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The marine laboratories in Plymouth have sampled at two principle sites in the Western English Channel for over a century in open-shelf (station E1; 50° 02′N, 4° 22′W) and coastal (station L4; 50° 15′N, 4° 13′W) waters. These stations are seasonally stratified from late-April until September, and the variable biological response is regulated by subtle variations in temperature, light, nutrients and meteorology. Station L4 is characterized by summer nutrient depletion, although intense summer precipitation, increasing riverine input to the system, results in pulses of increased nitrate concentration and surface freshening. The winter nutrient concentrations at E1 are consistent with an open-shelf site. Both stations have a spring and autumn phytoplankton bloom; at station E1, the autumn bloom tends to dominate in terms of chlorophyll concentration. The last two decades have seen a warming of around 0.6°C per decade, and this is superimposed on several periods of warming and cooling over the past century. In general, over the Western English Channel domain, the end of the 20th century was around 0.5°C warmer than the first half of the century. The warming magnitude and trend is consistent with other stations across the north-west European Shelf and occurred during a period of reduced wind stress and increased levels of insolation (+20%); these are both correlated with the larger scale climatic forcing of the North Atlantic Oscillation. [ABSTRACT FROM PUBLISHER]

The seasonality of the mass specific scattering and backscattering by different components of the suspended particulate matter (SPM) was investigated at a coastal station in the Western English Channel. This study considered different components of the SPM, including inorganic and organic suspended matter, chlorophyll a and six phytoplankton groups. The data set comprised a 9-year time series of observations at station L4. Scattering variability was best explained by the particulate organic matter content, with significant differences between spring–summer and autumn–winter. Particulate backscattering variability was best explained by the inorganic fraction, with no clear seasonal variation. The temporal variations of the spectral slopes of these optical properties were related to the particle composition and size distribution slope. [ABSTRACT FROM PUBLISHER]

The article discusses a study which investigates how physico-chemical and biological variables influence the concentrations of dimethyl sulphide (DMS) and its precursor β-dimethylsulphoniopropionate (DMSP) through a seasonal cycle in shelf seas of the western English Channel. The study aims to enhance the understanding on the functional roles of DMS and DMSP by determining the seasonal variations of its concentrations, identifying the relation of changes in concentrations to phytoplankton abundance and taxonomy, and comparing the seasonal patterns of DMS and DMSP in relation to irradiance levels. The results suggest that the classic DMS summer paradox of high concentrations of DMS in relation to DMSP might only exist in temperate shelf areas.

Abstract: This paper presents an optical classification of station L4 south of Plymouth in the western English Channel based on satellite and in situ observations. Weekly-sampled in situ data from 2003 and 2004 are presented on temperature and salinity with depth, together with surface measurements of absorption by phytoplankton, coloured dissolved organic matter (CDOM) and non-algal particles (NAP), phytoplankton counts, HPLC chl-a concentrations and suspended particulates. Measured absorption values are compared with a published optical model used to estimate the magnitude and natural variability of absorption by in-water optically active constituents in case 1 waters. Satellite retrievals of remote sensing reflectance, R rs, are compared with model-based limits. L4 is found to be classed as turbid for ~35% of the year (in winter) related to the vertical stability and suspended particulate matter. In situ absorption data reveal an excess of CDOM absorption over that expected in modelled case 1 waters, while phytoplankton and NAP absorption are more consistent with case 1. Time series of satellite and in situ surface chl-a and vertical diffuse attenuation coefficient at 490 nm agree well statistically, probably as a result of the range of chl-a encountered and the use of R rs ratio based satellite algorithms. There was no evidence of coastal adjacency effects upon the chl-a retrievals. L4 can be considered as case 1 or case 2 depending upon the time period, the optical parameter or even the wavelength. This suggests that simple labelling of the site as case 1 or 2 is misleading and instead the station should be characterised in terms of the individual inherent optical property of interest. [Copyright &y& Elsevier]

Abstract: We present over 900 carbonate system observations collected over four years (2007–2010) in the Western English Channel (WEC). We determined CO2 partial pressure (pCO2), Total Alkalinity (TA) and Dissolved Inorganic Carbon (DIC) along a series of 40km transects, including two oceanographic stations (L4 and E1) within a sustained coastal observatory. Our data follow a seasonal pattern of CO2 undersaturation from January to August, followed by supersaturation in September–October and a return to near-equilibrium thereafter. This pattern is explained by the interplay of thermal and biological sinks in winter and spring–summer, respectively, followed by the breakdown of stratification and mixing with deeper, high-CO2 water in autumn. The drawdown of DIC and inorganic N between March and June with a C:N ratio of 8.7–9.5 was consistent with carbon over-consumption during phytoplankton growth. Monthly mean surface pCO2 was strongly correlated with depth integrated chlorophyll a highlighting the importance of subsurface chlorophyll a maxima in controlling C-fluxes in shelf seas. Mixing of seawater with riverine freshwater in near-shore samples caused a reduction in TA and the saturation state of calcite minerals, particularly in winter. Our data show that the L4 and E1 oceanographic stations were small, net sinks for atmospheric CO2 over an annual cycle (−0.52±0.66molCm−2 y−1 and −0.62±0.49molCm−2 y−1, respectively). [Copyright &y& Elsevier]