Aphotic zone

Abstract: This paper presents a new coupled, one dimensional biological-physical model applied to the subtropical region near Bermuda. The physical component of the model, which is driven by smooth climatological forcing, successfully reproduces the long-term seasonal cycles of upper ocean temperature, salinity and boundary layer depth from Hydrostation S. The nitrogen-based biological model, which includes the effects of photoadaptation, phytoplankton aggregation, and particle remineralization in the aphotic zone, shows significant skill in capturing the major features of the annual chlorophyll field (e.g. spring bloom, deep chlorophyll maximum) and depth-integrated chlorophyll and primary production as exhibited by the U.S. JGOFS Bermuda Atlantic Time-series Study (BATS) data. The introduction of variable phytoplankton chlorophyll-to-nitrogen ratios is found to be important for simulating the subsurface chlorophyll maximum, and the model solutions show a realistic deep nitracline in the summer and a low annual average f -ratio of ∼0.21 compared to previous modeling work. The performance of the model solutions are weakest during the late summer, when the model can not supply enough nutrients to support the high production observed in the stratified near-surface waters. The coupled model has large winter production values, leading to a substantial export of organic material from the euphotic zone via downward turbulent mixing. The model predicts a total export production from the euphotic zone of 0.24 mol N m −2 year −1 , approximately equally partitioned between particle sinking and suspended matter detrainment. The bulk of the export production is remineralized in the shallow aphotic zone, and only a small fraction is transported below the depth of the maximum winter mixed layer and thus contributes to “biological pump”.

Abstract: Particulate organic matter (POM) has a central role in the vertical transport of material in the sea1. In the open ocean, POM is produced in the euphotic layer by phytoplankton and degraded in the aphotic layer during sinking to the sea floor. Isotopic abundance of biophilic elements such as C and N in POM is altered by isotopic fractionations associated with biochemical reactions2. Natural abundances of 13C or 15N thus provide useful information on biochemical behaviour of POM in the sea. We report here the first comprehensive data on the vertical distribution of 15N in suspended POM3 collected at a station in the northeastern Indian Ocean. Also, we discuss how nitrogen isotopic analysis could be used for the identification and quantification of the vertical transport processes.

Abstract: A mesoscale resolution biogeochemical survey was carried out in the vicinity of the U.S. Joint Global Ocean Flux Study Bermuda Atlantic Time-series Study (BATS) site during the summer of 1996. Real-time nowcasting and forecasting of the flow field facilitated adaptive sampling of several eddy features in the area. Variations in upper ocean nutrient and pigment distributions were largely controlled by vertical isopycnal displacements associated with the mesoscale field. Shoaling density surfaces tended to introduce cold, nutrient-rich water into the euphotic zone, while deepening isopycnals displaced nutrient-depleted water downward. Chlorophyll concentration was generally enhanced in the former case and reduced in the latter. Eddy-induced upwelling at the base of the euphotic zone was affected by features of two different types captured in this survey: (1) a typical mid-ocean cyclone in which doming of the main thermocline raised the near-surface stratification upward and (2) a mode water eddy composed of a thick lens of 18°C water, which pushed up the seasonal thermocline and depressed the main thermocline. Model hindcasts using all available data provide a four-dimensional context in which to interpret temporal trends at the BATS site and two other locations during the 2 weeks subsequent to the survey. Observed changes in near-surface structure at the BATS site included shoaling isopycnals, increased nutrient availability at the base of the euphotic zone, and enhanced chlorophyll concentration within the euphotic zone. These trends are explicable in terms of a newly formed cyclone that impinged upon the site during this time period. These observations reveal that eddy upwelling has a demonstrable impact on the way in which the nitrate-density relationship changes with depth from the aphotic zone into the euphotic zone. A similar transition is present in the BATS record, suggesting that eddy-driven upwelling events are present in the time series of upper ocean biogeochemical properties. The variability in main thermocline temperature and nitrate in this synoptic spatial survey spans the range observed in these quantities in the 10-year time series available at BATS to date (1988–1998).

Abstract: The permanent ice cover of Lake Vida (Antarctica) encapsulates an extreme cryogenic brine ecosystem (−13 °C; salinity, 200). This aphotic ecosystem is anoxic and consists of a slightly acidic (pH 6.2) sodium chloride-dominated brine. Expeditions in 2005 and 2010 were conducted to investigate the biogeochemistry of Lake Vida’s brine system. A phylogenetically diverse and metabolically active Bacteria dominated microbial assemblage was observed in the brine. These bacteria live under very high levels of reduced metals, ammonia, molecular hydrogen (H2), and dissolved organic carbon, as well as high concentrations of oxidized species of nitrogen (i.e., supersaturated nitrous oxide and ∼1 mmol⋅L−1 nitrate) and sulfur (as sulfate). The existence of this system, with active biota, and a suite of reduced as well as oxidized compounds, is unusual given the millennial scale of its isolation from external sources of energy. The geochemistry of the brine suggests that abiotic brine-rock reactions may occur in this system and that the rich sources of dissolved electron acceptors prevent sulfate reduction and methanogenesis from being energetically favorable. The discovery of this ecosystem and the in situ biotic and abiotic processes occurring at low temperature provides a tractable system to study habitability of isolated terrestrial cryoenvironments (e.g., permafrost cryopegs and subglacial ecosystems), and is a potential analog for habitats on other icy worlds where water-rock reactions may cooccur with saline deposits and subsurface oceans.