Hans Jakobsen

Aarhus University | AU · Ecoscience

The removal of organic particulate matter, predominantly phytoplankton, in eutrophic coastal seas and estuaries is considered an ecosystem service performed by large bivalve assemblages. Mussel farming has been proposed as a measure to mitigate eutrophication, as filtration directly reduces the concentration of chlorophyll a (Chl a), a primary ecol...

Marine harmful algal blooms (HABs), caused by various aquatic microalgae, pose significant risks to ecosystems, some socio-economic activities and human health. Traditionally managed as a public health issue through reactive control measures such as beach closures, seafood trade bans or closure of mollusc production areas, the multifaceted linkages...

Biological invasions, resulting from human activities, exert substantial impacts on ecosystems worldwide. This review focuses on marine invasive alien species (IAS) in Europe, examining the current state, proposing strategies to address the problem, and offering recommendations for enhanced management. Effective management of biological invasions r...

Food webs are complex networks of both feeding (trophic) and non-trophic interactions among species from different functional groups (primary producers, plankton, benthos, fish, birds and mammals) in ecosystems. Therefore, holistic and integrative approaches are needed to assess the ecological status of food webs and ecosystems. This model-deriv...

Plankton form the base of marine food webs, making them important indicators of ecosystem status. Changes in the abundance of plankton functional groups, or lifeforms, can affect higher trophic levels and can indicate important shifts in ecosystem functioning. Here, we extend this knowledge by combining data from Continuous Plankton Recorder and fi...

This is a technical guide for monitoring the taxonomic diversity of phytoplankton using environmental DNA metabarcoding, together with conventional phytoplankton monitoring techniques in marine and brackish waters. The guidelines focus on the detection of eukaryotic and prokaryotic phytoplankton with 18S and 16S rRNA primers, using high-throughput...

Non-indigenous species (NIS) are of concern for biodiversity conservation and ecosystem functioning. We present an updated list of NIS, including cryptogenic species, from Danish marine waters containing 123 species. Benthic invertebrates (36%) and phytoplankton (28%) dominate the list, but fish (15%) and macroalgae (13%) are also important. The Li...

Invasive alien species are a major worldwide driver of biodiversity change. The current study lists verified records of non-indigenous species (NIS) in European marine waters until 2020, with the purpose of establishing a baseline, assessing trends, and discussing appropriate threshold values for good environmental status (GES) according to the rel...

Pigmented microalgae inhabiting snow and ice environments lower the albedo of glacier and ice-sheet surfaces, significantly enhancing surface melt. Our ability to accurately predict their role in glacier and ice-sheet surface mass balance is limited by the current lack of empirical data to constrain their representation in predictive models. Here w...

Identification of planktonic flagellates is challenging, and multiple species are often lumped together, limiting our understanding of their diversity and ecology. Our aim was to investigate changes in the phytoplankton community composition over the annual cycle in Roskilde Fjord, Denmark, focusing on the identification of organisms smaller than 2...

European legislation requires monitoring of toxic algae in marine areas where shellfish are harvested for consumption. Monitoring assumes the existence of homogeneous water bodies, the definition of which have important implications for stakeholders and consumers. Yet, the definition of homogeneous water bodies remains unclear. Here we present a me...

Recommendations to frame problems and solutions for the pelagic habitats’ assessment

Ecological status of coastal seas in the Baltic and Northern Europe is assessed by a number of criteria, one of the primary being phytoplankton concentrations, indicated by chlorophyll-a fluorescence. Commercial mussel farming designed for mitigating eutrophication are undergoing development in the Western Baltic, and the intense filtration pressur...

Abstract Phaeocystis pouchetii (Hariot) Lagerheim, 1893 regularly dominates phytoplankton blooms in higher latitudes spanning from the English Channel to the Arctic. Through zooplankton grazing and microbial activity, it is considered to be a key resource for the entire marine food web, but the actual relevance of biomass transfer to higher trophic...

Growing evidence suggests that sexual reproduction might be common in unicellular organisms, but observations are sparse. Limited knowledge of sexual reproduction constrains understanding of protist ecology. Although Teleaulax amphioxeia and Plagioselmis prolonga are common marine cryptophytes worldwide, and are also important plastid donors for so...

