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1.
The Sea of Okhotsk is one of the most productive marine basins in the world ocean and plays an important role in transport of organic carbon and iron to the western subarctic Pacific. We report the first measurements of phytoplankton growth and microzooplankton grazing rates in the Sea of Okhotsk, in late summer of 2006. The study area can be divided into two areas: nutrient-sufficient waters on the continental shelf along the east coast of Sakhalin Island and in the vicinity of Bussol Strait, and surface nutrient-depleted waters beyond the shelf break and in the vicinity of Sakhalin Bay. Phytoplankton growth rate in the studied area was strongly affected by nutrient availability, with high phytoplankton growth rate (0.55±0.14 d?1) in the nutrient-replete region and severely depressed growth (0.03±0.05 d?1) in the nutrient-depleted region. On the other hand, microzooplankton grazing rates in both the nutrient-replete and nutrient-depleted regions were approximately the same (0.26±0.20 d?1 vs. 0.27±0.24 d?1). Consequently, microzooplankton grazing consumed <50% of the phytoplankton growth in nutrient-rich waters but >3 times the phytoplankton growth in nutrient-depleted waters. Phytoplankton physiological condition as measured by the maximum photochemical quantum efficiency (Fv/Fm) of algal photosystem II (PS II) showed a general trend in agreement with the in situ growth rate of phytoplankton. In contrast to the phytoplankton community, picophytoplankton, especially the cyanobacteria Synechococcus, showed no nutrient effect on their growth, and the growth and mortality rates were well balanced, suggesting that they have a low nutrient requirement and their biomass was controlled principally by microzooplankton grazing.  相似文献   

2.
Phytoplankton growth and microzooplankton grazing were studied during the 2007 spring bloom in Central Yellow Sea. The surveyed stations were divided to pre-bloom phase (Chl a concentration less than 2 μg L−1), and bloom phase (Chl a concentration greater than 2 μg L−1). Shipboard dilution incubation experiments were carried out at 19 stations to determine the phytoplankton specific growth rates and the specific grazing rates of microzooplankton on phytoplankton. Diatoms dominated in the phytoplankton community in surface waters at most stations. For microzooplankton, Myrionecta rubra and tintinnids were dominant, and heterotrophic dinoflagellate was also important in the community. Phytoplankton-specific growth rates, with an average of 0.60±0.19 d−1, were higher at pre-bloom stations (average 0.62±0.17 d−1), and lower at the bloom stations (average 0.59±0.21 d−1), but the difference of growth rates between bloom and pre-bloom stations was not statistically significant (t test, p=0.77). The phytoplankton mortality rate by microzooplankton grazing averaged 0.41±0.23 d−1 at pre-bloom stations, and 0.58±0.31 d−1 during the blooms. In contrast to the growth rates, the statistic difference of grazing rates between bloom and pre-bloom stations was significant (after removal of outliers, t test, p=0.04), indicating the importance of the top-down control in the phytoplankton bloom processes. Average potential grazing efficiency on primary productivity was 66% at pre-bloom stations and 98% at bloom stations, respectively. Based on our results, the biomass maximum phase (bloom phase) was not the maximum growth rate phase. Both phytoplankton specific growth rate and net growth rate were higher in the pre-bloom phase than during the bloom phase. Microzooplankton grazing mortality rate was positively correlated with phytoplankton growth rate during both phases, but growth and grazing were highly coupled during the booming phase. There was no correlation between phytoplankton growth rate and cell size during the blooms, but they were positive correlated during the pre-bloom phase. Our results indicate that microzooplankton grazing is an important process controlling the growth of phytoplankton in spring bloom period in the Central Yellow Sea, particularly in the “blooming” phase.  相似文献   

