David N. Thomas

University of Helsinki | HY · Department of Biological and Environmental Sciences

Antarctic pack ice is inhabited by a diverse and active microbial community reliant on nutrients for growth. Seeking patterns and overlooked processes, we performed a large-scale compilation of macro-nutrient data (hereafter termed nutrients) in Antarctic pack ice (306 ice-cores collected from 19 research cruises). Dissolved inorganic nitrogen and...

Over the past 20 years the study of the frozen Arctic and Southern Oceans and sub-arctic seas has progressed at a remarkable pace. This third edition of Sea Ice gives insight into the very latest understanding of the how sea ice is formed, how we measure (and model) its extent, the biology that lives within and associated with sea ice and the effec...

Terrestrial dissolved organic matter (DOM) undergoes significant changes during the estuarine transport from river mouths to the open sea. These include transformations and degradation by biological and chemical processes, but also the production of fresh organic matter. Since many of these processes occur simultaneously, properties of the DOM pool...

Finnish rivers exported annually on average 1.2 M t carbon, and total organic carbon (TOC) comprised the major share (nearly 80 %) of this export. The mean area specific carbon export was 4.5 g C m-2 year-1. The highest organic carbon export originated from peat dominated catchments, whereas rivers draining agricultural catchments had the highest a...

Network theory offers innovative tools to explore the complex ecological mechanisms regulating species associations and interactions. Although interest in ecological networks has grown steadily during the last two decades, the application of network approaches has been unequally distributed across different study systems: while some kinds of intera...

The Arctic Ocean is undergoing rapid change: sea ice is being lost, waters are warming, coastlines are eroding, species are moving into new areas, and more. This paper explores the many ways that a changing Arctic Ocean affects societies in the Arctic and around the world. In the Arctic, Indigenous Peoples are again seeing their food security threa...

Poly-3-hydroxyalkanoic acids (PHAs) are bacterial storage polymers commonly used in bioplastic production. Halophilic bacteria are industrially interesting organisms as their salinity tolerance and psychrophilic nature lowers sterility requirements and subsequent production costs. We investigated the PHA synthesis in two bacterial strains, Halomona...

The relative flow of carbon through the viral shunt and the microbial loop is a pivotal factor controlling the contribution of secondary production to the food web and to rates of nutrient remineralization and respiration. The current study examines the significance of these processes in the coastal waters of the Antarctic during the productive aus...

Basal melt of ice shelves is not only an important part of Antarctica's ice sheet mass budget, but it is also the origin of platelet ice, one of the most distinctive types of sea ice. In many coastal Antarctic regions, ice crystals form and grow in supercooled plumes of Ice Shelf Water. They usually rise towards the surface, becoming trapped under...

Marine hydrocarbon-degrading bacteria play an important role in natural petroleum biodegradation processes and were initially associated with man-made oil spills or natural seeps. There is no full clarity though on what, in the absence of petroleum, their natural niches are. Few studies pointed at some marine microalgae that produce oleophilic comp...

Seawater freezes below -2.0°C and therefore ice covers vast areas of the polar oceans for part of every year. First, ice crystals float to surface; then ice floes form to create a frozen landscape on the ocean surface. This pack ice moves with wind and currents and can form huge piles of ice rubble or expose open water when ice floes move apart. Wh...

Current concerns about climate change have led to intensive research attempting to understand how climate-driven stressors affect the performance of organisms, in particular the offspring of many invertebrates and fishes. Although stressors are likely to act on several stages of the life cycle, little is known about their action across life phases,...

The episodic freezing over of the surface waters is arguably the most striking feature of the Southern and Arctic Oceans as well as some subarctic oceans such as the Baltic and Okhotsk Seas. Millions of square kilometers of ocean are covered by ephemeral layers of ice that are on average less than 1 m thick. This pack ice is highly dynamic, driftin...

Historical sea ice core chlorophyll-a (Chla) data are used to describe the seasonal, regional, and vertical distribution of ice algal biomass in Antarctic landfast sea ice. The analyses are based on the Antarctic Fast Ice Algae Chlorophyll-a data set, a compilation of currently available sea ice Chla data from landfast sea ice cores collected at ci...

Diatoms are significant primary producers in sea ice, an ephemeral habitat with steep vertical gradients of temperature and salinity characterizing the ice matrix environment. To cope with the variable and challenging conditions, sea ice diatoms produce polysaccharide-rich extracellular polymeric substances (EPS) that play important roles in adhesi...

Sea ice is one the largest biomes on earth, yet it is poorly described by biogeochemical and climate models. In this paper, published and unpublished data on sympagic (ice-associated) algal biodiversity and productivity have been compiled from more than 300 sea-ice cores and organized into a systematic framework. Significant patterns in microalgal...

