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1.
An enhancement of aeolian inputs to the ocean due to a future increase in aridity in certain parts of the world is predicted from global change. We conducted an experimental simulation to assess the biological response of NW Mediterranean coastal surface waters to an episodic dust addition. On the assumption that planktonic growth was limited by phosphorus, dust effects were compared to those induced by equivalent enrichments of phosphate. The experiment analyzed the dynamics of several parameters during one week: inorganic nutrients, total and fractioned chlorophyll a, bacterial abundance, phytoplankton species composition, abundance of autotrophic and heterotrophic flagellates, particulate organic carbon and particulate organic nitrogen. The maximum addition of dust (0.5 g dust L−1) initiated an increase in bacterial abundance. After 48 h, bacterial numbers decreased due to a peak in heterotrophic flagellates and a significant growth of autotrophic organisms, mainly nanoflagellates but also diatoms, was observed. Conversely, lower inputs of dust (0.05 g dust L−1) and phosphate enrichments (0.5 μmol PO43− L−1) only produced increases in phototrophic nanoflagellates. In our experiment, dust triggered bacterial growth, changed phytoplankton dynamics and affected the ratio of autotrophic to heterotrophic biomass, adding to the variability in the sources that affect system dynamics, energy and carbon budgets and ultimately higher trophic levels of the coastal marine food web.  相似文献   

2.
This study compares intracellular Cd content (Cd:C) of cultured marine phytoplankton grown under various Fe levels, with estimated particulate Cd:P ratios derived from regression slopes of Cd versus PO43− relationships from a global dataset. A 66-fold difference in Cd:C ratios was observed among the seven species grown under identical Fe concentrations, with oceanic diatoms having the highest Cd quotas and prymesiophytes the lowest. Interestingly, all species significantly increased their Cd:C ratios under Fe-limitation (on average 2-fold). The global data set also showed that the mean estimated Cd:P ratio of surface water particulates in HNLC (high nutrient low chlorophyll) regions were approximately 2-fold higher than non-HNLC regions. A sequence of events are proposed to explain high Cd:P ratios in HNLC waters. First, the seasonal relief from Fe-limitation in HNLC regions leads to blooms of large chain forming diatoms with high intrinsic Cd:P ratios. These large blooms may, in theory, deplete surface water CO2 and Zn concentrations, which ultimately, would result in increased Cd uptake. Eventually these blooms will run out of Fe, which has been shown to further increase intercellular Cd via growth biodilution and increased Cd uptake through non-specific Fe(II) transporters. Ultimately, Fe-limited diatoms with enhanced Cd quotas will sink out of surface waters leading to pronounced regional differences in Cd:P ratios between HNLC and non-HNLC waters in the global ocean.  相似文献   

3.
Nutrient-enrichment bottle experiments in the northwestern Indian Ocean surface waters were conducted to investigate phytoplankton growth following enrichments with either NH4+, NO3−, Fe or Fe + NO3−. Stimulation of phytoplankton growth could be achieved by the addition of either NH4+ or NO3− under the ambient Fe concentrations, but the most significant increases in Chl a, POC, and cell densities were observed in the Fe + NO3−-amended culture. Iron addition caused more rapid responses of phytoplankton growth in the Fe + NO3− treatment than those in the NO3− and NH4− treatment. However, the Fe-enrichment treatment revealed minimal growth of phytoplankton because of severe major nutrient deficiency and was similar to the control treatment. Increases in the cell density of diatoms and spherical phytoplankton cells (< 10 μm) were significant in the NH4+-enriched samples, whereas NO3− enrichment alone had little effect on the diatoms. Simultaneous addition of Fe and NO3− stimulated maximal growth of phytoplankton, in particular in diatoms, coccolithophorids and Phaeocystis type colonies. However, the dominance of coccolithophorids and Phaeocystis type colonies in the Fe + NO3− treatment may be interpreted as resulting from Si-limitation. The high N/P ratio for phytoplankton nutrient uptake in the N-amended culture indicates the possibility of some P-limited growth. From these results, we conclude that in the northwestern Indian Ocean, Fe and major nutrients are co-limiting phytoplankton production during the northeast monsoon. Iron appeared to affect the ability of phytoplankton to respond quickly to transient nutrient inputs.  相似文献   