3.
Microzooplankton grazing impact on phytoplankton was assessed using the Landry–Hassett dilution technique in the Western Arctic Ocean during spring and summer 2002 and 2004. Forty experiments were completed in a region encompassing productive shelf regions of the Chukchi Sea, mesotrophic slope regions of the Beaufort Sea off the North Slope of Alaska, and oligotrophic deep-water sites in the Canada Basin. A variety of conditions were encountered, from heavy sea-ice cover during both spring cruises, moderate sea-ice cover during summer of 2002, and light to no sea ice during summer of 2004, with a concomitant range of trophic conditions, from low chlorophyll-a (Chl-a; <0.5 μg L−1) during heavy ice cover in spring and in the open basin, to late spring and summer shelf and slope open-water diatom blooms with Chl-a >5 μg L−1. The microzooplankton community was dominated by large naked ciliates and heterotrophic gymnodinoid dinoflagellates. Significant, but low, rates of microzooplankton herbivory were found in half of the experiments. The maximum grazing rate was 0.16 d−1 and average grazing rate, including experiments with no significant grazing, was 0.04±0.06 d−1. Phytoplankton intrinsic growth rates varied from the highest values of about 0.4 d−1 to the lowest values of zero to slightly negative growth, on average 0.16±0.15 d−1. Light limitation in spring and post-bloom senescence during summer were likely explanations of observed low phytoplankton growth rates. Microzooplankton grazing consumed 0–120% (average 22±26%) of phytoplankton daily growth. Grazing and growth rates found in this study were low compared to rates reported in another Arctic system, the Barents Sea, and in major geographic regions of the world ocean.  相似文献   

4.
Phytoplankton growth and microzooplankton grazing rates were measured by the dilution technique in the subarctic North Pacific Ocean along a west–east transect during summer 1999. Average phytoplankton growth rates without added nutrients (μ0) were 0.33, 0.41, 0.20 and 0.49 d−1 for the four regions sampled: the Western Gyre, the Bering Sea, the Gulf of Alaska gyre and stations along the Aleutian Trench. Average grazing mortality rates (m) were 0.34, 0.27, 0.20 and 0.49 d−1. Limitation of phytoplankton growth by macronutrients, such as NO3 and SiO2, was identified only at a few stations, with overall μ0/μn (μn is nutrient-enhanced growth rate) averaging 0.9. Phytoplankton growth and microzooplankton grazing were approximately balanced, as indicated by high m/μ0 ratio, except in the Bering Sea, where the m/μ0 ratio was 0.65, indicating the relative importance of the diatom-macrozooplankton grazing food chain and possible higher export flux to the deep layer. Flow cytometric analysis revealed that the growth rates of picoplankton (Synechococcus and picoeukaryotes) were usually much lower than the total phytoplankton community growth rates estimated from chlorophyll a, except for stations in the Gulf of Alaska Gyre, where the growth rates for different populations were about the same. Lower than community-average growth rate for picoplankton indicates larger phytoplankters, presumably diatoms, were growing at a much faster rate. Suppressed phytoplankton growth in the Gulf of Alaska was probably a result of iron limitation.  相似文献   

5.
The latitudinal distributions of phytoplankton biomass, composition and production in the Atlantic Ocean were determined along a 10,000-km transect from 50°N to 50°S in October 1995, May 1996 and October 1996. Highest levels of euphotic layer-integrated chlorophyll a (Chl a) concentration (75–125 mg Chl m−2) were found in North Atlantic temperate waters and in the upwelling region off NW Africa, whereas typical Chl a concentrations in oligotrophic waters ranged from 20 to 40 mg Chl m−2. The estimated concentration of surface phytoplankton carbon (C) biomass was 5–15 mg C m−2 in the oligotrophic regions and increased over 40 mg C m−2 in richer areas. The deep chlorophyll maximum did not seem to constitute a biomass or productivity maximum, but resulted mainly from an increase in the Chl a to C ratio and represented a relatively small contribution to total integrated productivity. Primary production rates varied from 50 mg C m−2 d−1 at the central gyres to 500–1000 mg C m−2 d−1 in upwelling and higher latitude regions, where faster growth rates (μ) of phytoplankton (>0.5 d−1) were also measured. In oligotrophic waters, microalgal growth was consistently slow [surface μ averaged 0.21±0.02 d−1 (mean±SE)], representing <20% of maximum expected growth. These results argue against the view that the subtropical gyres are characterized by high phytoplankton turnover rates. The latitudinal variations in μ were inversely correlated to the changes in the depth of the nitracline and positively correlated to those of the integrated nitrate concentration, supporting the case for the role of nutrients in controlling the large-scale distribution of phytoplankton growth rates. We observed a large degree of temporal variability in the phytoplankton dynamics in the oligotrophic regions: productivity and growth rates varied in excess of 8-fold, whereas microalgal biomass remained relatively constant. The observed spatial and temporal variability in the biomass specific rate of photosynthesis is at least three times larger than currently assumed in most satellite-based models of global productivity.  相似文献   