Microalgae underpin most foodwebs in polar regions as terrestrial primary production is too limited to support these complex and productive ecosystems. The success of microalgae in these extreme and highly variable ecosystems is rooted in their evolution and adaptation. The recent application of omics approaches in addition to biochemical and physi...

1.The formation of sea ice impacts directly on the physical dynamics of water masses (e.g. wind stress at the sea surface) and air-sea exchange processes (e.g. vertical heat fluxes). 2. The annual cycle of formation, consolidation and melting of sea ice has a major influence on the ecology of both the benthic and pelagic components of the Baltic Se...

Antarctic pack ice is inhabited by a diverse and active microbial community reliant on nutrients for growth. Seeking patterns and overlooked processes, we performed a large-scale compilation of macro-nutrient data (hereafter termed nutrients) in Antarctic pack ice (306 ice-cores collected from 19 research cruises). Dissolved inorganic nitrogen and...

The episodic freezing over of the surface waters is arguably the most striking feature of the Southern and Arctic Oceans as well as some subarctic oceans such as the Baltic and Okhotsk Seas. Millions of square kilometers are covered by ephemeral layers of ice that are on average less than 1 m thick. This pack ice is highly dynamic, drifting on the...

Over the past 20 years the study of the frozen Arctic and Southern Oceans and sub-arctic seas has progressed at a remarkable pace. This third edition of Sea Ice gives insight into the very latest understanding of the how sea ice is formed, how we measure (and model) its extent, the biology that lives within and associated with sea ice and the effec...

Estuarine bacteria are key modifiers of allochthonous matter entering the sea, and thereby control major biogeochemical processes such as nutrient cycling and organic matter transformation. In a highly dynamic estuarine environment, bacterial growth and activity are regulated by multiple factors including the availability of organic carbon, inorgan...

This special issue brought together a complementary set of studies describing the range of processes affecting the cycling and fate of terrigenous organic matter within aquatic systems. It focuses in particular on headwater streams, and on peat catchments as major global sources of freshwater dissolved organic matter and of particulate organic matt...

Given rapid sea ice changes in the Arctic Ocean in the context of climate warming, better constraints on the role of sea ice in CO2 cycling are needed to assess the capacity of polar oceans to buffer the rise of atmospheric CO2 concentration. Air-ice CO2 fluxes were measured continuously using automated chambers from the initial freezing of a sea i...

The objective of this study was to assess the O2 budget in the water under sea ice combining observations and modelling. Modelling was used to discriminate between physical processes, gas-specific transport (i.e., ice-atmosphere gas fluxes and gas bubble buoyancy) and bacterial respiration (BR) and to constrain bacterial growth efficiency (BGE). A...

These radiant images from the renowned 19th century biologist and illustrator Ernst Haeckel, featuring marine microorganisms, will enthrall fans of his previous collections and stimulate a renewed attention for Haeckel’s unparalleled artistry. From jewelry designers to scientists, graphic artists to naturalists, the range of people inspired by Erns...

Boreal coastal seas are reported to be subject of high carbon load from land, yet actual published results on the trends in carbon concentration in coastal waters are scarce.We examined a unique time series of total organic carbon (TOC) concentration at 20 sites along the Finnish coast of the northern Baltic Sea to identify linkages between TOC and...

The Arctic Ocean receives a large amount of terrestrial dissolved organic matter (DOM) from rivers and more than half of this is removed during its passage through the Arctic Ocean. Terrestrial DOM is generally believed to have a low bioavailability and recent studies point to physicochemical processes such as sea ice formation as the source of the...

Over the past two decades, with recognition that the ocean’s sea-ice cover is neither insensitive to climate change nor a barrier to light and matter, research in sea-ice biogeochemistry has accelerated significantly, bringing together a multi-disciplinary community from a variety of fields. This disciplinary diversity has contributed a wide range...

Sea ice is an active source or a sink for carbon dioxide (CO2), although to what extent is not clear. Here, we analyze CO2 dynamics within sea ice using a one-dimensional halo-thermodynamic sea ice model including gas physics and carbon biogeochemistry. The ice-ocean fluxes, and vertical transport, of total dissolved inorganic carbon (DIC) and tota...

We present the CH4 concentration [CH4], the partial pressure of CO2 (pCO2) and the total gas content in bulk sea ice from subarctic, land-fast sea ice in the Kapisillit fjord, Greenland. Fjord systems are characterized by freshwater runoff and riverine input and based on δ18O data, we show that > 30% of the surface water originated from periodic ri...

This study measured the effects of land use on organic matter released to surface waters in a boreal peat catchment using radiocarbon dating of particulate and dissolved organic carbon (POC & DOC), DOC concentration, stable carbon and nitrogen isotope composition and optical measurements. Undisturbed sites invariably released modern DOC & POC (< 20...