4.
Temporal changes in the abundance, community composition, and photosynthetic physiology of phytoplankton in surface waters were investigated during the second in situ iron (Fe) fertilization experiment in the NW subarctic Pacific (SEEDS-II). Surface chlorophyll a concentration was 0.75 mg m−3 on the day before the first Fe enrichment (i.e. Day 0), increased ca. 3-fold until Day 13 after two Fe additions, and thereafter declined with time. The photochemical quantum efficiency (Fv/Fm) and functional absorption cross-section (σPSII) of photosystem II for total phytoplankton in surface waters increased and decreased inside the Fe-enriched patch through Day 13, respectively. These results indicate that the photosynthetic physiological condition of the phytoplankton improved after the Fe infusions. However, the maximum Fv/Fm value of 0.43 and the maximum quantum yield of carbon fixation (φmax) of 0.041 mol C (mol photon)−1 during the development phase of the bloom were rather low, compared to their theoretical maximum of ca. 0.65 and 0.10 mol C (mol photon)−1, respectively. Diatoms, which were mainly composed of oceanic species, did not bloom, and autotrophic nanoflagellates such as cryptophytes and prasinophytes became predominant in the phytoplankton community inside the Fe-enriched patch. In ferredoxin/flavodoxin assays for micro-sized (20–200 μm in cell length) diatoms, ferredoxin was not detected but flavodoxin expressions consistently occurred with similar levels both inside and outside the Fe-enriched patch, indicating that the large-sized diatoms were stressed by Fe bioavailability inside the Fe-enriched patch even after the Fe enrichments. Our data suggest that the absence of a Fe-induced large-sized diatom bloom could be partly due to their Fe stress throughout SEEDS-II.  相似文献   

5.
Winter-spring phytoplankton blooms in Dabob Bay, Washington   总被引：4，自引：2，他引：4  
Scientific investigations in Dabob Bay, Washington State, USA, have been extensive since the early 1960s, but phytoplankton blooms have been studied mostly with regard to chlorophyll concentrations and little is known about the phytoplankton species themselves. Here we provide information on the species present, their abundances during blooms, their contribution to organic carbon concentrations and the ability of some phytoplankton species to produce toxic aldehydes that may impact metazoan grazers.Multiple blooms of phytoplankton, dominated by diatoms, occurred in the late winter-early spring period, with depth-integrated chlorophyll levels ranging from <20 to 230 mg m−2 and peaks in February and April. The major bloom species included Skeletonema costatum, Thalassiosira spp. and Chaetoceros spp; Phaeocystis cf. pouchetii occurred in 2002 and 2004. Other taxa or groups of organisms that were sometimes abundant included unidentified small flagellates <10 μm in size and unidentified heterotrophic dinoflagellates. Large diatoms usually comprised most of the cell carbon, but a large, heterotrophic dinoflagellate, identified only as Gyrodinium “tear” because of its shape, was a major contributor to the microplankton carbon when present even in small numbers. Five Thalassiosira species and S. costatum were found to produce polyunsaturated aldehydes (PUA) that are known to affect copepod reproduction and hatching success. Our findings are similar to the few previous studies in the last four decades that included phytoplankton species and suggest long-term similarities and relative stability in the phytoplankton species present and their timing in Dabob Bay.  相似文献   