6.
Phytoplankton community structure is expected to shift to larger cells (e.g., diatoms) with monsoonal forcing in the Arabian Sea, but recent studies suggest that small primary producers remain active and important, even in areas strongly influenced by coastal upwelling. To better understand the role of smaller phytoplankton in such systems, we investigated growth and grazing rates of picophytoplankton populations and their contributions to phytoplankton community biomass and primary productivity during the 1995 Southwest Monsoon (August–September). Environmental conditions at six study stations varied broadly from open-ocean oligotrophic to coastal eutrophic, with mixed-layer nitrate and chlorophyll concentrations ranging from 0.01 to 11.5 μM NO3 and 0.16 to 1.5 μg Chl a. Picophytoplankton comprised up to 92% of phytoplankton carbon at the oceanic stations, 35% in the diatom-dominated coastal zone, and 26% in a declining Phaeocystis bloom. Concurrent in situ dilution and 14C-uptake experiments gave comparable ranges of community growth rates (0.53–1.05 d−1 and 0.44–1.17 d−1, to the 1% light level), but uncertainties in C:Chl a confounded agreement at individual stations. Microzooplankton grazing utilized 81% of community phytoplankton growth at the oligotrophic stations and 54% at high-nutrient coastal stations. Prochlorococcus (PRO) was present at two oligotrophic stations, where its maximum growth approached 1.4 d−1 (two doublings per day) and depth-integrated growth varied from 0.2 to 0.8 d−1. Synechococcus (SYN) growth ranged from 0.5 to 1.1 d−1 at offshore stations and 0.6 to 0.7 d−1 at coastal sites. Except for the most oligotrophic stations, growth rates of picoeukaryotic algae (PEUK) exceeded PRO and SYN, reaching 1.3 d−1 offshore and decreasing to 0.8 d−1 at the most coastal station. Microzooplankton grazing impact averaged 90, 70, and 86% of growth for PRO, SYN, and PEUK, respectively. Picoplankton as a group accounted for 64% of estimated gross carbon production for all stations, and 50% at high-nutrient, upwelling stations. Prokaryotes (PRO and SYN) contributed disproportionately to production relative to biomass at the most oligotrophic station, while PEUK were more important at the coastal stations. Even during intense monsoonal forcing in the Arabian Sea, picoeukaryotic algae appear to account for a large portion of primary production in the coastal upwelling regions, supporting an active community of protistan grazers and a high rate of carbon cycling in these areas.  相似文献   

7.
There has been more attention to phytoplankton dynamics in nutrient-rich waters than in oligotrophic ones thus requiring the need to study the dynamics and responses in oligotrophic waters. Accordingly, phytoplankton community in Blanes Bay was overall dominated by Prymnesiophyceae, remarkably constant throughout the year (31 ± 13% Total chlorophyll a, Tchl a) and Bacillariophyta with a more episodic appearance (20 ± 23% Tchl a). Prasinophyceae and Synechococcus contribution became substantial in winter (Prasinophyceae = 30% Tchl a) and summer (Synechococcus = 35% Tchl a). Phytoplankton growth and grazing mortality rates for major groups were estimated by dilution experiments in combination with high pressure liquid chromatography and flow cytometry carried out monthly over two years. Growth rates of total phytoplankton (range = 0.30–1.91 d−1) were significantly higher in spring and summer (μ > 1.3 d−1) than in autumn and winter (μ ∼ 0.65 d−1) and showed a weak dependence on temperature but a significant positive correlation with day length. Microzooplankton grazing (range = 0.03–1.4 d−1) was closely coupled to phytoplankton growth. Grazing represented the main process for loss of phytoplankton, removing 60 ± 34% (±SD) of daily primary production and 70 ± 48% of Tchl a stock. Chla synthesis was highest during the Bacillarophyceae-dominated spring bloom (Chl asynt = 2.3 ± 1.6 μg Chl a L−1 d−1) and lowest during the following post-bloom conditions dominated by Prymnesiophyceae (Chl asynt = 0.23 ± 0.08 μg Chl a L−1 d−1). This variability was smoothed when expressed in carbon equivalents mainly due to the opposite dynamics of C:chl a (range = 11–135) and chl a concentration (range = 0.07–2.0 μg chl a L−1). Bacillariophyta and Synechococcus contribution to C fluxes was higher than to biomass because of their fast-growth rate. The opposite was true for Prymnesiophyceae.  相似文献   