Copepod swimming behavior is governed by chemical and hydro-mechanical cues. The environment of copepods, however, is frequently impacted by anthropogenic activities, in particular increased levels of suspended sediment due to coastal development. To better understand the effects of sediments on copepod behavior, we used video recordings to documen...

We assess the trends and influences of non-indigenous and cryptogenic species (hereafter simply referred to as "NIS") on Danish marine community compositions using three decades of quantitative monitoring data. Since the initiation of the Danish marine monitoring programmes in the 1980s, the number of marine NIS recorded in Denmark increased from 3...

Dissolved organic matter (DOM) is an important component of nutrient cycling, but the role of different organisms controlling the processing of autochthonous DOM remains poorly understood. Aiming to characterize phytoplankton-derived DOM and the effects of complex pelagic communities on its dynamics, we incubated natural plankton communities from a...

Blooms of Alexandrium spp. are a well-known phenomenon in Northern European waters. While A. tamarense/catenella, and A. pseudogonyaulax have been reported from marine waters, high densities of A. ostenfeldii are mainly observed at lower salinities in North Sea estuaries and the Baltic Sea, suggesting salinity as a driver of Alexandrium species com...

Refined baseline inventories of non-indigenous species (NIS) are set per European Union Member State (MS), in the context of the Marine Strategy Framework Directive (MSFD). The inventories are based on the initial assessment of the MSFD (2012) and the updated data of the European Alien Species Information Network, in collaboration with NIS experts...

Characterization of airborne bacterial cells requires efficient collection, concentration, and analysis techniques, particularly to overcome the challenge of their dilute nature in outdoor environments. This study aims to establish a rapid and reliable approach for quantification of bacteria in air samples. To do this, a high volume impingement sam...

Phytoplankton plays a key role as primary producers and mediating biogeochemical cycles in the water column. The understanding of the temporal dynamic of primary grazers channeling energy and carbon from primary producers is important for evaluating aquatic ecosystems functioning. This study investigates the coupling between phytoplankton and cilia...

Autotrophic microalgae are in general used as prey for copepods in laboratory experiments and in aquaculture mass culturing. We tested the suitability of using the osmotrophic thecate dinoflagellate Crypthecodinium cohnii as an alternative prey for the live prey organism for fish larvae, the planktonic calanoid copepod Acartia tonsa. We found that...

Incubation experiments were performed to examine the processing of fresh autochthonous dissolved organic matter (DOM) produced by coastal plankton communities in spring and autumn. The major driver of observed DOM dynamics was the seasonally variable inorganic nutrient status and characteristics of the initial bulk DOM, whereas the characteristics...

Coastal-estuarine systems are among the most productive marine ecosystems and their special role in producing harvestable fish and shellfish has been attributed to high primary production fueled by nutrient runoff from land and efficient trophic transfer. Here we ask if phytoplankton species composition and their food quality based on the percentag...

The phytoplankton community can vary within hours (physiology) to years (climatic and anthropogenic responses), and monitoring at different timescales is relevant for understanding community functioning and assessing changes. However, standard techniques used in monitoring programmes are time-consuming and/or expensive, limiting sampling frequency....

The motivation for this study was to analyse the productivity of Taiwanese aquaculture ponds while providing recommendations for a more effective copepod harvesting. Hence, variations in copepod species composition, biomass and productivity were investigated during four separate 3 weeks of intensive sampling campaigns across an annual season. The p...

We studied the predator–prey interactions between heterotrophic protists and endospores of Bacillus cereus group bacteria, in order to gain insight on survival and dispersal of B. cereus endospores in the environment. It has been hypothesised that the spore stage protects against digestion by predating protists. Therefore, experiments were carried...

Using copepod nauplii as live feed in aquaculture hatcheries could solve high mortality rates of first-feeding fish larvae due to malnutrition. However, implementing the use of copepod nauplii on an intensive production scale requires a stable production at preferably high densities, which is problematic for calanoid copepod species like Acartia to...

Turbot were reared from yolk sack larvae to juvenile in an outdoor semi-intensive system. Three production cycles were monitored from May to September. A pelagic food chain was established with phytoplankton, copepods and turbot larvae. Abiotic and biotic parameters of lower trophic levels together with turbot larval survival, development, prey ele...

Anthropogenic atmospheric loading of CO 2 raises concerns about combined effects of increasing ocean temperature and acidification, on biological processes. In particular, the response of appendicularian zoo-plankton to climate change may have significant ecosystem implications as they can alter biogeochemical cycling compared to classical copepod...

