Pablo Serret

Plankton respiration in the Eastern Atlantic Ocean

Concurrent measurements of dark community respiration (DCR), gross production (GP), size fractionated primary production (14C PP), nitrogen uptake, nutrients, chlorophyll a concentration, and heterotrophic and autotrophic bacterial abundance were collected from the upper 200 m of a latitudinal (32°S–48°N) transect in the Eastern Atlantic Ocean during May/June 1998. The mean mixed layer respiration rate was 2.5±2.1 mmol O2 m−3 d−1 (n=119) for the whole transect, 2.2±1.1 mmol O2 m−3 d−1 (n=32) in areas where chlorophyll a was <0.5 mg m−3 and 1.5±0.7 mmol O2 m−3 d−1 (n=10) where chlorophyll a was <0.2 mg m−3. These values lie within the range of published data collected in comparable waters, they co-vary with indicators of heterotrophic and autotrophic biomass (heterotrophic bacterial abundance, chlorophyll a concentration, beam attenuation and particulate organic carbon concentration) and they can be reconciled with accepted estimates of total respiratory activity. The mean and median respiratory quotient (RQ), calculated as the ratio of dissolved inorganic carbon production to dissolved oxygen consumption, was 0.8 (n=11). At the time of the study, plankton community respiration exceeded GP in the picoautotroph dominated oligotrophic regions (Eastern Tropical Atlantic [15.5°S–14.2°N] and North Atlantic Subtropical Gyre [21.5–42.5°N]), which amounted to 50% of the stations sampled along the 12,100 km transect. These regions also exhibited high heterotrophic: autotrophic biomass ratios, higher turnover rates of phytoplankton than of bacteria and low f ratios. However, the carbon supply mechanisms required to sustain the rates of respiration higher than GP could not be fully quantified. Future research should aim to determine the temporal balance of respiration and GP together with substrate supply mechanisms in these ocean regions.

Phytoplankton biomass and production in shelf waters off NW Spain: spatial and seasonal variability in relation to upwelling

SummaryChlorophyll-a and primary production on the euphotic zone of the N-NW Spanish shelf were studied at 125 stations between 1984 and 1992. Three geographic areas (Cantabrian Sea, Rías Altas and Was Baixas), three bathymetric ranges (20 to 60 m, 60 to 150 m and stations deeper than 200 m), and four oceanographic stages (spring and autumn blooms, summer upwelling, summer stratification and winter mixing) were considered. One of the major sources of variability of chlorophyll and production data was season. Bloom and summer upwelling stages have equivalent mean and maximum values. Average chlorophyll-a concentrations approximately doubled in every step of the increasing productivity sequence: winter mixing — summer stratification — high productivity (upwelling and bloom) stages. Average primary production rates increased only 60% in the described sequence. Mean (± sd) values of chlorophyll-a and primary production rates during the high productivity stages were 59.7 ± 39.5 mg Chl-a m−2 and 86.9 ± 44.0 mg C m−2 h−1, respectively. Significant differences in both chlorophyll and primary production resulted between geographic areas in most stages. Only 27 stations showed the effects of the summer upwelling that affected coastal areas in the Cantabrian Sea and Rías Baixas shelf, but also shelf-break stations in the Rías Altas area. The Rías Baixas area had lower chlorophyll than both the Rías Altas and the Cantabrian Sea areas during spring and autumn blooms, but higher during summer upwelling events. On the contrary, primary production rates were higher in the Rías Baixas area during blooms in spring and autumn. Mid-shelf areas showed the highest chlorophyll concentrations during high productivity stages, probably due to the existence of frontal zones in all geographic areas considered. The estimated phytoplankton growth rates were comparable to those of other coastal upwelling systems, with average values lower than the maximum potential growth rates. Doubling rates for upwelling and stratification stages in the northern and Rías Altas shelf areas were equivalent, despite larger biomass accumulations during upwelling events. Low turnover rates of the existing biomass in the Rías Baixas shelf in upwelling stages suggests that the accumulation of phytoplankton was due mainly to the export from the highly productive rías, while the contribution of in situ production to these accumulations was relatively lower.

