Ocean acidification decreases the light-use efficiency in an Antarctic diatom under dynamic but not constant light, link to supplementary data

There is increasing evidence that different light intensities strongly modulate the effects of ocean acidification (OA) on marine phytoplankton. The aim of the present study was to investigate interactive effects of OA and dynamic light, mimicking natural mixing regimes. The Antarctic diatom Chaetoceros debilis was grown under two pCO2 (390 and 1000 latm) and light conditions (constant and dynamic), the latter yielding the same integrated irradiance over the day. To characterize interactive effects between treatments, growth, elemental composition, primary production and photophysiology were investigated. Dynamic light reduced growth and strongly altered the effects of OA on primary production, being unaffected by elevated pCO2 under constant light, yet significantly reduced under dynamic light. Interactive effects between OA and light were also observed for Chl production and particulate organic carbon (POC) quotas. Response patterns can be explained by changes in the cellular energetic balance. While the energy transfer efficiency from photochemistry to biomass production (Phi_e,C) was not affected by OA under constant light, it was drastically reduced under dynamic light. Contrasting responses under different light conditions need to be considered when making predictions regarding a more stratified and acidified future ocean.

Measurements of formation, degradation, and sinking velocity of diatom aggregates from laboratory studies performed at 15 °C and 4 °C

Most deep ocean carbon flux profiles show low and almost constant fluxes of particulate organic carbon (POC) in the deep ocean. However, the reason for the non-changing POC fluxes at depths is unknown. This study presents direct measurements of formation, degradation, and sinking velocity of diatom aggregates from laboratory studies performed at 15 °C and 4 °C during a three-week experiment. The average carbon-specific respiration rate during the experiment was 0.12 ± 0.03 at 15 °C, and decreased 3.5-fold when the temperature was lowered to 4 °C. No direct influence of temperature on aggregate sinking speed was observed. Using the remineralisation rate measured at 4 °C and an average particle sinking speed of 150 m d**-1, calculated carbon fluxes were similar to those collected in deep ocean sediment traps from a global data set, indicating that temperature plays a major role for deep ocean fluxes of POC.

Three species distribution models for the marine diatom Fragilariopsis kerguelensis of the Southern Ocean, links to NetCDF files

Fragilariopsis kerguelensis, a dominant diatom species throughout the Antarctic Circumpolar Current, is coined to be one of the main drivers of the biological silicate pump. Here, we study the distribution of this important species and expected consequences of climate change upon it, using correlative species distribution modeling and publicly available presence-only data. As experience with SDM is scarce for marine phytoplankton, this also serves as a pilot study for this organism group. We used the maximum entropy method to calculate distribution models for the diatom F. kerguelensis based on yearly and monthly environmental data (sea surface temperature, salinity, nitrate and silicate concentrations). Observation data were harvested from GBIF and the Global Diatom Database, and for further analyses also from the Hustedt Diatom Collection (BRM). The models were projected on current yearly and seasonal environmental data to study current distribution and its seasonality. Furthermore, we projected the seasonal model on future environmental data obtained from climate models for the year 2100. Projected on current yearly averaged environmental data, all models showed similar distribution patterns for F. kerguelensis. The monthly model showed seasonality, for example, a shift of the southern distribution boundary toward the north in the winter. Projections on future scenarios resulted in a moderately to negligibly shrinking distribution area and a change in seasonality. We found a substantial bias in the publicly available observation datasets, which could be reduced by additional observation records we obtained from the Hustedt Diatom Collection. Present-day distribution patterns inferred from the models coincided well with background knowledge and previous reports about F. kerguelensis distribution, showing that maximum entropy-based distribution models are suitable to map distribution patterns for oceanic planktonic organisms. Our scenario projections indicate moderate effects of climate change upon the biogeography of F. kerguelensis.

Miocene to Pleistocene diatom biostratigraphy with a focus on Thalassiosira in sediments of ODP Leg 186 sites

Late Neogene biostratigraphy of diatoms has been investigated from two sites occupied during Ocean Drilling Program (ODP) Leg 186 off the coast of northeast Japan. A unique aspect of ODP Leg 186 was the installation of two permanent borehole geophysical observatories at the deep-sea terrace along the Japan Trench. The Neogene subsidence history of the forearc was documented from both Sites 1150 and 1151, and Quaternary to middle Miocene (16 Ma) sediments represent a nearly continuous stratigraphic sequence including numerous ash records, especially during the past 9 m.y. Diatoms are found in most samples in variable abundance and in a moderately well preserved state throughout the sequence. The assemblages are characterized consistently by age-diagnostic species of Denticulopsis and Neodenticula found in regions of high surface water productivity typical of middle to high latitudes. The Neogene North Pacific diatom zonation divides the Miocene to Quaternary sequences fundamentally well, except that the latest Miocene through early Pliocene Thalassiosira oestrupii Subzone is not applicable. Miocene and late Pliocene through Pleistocene diatom datum levels that have been proven to be of great stratigraphic utility in the North Pacific Ocean appear to be nearly isochronous within the level of resolution constrained by core catcher sample spacing. The taxonomy and stratigraphy of previously described species determined to be useful across the Miocene/Pliocene boundary have been investigated on the basis of the evolutionary changes within the Thalassiosira trifulta group. The biostratigraphically important forms belonging to the genus Thalassiosira have been illustrated with scanning electron micrographs.