The seasonal variation in phytoplankton activity is determined by analysing 1385 primary production (PP) profiles, chlorophyll a (Chl) concentration profiles and phytoplankton carbon biomass concentrations (C) from the period 1998–2012. The data was collected at six different stations in the Baltic Sea transition zone (BSTZ) which is a location wit...

Carbon-to-chlorophyll a ratios (C:Chl a; weight : weight) were analyzed for 7578 coastal seawater samples collected from Danish waters from 1990 to 2014. The aim was to identify the seasonal and spatial dynamics relative to nutrient richness and to study the effect of reduced nitrogen loadings over time. C:Chl a values were lowest during winter, ab...

The report investigates trends in the temporal and spatial changes of non-indigenous marine species in the Danish part of the OSPAR and HELCOM regions. The assessment is based on a quantitative analysis of data available in national monitoring databases and covers the period 1989 to 2014 and other documented records of non-indigenous marine species...

Mesozooplankton grazing selection in complex marine microbial communities is a poorly understood yet critical structuring component of marine microbial food webs. We wished to quantitatively assess how relative grazing by the calanoid copepod Calanus spp. changes as a function of prey abundance dynamics in a controlled experimental setting. Our stu...

Summer planktonic protist assemblages (>20 µm) were investigated in the East Greenland Young Sound fjord and offshore waters between 2009 and 2012. Samples were taken using 20 µm mesh nets and Niskin bottles, fixed in acidic Lugol’s solution, and analysed by microscopy. Comparison between the two methods showed that important colonial species prese...

Quantification of grazing losses of marine heterotrophic bacteria is critical for understanding nutrient and carbon pathways in aquatic systems. The dilution method is a commonly used experimental approach for quantifying bacterivory. However, valid estimates of grazing rates obtained using this method depend on several methodological assumptions i...

A nitrogen manipulation experiment was conducted in a semi-intensive outdoor system where turbot larvae feed on copepods. Nitrogen addition is hypothesized to stimulate a cascade reaction increasing phytoplankton biomass, copepods' productivity and larval fish survival. Triplicates were established for three treatments: a control with no additional...

Plankton food web dynamics were studied during a complete production season in a semi-intensive land-based facility for rearing of turbot (Scophthalmus maximus) larvae. The production season was divided into three production cycles of 3–5 weeks. Phytoplankton biomass (using chlorophyll a as biomass proxy) peaked in each production cycle. However, t...

Microzooplankton dilution grazing experiments conducted in phytoplankton rich waters, particularly in polar and subpolar seas, often result in calculation of nonsignificant or negative grazing coefficients. We hypothesized that preparation of filtered seawater (FSW) from water containing high biomass of phytoplankton results in release of alleloche...

In the 1980s, Danish coastal waters suffered from eutrophication and several nutrient management plans have been implemented during the years to improve ecological status. This study aims at giving a holistic ecosystem perspective on 25 years of mitigation measures. We report trends of nutrient inputs and the responses to these in various chemical...

The development of a marine phytoplankton community was studied in a series of mesocosm tanks exposed to different levels of nutrient inputs. Key ecosystem variables such as phytoplankton species development, ecosystem net production (NEP), pH and bacteria production were measured. The overall aim was to mimic the consequences of extreme weather ev...

The environmental chemodynamics of hydrophobic organic chemicals (HOCs) are often rate-limited by diffusion in stagnant boundary layers. This study investigated whether motile microorganisms can act as microbial carriers that enhance mass transfer of HOCs through diffusive boundary layers. A new experimental system was developed that allows (1) gen...

We studied the effects of future climate change scenarios on plankton communities of a Norwegian fjord using a mesocosm approach. After the spring bloom, natural plankton were enclosed and treated in duplicates with inorganic nutrients elevated to pre-bloom conditions (N, P, Si; eutrophication), lowering of 0.4 pH units (acidification), and rising...

The short video is accessible at: http://youtu.be/LeuOyBMY_YA The video is supporting material for the full paper in ES&T: http://pubs.acs.org/doi/abs/10.1021/es404793u (https://www.researchgate.net/publication/260808488_Co-transport_of_polycyclic_aromatic_hydrocarbons_by_motile_microorganisms_leads_to_enhanced_mass_transfer_under_diffusive_condit...

