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The impact of the physical environment and phytoplankton size on particle types (zooplankton, biogenic matter or phytodetritus) in the water column is poorly understood. Here, we investigate how hydrography (e.g. water column stratification) impacts phytoplankton size and photophysiology across a productive coastal bank area. Additionally, we investigate how the physical environment and phytoplankton size structure influence the concentrations of plankton (e.g. copepods and diatom chains), biogenic forms (fecal pellets) and other particles (minerals, aggregates or phytodetritus) using discrete samples and in-situ optical instruments. Microphytoplankton (>20 µm), including many chain-forming diatoms, dominated (average > 90% of total size fraction) in un-stratified waters of the bank. Phytoplankton within the bank region also required more irradiance to saturate photosynthesis, as indicated by the onset light saturation parameter (Ek, average 297 µmol photons m−2 s−1), suggesting high plasticity to a dynamic light environment. Conversely, the contribution of nano- and picophytoplankton ( 1 × 103 counts m−3), fecal pellets (>1 × 104 counts m−3) and ammonium (>0.5 µM) within the bank compared to off-bank regions, indicated that copepods were actively grazing in this region. Low stratification (average stratification index (SI) 5 × 105 counts m−3) within the bank when compared to off-bank (SI off-bank > 10 × 10−3 kg m−4). Our results, obtained using automated techniques measured in-situ, represent an innovative approach to demonstrate that phytoplankton size and stratification influence the nature of particles found in the water column (including aggregates, fecal pellets and grazer abundances).

Plankton are an extremely diverse and polyphyletic group, exhibiting a large range in morphological and physiological traits. Here, we apply automated optical techniques, provided by the pulse‐shape recording automated flow cytometer—CytoSense—to investigate trait variability of phytoplankton and plastidic ciliates in Arctic and Atlantic waters of the subpolar North Atlantic. We used the bio‐optical descriptors derived from the CytoSense (light scattering [forward and sideward] and fluorescence [red, yellow/green and orange from chlorophyll a , degraded pigments, and phycobiliproteins, respectively]) and translated them into functional traits to demonstrate ecological trait variability along an environmental gradient. Cell size was the master trait varying in this study, with large photosynthetic microplankton (> 20 μ m in cell diameter), including diatoms as single cells and chains, as well as plastidic ciliates found in Arctic waters, while small‐sized phytoplankton groups, such as the picoeukaryotes (μ m) and the cyanobacteria Synechococcus were dominant in Atlantic waters. Morphological traits, such as chain/colony formation and structural complexity (i.e., cellular processes, setae, and internal vacuoles), appear to favor buoyancy in highly illuminated and stratified Arctic waters. In Atlantic waters, small cell size and spherical cell shape, in addition to photo‐physiological traits, such as high internal pigmentation, offer chromatic adaptation for survival in the low nutrient and dynamic mixing waters of the Atlantic Ocean. The use of automated techniques that quantify ecological traits holds exciting new opportunities to unravel linkages between the structure and function of plankton communities and marine ecosystems.

