Shelf sea subsurface chlorophyll maximum thin layers have a distinct phytoplankton community structure

Michelle L Barnett,Alan E.S Kemp,Anna E Hickman,Duncan A Purdie

doi:10.1016/j.csr.2018.12.007

Abstract

The Western English Channel is a seasonally stratified temperate coastal sea where a subsurface chlorophyll maximum (SCM) is typically detectable within the seasonal thermocline. The SCM often develops as a thin layer (< 5 m) that may contain elevated concentrations of phytoplankton (subsurface chlorophyll maximum thin layer; SCMTL). During summer 2013 a study was conducted offshore of Falmouth, UK to assess spatial and short-term temporal variability in SCM thickness in relation to water column structure and physical conditions and to evaluate any associated changes in phytoplankton community structure. SCMTLs were observed in 18 of 52 vertical profiles, typically characterised by higher chlorophyll concentrations than broader SCMs. SCMTLs were generally associated with a ‘stepped’ thermocline, likely representing the presence of one or more shallow mixed layers forming above/within the seasonal thermocline, and related to increased stratification compared to broader SCMs. Pseudo-nitzschia was almost exclusively the dominant diatom taxon in SCMs, yet statistically distinct differences in community structure existed between SCMTLs and broader SCMs. Within the phytoplankton, the distinction was largely due to a greater biomass of Proboscia alata and other rhizosolenid diatoms, and the dinoflagellate Ceratium lineatum in SCMTLs, and a smaller population of the diatom Chaetoceros spp. There was also a distinction amongst heterotrophic dinoflagellates, with enhanced biomass of Gyrodinium spp. in SCMTLs and a reduction in Diplopsalis lenticula. We propose that this observed difference resulted from promotion of phytoplankton better adapted to environmental conditions more specific to SCMTLs compared to broader SCMs. With more intense and prolonged stratification projected for the NW European shelf, there may be increased prevalence of SCMTLs and the associated larger-sized specialised taxa, with implications for increased carbon export. This study adds to a growing body of evidence of the importance of SCMTLs in coastal and shelf seas, and highlights the requirement for improved understanding of physical forcing and the ecology and physiology of key taxa, particularly as predicted changes in stratification could alter the role of SCM phytoplankton in a future influenced by climate change.

Highlights

Subsurface chlorophyll maxima (SCM) are commonly identified in the seasonally stratified waters of temperate and high latitude coastal and shelf seas (Cullen, 1982, Holligan et al, 1984a, Martin et al, 2010)
subsurface chlorophyll maximum (SCM) development is promoted in stratified waters, so it is important to improve our understanding of SCM ecology in the shelf seas, in particular because these regions of the marine environment are highly significant for global biogeochemical cycling and trophic dynamics (Muller-Karger et al, 2005, Jahnke, 2010, Simpson and Sharples, 2012)
Subsurface chlorophyll maximum thin layers (SCMTL) were identified in 18 of the 52 water column profiles collected over an 11 day period in 2013 in the Western English Channel, adding to a growing body of evidence for the recurring and persistent nature of these features in coastal and shelf seas

Summary

Introduction

Subsurface chlorophyll maxima (SCM) are commonly identified in the seasonally stratified waters of temperate and high latitude coastal and shelf seas (Cullen, 1982, Holligan et al, 1984a, Martin et al, 2010). Biogeochemical models predict a decrease in primary production and export of particulate organic carbon with increasing stratification (Steinacher et al, 2010) These models are highly simplified, not accurately replicating the phytoplankton community or the physics within the marine ecosystem, and their predictions have been challenged by studies conducted in stratified regions of the modern ocean and by palaeoceanographic evidence (Kemp et al, 2006, Kemp and Villareal, 2013, Kemp and Villareal, 2018). SCM development is promoted in stratified waters, so it is important to improve our understanding of SCM ecology in the shelf seas, in particular because these regions of the marine environment are highly significant for global biogeochemical cycling and trophic dynamics (Muller-Karger et al, 2005, Jahnke, 2010, Simpson and Sharples, 2012). We focus on a region of the NW European continental shelf, where more intense and persistent seasonal stratification has been projected (Lowe et al, 2009, Sharples et al, 2013, Tinker et al, 2016)

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