6.
To verify the hypothesis that the growth of phytoplankton in the Western Subarctic Gyre (WSG), which is located in the northwest subarctic Pacific, is suppressed by low iron (Fe) availability, an in situ Fe fertilization experiment was carried out in the summer of 2001. Changes over time in the abundance and community structure of phytoplankton were examined inside and outside an Fe patch using phytoplankton pigment markers analyzed by high-performance liquid chromatography (HPLC) and flow cytometry (FCM). In addition, the abundance of heterotrophic bacteria was also investigated by FCM. The chlorophyll a concentration was initially ca. 0.9 μg l−1 in the surface mixed layer where diatoms and chlorophyll b-containing green algae (prasinophytes and chlorophytes) were predominant in the chlorophyll biomass. After the iron enrichment, the chlorophyll a concentration increased up to 9.1 μg l−1 in the upper 10 m inside the Fe patch on Day 13. At the same time, the concentration of fucoxanthin (a diatom marker) increased 45-fold in the Fe patch, and diatoms accounted for a maximum 69% of the chlorophyll biomass. This result was consistent with a microscopic observation showing that the diatom Chaetoceros debilis had bloomed inside the Fe patch. However, chlorophyllide a concentrations also increased in the Fe patch with time, and reached a maximum of 2.2 μg l−1 at 5 m depth on Day 13, suggesting that a marked abundance of senescent algal cells existed at the end of the experiment. The concentration of peridinin (a dinoflagellate marker) also reached a maximum 24-fold, and dinoflagellates had contributed significantly (>15%) to the chlorophyll biomass inside the Fe patch by the end of the experiment. Concentrations of 19′-hexanoyloxyfucoxanthin (a prymnesiophyte marker), 19′-butanoyloxyfucoxanthin (a pelagophyte marker), and alloxanthin (a cryptophyte marker) were only incremented a few-fold increment inside the Fe patch. On the contrary, chlorophyll b concentration reduced to almost half of the initial level in the upper 10 m water column inside the Fe patch at the end of the experiment. A decrease with time in the abundance of eukaryotic ultraphytoplankton (<ca. 5 μm in size), in which chlorophyll b-containing green algae were possibly included was also observed by FCM. Overall, our results indicate that Fe supply can dramatically alter the abundance and community structure of phytoplankton in the WSG. On the other hand, cell density of heterotrophic bacteria inside the Fe patch was maximum at only ca. 1.5-fold higher than that outside the Fe patch. This indicates that heterotrophic bacteria abundance was little respondent to the Fe enrichment.  相似文献   

7.
Microbial plankton biomass, primary production (PP) and phytoplankton growth rates (μ) were estimated along the NW Iberian margin during an upwelling relaxation event. Although the interaction between wind forcing and coastline singularities caused high spatial variability in PP (0.4-8.4 g C m−2 d−1), two domains (coastal and oceanic) could be distinguished regarding microbial plankton biomass and μ. At the coastal domain, with higher influence of upwelling, diatoms showed an important contribution (27 ± 17%) to total autotrophic biomass (AB). Nonetheless, AB was dominated by autotrophic nanoflagellates (ANF) at both realms, accounting for 62 ± 16% and 89 ± 6% of the integrated AB at the coastal and oceanic domain respectively. AB and heterotrophic biomass (HB) were significantly higher at the oceanic than at the coastal domain, with both biomasses covarying according to HB:AB = 0.33. Whereas the low phytoplankton carbon to chlorophyll a ratio (Cph:chl a = 38 ± 3) and the high μ = 0.54 ± 0.09 d−1 registered at the coastal stations suggest that phytoplankton was not nutrient limited at this domain, the values (Cph:chl a = 157 ± 8; μ = 0.17 ± 0.02 d−1) recorded at the oceanic domain point to severe nutrient limitation. However, the high Fv/Fm fluorescence ratios (0.56 ± 0.09) measured at the sea surface in the oceanic domain suggest that nutrient limitation did not occur. To reconcile these two apparently opposite views, it is suggested the occurrence of mixotrophic nutrition of ANF, with heterotrophic nutrition supplying about 75% of carbon requirements.  相似文献   

