Particulate fluxes in the subtropical convergence region and other marine environments of New Zealand
The thesis was designed to address two themes: (1) sediment trap methods and technique development; and (2) oceanic sediment fluxes within the Chatham Rise-Subtropical Convergence ecosystem, east of New Zealand. Accordingly, the principal objectives of Theme 1 were to design, construct and deploy a sediment trap system for sampling sinking marine particles and to evaluate hydrodynamic biases in sediment traps from field studies. Theme 2 focuses on biological and physical processes affecting modem particulate sedimentation in the vicinity of the Subtropical Convergence Zone, and the magnitude and composition of particulate fluxes within the Chatham Rise-Subtropical Convergence region. An overall conservative strategy was adopted for the sediment trap design criteria since critical aspects of the bio-physical environment of Chatham Rise-Subtropical Convergence were poorly known. A multiple arrangement of 8 to 12 baffled cylindrical traps deployed at set water depths on free-floating sediment trap arrays was the preferred design. Basal high density brines with formalin as a poison/preservative were employed for deployments scheduled to last 2-3 days in the winter and spring of 1993. Pilot studies in autumn 1992 and 1993 allowed practicalities of sediment trap deployments to be evaluated. Field experiments in Evans Bay, Wellington Harbour, showed that there were' minimal hydrodynamic interactions between traps on the same array. Furthermore, baffles did not significantly affect trapping efficiency, whereas brine volume had a profound effect. In the latter case, traps filled completely with a high-density salt brine collected 2-3 times less material than traps with basal brine thicknesses equal in height to 1- and 2.5-cylinder diameters. The resultant free-floating sediment trap arrays were deployed as part of the multi-disciplinary New Zealand Joint Global Ocean Flux Study (JGOFS) in winter and spring 1993. This study was designed to determine the trophic pathways and transferal rates of carbon within pelagic food webs in contrasting water masses (subantarctic and subtropical) on either side of the Subtropical Convergence. The sediment trap work was undertaken to evaluate the efficiency of the "biological pump" in removing particulate material from the surface layers of the ocean, as part of Theme 2 of the thesis. Despite substantial temporal and spatial differences in physical and biological parameters measured in subantarctic, convergence and subtropical waters, total mass and particulate phosphorus fluxes were not significantly different across the three water types in winter or spring. High levels of variability between trap samples are attributed to errors associated with subsampling procedures; a problem that appears to be widespread in other, more exhaustive sediment trap experiments in oligotrophic environments. In order to improve the statistical power of the experiments, more than two free-floating sediment trap moorings are required in similar studies to discriminate between water type differences. Particulate fluxes appeared to be decoupled from upper ocean primary production, and in the case of deployments within the Subtropical Convergence, affected markedly by resuspension of bottom sediments from the crest of Chatham Rise caused probably by strong tidal currents that are known to operate across the Rise. The presence of photosynthetic pigments in trap samples from 100 to 550 m depths suggests that sinking material must be transported rapidly out of well lit surface waters, probably as intact marine aggregates or mesozooplankton faecal pellets. The low concentrations of phaeopigments, normally attributed to zooplankton grazing or algal senescence, suggest that organic material has not been degraded in the upper ocean prior to sinking. This observation is perhaps attributable to the aforementioned rapid sedimentation, or the occurrence of unbleached pigments preserved in faecal pellets due to low conversion rates of chlorophyll a to phaeopigments in zooplankton guts. Furthermore, low rates of pigment export as a function of phytoplankton biomass and primary production ( <5%) suggest that other processes were operating in the upper water column to prevent pronounced sedimentation of organic material out of surface layers. Microzooplankton grazing of organic material is likely to be important in preferentially increasing particle residence times in the upper ocean. This mechanism may be concomitant with other processes including bacterial decomposition, mesozooplankton foraging activities and shallow surface stratification, especially in spring. The Southwest Pacific Ocean is recognised as an important regional, probably biologically mediated sink for atmospheric carbon dioxide. Sediment trap results from Chatham Rise Subtropical Convergence are statistically ambivalent in terms of assessing the efficiency of the "biological pump" in removing carbon in organic material from the upper ocean by sinking processes. However, the persistence of undegraded pigmented material at 550 m depth, the continental margin affinities suggested by relatively high mean mass fluxes, and the observation that particulate carbon can comprise up to 40% of mass flux, all suggest that "biological pump" efficiency is also enhanced in this region. More work is required, however, to determine the temporal and spatial variability of particulate fluxes in the Southwest Pacific Ocean in order to develop an understanding of how the "biological pump" functions in this region. Future studies will have to overcome the statistical design criteria suggested by the present study (i.e., three or more free-floating arrays to be deployed at any one time), and should investigate the collection of longer time-series measurements made possible by the use of deep ocean, bottom-moored, time-incremental sediment traps. In order to fully understand sediment trap interpretations, contemporaneous measurements of food web structure and processes must also be made in an oceanic region largely devoid of extensive biological oceanographic data-sets. In conclusion, despite the shortcomings of the sediment trap method (i.e., hydrodynamic biases, zooplankton contamination, lack of independent calibration), traps remain the only technique available to the oceanographic community that provides a quantifiable measurement of export fluxes from the upper to deep ocean. Until modem technology develops a truly neutrally buoyant sediment trap system with zooplankton exclusion devices, relatively simple sediment trap arrays, as utilised by the present study, will continue to provide the best initial estimates of oceanic particulate fluxes from previously uncharacterised regions of the global ocean.
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