Jorun K. Egge

A model system approach to biological climate forcing. The example of Emiliania huxleyi

Abstract Particulate inorganic carbon (calcium carbonate mineral) is produced by pelagic calcifying organisms in the upper layers of the open ocean, it sinks to the deep sea, is partly dissolved and partly stored in the geological archive. This phenomenon, known as the carbonate pump, is an important component of the global carbon cycle and exerts a major influence on climate. The amount of carbonate mineral produced depends on the evolutionary and ecological success of calcifying pelagic organisms. The formulation of adequate predictive carbonate pump modules raises the problem that the behaviour of this highly diverse set of organisms needs to be taken into account. To overcome this difficulty, we propose a “model system” approach, whereby a single representative organism, the coccolithophore Emiliania huxleyi , is investigated in detailed interactive experimental and modelling studies. To construct a comprehensive model of the carbonate pump, subsequent research is envisaged on additional representative organisms, but this work is likely to be facilitated by the experience gained with E. huxleyi . The model system approach permits (1) an emphasis on the non-linear character of the fluxes; (2) a focus on the coupling of the carbonate pump with other climatically important phenomena — the organic carbon pump and DMS production; and (3) exploitation of the experimental accessibility of the E. huxleyi system.

Counterintuitive carbon-to-nutrient coupling in an Arctic pelagic ecosystem

Predicting the ocean’s role in the global carbon cycle requires an understanding of the stoichiometric coupling between carbon and growth-limiting elements in biogeochemical processes. A recent addition to such knowledge is that the carbon/nitrogen ratio of inorganic consumption and release of dissolved organic matter may increase in a high-CO2 world. This will, however, yield a negative feedback on atmospheric CO2 only if the extra organic material escapes mineralization within the photic zone. Here we show, in the context of an Arctic pelagic ecosystem, how the fate and effects of added degradable organic carbon depend critically on the state of the microbial food web. When bacterial growth rate was limited by mineral nutrients, extra organic carbon accumulated in the system. When bacteria were limited by organic carbon, however, addition of labile dissolved organic carbon reduced phytoplankton biomass and activity and also the rate at which total organic carbon accumulated, explained as the result of stimulated bacterial competition for mineral nutrients. This counterintuitive ‘more organic carbon gives less organic carbon’ effect was particularly pronounced in diatom-dominated systems where the carbon/mineral nutrient ratio in phytoplankton production was high. Our results highlight how descriptions of present and future states of the oceanic carbon cycle require detailed understanding of the stoichiometric coupling between carbon and growth-limiting mineral nutrients in both autotrophic and heterotrophic processes.

Blooms of phytoplankton including Emiliania huxleyi (Haptophyta). Effects of nutrient supply in different N : P ratios

Abstract Enclosures containing natural phytoplankton communities were fertilized with nitrate and phosphate in duplicates in three different ratios (16 : 5, 16 : 1, 16 : 0.2) in order to initiate blooms of the coccolithophorid Emiliania huxleyi (Lohmann) Hay et Mohler under controlled environmental conditions. The development of the phytoplankton community was in addition followed in two unfertilized enclosures and in the surrounding sea water. The phytoplankton succession during the experiment (22 April - 29 May 1992) was mainly dinoflagellates-diatoms-E. huxleyil Phaeocystis sp. in all fertilized enclosures, but the importance of the different species/groups were different in enclosures with different N : P ratio. Diatom numbers decreased when the N : P ratio increased in the nutrient supply. From an initial concentration of 0.09 109 cells m−3 E. huxleyi increased to concentrations between 20 109 cells m−3 and 37 109 cells m−3 in the enclosures supplied with nitrate and phosphate in aN: P ratio of 16 : ...

Ecological modelling in coastal waters: Towards predictive physical-chemical-biological simulation models

Abstract A simple, but general, simulation model is specified according to the state-of-the-art within phytoplankton modelling: Process representations are based upon prevailing theoretical and empirical representations given in the literature, and a set of earlier published values of model coefficients that have demonstrated good fit to reliable observations was selected. The emerging phytoplankton model was then validated against data obtained from enclosure experiments with light-, N-, P- and Si-limitations. We applied no tuning of the coefficients as the purpose of this test was to estimate the predictive power of the proposed model. The general standard deviations between model predictions and observations were on the range 0.04–0.36 and 0.13–0.42 for the nutrient and phytoplankton state variables respectively. Not surprisingly, these values are higher than those obtained in tuned simulations. Nevertheless, several characteristics, such as the balance between diatoms and flagellates, were predicted b...

