Eddy-Driven Stratification Initiates North Atlantic Spring Phytoplankton Blooms
ABSTRACT Springtime phytoplankton blooms photosynthetically fix carbon and export it from the surface ocean at globally important rates. These blooms are triggered by increased light exposure of the phytoplankton due to both seasonal light increase and the development of a near-surface vertical density gradient (stratification) that inhibits vertical mixing of the phytoplankton. Classically and in current climate models, that stratification is ascribed to a springtime warming of the sea surface. Here, using observations from the subpolar North Atlantic and a three-dimensional biophysical model, we show that the initial stratification and resulting bloom are instead caused by eddy-driven slumping of the basin-scale north-south density gradient, resulting in a patchy bloom beginning 20 to 30 days earlier than would occur by warming.
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ABSTRACT: Interactive effects of temperature and light during deep convection: a case study on growth and condition of the diatom Thalassiosira weissflogii. Aim of this study was to expose phytoplankton to growth conditions simulating deep winter convection in the North Atlantic and thereby to assess changes in physiology enabling their survival. Growth rate, biochemical composition, and photosynthetic activity of the diatom Thalassiosira weiss-flogii were determined under two different light scenarios over a temperature range of 5 –158C to simulate conditions experienced by cells during winter deep convection. These metrics were examined under a low light scenario (20 mmol m 22 s 21 , 12/12 h light/dark), and compared with a scenario of short light pulses of a higher light intensity (120 mmol m 22 s 21 , 2/22 h light/dark). Both experimental light conditions offered the same daily light dose. No growth was observed at temperatures below 88C. Above 88C, growth rates were significantly higher under low light conditions compared with those of short pulsed light exposures, indicating a higher efficiency of light utilization. This could be related to (i) a higher content of Chl a per cell in the low light trial and/or (ii) a more efficient transfer of light energy into growth as indicated by constantly low carbohydrate levels. In contrast, pulsed intense light led to an accumulation of carbohydrates, which were catabolized during the longer dark period for maintaining metabolism. Light curves measured via Chl a fluorescence indicated low light assimilation for the algae exposed to short pulsed light. We postulate that our trial with short light pluses did not provide sufficient light to reach full light saturation. In general, photosynthesis was more strongly affected by temperature under pulsed light than under low light conditions. Our results indicate that model estimates of primary production in relation to deep convection, which are based on average low light conditions, not considering vertical transportation of algae will lead to an overestimation of in situ primary production.ICES Journal of Marine Science 12/2014; DOI:10.1093/icesjms/fsu218 · 2.53 Impact Factor
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ABSTRACT: A research cruise in the North Atlantic during the annual diatom bloom provided an ideal platform to study chlorophyll-a (chl-a) transformations associated with a large scale diatom bloom and export below the photic zone. On one deployment, Lagrangian sediment traps captured a significant flux of aggregated diatom cells produced during the termination of the main bloom. We examined the distribution of chl-a transformation products in sinking particles from the sediment traps and in suspended particles from the water column using high-resolution HPLC with multistage mass spectrometry (LC-MSn). There was a dramatic change in the distribution of chl-a and its transformation products between the pre-sinking period, when the average chl-a concentration integrated over the upper 50 m was 68 ± 36 mg m- 2, and the post-sinking period, when it was 30 ± 11 mg m- 2. Before the diatom bloom left the euphotic zone (pre-sinking), suspended particles contained a considerably higher percentage of pheophorbide-a and other chl-a transformation products (27%) than during the post-sinking period (10%). Despite high levels of spatial variability in the chl-a concentration, and despite sampling from both within and outside a main bloom patch, the chl-a transformation products in suspended particles did not exhibit spatial variability. Sinking particles associated with the diatom bloom export had low POC: chl-a ratios (52 - 97), suggesting undegraded phytoplankton cells. However, the samples with especially low POC: chl-a ratios exhibited similar distributions of chl-a transformation products to those with a higher ratio. The proportions of demetalated and de-esterified transformation products increased with depth of suspended particles, although significant levels of these products were also found in the uppermost 20 m during the bloom. This suggests processes in both surface waters and through the water column led to the formation of these products.Marine Chemistry 03/2015; 172:23-33. DOI:10.1016/j.marchem.2015.03.005 · 3.20 Impact Factor
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ABSTRACT: We show the application of a Spatially-Explicit Individual-Based Model (SEIBM) to understand the recruitment process of European anchovy. The SEIBM is applied to simulate the effects of inter-annual variability in parental population spawning behavior and intensity, and ocean dynamics, on the dispersal of eggs and larvae from the spawning area in the Gulf of Lions (GoL) towards the coastal nursery areas in the GoL and Catalan Sea (northwestern Mediterranean Sea). For each of seven years (2003-2009), we initialize the SEIBM with the real positions of anchovy eggs during the spawning peak, from an acoustics-derived eggs production model. We analyze the effect of spawners’ distribution, timing of spawning, and oceanographic conditions on the connectivity patterns, growth, dispersal distance and late-larval recruitment (14 mm larva recruits, R14) patterns. The area of influence of the Rhône plume was identified as having a high probability of larval recruitment success (64%), but up to 36% of R14 larvae end up in the Catalan Coast. We demonstrate that the spatial paths of larvae differ dramatically from year to year, and suggest potential offshore nursery grounds. We showed that our simulations are coherent with existing recruitment proxies and therefore open new possibilities for fisheries management.Progress In Oceanography 01/2015; In Press. DOI:10.1016/j.pocean.2015.01.011 · 3.99 Impact Factor