Plankton biomass, POC and PON concentrations, and sinking velocities during the 2008 spring diatom bloom in Disko Bay

Phytoplankton and copepod succession was investigated in Disko Bay, western Greenland from February to July 2008. The spring phytoplankton bloom developed immediately after the breakup of sea ice and reached a peak concentration of 24 mg chl a/m**3 2 wk later. The bloom was analyzed during 3 phases: the developing, the decaying, and the post-bloom phases. Grazing impact by the copepod community was assessed by 4 methods; gut fluorescence, in situ faecal pellet production, and egg and faecal pellet production from bottle incubations. Calanus spp. dominated the mesozooplankton community. They were present from the initiation of the bloom but only had a small grazing impact on the phytoplankton. Consequently, there was a close coupling between the spring phytoplankton bloom and sedimentation of particulate organic carbon (POC). Out of 1836 ±180 mg C/m**2/d leaving the upper 50 m, 60 % was phytoplankton based carbon (PPC). The composition and quality of the sedimenting material changed throughout the bloom succession from PPC dominance in the initial phase with a POC/PON ratio close to 6.6 to a dominance of amorphous detritus with a higher POC/PON ratio (>10) in the post-bloom phase. The succession and fate of the phytoplankton spring bloom was controlled by nitrogen limitation and subsequent sedimentation, while grazing-mediated flux by the Calanus-dominated copepod community played a minor role in the termination of the spring bloom of Disko Bay.

(Tables 2) The bPSi:POC ratio of the total diatom community, and abundance of full, empty, and broken diatom cells compared to heavily silicified diatoms during the EIFEX experiment

During the European Iron Fertilisation Experiment (EIFEX), performed in the Southern Ocean, we investigated the reactions of different phytoplankton size classes to iron fertilization, applying measurements of size fractionated pigments, particulate organic matter, microscopy, and flow cytometry. Chlorophyll a (Chl a) concentrations at 20-m depth increased more than fivefold following fertilization through day 26, while concentrations of particulate organic carbon (POC), nitrogen (PON), and phosphorus (POP) roughly doubled through day 29. Concentrations of Chl a and particulate organic matter decreased toward the end of the experiment, indicating the demise of the iron-induced phytoplankton bloom. Despite a decrease in total diatom biomass at the end of the experiment, biogenic particulate silicate (bPSi) concentrations increased steadily due to a relative increase of heavily silicified diatoms. Although diatoms >10 µm were the main beneficiaries of iron fertilization, the growth of small diatoms (2-8 mm) was also enhanced, leading to a shift from a haptophyte- to a diatom-dominated community in this size fraction. The total biomass had lower than Redfield C : N, N : P, and C : P ratios but did not show significant trends after iron fertilization. This concealed various alterations in the elemental composition of the different size fractions. The microplankton (>20 µm) showed decreasing C : N and increasing N : P and C : P ratios, possibly caused by increased N uptake and the consumption of cellular P pools. The nanoplankton (2-20 µm) showed almost constant C : N and decreasing N : P and C : P ratios. Our results suggest that the latter is caused by a shift in composition of taxonomic groups.

Radiocarbon ages and diatom isotope data for Lake El'gygytgyn sediment core Lz1029

Determining the response of sites within the Arctic Circle to long-term climatic change remains an essential pre-requisite for assessing the susceptibility of these regions to future global warming and Arctic amplification. To date, existing records from North East Russia have demonstrated significant spatial variability across the region during the late Quaternary. Here we present diatom d18O and d30Si data from Lake El'gygytgyn, Russia, and suggest environmental changes that would have impacted across West Beringia from the Last Glacial Maximum to the modern day. In combination with other records, the results raise the potential for climatic teleconnections to exist between the region and sites in the North Atlantic. The presence of a series of 2-3 per mil decreases in d18Odiatom during both the Last Glacial and the Holocene indicates the sensitivity of the region to perturbations in the global climate system. Evidence of an unusually long Holocene thermal maximum from 11.4 ka BP to 7.6 ka BP is followed by a cooling trend through the remainder of the Holocene in response to changes in solar insolation. This is culminated over the last 900 years by a significant decrease in d18Odiatom of 2.3 per mil, which may be related to a strengthening and easterly shift of the Aleutian Low in addition to possible changes in precipitation seasonality.

Radiocarbon ages and diatom abundance in samples obtained from Qarlikturvik Valley, Bylot Island

The formation of many arctic wetlands is associated with the occurrence of polygon-patterned permafrost. Existing scenarios to describe and explain surface landforms in arctic wetlands (low-center and high-center polygons and polygon ponds) invoke competing hypotheses: a cyclic succession (the thaw-lake hypothesis) or a linear succession (terrestrialization). Both hypotheses infer the predictable development of polygon-patterned wetlands over millennia. However, very few studies have applied paleoecological techniques to reconstruct long-term succession in tundra wetlands and thereby test the validity of existing hypotheses. This paper uses the paleoecological record of diatoms to investigate long-term development of individual polygons in a High Arctic wetland. Two landform processes were examined: (1) the millennial-scale development of a polygon-pond, and (2) the transition from low-center to erosive high-center polygons. Diatom assemblages were quantified from habitats associated with contrasting landforms in the present-day landscape, and used as an analog to reconstruct past transitions between polygon types. On the basis of this evidence, the paleoecological record does not support either of the existing models describing the predictable succession of polygon landforms in an arctic wetland. Our results indicate a need for greater paleoecological understanding, in combination with in situ observations in present-day geomorphology, in order to identify patterns of polygon wetland development and elucidate the long-term drivers of these landform transitions.