8.
The feeding behaviour of adults of the marine calanoid copepod Centropages hamatus was studied in laboratory experiments with ciliates and phytoplankton as food sources. The ingestion rate of algal (flagellates, diatoms) and ciliate prey (oligotrichs) as a function of prey concentration could be described by a Holling type III functional response, with close to zero ingestion rates at concentrations below 5 µg C l− 1. In general, ingestion of ciliates was higher than ingestion of algae, and maximum feeding rates by adult males reached were half the feeding rates of adult females at prey concentrations exceeding 50 µg C l− 1. When diatoms and ciliates were offered together C. hamatus (both sexes) fed exclusively on ciliates as long as they contributed with more than 5% to the mixture. This indicates the capability of active prey selection and switching between suspension feeding and ambush predation. Therefore, the feeding behaviour of adult C. hamatus can be characterised as omnivorous with a preference for larger motile prey. This implies a trophic level above two, if there is a sufficient abundance of protozoan food available.  相似文献   

9.
An investigation of pigments, phytoplankton types and absorption characteristics was conducted in the Delagoa and Natal Bights during late winter and spring in the southwest Indian Ocean. The study demonstrated that small flagellates dominated the phytoplankton communities in both bights and were ubiquitous across a temperature range of 18–24 °C. Diatoms were dominant in patches of cool water (<22 °C) related to upwelling processes and were associated with elevated levels of phytoplankton biomass, while prokaryotes were observed to increase in warm waters >22 °C. Absorption coefficients varied closely with variations in chlorophyll a and specific coefficients were lower for diatoms compared to flagellates. Chlorophyll-specific coefficients also provided useful information on the level of pigment packaging and were related to the proportion of chlorophylls and carotenoids in the pigment pool.  相似文献   

10.
THE EFFECT OF MINE TAILINGS ON THE PRODUCTION OF PLANKTON   总被引：4，自引：0，他引：4  
Mine tailings at levels of ca 30 and 300 ppm were added to two plankton communities contained in 60 ton Controlled Experimental Ecosystems (CEE); a third CEE was used as a control. The mine tailings caused (1) a delay in the primary productivity and chlorophyll a maxima; (2) a shift in the size spectrum of primary producers towards autotrophic flagellates compared with diatoms in the control; (3) a suppression of heterotrophic microflagellates; and (4) an increased production of zooplankton compared with the control. Zooplankton were observed to have consumed tailings but no changes were observed in the structure of the zooplankton community as a result of tailings additions.  相似文献   

11.
This paper reports the initial results of a study of groundwater and coastal waters of southern Brazil adjacent to a 240 km barrier spit separating the Patos Lagoon, the largest coastal lagoon in South America, from the South Atlantic Ocean. The objective of this research is to assess the chemical alteration of freshwater and freshwater–seawater mixtures advecting through coastal permeable sands, and the influence of the submarine discharge of these fluids (SGD) on the chemistry of coastal waters. Here we focus on dissolved iron in this system and use radium isotopic tracers to quantify SGD and cross-shelf fluxes. Iron concentrations in groundwaters vary between 0.6 and 180 μM. The influence of the submarine discharge of these fluids into the surf zone produces dissolved Fe concentrations as high as several micromolar in coastal surface waters. The offshore gradient of dissolved Fe, coupled with results for Ra isotopes, is used to quantify the SGD flux of dissolved Fe from this coastline. We estimate the SGD flux to be 2 × 106 mol day− 1 and the cross-shelf flux to be 3.2 × 105 mol day− 1. This latter flux is equal to about 10% of the soluble atmospheric Fe flux to the entire South Atlantic Ocean. We speculate on the importance of this previously unrecognized iron input to regional ocean production and on the potential significance of this source to understanding variations in glacial–interglacial ocean production.  相似文献   

