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 vertical gradients in phytoplankton community structure were made through the water column of the seasonally stratified Celtic Sea, including within the thermocline. A deep chlorophyll maximum (DCM) was located within the thermocline at all stations, coupled to the nitracline. Vertical gradients in phytoplankton community composition were routinely observed within the thermocline. The cell abundance maxima for Synechococcus occurred in the upper part of the DCM coincident with a picoeukaryote abundance minima. Picoeukaryote abundance typically increased at or just above the peak of the DCM. Diatoms were observed occasionally at the DCM peak. Pigment compositions and phytoplankton absorption spectra indicated that the different phytoplankton communities were chromatically well adapted to the spectral composition of irradiance at the depths where they occurred in the water column. Profiles of vertical eddy diffusivity revealed that timescales for mixing between the phytoplankton layers within the thermocline were in excess of typical phytoplankton growth rates. The observed vertical gradients in community structure could therefore result from selection and niche partitioning of phytoplankton types on the light and nutrient gradient within the thermocline. The data further indicate that the pigments, light absorption characteristics, and cell size contribute to the phytoplankton selection process.The Celtic Sea, part of the temperate Northwest European shelf, is a tidally dynamic environment where water column structure is strongly influenced by the balance of solar heating and tidally generated mixing (Simpson and Hunter 1974). Much of the region becomes thermally stratified in April, initiating the spring phytoplankton bloom (Pingree et al. 1976). The water column remains stratified throughout the summer, with the thermocline forming a boundary between nutrient-depleted surface mixed layer (SML) above and the nutrient-rich bottom mixed layer (BML) below (Pingree et al. 1977). During this seasonal stratification, a deep chlorophyll maximum (DCM) is present within the thermocline (Pingree et al. 1977). The DCM is typically located toward the base of the density gradient, lies within the euphotic zone, and is strongly coupled to the nitracline (Holligan et al. 1984a,b;Sharples et al. 2001).Energy from tidally induced turbulence is dissipated at the base of the thermocline, causing upward mixing of nitrate into the thermocline from the BML and downward mixing of phytoplankton from the base of the DCM (Sharples et al. 2001). Previous observations in the Celtic Sea have revealed a vertical flux of nitrate into the thermocline of around 2 mmol N m 22 d 21 with the DCM maintained at a depth corresponding to ,5% of surface irradiance (Sharples et al. 2001). The DCM is often observed to be a biomass and photosynthesis maximum as well as a pigment maximum (Holligan et al. 1984a,b;Pemberton et al. 2004;Moore et al. 2006). Monospecific blooms with chlorophyll a (Chl a) concentrations .10 mg Chl a m 23...

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In pycnoclines, the density differences can cause light scattering -schlieren -even though only few particulate scatterers may be present. This may pose problems for the interpretation of results obtained with instruments relying on light scattering and transmission, for example the LISST (Laser In Situ Scattering and Transmissometry) particle sizer, and various cameras. Here, the influence of schlieren on in situ forward light scattering, beam attenuation and image analysis is evaluated using a LISST-100 and a digital floc camera. Automated image analysis routines detect schlieren as particles, causing an apparent increase in particle size and volume. Re-analysis omitting schlieren-affected parts of the images reveals no increase. LISST beam attenuation and Volume Scattering Function (VSF) measurements indicate that schlieren can cause increases in beam attenuation due to a marked increase in the VSF at angles smaller than ~1.5°-2°, and falsely indicate accumulation of suspended particles in the pycnocline. Light scattering caused by density differences can also cause multiple scattering, which produces an apparent decrease in particle size derived from the LISST. Schlieren is visible in images when the buoyancy frequency exceeds ~0.12 s -1

Achieving urban flood resilience at local, regional and national levels requires a transformative change in planning, design and implementation of urban water systems. Flood risk, wastewater and stormwater management should be re-envisaged and transformed to: ensure satisfactory service delivery under flood, normal and drought conditions, and enhance and extend the useful lives of ageing grey assets by supplementing them with multi-functional Blue-Green infrastructure. The aim of the multidisciplinary Urban Flood Resilience (UFR) research project, which launched in 2016 and comprises academics from nine UK institutions, is to investigate how transformative change may be possible through a whole systems approach. UFR research outputs to date are summarised under three themes. Theme 1 investigates how Blue-Green and Grey (BG + G) systems can be co-optimised to offer maximum flood risk reduction, continuous service delivery and multiple co-benefits. Theme 2 investigates the resource capacity of urban stormwater and evaluates the potential for interoperability. Theme 3 focuses on the interfaces between planners, developers, engineers and beneficiary communities and investigates citizens’ interactions with BG + G infrastructure. Focussing on retrofit and new build case studies, UFR research demonstrates how urban flood resilience may be achieved through changes in planning practice and policy to enable widespread uptake of BG + G infrastructure.

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