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A comparison of the vertical distribution of Foraminifera at the seasonally hypoxic 140-m site and the permanently hypoxic 300-m site along the Pakistan continental margin provided some insight into the effect of food and bottom-water oxygen concentration on the vertical distribution of the live Foraminifera within the sediment. Table 4.16 summarises the Average Living Depth (ALD5) and the percentage of live Foraminifera in the upper 1 cm sediment layer for the 140-m and 300-m sites during the spring intermonsoon and the SW monsoon. An upward vertical migration of Foraminifera was observed in sediments at both sites following

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the SW monsoon. This was reflected in both a shallowing of the Average Living Depth of the foraminiferal assemblage, and an increase in the percentage abundance in the upper 1 cm of the sediment following the SW monsoon (Table 4.16).

Table 4.16 Mean vertical distribution of Foraminifera at 140 m and 300 m during spring intermonsoon (April 2003) and SW monsoon (October 2003). Data shown are Average Living Depth in the total sample (0-5 cm). Percentage of live Foraminifera in the upper 1 cm is shown in brackets.

140-m site 300-m site ALD5 spring intermonsoon 0.57 (89.0 %) 0.41 (93.5 %) ALD5 SW monsoon 0.39 (98.5 %) 0.33 (97.6 %)

In general, these results support the findings of the TROX model of Jorissen et al. (1995). This proposes that the depth to which foraminiferal species occur in the sediment is dependent on a balance between food and oxygen availability (see Chapter 2). The general concentration of Foraminifera in the surface layers of samples from 140 m and 300 m is consistent with this model. In particular, the restriction of most live individuals to the upper 1 cm of sediment at the persistently hypoxic 300-m site, is not unexpected. Presumably, they are forced to the surface by lack of oxygen. However, at the seasonally hypoxic 140-m site, live Foraminifera were still generally restricted to the upper 1 cm of the sediment, even during the spring when oxygen levels were 2.05 mll-1_{. The slightly deeper Average Living} Depth of Foraminifera at the 140 m site during the spring intermonsoon may be explained by the relatively high oxygen levels combined with a higher abundance of metazoa and consequent higher degree of bioturbation at this site. This may have facilitated a deeper oxygen penetration and downward biotransport of food (Levin et al. 1997) enabling Foraminifera to colonise deeper vertical layers within the sediment. The subsequent shallowing of the Average Living Depth at 140 m during the SW monsoon season could be interpreted as a response by the Foraminifera to the sharp decrease in bottom-water oxygen concentration at this time of year. At 300

m, there is a less pronounced seasonal shallowing of the Average Living Depth. Assuming that this change is real, it is probably a response to an increased food supply following the SW monsoon, since bottom water oxygen concentrations are relatively stable over the monsoonal cycle in the OMZ core. Thus, is seems likely that the vertical distribution of the Foraminifera at these sites is influenced by the increase in labile organic matter on the seafloor surface following the presumed phytodetrital flux associated with the SW monsoon and by oxygen profiles within the sediment.

The idea that food and oxygen exert a strong influence on sediment penetration by benthic Foraminifera is consistent with other studies. There is considerable evidence that food (carbon content) is the main factor driving the population dynamics of benthic Foraminifera (Jones and Ross 1979; Altenbach 1992; Gooday 1994; Rathburn and Corliss 1994; Basson and Murray 1995; Mackensen et al. 1995; Murray and Alve 2000; Altenbach et al. 2003). It has also been suggested that seasonal deposition of phytodetritus may cause some species to migrate towards the surface in order to exploit and utilize labile organic matter (Gooday 1988; Gooday and Lambshead 1989; Gooday and Turley 1990; Gooday 1993; Kitazato and Ohga 1995; Kitazato et al. 2000). On the other hand, seasonal vertical migration of Foraminifera at a bathyal site (1450 m) in Sagami Bay, Japan, is believed to reflect a change in thickness of the oxygenated sediment layer as a result of phytodetritus deposition, rather than, or in addition to, a direct response to the phytodetritus itself (Kitazato and Ohga 1995; Ohga and Kitazato 1997).

Further evidence of how food availability can influence the vertical distribution of Foraminifera has emerged from experimental studies. For example, Nomaki et al. (2005a) observed an upwards migration of Foraminifera following a simulated pulse of phytodetritus to the seafloor during in situ experiments using sediments from Sagami Bay. However, Heinz et al. (2001, 2002) observed no migration of deep-sea Foraminifera to the upper sediment layers following the addition of three different

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algal food sources (D. tertiolecta, Amphiphora sp. and Pyramimonas sp.) in laboratory feeding experiments on sediments collected from the western Mediterranean Sea (919 m water depth).

4.6.4.2 Species-level patterns

In oxygenated settings, benthic Foraminifera colonise deeper sediment layers with individual species tending to live in particular vertical microhabitats (Corliss 1985; Gooday 1986; Jorissen 1999), although the levels that they occupy are not consistent from place to place. It has been suggested that different microhabitats reflect feeding preferences as well as tolerances to oxygen (Nomaki et al. 2005a). In the Pakistan Margin OMZ, individual species were generally restricted to the upper 1 cm sediment layer and even typically deep-infaunal species such as Globobulimina cf. G. pyrula displayed relatively shallow Average Living Depths (ALD5 < 1.21).

A seasonal comparison of the Average Living Depth of individual species was relatively inconclusive. However, a few species displayed a consistent pattern of seasonal vertical migration. The Average Living Depth of Uvigerina ex. gr. semiornata and Cancris auriculus became shallower from the spring intermonsoon to the SW monsoon season (U. ex. gr. semiornata at 140 m and 300 m; C. auriculus at 140 m only). Although a lack of oxygen is probably one factor driving these species towards the sediment surface (especially at the seasonally hypoxic 140-m site), the shallowing of the Average Living Depth following the SW monsoon may also be a response to the presumed deposition of labile organic matter (phytodetritus) on the sediment surface during this season. Changes in the vertical distributions of other species were more puzzling. Globobulimina cf. G. pyrula underwent a shallowing of its Average Living Depth at the 300-m site following the SW monsoon, compared to a deepening of its Average Living Depth at the 140-m site. The direction of movement at 300 m suggests a response to the increased food availability on the sediment surface (similar to U. ex. gr. semiornata and C. auriculus). However, the deepening of the Average Living Depth of this species at

140 m is unexpected, since the 140-m site was seasonally hypoxic during the SW monsoon.