12.
A computer analysis was performed on experimental results obtained when mine tailings were added to seawater. The Mixed Upper-Layer Ecotrophie Simulation (MULES) model was tested by changing the extinction coefficient and the abundance of heterotrophic zooflagellates. Increasing the extinction coefficient resulted in a delay of phytoplankton growth, an increase in zooplankton standing stock and better growth of autotrophic flagellates compared with diatoms. Zooflagellates in the ecosystem influence the growth of zooplankton; secondary production by zooplankton was markedly depressed at low levels of zooflagellates. These results are believed to be of general significance for the diagnosis of suspended sediment effects on planktonic ecosystems.  相似文献   

13.
Whereas diatoms (class Bacillariophyceae) often dominate phytoplankton taxa in the Amazon estuary and shelf, their contribution to phytoplankton dynamics and impacts on regional biogeochemistry are poorly understood further offshore in the western tropical Atlantic Ocean (WTAO). Thus, relative contribution of diatoms to phytoplankton biomass and primary production rates and associated environmental conditions were quantified during three month-long cruises in January–February 2001, July–August 2001, and April–May 2003. The upper water column was sampled at 6 light depths (100%, 50%, 25%, 10%, 1% and 0.1% of surface irradiance) at 64 stations between 3° and 14°N latitude and 41° and 58°W longitude. Each station was categorized as ‘oceanic’ or ‘plumewater’, based on principal component analysis of eight physical, chemical and biological variables. All stations were within the North Brazil Current, and plumewater stations were characterized by shallower mixed layers with lower surface salinities and higher dissolved silicon (dSi) concentrations than oceanic stations. The major finding was a much greater role of diatoms in phytoplankton biomass and productivity at plumewater stations relative to oceanic stations. Mean depth-integrated bSi concentrations at the plumewater and oceanic stations were 14.2 and 3.7 mmol m−2, respectively. Mean depth-integrated SiP rates at the plumewater and oceanic stations were 0.17 and 0.02 mmol m−2 h−1, respectively. Based on ratios of SiP and PP rates, and typical Si:C ratios, diatoms contributed on average 29% of primary productivity at plumewater stations and only 3% of primary productivity at oceanic stations. In contrast, phytoplankton biomass (as chlorophyll a concentrations) and primary production (PP) rates (as 14C uptake rates) integrated over the euphotic zone were not significantly different at plumewater and oceanic stations. Chlorophyll a concentrations ranged from 8.5 to 42.4 mg m−2 and 4.0 to 38.0 mg m−2 and PP rates ranged from 2.2 to 11.2 mmol m−2 h−2 and 1.8 to 10.8 mmol m−2 h−2 at plumewater and oceanic stations, respectively. A conservative estimate of annual integrated SiP in offshore waters of Amazon plume between April and August is 0.59 Tmol Si, based on mean SiP rates in plumewaters and satellite-derived estimates of the area of the Amazon plume. In conclusion, river plumewaters dramatically alter the silicon dynamics of the WTAO, forming extensive diatom-dominated phytoplankton blooms that may contribute significantly to the global Si budget as well as contributing to energy and matter flow off of the continental shelf.  相似文献   

14.
Size and taxonomic structure of plankton community carbon biomass for the 0.2–2000 μm equivalent spherical diameter range were determined at the equator at 175°E in September 1990–1993 and April 1994. Total biomass of the plankton community ranged from 1944 to 3448 mg C m−2. Phytoplankton, zooplankton and bacteria carbon biomasses were 604–1669 mg C m-2, 300–797 mg C m2, and 968–1200 mg C m-2, and the percentages were 31–54%, 15–26%, and 29–54%, respectively. Biomass of heterotrophic bacteria was always the largest fraction andProchlorococcus biomass was second. Heterotrophic and autotrophic flagellates and dinoflagellates in the nanoplankton size range and copepods (adults and copepodites) in the mesoplankton range were also high. Relatively small biomass was observed in the microplankton size range. The differences in integrated biomass of plankton community for El Nin˜o type oligotrophic conditions of September 1990–1993 and non-El Nifio type mesotrophic conditions of April 1994 were generally small compared with the interannual difference during 1990–1993. However, the percentage ofProchlorococcus in phytoplankton carbon biomass was larger in non-El Nin˜o year. Biomasses of cyanobacteria, diatom, dinoflagellates, nauplii of copepods, and crustaceans other than copepods were larger in the non-El Nin˜o year. Primary production increased significantly from El Nin˜o to non-El Nin˜o years. Carbon flow through the plankton food chain was estimated using the plankton carbon biomass data, primary production measurements, and published empirical relationships.  相似文献   

