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Abstract: The CROZet natural iron bloom and EXport experiment (CROZEX) investigated the annual phytoplankton bloom that occurs in the vicinity of the Crozet plateau in the Polar Frontal Zone (PFZ) of the Southern Ocean. Shipboard manipulation experiments designed to investigate potential responses of phytoplankton community structure and elemental stoichiometry to iron (Fe) and/or light perturbation are compared to in situ data collected during CROZEX. The outcome of individual experiments was strongly influenced by initial phytoplankton community structure. For example Fe amendment of high (>8μM) silicic acid waters resulted in a strong response by medium-sized diatoms, including Eucampia antarctica. In contrast Phaeocystis antarctica dominated the community response to increased Fe within an experiment initiated during the early declining stage of the bloom. Conversely, small diatoms responded more strongly to increased irradiance, while the population of very large diatoms remained relatively static. Consistent with experimental results, the intense natural blooms north of the Crozet plateau were observed to be dominated by either P. antarctica or medium-sized diatoms while small phytoplankton and large diatoms dominated lower-chlorophyll waters in the south. In situ data and results from experiments supported previous observations of lower nitrate to phosphate removal ratios for diatoms compared to P. antarctica. Experimental and in situ data also supported previous work showing that silicic acid to nitrate removal ratios can be reduced under conditions of enhanced Fe availability. Higher irradiance decreased this ratio still further. Interactions between Fe and light were thus likely to have contributed to the observed decoupling of the major element cycles between the intense bloom that occurs north of Crozet and more typical PFZ conditions to the South. Specifically, silicic acid became exhausted in both systems, with drawdown in the south potentially resulting from co-limitation of diatom growth by Fe and light. In contrast, increased Fe supply in the north enhanced nitrogen, phosphorous and carbon drawdown in ratios that were dependent on the community composition of the resultant bloom. [Copyright &y& Elsevier]

The subsurface chlorophyll maximum (SCM) is increasingly recognised as an important but understudied locus of primary production particularly in shelf seas. Here we report the results of a 4 year, repeat station, summer sampling programme (2013–2016) of a seasonally recurrent SCM in the Western English Channel. Interannual variability in phytoplankton community structure and chlorophyll distribution and intensity was strongly related to water column stability at the depth interval of the SCM and also to water temperature. The phytoplankton community was statistically distinct in each year. High stability, as evidenced by large Richardson numbers and a well-developed strong thermocline appeared to favour the growth of larger dinoflagellates (autotrophs or mixotrophs) and diatoms. Such conditions led to development of the most intense SCMs and these were sometimes dominated by a single or a few key species most prominently in 2015 with near monospecific concentrations of the dinoflagellate Tripos fusus with average peak SCM chlorophyll concentrations of 7.3 ± 4.4 μg l−1. By contrast, in years with low water column stability and intermittent turbulence at the thermocline (2014, 2016) there was greater chlorophyll dispersal and less intense SCM. In these low stability conditions, red fluorescent nano-phytoplankton, such as naked dinoflagellates, chlorophytes and prymnesiophytes, made a greater contribution to the community, possibly as a result of the advantages that motility and enhanced light utilisation efficiency confer within an SCM exposed to turbulence. It is also likely that turbulence disrupted the stability required by the larger dinoflagellates and diatoms. Several of the key SCM taxa were absent from surface waters including the dinoflagellates Tripos fusus , Tripos lineatus , and most of the Rhizosolenia / Proboscia diatoms, consistent with adaptations more suited to survival at depth in stratified waters. These traits include luxury nutrient uptake and storage and survival in low light (both groups) and mixotrophy (dinoflagellates). On the other hand, in 2013, diatoms including Pseudo-nitzschia spp. were abundant in both surface, SCM and bottom waters. The relatively cooler waters (11.6–12.1 °C on average in 2013 and 2016) were characterised by smaller diatoms (Chaetoceros spp. and Pseudo-nitzschia spp.) whereas the warmer waters (13.1 °C on average in 2014) contained larger diatoms (large Rhizosolenia spp., Lauderia annulata and Leptocylindrus danicus). There did not appear to be continuity of key species between years, other than for the dinoflagellate Tripos lineatus , which was significant in both 2013 and 2014 and present in 2015. In any given year, there was no correspondence between the key spring bloom phytoplankton species as monitored in the nearby Western Channel Observatory L4 station and the key SCM taxa. [Display omitted] • Interannual (2013–2016) biological & physical survey of summer stratified shelf sea: Western English Channel. • Phytoplankton community structure of summer subsurface chlorophyll maxima (SCMs) assessed. • Interannual variability in environmental conditions influenced SCM chlorophyll structure and phytoplankton community. • Phytoplankton community structure strongly related to stability at the depth interval of the SCM. • High water column stability at the depth of the SCM, favoured the growth of larger dinoflagellates and diatoms. [ABSTRACT FROM AUTHOR]

