In the eastern North Water, most of the estimated annual new and net production of carbon (C) occ... more In the eastern North Water, most of the estimated annual new and net production of carbon (C) occurred during the main diatom bloom in 1998. During the bloom, at least 30% of total and new phytoplankton production occurred as dissolved organic carbon (DOC) and was unavailable for short-term assimilation into the herbivorous food web or sinking export. Based on particle interceptor traps and 234 Th deficits, 27% of the particulate primary production (PP) sank out of the upper 50 m, with only 7% and 1% of PP reaching the benthos at shallow (%200 m) and deep (%500 m) sites, respectively. Mass balance calculations and grazing estimates agree that %79% of PP was ingested by pelagic consumers between April and July. During this period, the vertical flux of biogenic silica (BioSi) at 50 m was equivalent to the total BioSi produced, indicating that all of the diatom production was removed from the euphotic zone as intact cells (direct sinking) or empty frustules (grazing or lysis). The estimated flux of empty frustules was consistent with rates of herbivory by the large, dominant copepods and appendicularians during incubations. Since the carbon demand of the dominant planktivorous bird, Alle alle, amounted to %2% of the biomass synthesized by its main prey, the large copepod Calanus hyperboreus, most of the secondary carbon production was available to pelagic carnivores. Stable isotopes indicated that the biomass of predatory amphipods, polar cod and marine mammals was derived from these herbivores, but corresponding carbon fluxes were not quantified. Our analysis shows that a large fraction of PP in the eastern North Water was ingested by consumers in the upper 50 m, leading to substantial carbon respiration and DOC accumulation in surface waters. An increasingly early and prolonged opening of the Artic Ocean is likely to promote the productivity of the herbivorous food web, but not the short-term efficiency of the particulate, biological CO 2 pump.
Communities of marine phytoplankton consist of cells of many different sizes. The size-structure ... more Communities of marine phytoplankton consist of cells of many different sizes. The size-structure of these communities often varies predictably with environmental conditions in aquatic systems. It has been hypothesized that physiological differences in nutrient and light requirements and acquisition efficiencies contribute to commonly observed correlations between phytoplankton community size structure and resource availability. Using physiological models we assess how light and nutrient availability can alter the relative growth rates of phytoplankton species of different cell sizes. Our models predict a change in the size dependence of growth rate depending on the severity of limitation by light and nutrient availability. Under conditions of growth-saturated resource supply, phytoplankton growth rate (mol C cell À1 time À1 ) scales with cell volume with a size-scaling exponent of 3 4 ; light limitation reduces the size-scaling exponent to approximately 2 3 , and nutrient limitation decreases the exponent to 1 3 as a consequence of the size-scaling of resource acquisition. Exponents intermediate between 1
The effects of colored dissolved organic matter (CDOM) from freshwater runoff and seasonal cycle ... more The effects of colored dissolved organic matter (CDOM) from freshwater runoff and seasonal cycle of temperature on the dynamic of phytoplankton and zooplankton biomass and production in the Gulf of St. Lawrence (GSL) are studied using a 3-D coupled physical-plankton ecosystem model. Three simulations are conducted: (1) the reference simulation based on Le , in which light attenuation by CDOM is not considered and maximum growth rate (m max ) of phytoplankton and zooplankton are not temperature-dependent (REF simulation); (2) light attenuation by CDOM is added to REF simulation (CDOM simulation); and (3) in addition to CDOM, the m max of phytoplankton and zooplankton are regulated by temperature (CDOM +TEMP simulation). CDOM simulation shows that CDOM substantially reduces phytoplankton biomass and production in the Lower St. Lawrence Estuary (LSLE), but slightly reduces overall primary production in the GSL. In the LSLE, the spring phytoplankton bloom is delayed from mid-March to mid-April, resulted from light attenuation by CDOM. The CDOM + TEMP simulation shows that the spring phytoplankton bloom in the LSLE is further delayed to July, which is more consistent with observations. Annual primary production is reduced by 33% in CDOM + TEMP simulation from REF and CDOM simulations. Zooplankton production is the same in all three simulations, and export of organic matter to depth is reduced in CDOM + TEMP simulation, suggesting that temperature controlled growth of phytoplankton and zooplankton enhances the coupling between primary production and zooplankton production under the seasonal temperature cycle of the GSL. (Z.-P. Mei), [email protected] (V. Le Fouest), [email protected] (B. Zakardjian), [email protected] (S. Sennville), [email protected] (H. Xie), [email protected] (M. Starr).
