Description: A simple two-size-class aggregation model is developed to describe the time-dependent carbon content of dissolved polysaccharides (PCHO) and of transparent exopolymer particles (TEP) during the bloom. A conservative estimate for the effective collision kernel is obtained from the Smoluchowski equation under the assumption that the growth of aggregates is controlled by a Brownian process near the scaling regime. In the model, PCHO are assumed to represent a fraction of the photosynthetic carbon, which is not used for net algal growth. Time dependence of chlorophyll a and of cellular carbon during the bloom is modelled in terms of algal growth and sinking of single and aggregated algal cells. The aggregation of exopolysaccharides into TEP may have important implications for the organic carbon cycle in the ocean, as TEP promote the aggregation of algae during a bloom.

Description: One particular task of marine ecosystem models is to simulate the biogenic transformation of dissolved inorganic carbon (DIC) into organic matter and hence to quantify the export of particulate organic carbon (POC) to deep oceanic layers. To date, environmental changes, such as increasing carbon dioxide concentrations (pCO_2) and temperature, are perceived to have an impact on the formation of organic carbon. However, well established nitrogen or phosphorus based ecosystem models are insensitive to variations in the carbonate system. In order to investigate biological responses to pCO_2 variations, ecosystem models need to distinguish between carbon, nitrogen, and/or phosphorus cycles. We present a simple biological model which decouples carbon from nitrogen fluxes such that carbon found in transparent exopolymer particles (TEP) is additionally accounted for. The model regards phytoplankton acclimation to varying environmental conditions, having included parameterizations for phytoplankton growth as proposed by Geider et al.~(1998, L&O). By means of data assimilation, an optimal parameter set is determined, which brings model results into agreement with experimental data. From the optimised model results it is infered that about 50% of dissolved organic carbon (DOC) exuded by phytoplankton is subsequently transformed into TEP, eventually influencing the amount of POC available for the export flux. Model sensitivity studies are performed at local sites and along a latitudinal transect (30^oN-60^oN at 19^oW) in the North Atlantic. As soon as CO_2 limitation for phytoplankton growth is explicitely considered in the model, the formation of POC shows great sensitivity to pCO_2 variations. Temperature variations alter remineralisation rates and growth efficiencies. With the current model version dependencies between biomass accumulation, the date of nutrient depletion to occur, and the exudation of organic compounds are acquired.

Description: The export of organic carbon to the deep ocean is mediated by sinking of large particles, such as marine snow, the formation of which is enhanced in the presence of transparent exopolymer particles (TEP) . TEP form from dissolved and colloidal polysaccharides by aggregation processes. Especially when running into nutrient limitation phytoplankton organisms are a source of TEP in pelagic ecosystems as the cells release a significant amount of the assimilated carbon in the form of polysaccharides. Because CO_2 concentration influences carbon assimilation rates, we hypothesized that polysaccharide exudation and aggregation into TEP is related to CO_2 concentration under nutrient limiting conditions. We tested this hypothesis in several lab and outdoor experiments with natural populations and cultures of phytoplankton exposed to various levels of CO_2 concentrations. Our results indicate that TEP production increases with CO_2 concentration and provides an enhanced sink for carbon during phytoplankton blooms.

Description: A fraction of the photosynthetically fixed carbon is not used for phytoplankton growth, but channelled to the outer medium via exudation. This fraction include carbon rich exopolymers that coagulate to particles, such as transparent exopolymer particles (TEP). Through aggregation with cells and debris, TEP are incorporated into large, rapidly settling marine snow and contribute to the vertical flux of organic matter to the deep sea. The influence of TEP formation on the C:N:P stoichiometry of marine phy- toplankton blooms was examined during two mesocosm studies. During the blooms, which were dominated by a natural assembly of marine diatoms and the calcifying coccolithophorid Emiliania huxleyi, respectively, an increase of TEP concentration was observed immediately after nutrient depletion, followed by the appearance of ma- rine snow. The contribution of TEP to the carbon flow during both blooms indicates that sinking of TEP-rich marine snow is a possible mechanism for the removal of carbon from surface waters above calculations based on Redfield stoichiometry.

Description: One of the most prominent consequences of human activities is the progressive rise in atmospheric pCO_2. This will also cause changes in seawater carbonate chemistry. However, to what extent anthropogenic perturbations in marine growth conditions will affect biological processes like species composition, ecosystem functioning, carbon production and vertical carbon export in the sea, as well as possible feedback mechanisms, is still under debate. In the present study, for the first time CO_2 effects were tested on a natural marine plankton community. In a series of floating mesocosms in a Norwegian fjord a phytoplankton bloom dominated by the coccolithophore Emiliania huxleyi was induced. By covering the enclosures by gas-tight domes, functioning as greenhouses, we were able to maintain CO_2 concentrations in the overlaying atmosphere ranging from pre-industrial to projected year 2100 levels over a three-week period. Here we present an overview of the experiment and report on the development of the bloom under different pCO_2 levels.

