Viability of pico- and nanophytoplankton in the Baltic Sea during spring

Phytoplankton cell death is an important process in marine food webs, but the viability of natural phytoplankton communities remains unexplored in many ecosystems. In this study, we measured the viability of natural pico- and nanophytoplankton communities in the central and southern parts of the Baltic Sea (55°21′ N, 17°06′ E–60°18′ N, 19°14′ E) during spring (4th–15th April 2016) to assess differences among phytoplankton groups and the potential relationship between cell death and temperature, and inorganic nutrient availability. Cell viability was determined by SYTOX Green cell staining and flow cytometry at a total of 27 stations representing differing hydrographic regimes. Three general groups of phytoplankton (picocyanobacteria, picoeukaryotes, and nanophytoplankton) were identified by cytometry using pigment fluorescence and light scatter characteristics. The picocyanobacteria and picoeukaryotes had significantly higher cell viability than the nanophytoplankton population at all depths throughout the study area. Viability correlated positively with the photosynthetic efficiency (Fv/Fm, maximum quantum yield of photosystem II) as measured on the total phytoplankton community. However, an anticipated correlation with dissolved organic carbon was not observed. We found that the abiotic factors suggested to affect phytoplankton viability in other marine ecosystems were not as important in the Baltic Sea, and other biotic processes, e.g. processes related to species succession could have a more pronounced role.peerReviewe

Respiration rate scales inversely with sinking speed of settling marine aggregates

Peer reviewe

Dataset from a mesocosm experiment on brownification in the Baltic Sea

Refers to Brownification affects phytoplankton community composition but not primary productivity in eutrophic coastal waters: A mesocosm experiment in the Baltic Sea Science of The Total Environment, Volume 841, 1 October 2022, Pages 156510 Kristian Spilling, Eero Asmala, Noora Haavisto, Lumi Haraguchi, Kaisa Kraft, Anne-Mari Lehto, Aleksandra M. Lewandowska, Joanna Norkko, Jonna Piiparinen, Jukka Seppälä, Mari Vanharanta, Anu Vehmaa, Pasi Ylöstalo, Timo TamminenClimate change is projected to cause brownification of some coastal seas due to increased runoff of terrestrially derived organic matter. We carried out a mesocosm experiment over 15 days to test the effect of this on the planktonic ecosystem. The experiment was set up in 2.2 m3 plastic bags moored outside the Tvärminne Zoological Station at the SW coast of Finland. We used four treatments, each with three replicates: control (Contr) without any manipulation; addition of a commercially available organic carbon additive called HuminFeed (Hum; 2 mg L−1); addition of inorganic nutrients (Nutr; 5.7 µM NH4 and 0.65µM PO4); and a final treatment of combined Nutr and Hum (Nutr+Hum) additions. Water samples were taken daily, and measured variables included water transparency, organic and inorganic nutrient pools, chlorophyll a (Chla), primary and bacterial production and particle counts by flow cytometry.Peer reviewe

Uptake of excess phosphate at low inorganic N:P ratio in a coastal sea afflicted with eutrophication

Following the spring bloom in the northern Baltic Sea, nitrogen limits phytoplankton growth and there is typically a residual phosphate concentration (>0.2 µmol l-1) remaining that is often assumed to induce the recurring blooms of nitrogen-fixing cyanobacteria. However, these cyanobacterial blooms typically occur 2-3 mo later during the summer, when the phosphate concentration has been depleted, and it is unclear what organisms take up the excess phosphate. We studied the removal potential of excess phosphate (0.55 µmol l-1) at different temperatures (10, 13, 16°C) and with or without nitrogen addition in an indoor 20 l tank experiment. In addition, we followed the element pools and plankton community composition. As expected, the phosphate uptake rate was up to 3-fold faster in nitrogen-amended than non-amended tanks, but complete drawdown of phosphate also occurred under severe nitrogen limitation. The uptake ratio of dissolved inorganic nitrogen to phosphorus was 4.6, which is substantially lower than the Redfield ratio (16) and indicates excessive phosphate removal potential relative to nitrogen. A large part of the excess phosphate ended up in the particulate pool, which has a higher potential to sink out from the surface. The nitrogen-fixing cyanobacteria Nodularia spumigena grew close to summer bloom concentrations only in the highest experimental temperature. However, the combined biovolume of all 3 major bloom-forming cyanobacteria accounted for only 5.3% of the total autotropic biovolume, and their potential phosphate uptake was calculated to bePeer reviewe

Effect of intensity and mode of artificial upwelling on particle flux and carbon export