8.
Dilution experiments were conducted to investigate microzooplankton grazing impact on phytoplankton of different taxonomic groups and size fractions (< 5, 5–20, 20–200 μm) during spring and summer bloom periods at two different sites (inner Tolo Harbour and Tolo Channel) in the Tolo Harbour area, the northeastern coastal area of Hong Kong. Experiments combined with HPLC pigment analysis in three phytoplankton size fractions measured pigment and size specific phytoplankton growth rates and microzooplankton grazing rates. Pigment-specific phytoplankton growth rates ranged between 0.08 and 3.53 d− 1, while specific grazing rates of microzooplankton ranged between 0.07 and 2.82 d− 1. Highest specific rates of phytoplankton growth and microzooplankton grazing were both measured in fucoxanthin in 5–20 μm size fraction in inner Tolo Harbour in summer, which coincided with the occurrence of diatom bloom. Results showed significant correlations between phytoplankton growth and microzooplankton grazing rates. Microzooplankton placed high grazing pressure on phytoplankton community. High microzooplankton grazing impact on alloxanthin (2.63–5.13) suggested strong selection toward cryptophytes. Our results provided no evidence for size selective grazing on phytoplankton by microzooplankton.  相似文献   

9.
Phytoplankton growth rates and mortality rates were experimentally examined at eight stations in the Arabian Sea along the U.S. JGOFS cruise track during the 1995 Northeast Monsoon (January) and Spring Intermonsoon (March–April). Instantaneous growth rates averaged over an entire cruise were approximately twice as high during the NE Monsoon than during the Spring Intermonsoon period (overall averages of 0.84±0.29 (s.d.) versus 0.44±0.19 d−1). Average herbivore grazing (mortality) rates, however, were quite similar for the two seasons (overall averages of 0.35±0.18 and 0.30±0.17 d−1 for the NE Monsoon and Spring Intermonsoon, respectively). The absolute amounts of phytoplankton biomass consumed during each season also were similar (29 and 25% of standing stock consumed d−1 for the January and March–April cruises, respectively), as were the geographical trends of this removal. These seasonal trends in growth and removal rates resulted in net phytoplankton growth rates that were considerably higher during the January cruise (0.48 d−1) than during the March–April cruise (0.14 d−1). That is, phytoplankton production was more closely balanced during the Spring Intermonsoon season (87% of daily primary production consumed) relative to the NE Monsoon season (49% of daily primary production consumed). Station-to-station variability was high for rate measurements during either cruise. Nevertheless, there was a clear onshore–offshore trend in the absolute rate of removal of phytoplankton biomass (μg chlorophyll consumed l−1 d−1) during both cruises. Coastal stations had removal rates that were typically 2–4 times higher than removal rates at oceanic stations.  相似文献   

10.
Size-fractionated chlorophyll-a and carbon incorporation rates were determined on a series of 13 cruises carried out from 1992 to 2001with the aim of investigating the patterns and causes of variability in phytoplankton chlorophyll and production in the Eastern North Atlantic Subtropical Gyral Province (NASE). Averaged (±SE) integrated chlorophyll-a concentration and primary production rate were 17±1 mg m−2 and 253±22 mg C m−2 d−1. Small-sized cells (<2 μm) formed the bulk of phytoplankton biomass (71%) and accounted for 54% of total primary production. A clear latitudinal gradient in these variables was not detected. By contrast, large seasonal variability was detected in terms of primary production, although integrated phytoplankton biomass, as estimated from chlorophyll-a concentration, remained rather constant and did not display significant changes with time. Variability in primary production (PP) was related mainly to variability in surface temperature and surface chlorophyll-a concentration. The control exerted by surface temperature was related to nutrient availability. By contrary, euphotic-zone depth, depth of maximum concentration of chlorophyll-a and integrated chlorophyll-a did not contribute significantly to the high variability in primary production observed in this oligotrophic region.  相似文献   