Biomass and production of plankton communities were investigated in two Chinese integrated fish c... more Biomass and production of plankton communities were investigated in two Chinese integrated fish culture ponds in August, Dianshanhu Pond (with high density of planktivorous carp) and Pingwang Pond (with low density of planktivorous carp). The plankton communities were composed of rotifers, protozoans, phytoplankton (<40 µm) and bacteria. The large phytoplankton (>40 µm), cladocerans and copepods were rare because of grazing pressure by the carp. The density or biomass of bacteria (1.93 × 107 and 2.20 × 107 cells ml−1 on average in Dianshanhu and Pingwang Ponds, respectively), picophytoplankton (24.6 and 18.5 mg m−3 Chla on average) and rotifers (5372 and 20733 ind. 1−1 on average) exceeded the maximum values reported for natural waters. The average [3H]thymidine uptake rates were 694 and 904 pmoles 1−1 h−1 (13.4 and 20.6 µgC 1−1) and the bacterial production by the >2 µm fraction amounted 21–28% of total [3H] thymidine uptake rate in both ponds. The mean chlorophylla concentrations were 59.1 and 183 mg m−3 in Dianshanhu and Pingwang Ponds, respectively. 82.4% and 65.3% of the total Chla was contributed by the <10 µm nano- and picophytoplankton in each pond, respectively. In particular, the picophytoplankton contribution amounted 41.2% of thtal Chla in Dianshanhu Pond. Primary production was 2.5 and 3.4 gC m−2 d−1 in each pond, respectively, and >50% of production was contributed by picophytoplankton. The mean biomasses of protozoa were 168 µg 1−1 and 445 µg 1−1 and those of rotifers were 763 µg 1−1 and 1186 µg 1−1 in Dianshanhu and Pingwang Ponds, respectively. The ecological efficiencies expressed in terms of the ratios of primary production to zooplankton production were 0.22 and 0.31, for the two ponds.
Size scaling of phytoplankton growth rates and size-dependent carbon to nitrogen (C:N) stoichiome... more Size scaling of phytoplankton growth rates and size-dependent carbon to nitrogen (C:N) stoichiometry determine phytoplankton size structure and coupling of carbon and nitrogen cycling of marine ecosystems. They are critical in predicting the growth of phytoplankton spanning a wide range of sizes and their consequences for the biological pump in marine ecosystem models. The size scaling of phytoplankton growth and size-dependent C:N stoichiometry are modelled by embedding size-dependent light-harvesting, nutrient acquisition and storage into Droop's quota-dependent phytoplankton growth model. The size-scaling exponent of maximum growth rate of phytoplankton is -0.17 (which is higher than the universal size-scaling exponent of -1 ⁄ 4 predicted by the metabolic theory of ecology) under saturated light and NO 3 . The size-scaling exponent of growth rate (µ) decreases with increasing light under saturated NO 3 , and decreases with decreasing NO 3 concentration under saturated light. The allometry of equilibrium cellular C and N quota varies with light and NO 3 concentrations. Under saturated light and NO 3 concentration, C:N increases slightly with cell size. Under limiting light, but saturated NO 3 , C:N is close to the Redfield ratio and is not size dependent. Under limiting NO 3 but saturated light, C:N is higher than the Redfield ratio and increases with cell size. We identified the uncertainty of the size-scaling exponent of µ associated with key parameters, for which more data need to be collected in the lab and field.