The Labrador Sea is an ideal region to study the biogeographical, physiological, and biogeochemical implications of phytoplankton community composition due to sharp transitions between distinct water masses across its shelves and central basin. We have investigated the multi-year (2005–2014) distributions of late spring and early summer (May to June) phytoplankton communities in the various hydrographic settings of the Labrador Sea. Our analysis is based on pigment markers (using CHEMTAX analysis), and photophysiological and biogeochemical characteristics associated with each phytoplankton community. Diatoms were the most abundant group, blooming first in shallow mixed layers of haline-stratified Arctic shelf waters. Along with diatoms, chlorophytes co-dominated at the western end of the section (particularly in the polar waters of the Labrador Current (LC)), whilst Phaeocystis co-dominated in the east (modified polar waters of the West Greenland Current (WGC)). Pre-bloom conditions occurred in deeper mixed layers of the central Labrador Sea in May, where a mixed assemblage of flagellates (dinoflagellates, prasinophytes, prymnesiophytes, particularly coccolithophores, and chrysophytes/pelagophytes) occurred in low-chlorophyll areas, succeeding to blooms of diatoms and dinoflagellates in thermally stratified Atlantic waters in June. Light-saturated photosynthetic rates and saturation irradiance levels were highest at stations where diatoms were the dominant phytoplankton group ( > 70 % of total chlorophyll a), as opposed to stations where flagellates were more abundant (from 40 up to 70 % of total chlorophyll a). Phytoplankton communities from the WGC (Phaeocystis and diatoms) had lower light-limited photosynthetic rates, with little evidence of photoinhibition, indicating greater tolerance to a high light environment. By contrast, communities from the central Labrador Sea (dinoflagellates and diatoms), which bloomed later in the season (June), appeared to be more sensitive to high light levels. Ratios of accessory pigments (AP) to total chlorophyll a (TChl a) varied according to phytoplankton community composition, with polar phytoplankton (cold-water related) having lower AP : TChl a. Polar waters (LC and WGC) also had higher and more variable particulate organic carbon (POC) to particulate organic nitrogen (PON) ratios, suggesting the influence of detritus from freshwater input, derived from riverine, glacial, and/or sea ice meltwater. Long-term observational shifts in phytoplankton communities were not assessed in this study due to the short temporal frame (May to June) of the data. Nevertheless, these results add to our current understanding of phytoplankton group distribution, as well as an evaluation of the biogeochemical role (in terms of C : N ratios) of spring phytoplankton communities in the Labrador Sea, which will assist our understanding of potential long-term responses of phytoplankton communities in high-latitude oceans to a changing climate.

The Labrador Sea is an important oceanic sink for atmospheric CO2 because of intensive convective mixing during winter and extensive phytoplankton blooms that occur during spring and summer. Therefore, a broad-scale investigation of the responses of phytoplankton community composition to environmental forcing is essential for understanding planktonic food-web organisation and biogeochemical functioning in the Labrador Sea. Here, we investigated the phytoplankton community structure (>4 ?m) from near surface blooms (1.2 mg chla m?3) occurred on and near the shelves in May and in offshore waters of the central Labrador Sea in June due to haline- and thermal-stratification, respectively. Sea ice-related (Fragilariopsis cylindrus and F. oceanica) and Arctic diatoms (Fossula arctica, Bacterosira bathyomphala and Thalassiosira hyalina) dominated the relatively cold (

The phytoplankton in coastal regions are responding to constant environmental changes, thus the use of proxies derived from in situ frequent time-series observations and validated from traditional microscopic or pigment methods can be a solution for detecting rapid responses of community dynamics and succession. In this study, we combined in situ high-frequency (every 30 min from May to September 2017) optical and hydrographic data from a moored buoy and weekly discrete samplings to track phytoplankton community dynamics and succession in Mausund Bank, a highly productive region of the coast of Norway. Three hydrographic regimes were observed: mixing period (MP) in spring, onset of stratification (transient period, TP) in summer and a stratified period (SP) in fall, with occasional strong winds that disrupted the surface stratification in the beginning of September. A bloom dominated by the diatom Skeletonema costatum was observed in the MP due to intense mixing and nutrient availability, while flagellates prevailed in nutrient-poor waters during the TP, followed by a bloom dominated by rhizosolenid diatoms (Proboscia alata and Guinardia delicatula), when stratification peaked. A mixed assemblage of diatoms (e.g. Pseudo-nitzschia), coccolithophores and dinoflagellates occurred during the SP, as strong winds reintroduced nutrients to surface waters. Through pigment (chemotaxonomy) and microscopic observations, we tested, for the first time in a coastal region, whether an ‘optical community index’ derived from in situ measurements of chlorophyll a fluorescence (Fchla) and optical particulate backscattering (bbp) is suitable to differentiate between diatom versus flagellate dominance. We found a negative relationship between Fchla:bbp and diatom:flagellate, contrary to previous observations, possibly because of the influence of non-algal contribution (e.g. zooplankton, fecal pellets and detritus) to the bbp pool in highly productive systems. This finding suggests that such relationship is not universal and that other parameters are needed to refine the optical community index in coastal regions. 0278-4343/© 2020 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/)