11.
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12.
Ninety-four stations were sampled in the Atlantic subtropical gyres during 10 cruises carried out between 1995 and 2001, mainly in boreal spring and autumn. Chlorophyll a (Chl-a) and primary production were measured during all cruises, and phytoplankton biomass was estimated in part of them. Picoplankton (<2 μm) represented >60% of total Chl-a concentration measured at the surface, and their contribution to this variable increased with depth. Phytoplankton carbon concentrations were higher in the upper metres of the water column, whereas Chl-a showed a deep maximum (DCM). At each station, the water column was divided into the upper mixed layer (ML) and the DCM layer (DCML). The boundary between the two layers was calculated as the depth where Chl-a concentration was 50% of the maximum Chl-a concentration. On average DCML extends from 67 to 126 m depth. Carbon to Chl-a (C:Chl-a) ratios were used to estimate phytoplankton carbon content from Chl-a in order to obtain a large phytoplankton carbon dataset. Total C:Chl-a ratios averaged (±s.e.) 103±7 (n=22) in the ML and 24±4 (n=12) in the DCML and were higher in larger cells than in picoplankton. Using these ratios and primary production measurements, we derived mean specific growth rates of 0.17±0.01 d−1 (n=173) in the ML and 0.20±0.01 d−1 (n=165) in the DCML although the differences were not significant (t-test, p>0.05). Our results suggest a moderate contribution of the DCML (43%) to both phytoplankton biomass and primary production in the Atlantic subtropical gyres.  相似文献   

13.
Mesozooplankton and 63–200 μm net-collected microzooplankton grazing on phytoplankton and protozoans was evaluated by 24-h incubations on a latitudinal transect in the Atlantic Ocean, from 35°N to 38°S (AMT-15; September–October 2004). The sampling area comprised contrasting ecosystems, including upwelling zones and oligotrophic subtropical gyres. Grazing impacts of mesozooplankton and 63–200 μm microzooplankton on total chlorophyll a (Chl a), >5 μm Chl a, ciliates, and dinoflagellates were low for both zooplankton size fractions, always removing<1.5% of the standing stocks of these groups. Grazing had a slightly greater impact upon primary production (up to 10% of primary production consumed daily), although on most occasions grazing removed<1% of primary production per day. To account for the reduction of micrograzers by predators in the experimental bottles and the consequent reduction of grazing pressure, the data were corrected with knowledge on the decrease of microzooplankton during incubations and global estimates of microzooplankton grazing. The corrected grazing rates for mesozooplankton ranged from 4% to 28% of the primary production consumed daily, and from 1% to 2% of the standing stock of Chl a removed every day. The 63–200 μm microzooplankton corrected grazing impact was always<5% of the primary production and standing stock consumed per day. The corrected grazing activity of 63–200 μm microzooplankton and mesozooplankton rendered daily rations ranging from 3% to 38% of the body carbon consumed daily, not sufficient for basal metabolism in most of the areas studied. Finally, the data on mesozooplankton grazing on primary production confirm the recent hypothesis of a decline of the relative importance of mesozooplankton grazing on primary producers with increasing primary production [Calbet, A., 2001. Mesozooplankton grazing effect on primary production: a global comparative analysis in marine ecosystems. Limnology and Oceanography 46, 1824–1830].  相似文献   

14.
Phytoplankton and bacterial abundance, size-fractionated phytoplankton chlorophyll-a (Chl-a) and production together with bacterial production, microbial oxygen production and respiration rates were measured along a transect that crossed the Equatorial Atlantic Ocean (10°N–10°S) in September 2000, as part of the Atlantic Meridional Transect 11 (AMT 11) cruise. From 2°N to 5°S, the equatorial divergence resulted in a shallowing of the pycnocline and the presence of relatively high nitrate (>1 μM) concentrations in surface waters. In contrast, a typical tropical structure (TTS) was found near the ends of the transect. Photic zone integrated 14C primary production ranged from ∼200 mg C m−2 d−1 in the TTS region to ∼1300 mg C m−2 d−1 in the equatorial divergence area. In spite of the relatively high primary production rates measured in the equatorial upwelling region, only a moderate rise in phytoplankton biomass was observed as compared to nearby nutrient-depleted areas (22 vs. 18 mg Chl-a m−2, respectively). Picophytoplankton were the main contributors (>60%) to both Chl-a biomass and primary production throughout the region. The equatorial upwelling did not alter the phytoplankton size structure typically found in the tropical open ocean, which suggests a strong top-down control of primary producers by zooplankton. However, the impact of nutrient supply on net microbial community metabolism, integrated over the euphotic layer, was evidenced by an average net microbial community production within the equatorial divergence (1130 mg C m−2 d−1) three-fold larger than net production measured in the TTS region (370 mg C m−2 d−1). The entire region under study showed net autotrophic community metabolism, since respiration accounted on average for 51% of gross primary production integrated over the euphotic layer.  相似文献   