In the eastern North Water, most of the estimated annual new and net production of carbon (C) occ... more In the eastern North Water, most of the estimated annual new and net production of carbon (C) occurred during the main diatom bloom in 1998. During the bloom, at least 30% of total and new phytoplankton production occurred as dissolved organic carbon (DOC) and was unavailable for short-term assimilation into the herbivorous food web or sinking export. Based on particle interceptor traps and 234 Th deficits, 27% of the particulate primary production (PP) sank out of the upper 50 m, with only 7% and 1% of PP reaching the benthos at shallow (%200 m) and deep (%500 m) sites, respectively. Mass balance calculations and grazing estimates agree that %79% of PP was ingested by pelagic consumers between April and July. During this period, the vertical flux of biogenic silica (BioSi) at 50 m was equivalent to the total BioSi produced, indicating that all of the diatom production was removed from the euphotic zone as intact cells (direct sinking) or empty frustules (grazing or lysis). The estimated flux of empty frustules was consistent with rates of herbivory by the large, dominant copepods and appendicularians during incubations. Since the carbon demand of the dominant planktivorous bird, Alle alle, amounted to %2% of the biomass synthesized by its main prey, the large copepod Calanus hyperboreus, most of the secondary carbon production was available to pelagic carnivores. Stable isotopes indicated that the biomass of predatory amphipods, polar cod and marine mammals was derived from these herbivores, but corresponding carbon fluxes were not quantified. Our analysis shows that a large fraction of PP in the eastern North Water was ingested by consumers in the upper 50 m, leading to substantial carbon respiration and DOC accumulation in surface waters. An increasingly early and prolonged opening of the Artic Ocean is likely to promote the productivity of the herbivorous food web, but not the short-term efficiency of the particulate, biological CO 2 pump.
Communities of marine phytoplankton consist of cells of many different sizes. The size-structure ... more Communities of marine phytoplankton consist of cells of many different sizes. The size-structure of these communities often varies predictably with environmental conditions in aquatic systems. It has been hypothesized that physiological differences in nutrient and light requirements and acquisition efficiencies contribute to commonly observed correlations between phytoplankton community size structure and resource availability. Using physiological models we assess how light and nutrient availability can alter the relative growth rates of phytoplankton species of different cell sizes. Our models predict a change in the size dependence of growth rate depending on the severity of limitation by light and nutrient availability. Under conditions of growth-saturated resource supply, phytoplankton growth rate (mol C cell À1 time À1 ) scales with cell volume with a size-scaling exponent of 3 4 ; light limitation reduces the size-scaling exponent to approximately 2 3 , and nutrient limitation decreases the exponent to 1 3 as a consequence of the size-scaling of resource acquisition. Exponents intermediate between 1
The effects of colored dissolved organic matter (CDOM) from freshwater runoff and seasonal cycle ... more The effects of colored dissolved organic matter (CDOM) from freshwater runoff and seasonal cycle of temperature on the dynamic of phytoplankton and zooplankton biomass and production in the Gulf of St. Lawrence (GSL) are studied using a 3-D coupled physical-plankton ecosystem model. Three simulations are conducted: (1) the reference simulation based on Le , in which light attenuation by CDOM is not considered and maximum growth rate (m max ) of phytoplankton and zooplankton are not temperature-dependent (REF simulation); (2) light attenuation by CDOM is added to REF simulation (CDOM simulation); and (3) in addition to CDOM, the m max of phytoplankton and zooplankton are regulated by temperature (CDOM +TEMP simulation). CDOM simulation shows that CDOM substantially reduces phytoplankton biomass and production in the Lower St. Lawrence Estuary (LSLE), but slightly reduces overall primary production in the GSL. In the LSLE, the spring phytoplankton bloom is delayed from mid-March to mid-April, resulted from light attenuation by CDOM. The CDOM + TEMP simulation shows that the spring phytoplankton bloom in the LSLE is further delayed to July, which is more consistent with observations. Annual primary production is reduced by 33% in CDOM + TEMP simulation from REF and CDOM simulations. Zooplankton production is the same in all three simulations, and export of organic matter to depth is reduced in CDOM + TEMP simulation, suggesting that temperature controlled growth of phytoplankton and zooplankton enhances the coupling between primary production and zooplankton production under the seasonal temperature cycle of the GSL. (Z.-P. Mei), [email protected] (V. Le Fouest), [email protected] (B. Zakardjian), [email protected] (S. Sennville), [email protected] (H. Xie), [email protected] (M. Starr).