Reduction of anthropogenic CO2 emissions alone will not sufficiently restrict global warming and enable the 1.5 degrees C goal of the Paris agreement to be met. To effectively counteract climate change, measures to actively remove carbon dioxide from the atmosphere are required. Artificial upwelling has been proposed as one such carbon dioxide removal technique. By fueling primary productivity in the surface ocean with nutrient-rich deep water, it could potentially enhance downward fluxes of particulate organic carbon (POC) and carbon sequestration. In this study we investigated the effect of different intensities of artificial upwelling combined with two upwelling modes (recurring additions vs. one singular addition) on POC export, sinking matter stoichiometry and remineralization depth. We carried out a 39 day-long mesocosm experiment in the subtropical North Atlantic, where we fertilized oligotrophic surface waters with different amounts of deep water. The total nutrient inputs ranged from 1.6 to 11.0 mu mol NO3- L-1. We found that on the one hand POC export under artificial upwelling more than doubled, and the molar C:N ratios of sinking organic matter increased from values around Redfield (6.6) to similar to 8-13, which is beneficial for potential carbon dioxide removal. On the other hand, sinking matter was remineralized at faster rates and showed lower sinking velocities, which led to shallower remineralization depths. Particle properties were more favorable for deep carbon export in the recurring upwelling mode, while in the singular mode the C:N increase of sinking matter was more pronounced. In both upwelling modes roughly half of the produced organic carbon was retained in the water column until the end of the experiment. This suggests that the plankton communities were still in the process of adjustment, possibly due to the different response times of producers and consumers. There is thus a need for studies with longer experimental durations to quantify the responses of fully adjusted communities. Finally, our results revealed that artificial upwelling affects a variety of sinking particle properties, and that the intensity and mode with which it is applied control the strength of the effects.Peer reviewe

KOSMOS 2021 Gran Canaria mesocosm study on ocean alkalinity enhancement: remineralization

The data presented herein originates from a mesocosm study conducted as part of the EU H2020 OceanNETs project, aimed at investigating the ecological ramifications of ocean alkalinity enhancement. Nine mesocosms were deployed in Taliarte Harbour, Gran Canaria, Spain, and systematically sampled using integrated water samplers over the period spanning from September 10th to October 25th, 2021. Alkalinity was employed in a gradient design, ranging from ambient (0 µeq kg-1 added alkalinity, OAE0) to elevated levels of 2400 µeq kg-1 additional alkalinity (OAE2400) in increments of 300 µeq kg-1. The dataset encompasses a spectrum of sediment trap particle flux data, water column biogeochemistry variables, including inorganic nutrients, carbonate chemistry parameters, and particulate matter, alongside chlorophyll a concentrations. The study and data set offer insights into impacts of alkalinity enhancement on marine ecosystems and their associated biogeochemistry

KOSMOS 2021 Gran Canaria mesocosm study on ocean alkalinity enhancement: carbonate chemistry

The data presented herein originates from a mesocosm study conducted as part of the EU H2020 OceanNETs project, aimed at investigating the ecological ramifications of ocean alkalinity enhancement. Nine mesocosms were deployed in Taliarte Harbour, Gran Canaria, Spain, and systematically sampled using integrated water samplers over the period spanning from September 10th to October 25th, 2021. Alkalinity was employed in a gradient design, ranging from ambient (0 µeq kg-1 added alkalinity, OAE0) to elevated levels of 2400 µeq kg-1 additional alkalinity (OAE2400) in increments of 300 µeq kg-1. The dataset encompasses a spectrum of sediment trap particle flux data, water column biogeochemistry variables, including inorganic nutrients, carbonate chemistry parameters, and particulate matter, alongside chlorophyll a concentrations. The study and data set offer insights into impacts of alkalinity enhancement on marine ecosystems and their associated biogeochemistry

KOSMOS 2018 Gran Canaria mesocosm study: particle flux data from sediment trap

The data set compiles sinking flux data collected during a KOSMOS mesocosm experiment carried out in the frame work of the Ocean Artificial Upwelling project. The experiment was performed in the North-East Atlantic Ocean off the coast of Gran Canaria in autumn 2018 and lasted for 39 days. In this study we investigated the effect of different intensities of artificial upwelling combined with two upwelling modes (recurring additions versus one singular addition) on POC export and its potential transfer efficiency to depth. The data set includes the amounts of surface water that were exchanged with nutrient-rich deep water (from ~300 m depth). It also contains particle flux data, i.e. POC flux, PON flux, BSi flux and the corresponding C:N and C:Si ratios, as well as the carbon-specific remineralization rates, sinking velocities, porosities and remineralization length scales of sinking particles

KOSMOS 2018 Gran Canaria mesocosm study: water column biogeochemistry

The data set compiles biogeochemical water column collected during a KOSMOS mesocosm experiment carried out in the frame work of the Ocean Artificial Upwelling project. The experiment was performed in the North-East Atlantic Ocean off the coast of Gran Canaria in autumn 2018 and lasted for 39 days. In this study we investigated the effect of different intensities of artificial upwelling combined with two upwelling modes (recurring additions versus one singular addition) on POC export and its potential transfer efficiency to depth. The data set includes the amounts of surface water that were exchanged with nutrient-rich deep water (from ~300 m depth), primary production and chlorophyll a, elemental composition of suspended particulate matter (POC, PON, C:N) and prokaryotic heterotrophic production