15.
IronEx I demonstrated a rapid and marked response by grazers to Fe-induced increases in phytoplankton stocks, which was thought to be due, in part, to arrested vertical migration by mesozooplankton. These observations prompted an investigation of the relative roles of Fe enrichment and grazing pressure in controlling the magnitude of phytoplankton stocks in the NE subarctic Pacific. The grazing impact of increased mesozooplankton abundance in response to a localised Fe-induced enhancement of algal biomass was simulated by performing in vitro (6 d) grazer perturbation experiments in May 1994 and September 1995 at Ocean Station Papa (OSP), when pelagic mesozooplankton stocks are usually at their annual maximum and submaximal, respectively. Manipulations were designed to increase mesozooplankton stocks in 25L carboys after various lag-times corresponding to grazing pressure greater or equal to that in situ, and to monitor changes in chlorophyll a levels as a proxy for grazing pressure. At the onset of the experiments, in vitro mesozooplankton abundances were comparable to those in situ. Despite the addition of mesozooplankton to selected Fe-enriched carboys in May after 24, 48 and 72 h, corresponding to ca. two-fold increases in their abundances, chlorophyll a increased to ca. 2 μg l−1 in all treatments. In September, chlorophyll a levels increased five-fold to 2 μg l−1 after 4 days – but little thereafter – in the presence of up to ten-fold higher animal abundances (added at t=0) than observed in situ. Thus, Fe-induced increases in diatom growth rates were sufficiently high to escape both initial and additional grazing pressure. If and when Fe is supplied to this region, it is unlikely that mesozooplankton can respond and graze down the resulting elevated algal abundance. Theoretical calculations, based on algal growth and grazing rate data from May in this study, suggested that a greater than five-fold increase in mesozooplankton abundance, after a 48-h lag, is required to exert sufficient grazing pressure to prevent Fe-mediated increases in algal biomass. These findings are discussed in relation to the scale dependency of such events, and the pelagic ecology of other High Nitrate Low Chlorophyll regions.  相似文献   

16.
Microzooplankton species composition and grazing rates on phytoplankton were investigated along a transect between ∼46 and 67°S, and between 140 and 145°E. Experiments were conducted in summer between November 2nd and December 14th in 2001. The structure of the microbial food web changed considerably along the transect and was associated with marked differences in the physical and chemical environment encountered in the different water masses and frontal regions. On average microzooplankton grazing experiments indicated that 91%, 102%, and 157%, (see results) of the phytoplankton production would be grazed in the <200, <20 and <2 μm size fractions, respectively, indicating microzooplankton grazing was potentially constraining phytoplankton populations (<200 μm) along most of the transect. Small ciliates in general and especially oligotrich species declined in importance from the relatively warm, Southern Subtropical Front waters (6.8 μg C/L) to the colder waters of the southern branch of the Polar Front (S-PF), (∼0.5 μg C/L) before increasing again near the Antarctic landmass. Large changes in microzooplankton dominance were observed, with heterotrophic nanoflagellates (HNF), ciliates and larger dinoflagellates having significant biomass in different water masses. HNF were the dominant grazers when chlorophyll a was low in areas such as the Inter-Polar Frontal Zone (IPFZ), while in areas of elevated biomass such as the S-PF and Southern Antarctic Circumpolar Current (SACC), a mix of copepod nauplii and large heterotrophic and mixotrophic dinoflagellates tended to dominate the grazing community. In the S-PF and SACC water masses the tight coupling observed between the microzooplankton grazers and phytoplankton populations over most of the rest of the transect was relaxed. In these regions grazing was low on the >20 μm size fraction of chlorophyll a, which dominated the biomass, while smaller diatoms and nanoplankton in the <20 μm size fraction were still heavily grazed. The lack of grazing pressure on large phytoplankton contributes to this region's potential to export carbon with larger cells known to have higher sinking rates.  相似文献   