Numerous biological activities such as grazer defense and intraspecific signaling have been described for diatom oxylipins, fatty acid derived secondary metabolites produced by some diatom species. As the function and importance of these compounds are still controversial, the production of a subclass of these molecules, nonvolatile oxylipins, was s...

We present a study on the protozooplankton >5µm and copepods larger than 50 µm at a series of contrasting stations across the Southern Indian Ocean (SIO). Numerically, over 80% of the copepod community across the transect was less than 650 µm in size, dominated by nauplii, and smaller copepods, while 80% of the biomass (as mgC m-3) was larger than...

Increasing pCO2 is hypothesized to induce shifts in plankton communities toward smaller cells, reduced carbon export rates and increased roles of gelatinous zooplankton. Appendicularians, among the most numerous pan-global “gelatinous” zooplankton, continuously produce filter-feeding houses, shortcutting marine food webs by ingesting submicron part...

Studies on intraspecific variation in Heterocapsa triquetra and Karlodinium veneficum

We show that Skeletonema marinoi suppresses chain formation in response to copepod cues. The presence of three different copepod species (Acartia tonsa, Centropages hamatus, or Temora longicornis) significantly reduced chain length. Furthermore, chain length was significantly reduced when S. marinoi was exposed to chemical cues from caged A. tonsa...

We studied the effect of a developing Skeletonema marinoi/Phaeocystis spp. bloom on Calanus finmarchicus hatching success, early naupliar survival and metabolism. Our focus was (1) on the development of reproductive rates during a bloom initiation, peak and decline in relation to the production of potentially toxic algal metabolites and (2) on the...

A survey of the production of polyunsaturated aldehydes (PUA) of manipulated plankton communities is presented here. PUA are phytoplankton-derived metabolites that are proposed to play an important role in chemically mediated plankton interactions. Blooms of different intensities of the diatom Skeletonema marinoi were generated in eight mesocosms f...

Instruments for in vivo identification and quantification of marine organisms are becoming more common, and the interpretation of data from these instruments is still evolving. In the present study, we compare the sizing performance of 3 instruments: (1) a black and white (B/W) FlowCAM II; (2) a BeckmanCoulter Multisizer III (MIII); and (3) an inve...

Egg and faecal pellet production and egg hatching success of the calanoid copepod Calanus finmarchicus were monitored over a period of 14days (14–28 April, 2008) while fed water from 4 differently treated mesocosms and ambient water. Two of the mesocosms used were inoculated with the polyunsaturated aldehyde (PUA)-producing diatom Skeletonema marin...

The oligotrophic Sargasso Sea in the western part of the North Atlantic Ocean is influenced by a complex set of oceanographic features that might introduce nutrients and enhance productivity in certain areas. To increase our understanding of the variability in plankton communities and to determine the potential reasons why Atlantic eels Anguilla sp...

Elevated levels of biomass and productivity are often associated with ocean frontal systems. The Subtropical Convergence Zone (STCZ) in the southern Sargasso Sea shows pronounced and stable thermal fronts, but little is known about the ecological consequences of these hydrographic features. With the aim of improving the understanding of physical an...

We studied the effect of a developing Skeletonema marinoi/Phaeocystis spp. bloom on Calanus finmarchicus hatching success, early naupliar survival and metabolism. Our focus was (1) on the development of reproductive rates during a bloom initiation, peak and decline in relation to the production of potentially toxic algal metabolites and (2) on the...

Chain formation is common among phytoplankton organisms but the underlying reasons and consequences are poorly understood. Here we show that chain formation is strongly impaired by waterborne cues from copepod grazers in the dinoflagellate Alexandrium tamarense. Chains of Alexandrium cells exposed to copepod cues responded by splitting into single...

Phytoplankton composition and biomass was investigated across the southern Indian Ocean. Phytoplankton composition was determined from pigment analysis with subsequent calculations of group contributions to total chlorophyll a (Chl a) using CHEMTAX and, in addition, by examination in the microscope. The different plankton communities detected refle...

Increased free CO2 and ocean acidification are among the consequences of anthropogenic carbon emissions. Responses of marine protists to increased levels of CO2 are highly species-specific, and this has been suggested to cause an alteration in plankton species composition, community functions and ultimately biogeochemical cycles. This study aims to...