Production of DMSP and DMS during a mesocosm study of an Emiliania huxleyi bloom: influence of bacteria and Calanus finmarchicus grazing

We investigated the influence of bacteria and metazooplankton on the production of dimethylsulfoniopropionate (DMSP) and dimethylsulfide (DMS) during blooms of Emiliania huxleyi (Lohmann) Hay and Mohler in seawater mesocosms. The phytoplankton succession was marked by the rapid collapse of an initial Skeletonema costatum (Greville) Cleve bloom followed by a small E. huxleyi bloom. The collapse of the diatom bloom was accompanied by an increase in concentrations of dissolved DMSP (DMSPd) and bacterial abundance and activity (as determined by the thymidine incorporation technique). The increase in bacterial activity was followed by a rapid decrease in DMSPd concentrations which remained low for the rest of the experiment, even during the subsequent collapse of the E. huxleyi blooms. The absence of DMSPd and DMS peaks during the declining phase of the E. huxleyi blooms was attributed to the high bacterial activity prevailing at that time. The influence of metazooplankton grazing on DMSP and DMS production was investigated by adding moderate (24 mg dry weight m-3) and high (520 mg dry weight m-3) concentrations of Copepodite Stage V and adults of Calanus finmarchicus to two of four filtered (200 μm mesh net) enclosures during the E. huxleyi blooms. The addition of C. finmarchicus, even in high concentrations, had no apparent effect on the dynamics of E. huxleyi, suggesting that the copepods were not grazing significantly on nanophytoplankton. The addition of copepods in high concentrations favored an accumulation of chlorophyll a and particulate DMSP. These results suggest that copepods were preying on the herbivorous microzooplankton which, in turn, was controlling the biomass of nanophytoplankton. DMS production was also enhanced in the enclosure with maximum metazooplankton biomass, suggesting that the grazing of C. finmarchicus on microzooplankton containing DMSP may contribute to DMS production. These results provide strong support to the emerging idea that bacteria and metazooplankton grazing play a dominant role in determining the timing and magnitude of DMS pulses following phytoplankton blooms.

Effects of increased concentration of nitrate and phosphate during a springbloom experiment in mesocosm

A mesocosm experiment was carried out in the early spring of 1991 (19 February to 20 March) in Raunefjorden, Norway. The experiment consisted of four enclosures of which two were initially supplied with nitrate and phosphate corresponding to an increase in concentration of 6 and 0.2 μM, respectively. Effects of an increased concentration of nitrate and phosphate on the development of the annual phytoplankton springbloom was investigated. Measurement of light, temperature, salinity, nutrients, chlorophyll (chl) a, primary production, phytoplankton enumeration and identification were performed daily or every other day. Average daily irradiance during the experiment was low (4.8 mol · m−2 · d−1). Maximum concentrations of biomass (chl a) and primary production were 11.5 μg chl a · 1−1 and 109 μg C ·1−1 · d−1. The initial phytoplankton community in all enclosures were dominated by diatoms, mainly Skeletonema costatum (Grev.) Cleve, and maximum cell number of this species was 11.2 · 106 1−1. After 1 wk, the diatoms were replaced by flagellates, due to silicate deficiency (< 2 μM). The fertilized enclosures were dominated by the haptophyte Phaeocystis cf. pouchetii (18 · 106 cells 1−1), and the non-fertilized enclosures were dominated by unidentified flagellates, together with cryptophytes and prasinophytes. High abundance of choanoflagellates and microzooplankton were also registered in the fertilized enclosures (3.3 · 106 1−1 and 11 · 106 1−1, respectively). This indicates that the microzooplankton may have controlled the growth of flagellates by grazing in the fertilized enclosures. The effect of an increased concentration of nitrate and phosphate was not an increase in biomass (chl a) or primary production, but a change in the species composition. The species composition changed from a diatom community dominated by S. costatum to a flagellate community dominated by P. cf. pouchetii. These results also suggest that major limiting factors for the biomass and primary production have been silicate deficiency, low irradiance and temperature.