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. Graphical abstract fx1 Highlights • Biological & physical survey of summer stratified shelf sea: Western English Channel (84). • Phytoplankton community structure of subsurface chlorophyll maxima (SCMs) assessed (81). • SCM thin layers (<5 m thick) occur with greatest stratification (64). • SCM thin layers have distinct phytoplankton community structure from broader SCMs (80). • SCM thin layers may expand in importance with predicted increase in stratification (82). [ABSTRACT FROM AUTHOR]

Despite the importance of Agulhas Bank (AB) marine productivity in supporting South African coastal fisheries and shelf ecosystems, there are relatively few regional-scale assessments of its spatial and temporal variability, and most productivity studies have been limited in scale. Here we use satellite-derived Net Primary Production (NPP) rates calculated using the Vertically Generalized Production Model (VGPM) to examine the spatial and temporal dynamics of NPP over the 21-year satellite record (1998–2018) on the AB. In calculating VGPM NPP we used the OCCI Chlorophyll- a product, SST from Operational-Sea-Surface-Temperature-and-Sea-Ice-Analysis (OSTIA) and PAR from GlobColour level-3 mapped products as these represent the longest datasets that fit our extended study period. We examine spatial trends between the eastern and central AB, as well as three areas of the bank (around Port Alfred, the Tsitsikamma coast, and the 'cold ridge') that have been previously identified as contributing significantly to the overall productivity of the AB. The AB shows only a moderate degree of seasonality in NPP calculated from the VGPM, with NPP being highest in austral summer (1.7–1.8 g C m−2 d−1) and lowest in winter (0.9–1.0 g C m−2 d−1), and remains relatively high (>1 g C m−2 d−1) throughout the year, contrasting sharply with other shelf systems. Considered annually, NPP on the bank was 516 g C m−2 yr−1 (38 Mt C yr−1 when scaled to the total shelf area) which is higher than many other shelf systems though lower than the neighbouring Benguela system and is indicative of a moderately productive shelf system fuelled by perennial NPP. Comparing different sections of the AB from east to central bank, and including the three upwelling areas, highlighted that spatial differences in NPP were relatively limited; that these three upwelling areas made similar contributions to their relative proportion of the total shelf area, and that average rates of NPP are spatially similar across the bank, though notable high rates occur in some coastal upwelling areas. Interannual variability in NPP was relatively modest, varying between years by only ∼15% over the two decades assessed. Over the 21-year data set, there was a slight (∼0.26% yr−1) statistically-significant decline in calculated NPP over time for the AB as a whole, which, when examined on a pixel-by-pixel basis, indicated that most of the decline was on the central bank between 100 m and 200 m isobaths. In summer, an increase in NPP occurred on the EAB (26.5–28°E). In conclusion, the AB is a significant site of perennial moderate levels of NPP, varying little interannually and with only a slight decline in NPP over time. These factors lead to a stable environment in terms of ecosystem productivity so that the AB makes a significant contribution to the productivity of South African regional fisheries. [ABSTRACT FROM AUTHOR]

Abstract: The CROZet natural iron bloom and EXport experiment (CROZEX) investigated phytoplankton blooms in the vicinity of the Crozet Plateau in the polar frontal zone (PFZ) of the Southern Ocean. Peak chlorophyll concentrations reached during an intense bloom within naturally iron (Fe)-fertilised regions north of the plateau were an order of magnitude higher than those observed in deeper mixed layers and low-Fe waters to the south. To establish the factors influencing phytoplankton dynamics, a suite of in situ phytoplankton physiological measurements and shipboard Fe–light perturbation experiments was performed. Addition of Fe in experiments performed during bloom decline north of the plateau resulted in increased accumulation of phytoplankton biomass and changes in a number of phytoplankton physiological characteristics. In particular photosystem II (PSII) photochemical efficiencies (F v/F m) measured by fast repetition rate fluorometry increased above in situ values within 24h of Fe amendment, suggesting that Fe stress had contributed to bloom termination. In contrast, responses to Fe amendment were minor within an experiment initiated in low-silicic acid, post-bloom waters south of the Plateau. Within the intense bloom in the north, light limitation due to self-shading may have constrained the peak phytoplankton standing stock. However, in the absence of Fe amendment, incubation at higher than in situ irradiance levels had little influence on phytoplankton biomass accumulation for declining bloom populations. Instead reduced F v/F m, reflecting increased photoinhibitory damage to PSII, was observed in high-light incubations and was also apparent in situ. Interactions between Fe and light availability thus influenced phytoplankton physiology and growth and potentially contributed to bloom longevity during CROZEX. [Copyright &y& Elsevier]