The subtidal salt balance and the mechanisms driving the downgradient salt flux in the Hudson River estuary are investigated using measurements from a cross-channel mooring array of current meters, temperature and conductivity sensors, and cross-channel and along-estuary shipboard surveys obtained during the spring of 2002. Steady (subtidal) vertical shear dispersion, resulting from the estuarine exchange flow, was the dominant mechanism driving the downgradient salt flux, and varied by over an order of magnitude over the spring-neap cycle, with maximum values during neap tides and minimum values during spring tides. Corresponding longitudinal dispersion rates were as big as 2500 m 2 s Ϫ1 during neap tides. The salinity intrusion was not in a steady balance during the study period. During spring tides, the oceanward advective salt flux resulting from the net outflow balanced the time rate of change of salt content landward of the study site, and salt was flushed out of the estuary. During neap tides, the landward steady shear dispersion salt flux exceeded the oceanward advective salt flux, and salt entered the estuary. Factor-of-4 variations in the salt content occurred at the spring-neap time scale and at the time scale of variations in the net outflow. On average, the salt flux resulting from tidal correlations between currents and salinity (tidal oscillatory salt flux) was an order of magnitude smaller than that resulting from steady shear dispersion. During neap tides, this flux was minimal (or slightly countergradient) and was due to correlations between tidal currents and vertical excursions of the halocline. During spring tides, the tidal oscillatory salt flux was driven primarily by oscillatory shear dispersion, with an associated longitudinal dispersion rate of about 130 m 2 s Ϫ1.

Marine phytoplankton account for approximately half of global primary productivity , making their fate an important driver of the marine carbon cycle. Viruses are thought to recycle more than one-quarter of oceanic photosynthetically fixed organic carbon , which can stimulate nutrient regeneration, primary production and upper ocean respiration via lytic infection and the 'virus shunt'. Ultimately, this limits the trophic transfer of carbon and energy to both higher food webs and the deep ocean . Using imagery taken by the Moderate Resolution Imaging Spectroradiometer (MODIS) onboard the Aqua satellite, along with a suite of diagnostic lipid- and gene-based molecular biomarkers, in situ optical sensors and sediment traps, we show that Coccolithovirus infections of mesoscale (~100 km) Emiliania huxleyi blooms in the North Atlantic are coupled with particle aggregation, high zooplankton grazing and greater downward vertical fluxes of both particulate organic and particulate inorganic carbon from the upper mixed layer. Our analyses captured blooms in different phases of infection (early, late and post) and revealed the highest export flux in 'early-infected blooms' with sinking particles being disproportionately enriched with infected cells and subsequently remineralized at depth in the mesopelagic. Our findings reveal viral infection as a previously unrecognized ecosystem process enhancing biological pump efficiency.

Abstract.Observations from the Hudson River estuary are presented to discuss the interaction between secondary flows and stratification in the vicinity of a headland during highly stratified conditions. Analysis of the cross-stream momentum balance suggests that centrifugal accelerations are in near balance with the cross-stream baroclincity. Results emphasize that the interaction between stratification and secondary flows can set up a cross-stream internal seiche. This results in a reduction and subsequent reversal of the secondary flows downstream of a headland. Simple scaling arguments suggest that the spin-down time for tidally driven eddies is related to stratification. 23,207

[1] Observations taken during the Lagrangian Transport and Transformation Experiment (LaTTE) in 2005 indicated that the Hudson's river outflow formed a bulge of recirculating fluid that limits the volume of fresh water that is advected away in a coastal current. Focusing on an event that began with downwelling winds we made estimates of the freshwater flux in the coastal current and the fresh water inventory of the bulge. The coastal current was characterized by a surface advected plume in thermal wind balance. However, the freshwater transport in the coastal current was less than 1/2 of the total freshwater outflow. The bulge extended 30 km from the coast and 40 km in the alongshore direction and was evident in ocean color imagery. Recirculation in the bulge region was also apparent in daily averaged surface current radar data, but this flow pattern was obscured in the hourly data by tidal and wind-forcing even in the diurnal band. Nevertheless, many aspects of the Hudson's outflow are consistent with recent laboratory experiments and numerical simulations of buoyant discharges. The growing bulge transports the river's outflow to the head of the Hudson shelf valley where it crosses the 50 m isobath. Previous work in this region indicates that frontal features reside along this isobath. We observed fresh water being transported along this isobath and is suggestive of a rapid cross-shelf transport pathway for fresh water. Both the bulge formation and cross-shelf transport have significant biogeochemical implications.

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