15.
This paper discusses the seasonal evolution of the hydrographic and biogeochemical properties in the Antarctic Circumpolar Current (ACC) during the US Joint Global Ocean Flux (JGOFS) Antarctic Environment and Southern Ocean Process Study (AESOPS) in 1997–1998. The location of the study region south of New Zealand along 170°W was selected based on the zonal orientation and meridional separation of the physical and chemical fronts found in that region. Here we endeavor to describe the seasonal changes of the macronutrients, fluorescence chlorophyll, particulate organic carbon (POC), and carbon dioxide (CO2) in the upper 400 m of the ACC during the evolution of the seasonal phytoplankton bloom found in this area. While the ACC has extreme variability in the meridional sense (due to fronts, etc.), it appears to be actually quite uniform in the zonal sense. This is reflected by the fact that a good deal of the seasonal zonal changes in nutrients distributions at 170°W follow a pattern that reflects what would be expected if the changes are associated with seasonal biological productivity. Also at 170°W, the productivity of the upper waters does not appear to be limited by availability of phosphate or nitrate. While there is a significant decrease (or uptake) of inorganic nitrogen, phosphate and silicate associated with the seasonal phytoplankton bloom, none of the nutrients, except perhaps silicate (north of the silicate front) are actually depleted within the euphotic zone. At the end of the growing season, nutrient concentrations rapidly approached their pre-bloom levels. Inspection of the ratios of apparent nutrient drawdown near 64°S suggests N/P apparent drawdowns to have a ratio of 10 and N/Si apparent drawdowns to have a ratio of >4. These ratios suggest a bloom that was dominated by Fe limited diatoms. In addition, the surface water in the Polar Front (PF) and the Antarctic Zone (AZ) just to the south of the PF take up atmospheric CO2 at a rate 2–3 times as fast as the mean global ocean rate during the summer season but nearly zero during the rest of year. This represents an important process for the transport of atmospheric CO2 into the deep ocean interior. Finally, the net CO2 utilization or the net community production during the 2.5 growing months between the initiation of phytoplankton blooms and mid-January increase southward from 1.5 mol C m−2 at 55°S to 2.2 mol C m−2 to 65°S across the Polar Frontal Zone (PFZ) into the AZ.  相似文献   

16.
As part of the US Joint Global Ocean Flux Study (US JGOFS) Southern Ocean Program, flow cytometry and epifluorescent microscopy were utilized to determine abundance, distribution and size structure of the microbial community in the Polar Front region during the summer biomass maximum. Surface samples were collected approximately every 10 km along 170°W during two N–S transects, separated in time by two weeks. Phytoplankton abundance and size structure varied with distinct latitudinal trends. Autotrophic biomass was lowest north of the Polar Front reflecting the dominance of small cells. The highest biomass (170 μg C l−1) occurred at 65°S where the composition was strongly influenced by large centric diatoms. Farther south, the diatom community shifted to the dominance of smaller pennate diatoms. Total grazer biomass and size distributions followed similar patterns, ranging from 4 μg C l−1 in the north to 52 μg C l−1 in the south where larger (>20 μm) grazers were more abundant. Heterotrophic bacteria varied over an order of magnitude in abundance across the study site, with size generally increasing from north to south. In the second transect, phytoplankton biomass at 65°S was 50% lower, and grazer biomass and bacterial populations were slightly greater, indicating the decline of the bloom. The changes in biomass and community structure along 170°W and the reduction of phytoplankton standing stock at 65°S over time suggests adjacent, yet different, microbial systems in terms of carbon flux, spanning from primarily recycling to export-dominated.  相似文献   