Description: Particulate organic matter which displays a fairly constant C:N ratio (Redfield ratio) consists of different particle types, including organisms, detritus and transparent exopolymer particles (TEP). Whereas the C:N-ratio of cells frequently lies below Redfield, the C:N ratio of TEP, which are formed abiotically from exuded polysaccharides is higher. The mean molar C:N ratios of TEP derived from cultures and natural diatom populations grown under different conditions exhibited a mean value of 26. This is considered a low estimate, as the nitrogen fraction can be explained through adsorption of amino acids onto TEP. The TEP-derived carbon concentration was calculated in 6 marine environments, based on the derived relationship between carbon and TEP. The contribution of TEP-carbon to the total carbon pool was large, in some cases as high as the POC contribution. TEP are also an essential component of marine snow and contribute appreciably to carbon flux. Because TEP are enriched in carbon over nitrogen, an increase in the formation rate of TEP due to global warming would provide a pathway for the sequestration of excess carbon to the deeper water column.

Description: According to a recent study, C:N ratios of sinking particulate organic matter (POM) in the ocean appear to be higher than Redfield (7.1 instead of 6.6) and depth dependent (increase +0.2/km). Here we investigate the effects of vertically variable C:N element ratios on marine carbon fluxes and the air-sea exchange of CO2 using a global ocean carbon cycle model (AAMOCC). For a steady-state ocean, the results show that models using the constant classical Redfield ratio underestimate both, total inventory and vertical gradients of dissolved inorganic carbon (DIC). While the amount of additional DIC (+150 Gt C) is negligible compared to the high marine carbon inventory, the C:N depth dependence can reduce the ambient atmospheric pCO2 by 20 ppm, permanently. Moreover, the simulation of a future scenario, estimating a possible effect of CO2-dependent C:N ratios of POM on the marine carbon cycle, has shown that even a moderate rise in the C:N element ratio of sinking POM, which is on the order of magnitude of natural variability, yields a considerably higher oceanic uptake of anthropogenic CO2 on timescales of decades to centuries. The assumption is based on a predicted increase in the production of highly carbon enriched transparent exopolymer particles (TEP) caused by rising atmospheric CO2 concentrations and enhanced nutrient limitation. However, counteracting a predicted decrease of the physical (solubility) CO2 pump as a consequence of global change, the effect in our scenario will alleviate further rising atmospheric CO2 concentrations rather than compensate a reduced physical uptake.

Description: Flows of the major biogeochemical elements (C, N, P, Si) and of transparent exopolymer particles (TEP) were traced during a bloom of a natural assemblage of marine diatoms in a mesocosm (l m(3)) to determine whether the exudation and subsequent gelation of carbon-rich phytoplankton exopolymers can account for the formation and potential export of carbon in excess of that predicted by Redfield ratios. Exponential growth of the phytoplankton community in the mesocosm extended for 10 d until nitrate concentration fell below detection and concentrations of dissolved inorganic and particulate organic nitrogen and phosphorus remained stable. Tight covariation of particulate organic elements occurred as long as nutrients were replete. But, after nitrate depletion, decoupling of carbon dynamics from that of nitrogen and phosphorus was observed, with a large flow of carbon into TEP An uptake of 72% more dissolved inorganic carbon (DIC) than inferred from nitrate supply and Redfield stoichiometry (referred to as carbon overconsumption) occurred during the study, largely during the postbloom phase, and was almost entirely traced to the particulate organic matter (POM) pool. Marine snow (aggregates 〉0.5 mm) appeared at the onset of nitrate depletion and coincided with rapid increase in TEP concentrations. Elemental composition of marine snow differed from the Redfield ratio by an enrichment in carbon and a depletion in phosphorus relative to nitrogen. It is suggested that sinking of TEP-rich marine snow could be a possible mechanism for export of carbon above calculations that are based on the Redfield stoichiometry.

Description: Observations that the majority of silica dissolution occurs within the upper 200 m of the ocean, and that sedimentation rates of diatom frustules generally do not decrease significantly with depth, suggested reduced dissolution rates of diatoms embedded within sinking aggregates. To investigate this hypothesis, silica dissolution rates of aggregated diatom cells were compared to those of dispersed cells during conditions mimicking sedimentation below the euphotic zone. Changes in the concentrations of biogenic silica, silicic acid, cell numbers, chlorophyll a and transparent exopolymer particles (TEP) were monitored within aggregates and in the surrounding seawater (SSW) during two 42-day experiments. Whereas the concentration of dispersed diatoms decreased over the course of the experiment, the amount of aggregated cells remained roughly constant after an initial increase. Initially only 6% of cells were aggregated and at the end of the experiment more than 60% of cells were enclosed within aggregates. These data imply lower dissolution rates for aggregated cells. However, fluxes of silica between the different pools could not be constrained reliably enough to unequivocally prove reduced dissolution for aggregated cells.