Phytoplankton communities of subsurface chlorophyll maxima in the summer stratified waters of the Western English Channel

Abstract

In the summer stratified temperate shelf waters of the Western English Channel, a subsurface chlorophyll maximum (SCM) is typically observable within the seasonal thermocline. Surveys were conducted in these summer (June/July) stratified waters from 2013 to 2016 investigating SCM phytoplankton communities. A SeaBird CTD with fluorometer and Niskin rosette sampler was routinely used to collect water column profiles and discrete water samples, and an Acoustic Doppler Current Profiler (ADCP) measured current profiles. In 2013 SCM thin layers (SCMTL; < 5 m thick) and broader SCM were studied in detail. SCMTL were detected in 18 of 52 profiles, typically characterised by higher maximum chlorophyll concentrations than broader SCM. Water column structure and physical forcing governed SCM chlorophyll structure, with SCMTL generally associated with a stepped thermocline and greater stratification. Community structure within SCMTL was statistically distinct from that of broader SCM, proposed to result from promotion of phytoplankton better adapted to conditions more specific to SCMTL compared to broader SCM. Findings suggest that with more intense stratification projected for the NW European shelf there may be increased prevalence of SCMTL and associated specialised flora. In 2015, a digital in-line holographic camera (holocam) was deployed, the data from which identified a transition of phytoplankton through the stratified water column, showed how one taxon can be dominant over a small part of the water column, and provided evidence of the SCM as a considerable source of carbon flux. Following methodological developments, in situ holography has the potential to be powerful in the assessment of the phytoplankton community on a large spatial and temporal scale, and as a tool for obtaining quantitative carbon flux data. Primary production was also assessed in 2015, using a combination of 13C incubation experiments and Fluorescence Induction and Relaxation (FIRe) measurements. The SCM was estimated to account for, on average, 50 % of total water column primary production and was identified as a key site of new production. Variation in the magnitude of water column production was driven by changes in SCM layer production, which was governed by factors including chlorophyll concentration, irradiance, and the photophysiology and structure of the phytoplankton community, with increases in SCM carbon fixation associated with greater contributions of red fluorescing nano-phytoplankton due to their enhanced light utilisation efficiency. Greater percentages of these nano-phytoplankton generally coincided with stronger stratification, which may have implications for our understanding of the relationship between stratification and primary production. Finally, data from all four years was gathered to study environmental controls of interannual variability in SCM chlorophyll and phytoplankton community structure at one location in the Western Channel. Stability (given by the instantaneous index of stability; IIS) and temperature were found to be key governing factors. Changes in stability were related to interannual variation in SCM maximum chlorophyll, the 50 m:SCM maximum chlorophyll ratio, and proportions of red fluorescing nano-phytoplankton and dinoflagellates within the SCM community. Changes in water temperature were associated with interannual variation in SCM maximum chlorophyll and proportions of small versus large diatoms within the SCM. These findings may have implications, in particular, for the silica cycle, carbon export to depth and the microbial loop. Possible causes and implications of all results, as described above, are discussed in detail within this thesis.

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Identifiers

ORCID for Michelle Barnett: orcid.org/0000-0002-3882-3649

ORCID for Duncan Purdie: orcid.org/0000-0001-6672-1722

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