Biomass and production of plankton communities were investigated in two Chinese integrated fish c... more Biomass and production of plankton communities were investigated in two Chinese integrated fish culture ponds in August, Dianshanhu Pond (with high density of planktivorous carp) and Pingwang Pond (with low density of planktivorous carp). The plankton communities were composed of rotifers, protozoans, phytoplankton (<40 µm) and bacteria. The large phytoplankton (>40 µm), cladocerans and copepods were rare because of grazing pressure by the carp. The density or biomass of bacteria (1.93 × 107 and 2.20 × 107 cells ml−1 on average in Dianshanhu and Pingwang Ponds, respectively), picophytoplankton (24.6 and 18.5 mg m−3 Chla on average) and rotifers (5372 and 20733 ind. 1−1 on average) exceeded the maximum values reported for natural waters. The average [3H]thymidine uptake rates were 694 and 904 pmoles 1−1 h−1 (13.4 and 20.6 µgC 1−1) and the bacterial production by the >2 µm fraction amounted 21–28% of total [3H] thymidine uptake rate in both ponds. The mean chlorophylla concentrations were 59.1 and 183 mg m−3 in Dianshanhu and Pingwang Ponds, respectively. 82.4% and 65.3% of the total Chla was contributed by the <10 µm nano- and picophytoplankton in each pond, respectively. In particular, the picophytoplankton contribution amounted 41.2% of thtal Chla in Dianshanhu Pond. Primary production was 2.5 and 3.4 gC m−2 d−1 in each pond, respectively, and >50% of production was contributed by picophytoplankton. The mean biomasses of protozoa were 168 µg 1−1 and 445 µg 1−1 and those of rotifers were 763 µg 1−1 and 1186 µg 1−1 in Dianshanhu and Pingwang Ponds, respectively. The ecological efficiencies expressed in terms of the ratios of primary production to zooplankton production were 0.22 and 0.31, for the two ponds.
Size scaling of phytoplankton growth rates and size-dependent carbon to nitrogen (C:N) stoichiome... more Size scaling of phytoplankton growth rates and size-dependent carbon to nitrogen (C:N) stoichiometry determine phytoplankton size structure and coupling of carbon and nitrogen cycling of marine ecosystems. They are critical in predicting the growth of phytoplankton spanning a wide range of sizes and their consequences for the biological pump in marine ecosystem models. The size scaling of phytoplankton growth and size-dependent C:N stoichiometry are modelled by embedding size-dependent light-harvesting, nutrient acquisition and storage into Droop's quota-dependent phytoplankton growth model. The size-scaling exponent of maximum growth rate of phytoplankton is -0.17 (which is higher than the universal size-scaling exponent of -1 ⁄ 4 predicted by the metabolic theory of ecology) under saturated light and NO 3 . The size-scaling exponent of growth rate (µ) decreases with increasing light under saturated NO 3 , and decreases with decreasing NO 3 concentration under saturated light. The allometry of equilibrium cellular C and N quota varies with light and NO 3 concentrations. Under saturated light and NO 3 concentration, C:N increases slightly with cell size. Under limiting light, but saturated NO 3 , C:N is close to the Redfield ratio and is not size dependent. Under limiting NO 3 but saturated light, C:N is higher than the Redfield ratio and increases with cell size. We identified the uncertainty of the size-scaling exponent of µ associated with key parameters, for which more data need to be collected in the lab and field.
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