Currents, wind, bathymetry, and freshwater runoff are some of the factors that make coastal waters heterogeneous, patchy, and scientifically interesting—where it is challenging to resolve the spatiotemporal variation within the water column. We present methods and results from field experiments using an autonomous underwater vehicle (AUV) with embedded algorithms that focus sampling on features in three dimensions. This was achieved by combining Gaussian process (GP) modeling with onboard robotic autonomy, allowing volumetric measurements to be made at fine scales. Special focus was given to the patchiness of phytoplankton biomass, measured as chlorophyll a (Chla), an important factor for understanding biogeochemical processes, such as primary productivity, in the coastal ocean. During multiple field tests in Runde, Norway, the method was successfully used to identify, map, and track the subsurface chlorophyll a maxima (SCM). Results show that the algorithm was able to estimate the SCM volumetrically, enabling the AUV to track the maximum concentration depth within the volume. These data were subsequently verified and supplemented with remote sensing, time series from a buoy and ship-based measurements from a fast repetition rate fluorometer (FRRf), particle imaging systems, as well as discrete water samples, covering both the large and small scales of the microbial community shaped by coastal dynamics. By bringing together diverse methods from statistics, autonomous control, imaging, and oceanography, the work offers an interdisciplinary perspective in robotic observation of our changing oceans. © 2019. This is the authors' accepted and refereed manuscript to the article. The final authenticated version is available online at: http://dx.doi.org/10.1126/scirobotics.aav3041

Diatoms are a keystone algal group, with diverse cell morphology and a global distribution. The biogeography of morphological, functional, and life-history traits of marine diatoms were investigated in Arctic and Atlantic waters of the Labrador Sea during the spring bloom (2013-2014). In this study, trait-based analysis using community-weighted means showed that low temperatures (< 0°C) in Arctic waters correlated positively with diatom species that have traits such as low temperature optimum growth and the ability to produced ice-binding proteins, highlighting their sea ice origin. High silicate concentrations in Arctic waters, as well as sea ice cover and shallow bathymetry, favoured diatom species that were heavily silicified, colonial and capable of producing resting spores, suggesting that these are important traits for this community. In Atlantic waters, diatom species with large surface area to volume ratios were dominant in deep mixed layers, whilst low silicate to nitrate ratios correlated positively with weakly silicified species. Sharp cell projections, such as processes or spines, were positively correlated with water-column stratification, indicating that these traits promote positive buoyancy for diatom cells. Our trait-based analysis directly links cell morphology and physiology with diatom species distribution, allowing new insights on how this method can potentially be applied to explain ecophysiology and shifting biogeographical distributions in a warming climate. Copyright © 2018 Fragoso, Poulton, Yashayaev, Head, Johnsen and Purdie. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

The picoplanktonic cyanobacteria, Synechococcus spp., (Nägeli) are important contributors to global ocean primary production that can be stressed by solar radiation, both in the photosynthetically active (PAR) and ultraviolet (UV) range. We studied the responses of PSII quantum yield (active fluorescence), carbon fixation ((14)C assimilation) and oxygen evolution (membrane inlet mass spectrometry) in Synechococcus WH8102 under moderate UV and PAR. PSII quantum yield decreased during exposure to moderate UV and UV+PAR, with response to the latter being faster (6.4 versus 2.8 min, respectively). Repair processes were also faster when UV+PAR exposure was followed by moderate PAR (1.68 min response time) than when UV was followed by very low PAR (10.5 min response time). For the UV+PAR treatment, the initial decrease in quantum yield was followed by a 50% increase ("rebound") after 7 min exposure, showing an apparent photoprotection induction. While oxygen uptake increased with PAR, it did not change under UV, suggesting that this oxygen-dependent mechanism of photoprotection, which may be acting as an electron sink, is not an important strategy against UV. We used propyl gallate, an antioxidant, to test for plastid terminal oxidase (ptox) or ptox-like enzymes activity, but it caused nonspecific and toxic effects on Synechococcus WH8102.

