Variations in Biogeochemical Parameters

Settling POM was enriched in AAs in M-series traps compared with in N-series traps. In all cases, AA content decreased from the shallow trap to the deep trap at the same site. This vertical decrease is commonly interpreted as a greater loss of labile AAs relative to other metabolites because of ongoing microbial degradation of settling POM. The deep traps at all sites clearly recorded higher b-Ala+g-Aba contents relative to the shallow traps. In the hemipelagic zone, the average b-Ala+g-Aba content was higher in the Ml traps than in the N2 traps. The shallow traps at sites M3 and N10 in the warm pool zone had nearly equal b-Ala+g-Aba concentrations, while at the eastern end of the pool (site N3) b-Ala+g-Aba content was relatively low in the shallow trap. In the equatorial upwelling zone, the shallow trap at site M5 recorded a b-Ala+g-Aba content comparable to that recorded in the shallow traps at the other sites.

Amino acid carbon and nitrogen contents relative to OC and TN (AA-C and AA-N, respectively) showed a vertical decrease between the shallow and deep traps at all sites (Fig. 5). Also, the M-series traps recorded considerably higher AA-C and AA-N contents than the N-series traps. In all three zones, the ratios Asp/b-Ala, Glu/g-Aba and Glc-NH2/Gal-NH2 showed a vertical decrease between the shallow and deep traps at the same site. The site M1 traps in the hemipelagic zone recorded lower Asp/b-Ala ratios than the N2 traps in the same zone. In the warm pool zone, the M3 trap recorded a slightly higher Asp/b-Ala ratio than the N10 trap, but the N3 traps in this zone recorded the highest values. In the warm pool and equatorial upwelling zones, the Glc-NH2/Gal-NH2 ratio was much larger in the M-series traps than in the N-series traps, but in the hemipelagic zone, the difference between the M1 and N2 traps was relatively small. The M-series traps recorded slightly higher values for the AA/HA ratio than the N-series traps.

At site Sll, samples from the end of December 1995 and the beginning of January 1996 showed the highest Glc-NH2/Gal-NH2 ratios, which were associated with relatively low AA/HA ratios. These ratios indicate that zooplanktonic debris and faecal pellets were a major vehicle for POM transport to the deep ocean during the austral summer. At site S12, the Asp/b-Ala, Glu/g-Aba and AA/HA ratios suggest that higher fluxes of AA and HA brought microbially less-degraded POM to the trap depth. The indicator ratios Asp/b-Ala and Glu/g-Aba in the deep trap suggest that the samples with lower AA contents had microbially more-degraded POM compared with those with higher AA content. The site S12 deep trap was located near the Queensland Plateau, which might be a source of laterally advected material,

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Figure 5: Regional variations in average values of amino acid carbon and nitrogen with respect to total organic carbon and total nitrogen, respectively, in particles settling in the western equatorial and south-eastern Pacific Ocean.

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Figure 5: Regional variations in average values of amino acid carbon and nitrogen with respect to total organic carbon and total nitrogen, respectively, in particles settling in the western equatorial and south-eastern Pacific Ocean.

especially under the influence of the EAC. Moreover, mid-depth circulation may re-suspend sediments lying on the sea floor (Davis, 1998). Re-suspended POM is generally depleted in labile components and therefore has a high C/Natomic ratio. The samples collected during the second half of June 1995 showed a sharp increase in the C/Natomic ratio between the shallow (6.3) and deep trap (8.6), while the AA and HA contents decreased from 92.5 and 5.2 mgg-1 to 32.7 and 3.5 mgg-1, respectively. Although these changes are to some degree expected because of the ongoing degradation of settling POM, their abruptness indicates mixing of resuspended POM with settling POM. The samples collected by the deep trap towards the end of July and in

December 1995 had much higher indicator ratios compared with the shallow trap samples corresponding to the same period. The higher values indicate a possible contribution of relatively fresh POM laterally advected from more productive surface waters.

Arrival of relatively fresh POM at trap depth is possible through the formation of large, rapidly settling aggregates of phytoplankton, which have been observed to exist under low zooplanktonic grazing pressure (Peinert et al., 1989). Modelling and experimental studies by Sarnelle (1999) have also shown that herbivorous zooplankton tend to reduce the vertical flux of phytoplanktonic biomass in lakes and possibly in other aquatic systems. In addition, rapid settling of particles during periods of high primary productivity in surface waters of the north-eastern subarctic Pacific Ocean (site OSP) has been observed to reduce loss of organic components into the water column (Wong et al., 1999). In contrast, the low C/Natomic ratio at site S13 during September 1995 was associated with high Glc-NH2/Gal-NH2 and low AA/HA ratios, indicating considerable influence of zooplanktonic debris; thus, POM became enriched in proteinaceous material, bringing the ratio down to 5.9. The C/Natomic ratio at site S13 was, in general, slightly lower than that at sites S11 and S12. The lower ratios (implying fresh POM) at site S13 may be attributed to rapid settling or to a reduced rate of microbial degradation of POM. In the mesopelagic environment, free-living bacteria are the principal mediators of particle decomposition (Cho and Azam, 1988), achieved by extracellular enzymatic hydrolysis exceeding the growth requirements of the bacteria (Smith et al., 1992). The enzymatic degradation is dependent on ambient temperature, which leads to enhanced export of relatively unhydrolysed POM in low-temperature waters (Vetter and Deming, 1994), e.g., at site S13. In addition, SST has been shown to influence primary productivity (Prasanna Kumar et al., 2001), which in turn influences particle flux to the deep ocean (Pace et al., 1987). Mulhearn et al. (1986) have reported the presence of eddies in the Tasman Sea whose influence extends to abyssal depths. Such eddies can redistribute suspended particles in the mid-ocean and consequently lead to higher particle fluxes, such as those recorded by the site S13 trap.

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