussen et al, 1996Rasmussen et al, , 2002Gupta and Thomas, 2003;Ujiié, 2003;Hayward et al, 2004aHayward et al, , 2004bWollenburg et al, 2001Wollenburg et al, , 2004.…”