Pelagic production at the Celtic Sea shelf break

Abstract

This paper reviews the data obtained in the OMEX I Project on biological production in the surface waters of the Celtic Sea shelf break. The study focused on two regions— the Goban Spur and La Chapelle Bank. Satellite images of the Celtic Sea frequently show a region of cooler water at the shelf break, which results in the mixing of cooler, nutrient-rich waters to the sea surface. To examine the hypothesis that the Celtic Sea shelf break might be a region of enhanced production and sedimentation, observations were made at five regions. These were four sites along a transect of the Goban Spur, from the Celtic Sea shelf (water depth <200 m), through stations at water depths of 500–1000, 1500, and 3600 m; the fifth region was at La Chapelle Bank, which offered a contrasting site where the slope is steeper and influenced by canyons. Estimates are made of seasonal production of phytoplankton, bacterioplankton, microzooplankton, and mesozooplankton. The region has a spring bloom which is of short duration at the oceanic sites and occurs earliest on the Celtic Sea shelf; phytoplankton biomass in the summer months is greatest at La Chapelle Bank. Photosynthetic pigments analyses indicate that prymnesiophytes are present throughout the year and are often the dominant group of phytoplankton; diatoms are most abundant in the spring bloom. Primary production is estimated to be ca. 160 gC m−2 a−1, with cells <5 μm in diameter accounting for almost half of the annual primary production. New production is estimated to be equivalent to 80 g C m−2 a−1; the f-ratio is generally <0.25 during the summer and autumn months, 0.7–0.8 during the spring bloom, and ca. 0.5 during the winter. Microzooplankton biomass and herbivory were measured from April to October at the Goban Spur regions. The biomass of mesozooplankton was determined from the records of the Continuous Plankton Recorder (CPR) survey, and was used to estimate the amount of primary production removed by mesozooplankton grazing. Bacterial production is estimated to be ca. 12 g C m−2 a−1. The sum of microzooplankton and mesozooplankton grazing and the carbon demands of bacteria were significantly lower than primary production from November through May, but heterotrophic processes were quantitatively greater than phytoplankton production from July to October. The data suggest that up to 62 g C m−2 a−1 of primary production was not grazed by micro- or mesozooplankton in the surface mixed layer, or utilised directly by bacteria. Depending on the region, up to 38% of the primary production at the Celtic Sea margin was apparently not grazed in the surface mixed layer and would be available for heterotrophic organisms in mid-water and the benthos. The estimated respiration of the heterotrophic community of the surface mixed layer estimated also suggested that between 37% and 60% of the carbon fixed by photosynthesis in the euphotic zone was not remineralised in the surface mixed layer. Data from satellite remote sensing are used in conjunction with the experimental data to extend the seasonal coverage of the observations made in OMEX I. The archive of the coastal zone color scanner provides mean monthly values of chlorophyll concentration, and these agree well with the seasonal variation of “green colour” of the CPR survey. Primary production has been estimated from the satellite-derived chlorophyll concentrations for the period April–September and is calculated to be 90 g C m−2 for the 6-month period; the estimated production for the same period from in situ experiments suggests that primary production was ca. 116 g C m−2. Nitrate concentrations in the surface water were correlated with sea-surface temperature, and this relationship was applied to temperature measurements from the advanced very high resolution radiometer sensor to estimate the potential nitrate concentrations over the region. The f-ratio was related to nitrate concentration by a simple hyperbolic function (r2=0.73). which was applied to the images of potential nitrate concentration for the region to estimate new production based on satellite data. For the period April through September, new production was calculated to be 46 g C m−2 from satellite estimates of temperature, nitrate, and f-ratio, which compares favourably with the estimated new production of 57 g C m−2 by direct measurement.