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Rezultaty
A simple food web model has been developed to test the effects of small-scale turbulence on plankton population dynamics. Organisms are grouped into six boxes: bacteria, autotrophic flagellates, diatoms, heterotrophic flagellates, ciliates and meso-zooplankton. The development of the populations of the different taxa is represented by differential equations in the population concentrations. These differential equations are integrated numerically using a fourth-order Runge-Kutta scheme with adaptive step-sizing, written in C. The routine is flexible, allowing for different input conditions and possible future modifications of the model. It has a user-friendly interface written in tcl/tk (this is freely availably software which should be portable between different platforms).
The input parameters are the initial population concentrations; maximum nutrient uptake/grazing rates and half-saturation coefficients for the different species; efficiency factors for the predators, a death rate for the meso-zoplankton and a sedimentation rate for the diatoms. The parameters that change with turbulence are the half-saturation coefficients for diatoms and meso-zooplankton, and the sedimentation rate for diatoms.
We have used experimental data from the NTAP project to choose parameter values for the model. We have calculated the change in the parameter values under different turbulent conditions. The model predictions agree well with the experimental results and the model is also a good tool for investigating individual effects of turbulence and "what-if" scenarios.
Datasets have been collected on the specific affinity constant for phosphate uptake in differently sized osmotrophic micro-organisms; heterotrophic bacteria, autotrophic flagellates, and diatoms. The affinity constant is a central parameter for mineral nutrient competition at permanently low concentrations and thus a crucial parameter in P-cycle models. Theoretical affinity constants can be calculated based on the assumption of diffusion limitation. Our results for heterotrophic bacteria fit well with such theoretical estimates. For autotrophic flagellates, the values are similar to, or lower than, the theoretical estimates, while the result for the investigated diatom confirms the hypothesis of a high affinity constant in diatoms. This result will be used in the scientific community to advance in our understanding of the competition for nutrients between different osmotrophic organisms.
A customized solution to measure flow velocity fluctuations within small laboratory containers has been developed for use within the project NTAP. These containers are subject to grid generated turbulence and hence have constraints as to the type of sensors that can be put in them. We developed systems of embedded acoustic emitters and receivers mounted flush with the internal wall of the containers. Each array of transducers (1 emitter and 4 receivers) can measure flow velocity fluctuation at a single point in space and their placement have a series of geometrical constraints. Velocity is measured by the Doppler effect of sound bouncing back from particles in the flow. Turbulence parameters are then estimated by analysing the velocity time series.
This development is needed to characterize the flow in such small (2-15 l) laboratory containers used for biological experiments. Flow characterization within the containers will be disseminated to the scientific community through scientific journals. This result is not expected to be developed further into commercial products to due its high specificity of container shape and dimensions. The system was designed as a custom chamber and it would not be practical as a general-purpose product.
This result re-examines from first principles how nutrient uptake of individual cells is affected by different types of flows and how to use this information to derive the relevant terms for inclusion in macroscopic models of plankton population dynamics. For nutrient uptake by "unsteady squirmers", we have found that the mean nutrient concentration field, in a high frequency oscillatory flow, satisfies a steady advection-diffusion equation, but the relevant advective velocity field is different from the mean velocity field in the flow. In particular, the effective velocity field has small recirculation regions on the surface of the model organism, which tend to inhibit the enhancement of nutrient uptake that results from the stirring.
A more realistic model of the stirring by a flagellated microorganism is gradually being developed, so that the effect on nutrient uptake can be computed. We are considering a biflagellate algal cell, which swims using a high-frequency, low-Reynolds number breast-stroke. Slender-body theory is used to generate the velocity field numerically, and then the unsteady advection-diffusion equation will also be solved by entirely computational means. This is a very challenging computational problem.Hydrodynamic interactions between randomly oriented squirming organisms have been modelled and their effect on the distribution of mechanical stress in a population is being computed.
This result will aid in the implementation of appropriate equations in models assessing the effect of turbulence on planktonic systems.
This result will improve the scientific understanding of the effects of turbulence on phototrophic, heterotrophic and mixotrophic dinoflagellates. Growth rates of phototrophic and mixotrophic dinoflagellates have been assessed under turbulence conditions. In addition, ingestion and clearance rates of heterotrophic and mixotrophic flagellates are also being tested under turbulence.
There does not seem to be a generalized trend of responses in front of turbulence. Challenges in this study include the specifity of responses by the different dinoflagellate species and the maintenance of homogeneous cell concentration conditions in experimental containers.
Since several dinoflagellate species produce harmful algal blooms, this result will increase our present knowledge on why dinoflagellate blooms develop and dissipate, that may aid managers in applying solutions.