17.
We report the first application of a biogeochemical model in which the major elemental composition of the phytoplankton is flexible, and responds to changing light and nutrient conditions. The model includes two phytoplankton groups: diatoms and non-siliceous picoplankton. Both fix C in accordance with photosynthesis-irradiance relationships used in other models and take up NO3− and NH4+ (and Si(OH)4 for diatoms) following Michaelis-Menten kinetics. The model allows for light dependence of photosynthesis and NO3− uptake, and for the observed near-total light independence of NH4+ uptake and Si(OH)4 uptake. It tracks the resulting C/N ratios of both phytoplankton groups and Si/N ratio of diatoms, and permits uptake of C, N and Si to proceed independently of one another when those ratios are close to those of nutrient-replete phytoplankton. When the C/N or Si/N ratio of either phytoplankton group indicates that its growth is limited by N, Si or light, uptake of non-limiting elements is controlled by the content of the limiting element in accordance with the cell-quota formulation of Droop (J. Mar. Biol. Ass. U.K 54 (1974) 825).We applied this model to the Bermuda Atlantic Time-series Study (BATS) site in the western Sargasso Sea. The model was tuned to produce vertical profiles and time courses of [NO3−], [NH4+] and [Si(OH)4] that are consistent with the data, by adjusting the kinetic parameters for N and Si uptake and the rate of nitrification. The model then reproduces the observed time courses of chlorophyll-a, particulate organic carbon and nitrogen, biogenic silica, primary productivity, biogenic silica production and POC export with no further tuning. Simulated C/N and Si/N ratios of the phytoplankton indicate that N is the main growth-limiting nutrient throughout the thermally stratified period and that [Si(OH)4], although always limiting to the rate of Si uptake by diatoms, seldom limits their growth rate. The model requires significant nitrification in the upper 200 m to yield realistic time courses and vertical profiles of [NH4+] and [NO3−], suggesting that NO3− is not supplied to the upper water column entirely by physical processes. A nitrification-corrected f-ratio (fNC), calculated for the upper 200 m as: (NO3− uptake—nitrification)/(NO3− uptake+NH4+ uptake) has annual values ranging from only 0.05–0.09, implying that 90–95% of the N taken up annually by phytoplankton is supplied by biological regeneration (including nitrification) in the upper 200 m. Reported discrepancies between estimates of organic C export based on seasonal chemical changes and POC export measured at the BATS site can be almost completely resolved if there is significant regeneration of NO3− via organic-matter decomposition in the upper 200 m.  相似文献   

18.
Recent discrepancies between geochemical and biological approaches for determining whether ocean ecosystems are net heterotrophic or net autotrophic have led to uncertainty in the net metabolic state of open ocean ecosystems. Geochemical approaches indicate that the oceans are net positive autotrophic, but direct observations based on short-term incubation techniques suggest that the ocean is in a state of net heterotrophy. One hypothesis for the apparent discrepancy is that net autotrophic production occurs in aperiodic “bursts,” which are superimposed on a more constant background state of net heterotrophy. Mixing events, which introduce new nutrients to the surface ocean, provide one mechanism for fueling such aperiodic bursts of net production. In conjunction with the Eddy Flux (E-Flux) program in the lee of the Hawaiian Islands during winter 2004–2005, we examined the relationship between photosynthesis and irradiance (P vs. E) in surface waters inside and outside of two cold-core, cyclonic eddies, and conducted five incubation experiments to examine the metabolic response of mixed-layer plankton communities to nutrient-rich deep-sea water additions. Our results showed that in the mixed layer, maximum rates of light-saturated photosynthesis, derived from photosynthesis–irradiance experiments were not significantly different inside vs. outside the eddies (p=0.35 and 0.44 for E-Flux I and E-Flux III, respectively). Addition of nutrients to mixed-layer water showed that (1) gross primary production (GPP) became decoupled from a more constant rate of respiration and (2) net system metabolism shifted from approximate balance, or slight net heterotrophy, to a demonstrably net autotrophic system. From these results, we determined that the threshold GPP for net autotrophic production for the mixed layer of the study region was 1.65 mmol O2 m−3 d−1, which is consistent with previous estimates for the oligotrophic open ocean.  相似文献   