17.
How pico- and nanophytoplankton responded to artificial iron infusions was investigated using flow cytometry during SEEDS II, the second mesoscale in situ iron enrichment experiment in the western subarctic North Pacific. Two iron infusions on days 0 and 7 caused a remarkable increase in cellular chlorophyll fluorescence and cell size of all the four phytoplankton groups investigated: Synechococcus, cryptophytes, picoeucaryotes and nanoeucaryotes other than cryptophytes. After the second infusion, the abundance of three phytoplankton groups, excluding Synechococcus, also started to increase. After surface dissolved iron concentration decreased to <0.2 nM on day 11, chlorophyll fluorescence of all the four groups returned to a level observed before the iron infusions, suggesting that pico- and nanophytoplankton were physiologically stressed by iron deficiency. Cell concentrations of pico- and nanoeucaryotes decreased to the pre-infusion level by day 23, while that of cryptophytes remained high until day 25. Flow cytometric diagnosis showed that cryptophytes were physiologically limited during this period and effective iron uptake from suspended particles, as reported for freshwater cryptophytes, was not observed. Thus their prosperity may have been due to alleviation from grazing. Cell concentration of Synechococcus started to increase at a net specific growth rate of 0.13 d−1 after day 12, reaching more than 6 times that of the pre-infusion level on day 24. This may have been due to the elevation of surface water temperature observed during the survey period, together with trophic cascading effects of increased copepod grazing.  相似文献   

18.
Extensive analyses of particulate lipids and lipid classes were conducted to gain insight into lipid production and related factors along the biogeochemical provinces of the Eastern Atlantic Ocean. Data are supported by particulate organic carbon (POC), chlorophyll a (Chl a), phaeopigments, Chl a concentrations and carbon content of eukaryotic micro-, nano- and picophytoplankton, including cell abundances for the latter two and for cyanobacteria and prokaryotic heterotrophs. We focused on the productive ocean surface (2 m depth and deep Chl a maximum (DCM)). Samples from the deep ocean provided information about the relative reactivity and preservation potential of particular lipid classes. Surface and DCM particulate lipid concentrations (3.5–29.4 μg L−1) were higher than in samples from deep waters (3.2–9.3 μg L−1) where an increased contribution to the POC pool was observed. The highest lipid concentrations were measured in high latitude temperate waters and in the North Atlantic Tropical Gyral Province (13–25°N). Factors responsible for the enhanced lipid synthesis in the eastern Atlantic appeared to be phytoplankton size (micro, nano, pico) and the low nutrient status with microphytoplankton having the most expressed influence in the surface and eukaryotic nano- and picophytoplankton in the DCM layer. Higher lipid to Chl a ratios suggest enhanced lipid biosynthesis in the nutrient poorer regions. The various lipid classes pointed to possible mechanisms of phytoplankton adaptation to the nutritional conditions. Thus, it is likely that adaptation comprises the replacement of membrane phospholipids by non-phosphorus containing glycolipids under low phosphorus conditions. The qualitative and quantitative lipid compositions revealed that phospholipids were the most degradable lipids, and their occurrence decreased with increasing depth. In contrast, wax esters, possibly originating from zooplankton, survived downward transport probably due to the fast sinking rate of particles (fecal pellets). The important contribution of glycolipids in deep waters reflected their relatively stable nature and degradation resistance. A lipid-based proxy for the lipid degradative state (Lipolysis Index) suggests that many lipid classes were quite resistant to degradation even in the deep ocean.  相似文献   

19.
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20.
The biomass and production of phytoplankton and bacterioplankton was investigated in relation to the mesoscale structures found in the Algerian Current during the ALGERS'96 cruise (October 1996). Biological determinations were carried out in three transects between 0° and 2°E aimed at crossing a so-called event, formed by a coastal anticyclonic eddy associated with an offshore cyclonic eddy to the west. The concentration of chlorophyll a (Chl) was maximum (>1.2 mg m−3) within the cyclonic eddy and at the frontal zones between the Modified Atlantic Water (MAW) of the Algerian Current and the Mediterranean waters further north. Chl (total and >2 μm) was significantly correlated with proxies of nutrient flux into the upper layers. Autotrophic picoplankton and heterotrophic bacterial abundance and production presented clear differences between MAW and Mediterranean water, with higher values at those stations under the influence of the Algerian Current. In general, greater differences were observed in production than in biomass variables. The photosynthetic parameters (derived from P–E relationships) and integrated primary production (range 189–645 mg m−2 d−1) responded greatly to the different hydrological conditions. The mesoscale phenomena inducing fertilization caused a 2 to 3-fold increase in primary production rates. The relatively high values found within the cyclonic eddy suggest that, although short-lived in comparison with anticyclonic eddies, these eddies may produce episodic increases of biological production not accounted for in previous surveys in the region.  相似文献   

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