The hydrography and biology of a bloom of the coccolithophorid Emiliania huxleyi in the northern north sea

In June=July 1994 a study was made of a small bloom of the coccolithophorid Emiliania huxleyi in an area of the North Sea to the east of the Shetland Islands. Observations on the hydrography of the study area indicated the bloom was associated with Atlantic water and was confined to an area in which a stable shallow mixed layer had formed. There was no evidence to suggest association of horizontal physical structure with the bloom development. High cell densities of >1‐6 10 6 cells dm 3 , together with low concentrations of PIC (<50 m gd m 3 ) and detached liths (2‐3 10 4 liths cm 3 ) indicated that the bloom was studied at an early stage of development. Biochemical and physiological observations indicated active growth was taking place. The results presented are discussed in comparison with previous studies carried out in both oceanic and shelf seas.

Representation of Emiliania huxleyi in phytoplankton simulation models. A first approach

Abstract Emiliania huxleyi (Lohmann) Hay et Mohler is represented as a state variable in a general phytoplankton model that also includes one diatom group and one phytoplankton group encompassing the ‘other flagellates’ than E. huxleyi. Furthermore, three nutrient variables (nitrogen-, phosphorus-, and silicon-nutrients) are included. The main features of the model are that E. huxleyi has been given a higher growth affinity for orthophosphate than the two other groups, and that diatoms, besides being dependent on silicon, have been given a higher maximal growth rate than the two flagellate groups. The output from the simulation model is compared with observations made in mesocosm experiments loaded with different amounts of nitrate and orthophosphate. The simulated E. huxleyi is not able to grow as well as the real E. huxleyi, and this applies especially to the experiments low in orthophosphate. It is difficult to account for the high observed numbers of E. huxleyi in terms of the available inorganic otho...

Standing stocks of PIC, POC, PON and Emiliania huxleyi coccospheres and liths in sea water enclosures with different phosphate loadings

Abstract Time series of the standing stocks of particulate inorganic carbon (PIC), particulate organic carbon (POC) and organic nitrogen (PON), and the numbers and type of Emiliania huxleyi coccospheres and detached coccoliths were determined in sea water enclosures with low, intermediate and high phosphate loadings. In the enclosures with the low and the intermediate phosphate loadings, intense blooms of E. huxleyi type A developed. PIC increased exponentially (µ = 0.3 day−1) and reached maxima of 0.3 g C m−3. PIC increased both during night and day. At the peak of the blooms, E. huxleyi made up 80 and 50 % of total POC in the enclosure with the low and the intermediate phosphate loading, respectively. High phosphate loading favoured other phytoplankton species and had negative effects on the abundance of E. huxleyi pO % of total POC) and the PIC standing stock (µ = 0.2 day−1; maximum 0.09 g C m−3). PIC per E. huxleyi cell differed between the enclosures and increased with decreasing net specific growth ...

Influence of dissolved silicate on vertical flux of particulate biogenic matter

The influence of dissolved silicate (DSi) addition on primary production, phytoplankton development and subsequent vertical export of particulate matter was studied in enclosures. Blooms of different phytoplankton communities were initiated in the upper part of 10 m deep enclosures supplied with nitrate and phosphate (NP) and nitrate, phosphate and silicate (NPS). Primary production was 31% higher in the NPS enclosure as compared to the NP enclosure over the experimental period of 27 days. Increased phytoplankton growth was mainly caused by mass development of diatoms in the NPS enclosure. Enhanced growth was accompanied by an increased vertical flux of organic matter (86, 15.9 and 16.9% in terms of chlorophyll, particulate nitrogen and particulate carbon, respectively) and was dominated by diatoms. The present study indicates that for each gram of DSi added, vertical flux was enhanced by 3.6 g C, implying that the ratio of DSi added/carbon exported was close to the Redfield ratio. Thus DSi presence appears to decrease the nutrient turn-over time in the euphotic zone by increasing vertical export. This may improve water quality of the surface layer of eutrophicated environments, but can lead to oxygen depletion of bottom waters.