Variability and seasonality of physical and biological fields at the Great Meteor Tablemount (subtropical NE Atlantic)

Abstract Five oceanographic surveys were conducted at the Great Meteor Tablemount (subtropical NE Atlantic; 30.0oN, 28.5oW) throughout the 1992–1999 period to investigate temporal variability in the relationship between the physical structure of the water column associated with the seamount and phytoplankton biomass and/or production rates. Local increases in chlorophyll a , enhanced carbon incorporation rates and changes in phytoplankton species composition were associated with the seamount. These effects were subjected to a large degree of temporal and spatial variability both at seasonal and shorter time scales.

BIOGEOGRAPHIC DIFFERENCES IN THE NET ECOSYSTEM METABOLISM OF THE OPEN OCEAN

In vitro measurements, predictions, and geochemical estimates of the balance between gross primary production (GPP) and community respiration (CR) in the open ocean do not agree. This has generated an active debate about the carbon balance in unproductive pelagic marine ecosystems. The analysis of generalized GPP:CR relationships that sustains this debate assumes a continuous trophic gradient or the simple partition of the World Ocean into productive and unproductive regimes. We measured euphotic zone GPP and CR along a latitudinal (40° N–30° S) transect across the Atlantic Ocean, which included two open-ocean oligotrophic provinces: the eastern area of the North Atlantic Subtropical Gyre (NAST-E) and the center of the South Atlantic Gyre (SATL). Net heterotrophy prevailed in the euphotic zone of the NAST-E, while the plankton community in the central SATL was net autotrophic. A review of published studies of net plankton metabolism in the open ocean substantiates this geographic pattern, suggesting the e...

Biological and physical forcing of carbonate chemistry in an upwelling filament off northwest Africa: Results from a Lagrangian study

The Mauritanian upwelling system is one of the most biologically productive regions of the worlds oceans. Coastal upwelling transfers nutrients to the sun-lit surface ocean, thereby stimulating phytoplankton growth. Upwelling of deep waters also supplies dissolved inorganic carbon (DIC), high levels of which lead to low calcium carbonate saturation states in surface waters, with potentially adverse effects on marine calcifiers. In this study an upwelled filament off the coast of northwest Africa was followed using drifting buoys and sulphur hexafluoride to determine how the carbonate chemistry changed over time as a result of biological, physical and chemical processes. The initial pHtot in the mixed layer of the upwelled plume was 7.94 and the saturation states of calcite and aragonite were 3.4 and 2.2, respectively. As the plume moved offshore over a period of 9 days, biological uptake of DIC (37 ?mol kg?1) reduced pCO2 concentrations from 540 to 410 ?atm, thereby increasing pHtot to 8.05 and calcite and aragonite saturation states to 4.0 and 2.7 respectively. The increase (25 ?mol kg?1) in total alkalinity over the 9 day study period can be accounted for solely by the combined effects of nitrate uptake and processes that alter salinity (i.e., evaporation and mixing with other water masses). We found no evidence of significant alkalinity accumulation as a result of exudation of organic bases by primary producers. The ongoing expansion of oxygen minimum zones through global warming will likely further reduce the CaCO3 saturation of upwelled waters, amplifying any adverse consequences of ocean acidification on the ecosystem of the Mauritanian upwelling system.

Both respiration and photosynthesis determine the scaling of plankton metabolism in the oligotrophic ocean.

Despite its importance to ocean–climate interactions, the metabolic state of the oligotrophic ocean has remained controversial for >15 years. Positions in the debate are that it is either hetero- or autotrophic, which suggests either substantial unaccounted for organic matter inputs, or that all available photosynthesis (P) estimations (including 14C) are biased. Here we show the existence of systematic differences in the metabolic state of the North (heterotrophic) and South (autotrophic) Atlantic oligotrophic gyres, resulting from differences in both P and respiration (R). The oligotrophic ocean is neither auto- nor heterotrophic, but functionally diverse. Our results show that the scaling of plankton metabolism by generalized P:R relationships that has sustained the debate is biased, and indicate that the variability of R, and not only of P, needs to be considered in regional estimations of the oceans metabolic state.