The flocculation of dissolved organic matter (DOM) was studied along transects through three boreal estuaries. Besides the bulk concentration parameters, a suite of DOM quality parameters were investigated, including colored DOM (CDOM), fluorescent DOM and the molecular weight of DOM as well as associated dissolved iron concentrations. We observed...

We report bulk gas concentrations of O2, N2 and Ar, as well as their transport coefficients, in natural landfast subarctic sea ice in southwest Greenland. The observed bulk ice gas composition was 27.5% O2, 71.4% N2 and 1.09% Ar. Most previous studies suggest that convective transport is the main driver of gas displacement in sea ice and have negle...

The effect of long-term (2 years) storage on dissolved organic carbon concentrations of filtered river water was investigated. Replicate samples were stored at 3 temperatures 20°C, 4°C or -20°C, either acidified or without acidification. The effects of sample volume were tested by storing 4 ml or 20ml samples at each temperature and again acidified...

The kinetics of calcium carbonate hexahydrate (ikaite) precipitation and dissolution were investigated in seawater and seawater-derived brines at sub-zero temperatures using the constant addition experimental technique. The steady state rate of these two processes was found to be a function of the deviation of the solution from equilibrium with res...

This is a report from the INTERICE 5 project that used the Arctic Environment Test Basin at HSVA from 21 May to 19 June 2012. The overarching aim was to investigate the physical and biological controls of dissolved organic matter incorporation into growing sea ice and the effect of melting once the ice had consolidated. Measurements were also made...

The bioavailability of predegraded dissolved organic matter (DOM) from a humic-rich, boreal river to estuarine bacteria from the Baltic Sea was studied in 39-day bioassays. The river waters had been exposed to various degrees of bacterial degradation by storing them between 0 and 465 days in dark prior to the bioassay. The resulting predegraded DOM...

The cycle of metal carbonate solids, such as CaCO3, is a key component of the global carbon cycle and is controlled by their solubility and reaction kinetics in aquatic systems. The presence of CaCO3 6H2O (ikaite) as discrete crystals in the sea ice of the Arctic and Antarctic Oceans indicates that there is a seasonal precipitation-dissolution cycl...

The radiocarbon age and biodegradability of dissolved organic matter (DOM) from a northern peat-dominated river system was studied and the effects of land-use were compared. Samples were obtained from streams and ditches comprising sub-catchments of the Kiiminki River, Northern Finland. Sample sites included areas of natural mire, areas subjected t...

According to previous studies, pCO2 fluxes measured over Arctic sea ice are higher than those measured over Antarctic sea ice. We hypothesized that this was due to enhanced respiration in Arctic sea ice, as a consequence of higher riverine inputs of dissolved organic carbon (DOC) into Arctic seawater. We tested this hypothesis during the Interice V...

We present CH4 concentration [CH4] and the partial pressure of CO2 (pCO2) in bulk sea ice from subarctic, land-fast sea ice in the Kapisillit fjord, Greenland. The bulk ice [CH4] ranged from 1.8 to 12.1 nmol L-1, which corresponds to a partial pressure range of 3 to 28 ppmv. This is markedly higher than the average atmospheric methane content of 1....

The structure of sea-ice bacterial communities is frequently different from that in seawater. Bacterial entrainment in sea ice has been studied with traditional microbiological, bacterial abundance, and bacterial production methods. However, the dynamics of the changes in bacterial communities during the transition from open water to frozen sea ice...

Rarefaction analysis of 16s rRNA gene clone library sequences from (A) unenriched North Sea water (day 0), (B) unenriched bottom ice (day 5), and (C) DOM-enriched bottom ice (day 5) samples.

Knowledge on the relative effects of biological activity and precipitation/ dissolution of calcium carbonate (CaCO3) in influencing the air-ice CO2 exchange in sea-ice covered season is currently lacking. Furthermore, the spatial and temporal occurrence of CaCO3 and other biogeochemical parameters in sea ice are still not well described. Here we in...

Sea ice is a biome of immense size and provides a range of habitats for diverse microbial communities, many of which are adapted to living at low temperatures and high salinities in brines. We measured simultaneous incorporation of thymidine (TdR) and leucine (Leu), bacterial cell abundance and cell population properties (by flow cytometry) in suba...

The microbial degradation of dissolved organic carbon and nitrogen (DOC, DON) was studied in three Finnish boreal estuaries with contrasting land use patterns (Kiiminkijoki – natural forest and peatland; Kyrönjoki – agricultural; Karjaanjoki – mixed/urban). Bioassays of 12–18 d long durations were used in 3 seasons at in situ temperatures. Besides...

The use of higher plants in constructed wetlands and hydroponic systems for remediation of wastewater from aquaculture is well established but application in marine systems is limited by plant salt tolerance. A combination of plant-scale experiments and pilot-scale constructed wetland and hydroponic systems have shown the potential of the halophyte...