Measurements of the intra-tidal and spring-neap variation in the vertical flux of nitrate into the base of the sub-surface chlorophyll maximum (SCM) were made at the shelf edge of the Celtic Sea, a region with strong internal mixing driven by an internal tide. The neap tide daily mean nitrate flux was 1.3 (0.9-1.8, 95% confidence interval) mmol m 22 d 21 . The spring tide flux was initially estimated as 3.5 (2.3-5.2, 95% confidence interval) mmol m 22 d 21 . The higher spring tide nitrate flux was the result of turbulent dissipation occurring within the base of the SCM as compared to deeper dissipation during neap tides and was dominated by short events associated with the passage of internal solitons. Taking into account the likely under-sampling of these short mixing events raised the spring tide nitrate flux estimate to about 9 mmol m 22 d 21 . The neap tide nitrate flux was sufficient to support substantial new production and a considerable fraction of the observed rates of carbon fixation. Spring tide fluxes were potentially in excess of the capacity of the phytoplankton community to uptake nitrate. This potential excess nitrate flux during spring tides may be utilized to support new production during the lower mixing associated with the transition toward neap tide. The shelf edge is shown to be a region with a significantly different phytoplankton community as compared to the adjacent Celtic Sea and northeast Atlantic Ocean, highlighting the role of gradients in physical processes leading to gradients in ecosystem structure.3 Present address: Proudman Oceanographic Laboratory, 6 Brownlow Street, Liverpool, L3 5DA, United Kingdom. AcknowledgmentsOur thanks to the crew of the RRS Charles Darwin (cruise CD173) and the technical staff of the U.K. National Marine Facilities. We are grateful for the constructive comments from two anonymous reviewers, which helped improve this paper.

Observations of the vertical structure of density, concentrations of chlorophyll a and nitrate, and turbulent dissipation rates were made over a period of 25 h in a well-stratified shelf region in the Western English Channel, between neap and spring tides. Maximum turbulent dissipation at the base of the thermocline occurred almost 5 h after maximum tidal currents. This turbulence aids phytoplankton growth by supplying bottom-layer nutrients into the subsurface chlorophyll maximum but reduces phytoplankton concentrations in the thermocline by mixing cells from the base of the subsurface maximum into the bottom mixed layer. The turbulent dissipation observations were used to estimate an average nitrate flux into the thermocline of 2.0 (0.8-3.2, 95% confidence interval) mmol m, which is estimated to have been capable of supporting new phytoplankton growth at a rate of 160 (64-256) mg C m Ϫ2 d Ϫ1 . Turbulent entrainment of carbon from the base of the subsurface biomass maximum into the bottom mixed layer was observed to be 290 (120-480) mg C m Ϫ2 d Ϫ1 . This apparent excess export from the chlorophyll maximum is suggested to be a feature of the spring-neap cycle, with export dominating as the tidal turbulence increases toward spring tides and erodes the base of the thermocline. The observed rate of carbon export into the bottom mixed layer could account for as much as 25% of the gross annual primary production in stratifying shelf seas. Such turbulent losses, combined with grazing losses and low light levels, suggest that phytoplankton need to be highly adapted to environmental conditions within the thermocline in order to survive.The seasonal thermocline is an important physical barrier in the ocean, separating the surface mixed layer from the deeper water. The stability of the vertical temperature (density) gradient inhibits diapycnal transfer of properties (e.g., momentum, heat, nutrients, algal cells, and oxygen). As the transition region between the nutrient-poor, well-lit surface layer and the darker, nutrient-rich deeper water, the thermocline plays a role in determining the biological properties of the water column. Since the development of continuous fluorescence measurements as a technique for observing the vertical structure of algae (Lorenzen 1966), the thermocline has been observed to be a region of enhanced chlorophyll concentration (e.g., Anderson 1969;Cullen and Eppley 1981;Holligan et al. 1984). Observed concentrations of subsurface chlorophyll range between 0.5 mg mϪ3 in the open ocean up to 100 mg m Ϫ3 in shelf seas. Although in some cases this higher chlorophyll concentration has been shown to be the result of a lower carbon : chlorophyll ratio (Steele 1 Corresponding author (j.sharples@soc.soton.ac.uk). AcknowledgmentsRay Wilton (University of Wales, Bangor) provided invaluable technical support for FLY. Our thanks to the crew and RVS support staff on RRS Challenger during cruise CH145.

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[1] We present a new technique for the estimation of profiles of the rate of dissipation of turbulent kinetic energy (e, TKE) in the marine environment using a standard acoustic Doppler current profiler (ADCP). The technique is based on the structure function method used in radar meteorology. The new method is validated through comparisons of e estimates from a structure function with simultaneous measurements of profiles of e made using a freefall profiler, and estimates of the rate of production of TKE using the ADCP variance method. There is a good agreement between the estimates, although some differences in absolute values. A difference in e estimates between the upstream and downstream beams is attributed to the presence of a significant Reynolds stress.Citation: Wiles, P. J., T. P. Rippeth, J. H. Simpson, and P. J. Hendricks (2006), A novel technique for measuring the rate of turbulent dissipation in the marine environment, Geophys. Res. Lett., 33, L21608,

A 15-year duration record of mooring observations from the eastern (>70°E) Eurasian Basin (EB) of the Arctic Ocean is used to show and quantify the recently increased oceanic heat flux from intermediate-depth (∼150-900 m) warm Atlantic Water (AW) to the surface mixed layer (SML) and sea ice. The upward release of AW heat is regulated by the stability of the overlying halocline, which we show has weakened substantially in recent years. Shoaling of the AW has also contributed, with observations in winter 2017-2018 showing AW at only 80 m depth, just below the wintertime surface mixed layer (SML), the shallowest in our mooring records. The weakening of the halocline for several months at this time implies that AW heat was linked to winter convection associated with brine rejection during sea ice formation. This resulted in a substantial increase of upward oceanic heat flux during the winter season, from an average of 3-4 W/m2 in 2007-2008 to >10 W/m2 in 2016-2018. This seasonal AW heat loss in the eastern EB is equivalent to a more than a two-fold reduction of winter ice growth. These changes imply a positive feedback as reduced sea ice cover permits increased mixing, augmenting the summer-dominated ice-albedo feedback.

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A detailed set of observations are presented of the tidal forcing and basin response of Loch Etive, a jet-type fjordic system on the west coast of Scotland. The characteristics of the tidal jet observed during a spring tide are discussed in detail, and with reference to laboratory studies of Baines and Hoinka (1985). Although the system is categorized as a jet basin during spring tides (when the mode-1 densimetric Froude number exceeds 1) and a wave basin during neap tides (when the Froude number remains below 1), a mode-1 baroclinic wave response is observed throughout the spring/neap cycle. Of the total incident tidal energy, 16% is lost from the barotropic tide. The ratio between loss to bottom friction, barotropic form drag and baroclinic wave drag is estimated to be 1:4:1 (1:4:3.3) at springs (neaps). Despite this, during a spring tide, a 20-m amplitude baroclinic mode-1 wave is observed to propagate along the full length of the basin at a speed of 0.2 m s -1 , somewhat slower than the predicted linear mode-1 phase speed. A hydrographic section supports the implication of the dissipation of the baroclinic wave towards the loch head. The stratification of the upper layers is observed to decrease rapidly landward of the 40-m isobath, a possible signature of enhanced diapycnal mixing in the shallower reaches towards the loch head.