- We investigated the effects of turbulence, using an oscillating grid apparatus, on growth and ingestion in Strombidium sulcatum feeding on picoplankton-sized prey. In batch cultures of ciliates feeding on bacteria, subjected to 4 levels of turbulence ranging from epsilon = 0.005 2.0cm{2} sec{-3} or still water, we found a negative, near-linear, effect of turbulence on growth rate. Examination of turbulence-incubated cells showed no evidence of arrested cell division, known from some dinoflagellate species. Ingestion rates, measured using fluorescent microspheres, were lower under turbulent conditions. A prey selection experiment with microspheres of different surface qualities showed similar, previously established, patterns of selective ingestion but at lower rates under turbulent conditions. Based on a mathematical model we believe that turbulence affects predator-prey contact rates and suggest that turbulence causes changes in swimming speed or pattern in planktonic ciliates which results in lower ingestion rates, leading to lower growth rates.
- To test the effect of turbulence on respiration rates we incubated batch cultures of Strombidium sulcatum feeding on heterotrophic nanoflagellates (HNF) and bacteria as well as pre-screened cultures in BOD bottles, using an orbital shaker. Respiration rates were significantly higher under turbulence and correlated significantly just with HNF. Confirming previous results, turbulence had a negative effect on ciliates through higher respiration rates and lower growth with respect to the still control.
- Turbulence effects were examined with natural communities. Natural communities of ciliates and their prey were incubated either unaltered or diluted with filtered (algae-free) seawater. No significant effects of turbulence were evident.
- Data on the diversity of a sub-group of marine planktonic ciliates, tintinnid ciliates, were analysed to determine if a direct effect of turbulence (in the form of water column structure) could be demonstrated. We found that turbulence could not be shown to have a direct effect on tintinid diversity.
In summary, we discovered negative effects of turbulence on a marine ciliate but found no significant effect of turbulence, in and of itself, on natural, mixed species assemblages of marine ciliates. In the light of these findings we conclude that direct effects of turbulence on the growth of the natural ciliate communities need not be considered in models of planktonic food webs in which mixed species assemblages of ciliates are treated as a uniform group. However, our results with species of Strombidium sulcatum showed that turbulence could have a strong negative effect on certain ciliate species. Therefore, in systems in which the ciliate community is composed of a few species, for example in wastewater treatment tanks, turbulence may, through a particular species have a large effect on any process (bactivory, etc.) mediated through ciliates.
This project has focused on developing a 3D water velocity sensor that can be used to measure turbulence in laboratory flasks and culture vessels that have size and shape constrains. To achieve this objective, the starting point was the existing acoustic Doppler velocimeter (ADV); a well-established sensor based on the use of transmitted and received sound waves at frequencies around 10MHz. The ADV is in worldwide use today, mostly in hydraulics laboratories but also in field applications such as river flow.
The development focused on miniaturizing the sensor elements and the electronics of the ADV. First, we looked at alternative sensing elements (pvdf materials) that are easy to shape and thus may have the potential for use with external mounting, which would make measurement in small containers simpler to achieve. Unfortunately, this avenue also meant that the ability to measure in clear water would be reduced, a real problem for biological cultures since it would have required seeding particles to be added to the cultures. The idea of flexible sensor elements was thus abandoned. Instead, a probe was designed with very minimum mechanical dimensions limited only by the size of the acoustic ceramic elements that are used to send and received the sound waves. This significantly reduced the probe size relative to the existing designs but maybe not by as much as could be desired for the smallest flasks.
For the electronics design, all the objectives of the project were met (or exceeded) and the total size of the new electronics is reduced by a factor of 10 relative to the ADV designs. In addition, several performance enhancing elements were added:
- A new fourth receiver element was added to the probe. This improves the ability to measure turbulence by providing two independent estimates of the vertical flow component. This improves both dissipation and Reynold's stress estimates, especially at low to moderate turbulence levels.
- A temperature sensor was integrated with the probe. This allows the software to calculate the speed of sound on the fly and removes the need to enter the temperature manually when operating the instrument.
- All four receivers were parallelized. This means that it is possible to measure all four receivers at the same time, rather than alternating from one to the other. This increases the number of samples by a factor of 4.
- The maximum velocity that the system can measure was increased by a factor of 2.
- An echo sounder function was added so that the bottom contour of sampling contained can be measured with sub-mm resolution.
The new and smaller probe and the new electronics have been combined in a new Nortek product called "Vectrino". This product replaces that existing Nortek ADV and it expands the scales at which this technology is suitable. In other words, relative to the ADV, the Vectrino can measure in smaller measurement volumes, it can sample faster, it is more precise, and it can measure turbulence at lower turbulence levels. For example, the output rate for the 3D velocity measurement has been increased from 25Hz to 200Hz, and the number of possible applications is increased correspondingly.
The Vectrino is a new product with no common elements with the ADV other than the general principle. It has been developed in the context of biological research and it has been used within the project to help standardize literature estimates of turbulence in laboratory chambers. However, the applications of the Vectrino span well outside biological research. We expect that the Vectrino will replace the ADV for all practical flow research conducted on scales from a few mm to 1m.