The allometry of the smallest: superlinear scaling of microbial metabolic rates in the Atlantic Ocean

Prokaryotic planktonic organisms are small in size but largely relevant in marine biogeochemical cycles. Due to their reduced size range (0.2 to 1 μm in diameter), the effects of cell size on their metabolism have been hardly considered and are usually not examined in field studies. Here, we show the results of size-fractionated experiments of marine microbial respiration rate along a latitudinal transect in the Atlantic Ocean. The scaling exponents obtained from the power relationship between respiration rate and size were significantly higher than one. This superlinearity was ubiquitous across the latitudinal transect but its value was not universal revealing a strong albeit heterogeneous effect of cell size on microbial metabolism. Our results suggest that the latitudinal differences observed are the combined result of changes in cell size and composition between functional groups within prokaryotes. Communities where the largest size fraction was dominated by prokaryotic cyanobacteria, especially Prochlorococcus, have lower allometric exponents. We hypothesize that these larger, more complex prokaryotes fall close to the evolutionary transition between prokaryotes and protists, in a range where surface area starts to constrain metabolism and, hence, are expected to follow a scaling closer to linearity.

Temperature dependence of plankton community metabolism in the subtropical and tropical oceans: Temperature Dependence of Subtropical Plankton Metabolism

Here we assess the temperature dependence of the metabolic rates (gross primary production - GPP, community respiration - CR and the ratio GPP/CR) of oceanic plankton communities. We compile data from 133 stations of the Malaspina 2010 Expedition, distributed among the subtropical and tropical Atlantic, Pacific and Indian oceans. We used the in vitro technique to measured metabolic rates during 24 h incubations at three different sampled depths: surface, 20% and 1% of the photosynthetically active radiation measured at surface. We also measured the % of ultraviolet B radiation (UVB) penetrating at surface waters. GPP and CR rates increased with warming, albeit different responses were observed for each sampled depth. The overall GPP/CR ratio declined with warming. Higher activation energies (Ea´s) were derived for both processes (GPPChla = 0.97; CRChla = 1.26; CRHPA= 0.95 eV) compared to those previously reported. The Indian Ocean showed the highest Ea (GPPChla = 1.70; CRChla = 1.48; CRHPA= 0.57 eV), while the Atlantic Ocean showed the lowest (GPPChla = 0.86; CRChla = 0.77; CRHPA= -0.13 eV). We believe that the difference between previous assessments and the ones presented here can be explained by the overrepresentation of Atlantic communities in the previous data sets. We found that UVB radiation also affects the temperature dependence of surface GPP, which decreased rather than increased under high levels of UVB. Ocean warming, which causes stratification and oligotrophication of the subtropical and tropical oceans, may lead to reduced surface GPP as a result of increased penetration of UVB radiation.

Corrigendum: Both respiration and photosynthesis determine the scaling of plankton metabolism in the oligotrophic ocean.

Nature Communications 6:6961 doi: 10.1038/ncomms7961 (2015); Published April242015; Updated 2016 The original version of this Article failed to fully credit the use of the Ocean Data View software in figure 3, which appears below: Schlitzer, R., Ocean Data View, http://odv.awi.de, 2016.

Biogeochemical measurements at the Antarctic Peninsula

Size-fractionated chlorophyll a and photosynthetic carbon incorporation, microbial oxygen production and respiration and particulate vertical flux were measured in January 1996 at three regions, characterized by distinct hydrographic fields and planktonic communities, of the Antarctic Peninsula: (1) a diatom-Phaeocystis sp., dominated community associated with the relatively stratified waters of the Gerlache Strait, (2) a nanoplankton-Cryptomonas sp. dominated assemblage at the Gerlache-Bransfield confluence; and (3) a nano- and picoplankton community in mixed waters of the Bransfield Strait. Despite the marked differences in both community structure and total phytoplankton biomass and primary production, and against predictions from models about trophic control of C export, the lowest respiration rates were measured at Bransfield (pico- and nanoplankton), and no difference was observed between the Gerlache (large diatoms) and Bransfield stations in relative vertical particle flux (6.4 vs. 5.1 % of suspended C; 14.9 vs. 10.4 % of net community production, respectively). Growth and loss rates of the phytoplankton population studied for each community indicate that microbial populations can be explained by in situ growth, but spatial (diatom-Phaeocystis sp., bloom) and temporal (diatom-Phaeocystis sp. bloom and nanoplankton communities) scales of study were shown to be insufficient for addressing the coupling between primary production and biogenic carbon export, especially after the appreciation of the accumulation of dissolved organic carbon in the water column. This would explain the unexpected results and highlights the necessity of including the mechanisms controlling accumulation and consumption of dissolved organic matter into conceptual models about the trophic control of C export.