Search

Lagrangian studies of virus activity in pelagic environments over extended temporal scales are rare. To address this, viruses and bacteria were examined during the course of a natural phytoplankton bloom in the pelagic South Pacific Ocean east of New Zealand. Daily samples were collected in a mesoscale eddy from year days 263-278 ( September 19th- October 4th, 2008). The productive bloom transitioned from a diatom to a pico- and nanoplankton-dominated system, resulting in chlorophyll a concentrations up to 2.43 μg L−1. Virus abundances fluctuated c. 10-fold (1.8 × 1010-1.3 × 1011 L−1) over 16 days. The production rates of virus particles were high compared with those reported in other marine systems, ranging from 1.4 × 1010 to 2.1 × 1011 L−1 day−1. Our observations suggest viruses contributed significantly to the mortality of bacteria throughout the bloom, with 19-216% of the bacterial standing stock being lysed daily. This mortality released nutrient elements ( N, Fe) that likely helped sustain the bloom through the sampling period. Parametric analyses found significant correlations with both biotic (e.g. potential host abundances) and abiotic parameters (e.g. nutrient concentrations, temperature). These observations demonstrate that viruses may be critical in the extended maintenance of regeneration-driven biological production. [ABSTRACT FROM AUTHOR]

Abstract: Upper ocean cycling of dissolved and particulate iron was investigated within an eddy during a study of the annual subtropical phytoplankton bloom east of New Zealand in the austral spring of 2008. During this GEOTRACES process study, dissolved iron surface mixed layer concentrations were initially high at ~0.6nmolkg−1 and declined to ~0.03nmolkg−1 due to biological consumption during a diatom-dominated bloom. The consequent iron limitation of the phytoplankton assemblage resulted in the decline and downward export of the bloom. Particulate iron concentrations were high in the surface mixed layer and varied between 3.9nmolL−1 and 12.1nmolL−1 during the onset and export of the bloom, respectively. The particulate iron and manganese results, along with a tracer model for the establishment of the eddy, suggest that the likely origin of the upper ocean particulate and dissolved iron pools within the eddy was from the continental margins along eastern New Zealand. Iron to aluminium (Fe:Al) ratios for suspended particulate material collected using McLane pumps deployed at 100m and 200m were significantly higher (range 0.16 to 2.90) than Fe:Al ratios for sinking particulate material intercepted using free-floating sediment traps (range 0.19 to 0.23) at corresponding deployment depths to the pumps. Based on the particulate Fe:Al ratios obtained for McLane pump collected samples, greater than 70% iron within the mixed layer was biogenic, indicating that either the resident biota are efficiently retaining iron or iron is associated with organic detritus. Iron budgets, based on the dissolved, suspended and sinking particulate iron datasets, were constructed for the evolution and subsequent decline phases of the diatom bloom. The budgets reveal that the turnover time for dissolved iron within the mixed layer was on the order of days, suggesting that iron was rapidly exchanged from the dissolved pool to the particulate biogenic and lithogenic pools. In contrast, the residence time for iron in the particulate biogenic pool was in the order of 5–8months indicating that iron was rapidly recycled but strongly retained by the biological community compared to the pelagic residence time for lithogenic iron, which was around 10–24days. The regeneration of iron from biogenic particles was highest immediately below the euphotic zone (~50m) and decreased with depth. Estimated regeneration fluxes for dissolved iron released from biogenic particles at 100m depth were 0.34±0.26nmolm−3 d−1 during the development phase of the bloom and 7.1±3.9nmolm−3 d−1 at the peak of the bloom, when the overall export flux for particulate iron had increased 2-fold. Our results suggest a dynamic balance between iron regeneration from particulate organic material below the euphotic zone and the lability of this organic matter. Finally, the different Fe:Al ratios obtained for suspended and sinking particulate matter indicate that these pools are biogeochemically different. Future GEOTRACES process study work should focus on determining the biogeochemical differences of iron between these pools because this will assist with interpreting GEOTRACES sections and evaluating the impact of iron biogeochemistry on pelagic production and remineralisation within the global ocean. [Copyright &y& Elsevier]

Ocean acidification and greenhouse warming will interactively influence competitive success of key phytoplankton groups such as diatoms, but how long-term responses to global change will affect community structure is unknown. We incubated a mixed natural diatom community from coastal New Zealand waters in a short-term (two-week) incubation experiment using a factorial matrix of warming and/or elevated pCO2 and measured effects on community structure. We then isolated the dominant diatoms in clonal cultures and conditioned them for 1 year under the same temperature and pCO2 conditions from which they were isolated, in order to allow for extended selection or acclimation by these abiotic environmental change factors in the absence of interspecific interactions. These conditioned isolates were then recombined into 'artificial' communities modelled after the original natural assemblage and allowed to compete under conditions identical to those in the short-term natural community experiment. In general, the resulting structure of both the unconditioned natural community and conditioned 'artificial' community experiments was similar, despite differences such as the loss of two species in the latter. pCO2 and temperature had both individual and interactive effects on community structure, but temperature was more influential, as warming significantly reduced species richness. In this case, our short-term manipulative experiment with a mixed natural assemblage spanning weeks served as a reasonable proxy to predict the effects of global change forcing on diatom community structure after the component species were conditioned in isolation over an extended timescale. Future studies will be required to assess whether or not this is also the case for other types of algal communities from other marine regimes.