The Vectrino is designed to be backward compatible with the existing ADV, thereby providing an affordable upgrade path for existing users. This means that the instrument will be used by researchers and engineers all over the world in applications from basic fluid mechanics to ocean wave motion. The Vectrino also has a lower cost than the ADV, thereby increasing the ability of scientists without large equipment budgets to use this technology in their research.
Data on the effects of different hydrodynamic regimes upon organisms, communities and biological processes have been compiled into a database. These data include the intensity of the hydrodynamic regime, temperature, salinity, organism size, nutrient and/or prey particle concentration and size, organism generation time and other characteristic processing times derived from bulk uptake or ingestion data, swimming or sinking velocity, the turbulence generation and measurement methodology, and others. When data were incomplete in the source paper we have filled in gaps, mostly by using accepted empirical relationships. In order to estimate the physical data, the fluid motion has to be characterised in the source paper. If it is not, we have characterised it from the indications in the paper and accepted formulas.
Database structure. A relational database has been constructed based on the computer program FileMaker. Each data point addressing the effect of a certain level of turbulence on an organism or process is entered as a unique record that contains the information specified above in many fields.
Web connection. FileMaker allows integration with web pages. The database can currently be accessed and searched through the web for internal project use and for the general public with restrictions.
We expect to have an open version of the database for the scientific community to run with a web interface well beyond the end of the NTAP project. It will also be of use to other areas of society as administrations, managers, and others that need information on effects of turbulence on plankton.
The invention consists of a system to generate turbulence in a fluid inside laboratory containers. Turbulence is generated by vertically oscillating grids. The system allows for the simultaneous generation of an array of different turbulence levels thanks to the use of independent gear head motors of different nominal rotation frequency and a velocity regulation. Grid stroke can be changed by the use of the adjustable eccentric arm that transfers the rotational motion of the motor into the vertically translational motion. Turbulence can be generated in multiple containers.
The materials in contact with the fluid are adequate for use with life organisms and comply with food industry standards. The system is scalable both in the number of motor units needed as in the volume of the containers.
This system will be of use to laboratories studying the effect of turbulence on plankton, especially at the individual species level. Where most systems are quite rigid in their design, this system allows for a range of turbulence levels to be tested simultaneously.
In three mesocosm experiments conducted in a coastal Norwegian environment and several microcosm experiments conducted with Mediterranean coastal waters, datasets have been collected on the effect of nutrients and turbulence on mass transfer through the pelagic food web (up to mesozooplankton). The dataset contains information about rates (bacterial and primary production, phosphate uptake in size-fractions and turnover-time), on biomasses (chlorophyll and particulate-P in size-fractions, particulate C and N), and microscopic and flow-cytometric counts of specific groups (heterotrophic bacteria, cyanobacteria, diatoms, heterotrophic dinoflagellates). The dataset includes observations of the effect of one level of nutrient addition for different levels of turbulence, the effect of different levels of nutrient addition for two levels of turbulence, and the effect of zooplankton and silicate enrichments. The experiments have proven the technical feasibility of controlling nutrients and turbulence in these systems, and thus allowed us to study the effects of these factors in systems containing complete food webs (up to mesozooplankton).
The interactive effects of turbulence and nutrients appear to have been different in the different experiments. This suggests that the effects of turbulence and nutrients on the food web vary with the composition of the initial system present in the water mass filled into the experimental containers. Overall, our findings indicate that:
- Turbulence modifies the response of plankton to nutrient enrichment;
- The effect of turbulence depends on plankton structure (size distribution);
- The effect of turbulence on sedimentation of particulate organic matter needs to be taken into account;
- Turbulence can change the chemical composition of organic matter;
- There are particularities of the systems studied and of initial conditions when evaluating the response of a coastal system to turbulence and nutrient load;
- Turbulence should be included in the models to improve the predictions of plankton responses to eutrophication.
In a coastal ecosystem scenario with elevated (but below an upper threshold) nutrient load, turbulence will optimise the incorporation of nutrients into particulate matter, provided nutrients are more or less balanced. The ultimate fate of this particulate matter will depend on the timing of a number of variables.
The interaction between nutrients and turbulence seems to depend on the area or system. This shows the need to create location specific legislation for nutrient loading and also evidences the need for considering hydrodynamic characteristics when defining biogeochemical provinces. With these results we are enlarging the data set on turbulence responses and parameterisations of nutrient affinities at different mixing conditions. Predictions based in our experimental and modelling research will be useful for defining recommendations and aiding decision-making and policy implementation about human impacts in coastal areas that alter the patterns of turbulence and/or nutrient load (e.q. placement of waste outfall sites, optimisation of aquaculture sites, harbour construction, etc.).
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