19.
The first oceanographic research (hydrography, nutrient salts, chlorophyll, primary production and phytoplankton assemblages) in a Middle Galician Ria was carried out in Corme-Laxe during 2001, just a year before the Prestige oil spill, being the only reference to evaluate eventual changes in the phytoplankton community. Due to the small size of this ria (6.5 km2), oceanographic processes were driven by the continental water supplied by Anllons River during the wet season (20–30 m3 s−1 in winter), and the strong oceanic influence from the nearby shelf during the dry season. The annual cycle showed a spring bloom with high levels of chlorophyll (up to 14 μg Chl-a L−1) and primary production (3 g C m−2 d−1) and a summer upwelling bloom (up to 8 μg Chl-a L−1 and 10 g C m−2 d−1) where the proximity of the Galician upwelling core (<13.5 °C at sea surface) favors the input of upwelled seawater (up to 9 μM of nitrate and silicate) to the bottom ria layer, even during summer stratification events (primary production around 2 g C m−2 d−1). Thus, phytoplankton assemblages form a “continuum” from spring to autumn with a predominance of diatoms and overlapping species between consecutive periods; only in autumn dinoflagellates and flagellates characterized the phytoplankton community. In the Middle Rias as Corme-Laxe, the nutrient values, Chl-a, primary production and phytoplankton abundance for productive periods were higher than those reported for the Northern (Ria of A Coruña) and Southern Rias (Ria of Arousa) for year 2001; this suggests the importance of the hydrographic events occurring in the zone of maximum upwelling intensity of the Western Iberian Shelf, where a lack of annual cycles studies exists.  相似文献   

20.
The photosynthetic properties of phytoplankton populations as related to physical–chemical variations on small temporal and spatial scales and to phytoplankton size structure and pigment spectra were investigated in the Northern Adriatic Sea off the Po River delta in late winter 1997. Large diatoms (fucoxanthin) dominated the phytoplankton in the coastal area whereas small phytoflagellates (mainly 19′-hexanoyloxyfucoxanthin, chlorophyll b, 19′-butanoyloxyfucoxanthin) occurred outside the front. The front was defined by the steep gradient in density in the surface layer separating low-salinity coastal waters from the offshore waters.Physical features of the area strongly influenced phytoplankton biomass distributions, composition and size structure. After high volumes of Po River discharge several gyres and meanders occurred in the area off the river delta in February. Decreasing river discharge and the subsequent disappearance of the gyres and the spreading dilution of the river plume was observed in March. The dynamic circulation of February resulted in high photosynthetic capacity of the abundant phytoplankton population (>3.40 mg m−3). In March, the slow circulation and an upper low-salinity water layer, segregated from the deeper layers, resulted in lack of renewal of this water mass. The huge phytoplankton biomass, up to 15.77 mg chl a m−3, became nutrient depleted and showed low photosynthetic capacity. In February, an exceptionally high PmaxB, 20.11 mg C (mg chl a)−1 h−1 was recorded in the Po River plume area and average PmaxB was three-fold in February as compared to the March recordings, 10.50 mg C (mg chl a)−1 h−1 and 3.22 mg C (mg chl a)−1 h−1, respectively.The extreme variability and values of phytoplankton biomass in the innermost plume area was not always reflected in primary production. Modeling of circulation patterns and water mass resilience in the area will help to predict phytoplankton response and biomass distributions. In the frontal area, despite a considerable variability in environmental conditions, our findings have shown that the phytoplankton assemblages will compensate for nutrient depression and hydrographic constraints, by means of size and taxonomic composition and, as a result, the variability in the photosynthetic capacity was much less pronounced than that observed for other parameters.  相似文献   

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