The phytoplankton of the Ross Sea have been intensively studied, in contrast to that of the Amundsen Sea. This study focused on understanding the environmental variables that influence the spatial patterns of assemblages during late summer, 2007, and late spring–early summer, 2008 in the Amundsen and Ross Seas. Blooms of the prymnesiophyte Phaeocystis antarctica, and the silicoflagellate Dictyocha speculum occurred in the southwestern to eastern parts of the Ross Sea, respectively, whereas diatoms dominated in southeastern Ross and the Amundsen Sea. Shallow mixed layers supported the growth of diatoms, but were not the only factor required for diatom bloom development. Modified Circumpolar Deep Water intruded into the subsurface waters (< 200 m) in the southwestern Ross Sea during February 2007, and possibly favored the formation of P. antarctica blooms. Photosynthetic quantum yield data suggest that blooms from the southwestern Ross Sea were approaching stress during January 2008, likely due to iron limitation, in contrast to blooms close to the ice edge in the Amundsen Sea, where iron may be more available to the phytoplankton. A detailed comparison between the Amundsen and Ross Seas may allow a greater understanding of the environmental-induced impacts on phytoplankton distribution and regional biogeochemical cycles.

Arctic primary production is sensitive to reductions in sea ice cover, and will likely increase into the future. Whether this increased primary production (PP) will translate into increased export of particulate organic carbon (POC) is currently unclear. Here we report on the POC export efficiency during summer 2012 in the Atlantic sector of the Arctic Ocean. We coupled 234-Thorium based estimates of the export flux of POC to onboard incubation based estimates of PP. Export efficiency (defined as the fraction of PP that is exported below 100 m depth: ThE-ratio) showed large variability (0.09?±?0.19 to 1.3?±?0.3). The highest ThE-ratio (1.3?±?0.3) was recorded in a mono-specific bloom of Phaeocystis pouchetii located in the ice edge. Blooming diatom dominated areas also had high ThE-ratios (0.1?±?0.1 to 0.5?±?0.2), while mixed and/or pre-bloom communities showed lower ThE-ratios (0.10?±?0.03 to 0.19?±?0.05). Furthermore, using oxygen saturation, bacterial abundance, bacterial production, and zooplankton oxygen demand, we also investigated spatial variability in the degree to which this sinking material may be remineralised in the upper mesopelagic (< 300 m). Our results suggest that blooming diatoms and P. pouchetii can export a significant fraction of their biomass below the surface layer (100 m) in the open Arctic Ocean. Also, we show evidence that the material sinking from a P. pouchetii bloom may be remineralised (>100m) at a similar rate as the material sinking from diatom blooms in the upper mesopelagic, contrary to previous findings.

The Southern Ocean (SO) is an important CO2 reservoir, some of which enters via the production, sinking, and remineralization of organic matter. Recent work suggests that the fraction of production that sinks is inversely related to production in the SO, a suggestion that we confirm from 20 stations in the Scotia Sea. The efficiency with which exported material is transferred to depth (transfer efficiency) is believed to be low in high-latitude systems. However, our estimates of transfer efficiency are bimodal, with stations in the seasonal ice zone showing intense losses and others displaying increases in flux with depth. Zooplankton fecal pellets dominated the organic carbon flux and at stations with transfer efficiency >100% fecal pellets were brown, indicative of fresh phytodetritus. We suggest that active flux mediated by zooplankton vertical migration and the presence of sea ice regulates the transfer of organic carbon into the oceans interior in the Southern Ocean.