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1.
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   

2.
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   

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.
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   

5.
The carbon flux through major phytoplankton groups, defined by their pigment markers, was estimated in two contrasting conditions of the Northwestern Mediterranean open ocean ecosystem: the spring bloom and post-bloom situations (hereafter Bloom and Post-bloom, respectively). During Bloom, surface chlorophyll a (Chl a) concentration was higher and dominated by diatoms (53% of Chl a), while during Post-bloom Synechococcus (42%) and Prymnesiophyceae (29%) became dominant. The seawater dilution technique, coupled to high pressure liquid chromatography (HPLC) analysis of pigments and flow cytometry (FCM), was used to estimate growth and grazing rates of major phytoplankton groups in surface waters. Estimated growth rates were corrected for photoacclimation based on FCM-detected changes in red fluorescence per cell. Given the 30% average decrease in the pigment content per cell between the beginning and the end of the incubations, overlooking photoacclimation would have resulted in a 0.40 d?1 underestimation of phytoplankton growth rates. Corrected average growth rates (μo) were 0.90±0.20 (SD) and 0.40±0.14 d?1 for Bloom and Post-bloom phytoplankton, respectively. Diatoms, Cryptophyceae and Synechococcus were identified as fast-growing groups and Prymnesiophyceae and Prasinophyceae as slow-growing groups across Bloom and Post-bloom conditions. The higher growth rate during Bloom was due to dominance of phytoplankton groups with higher growth rates than those dominating in Post-bloom. Average grazing rates (m) were 0.58±0.20 d?1 (SD) and 0.31±0.07 d?1. The proportion of phytoplankton growth consumed by microzooplankton grazing (m/μo) tended to be lower in Bloom (0.69±0.34) than in Post-bloom (0.80±0.08). The intensity of nutrient limitation experienced by phytoplankton indicated by μo/μn (where μn is the nutrient-amended growth rate), was similar during Bloom (0.78) and Post-bloom (0.73). Primary production from surface water (PP) was estimated with 14C incubations. A combination of PP and Chl a synthesis rate yielded C/Chl a ratios of 34±21 and 168±75 (g:g) for Bloom and Post-bloom, respectively. Transformation of group-specific Chl a fluxes into carbon equivalents confirmed the dominant role of diatoms during Bloom and Synechococcus and Prymnesiophyceae during Post-bloom.  相似文献   

6.
The source and significance of three nutrients – nitrogen, phosphorous and silicon – were investigated by a modified dilution method performed on seawater samples from the Central Yellow Sea (CYS), in spring blooming period of 2007. This modified dilution method accounted for the phytoplankton growth rate, microzooplankton grazing mortality rate, the internal and external nutrient pools, as well as nutrients supplied through remineralization by microzooplankton grazing. The results indicate that phytoplankton growth during the bloom is mostly contributed by internal nutrient pools (KI=0.062–1.730). The external nutrient pools (KE=<0–0.362) are also of importance for phytoplankton growth during the bloom at some sampling sites. Furthermore, the contribution of the recycled-nutrient pool by remineralization (KR=<0–0.751) is significant when microzooplankton grazing rate was higher than 0.5 d−1 during the spring phytoplankton blooms in the Central Yellow Sea. Compared with internal phosphorus, internal nitrogen and silicon contribute more to the phytoplankton production at most sampling stations.  相似文献   

7.
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   

8.
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   

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.
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   

11.
Microzooplankton herbivory in the Arabian Sea was measured using dilution experiments towards the end of the SW monsoon in September and during the intermonsoon to NE monsoon period in November–December 1994. Microzooplankton grazing resulted in a turnover of phytoplankton stocks that ranged from 11 to 49% per day. This was equivalent to grazing fluxes of between 1 and 17 mg C m-3 d-1. Depth-integrated microzooplankton herbivory ranged between 161 and 415 mg C m-2 d-1 during the SW monsoon cruise, and between 110 and 407 mg C m-2 d-1 during the intermonsoon period. Microzooplankton grazed between 4 and 60% of daily primary production, with higher percentages found during the intermonsoon season. Phytoplankton growth rates during the SW monsoon ranged from 0.3 to 1.8 d-1, with lower values in upwelling waters and higher values in downwelling and oligotrophic areas. During the intermonsoon period, phytoplankton growth was more uniform across the basin and averaged 0.68±0.15 d-1. Microzooplankton abundance in experimental samples varied between 2800 and 16 162 cells l-1, equivalent to a biomass of between 1.1 and 7.2 mg C m-3. The mean cell carbon content of microzooplankton was similar in both periods and ranged from 0.33 to 0.55 ng C cell-1. Microzooplankton were smallest in downwelling waters and largest in oligotrophic waters. Average clearance rates in those taxa that took up fluorescently-labelled algae ranged from 0.2 to 14 μl ind-1 hr-1. Average mesozooplankton grazing rates, derived from biomass data, varied from 19 to 92 mg C m-2 d-1; these rates accounted for removal of between 4 and 12% of the daily primary production. Mesozooplankton herbivory was most pronounced in upwelling and downwelling waters and reduced in stratified oligotrophic waters during the SW monsoon period. Microzooplankton herbivory was greater than the average mesozooplankton herbivory at all stations, during both the SW monsoon and intermonsoon periods.  相似文献   

12.
The Drake Passage region near Elephant Island in the Southern Ocean displays patchy phytoplankton blooms. To test the hypothesis that natural Fe addition from localized sources promoted phytoplankton growth here, a grid of stations (59°S to 62°S, 59°W to 53°W, as well as four stations in the eastern Bransfield Strait) were occupied from 12 February–24 March 2004. Phytoplankton abundance was measured using shipboard flow cytometry (70 stations), with abundances conservatively converted to biomass, and compared with measurements of dissolved iron (dFe) at a subset of stations (30 stations). Based on T–S property plots, stations were divided into Antarctic Circumpolar Current (ACC), Water On Shelf (WOS), Bransfield Strait (BS), and Mixed water stations, the latter representing locations with T–S properties intermediate between ACC and WOS stations. The highest integrated phytoplankton biomass was found at Mixed water stations, however, the highest integrated abundance was found at WOS stations, demonstrating that abundance and biomass do not necessarily show the same patterns. The distributions of nano- and micro-phytoplankton (<20 and >20 μm diameter cells, respectively) were also examined, with nano- and micro-plankton contributing equally to the total biomass at WOS and BS stations, but micro-plankton representing ∼2/3 of the biomass at Mixed and ACC stations. Increased inventories of dFe did not always correspond to increases in phytoplankton biomass – rather stations with lower mean light levels in the mixed layer (<110 μEinsteins m−2 s−1) had lower biomass despite higher ambient dFe concentrations. However, where the mean light levels in the mixed layer were >110 μEinsteins m−2 s−1, total biomass shows a positive trend with dFe, as does micro-phytoplankton biomass, but neither regression is significant at the 95% level. In contrast, if just nano-phytoplankton biomass is considered as a function of dFe, there is a significant correlation (r2=0.62). These data suggest a dual mechanism for the patterns observed in biomass: an increasing reservoir of dFe allows increased phytoplankton biomass, but biomass can only accumulate where the light levels are relatively high, such that light is not limiting to growth.  相似文献   

13.
To illustrate areal differences in the structure of lower trophic levels of the pelagic ecosystems in the subarctic Pacific, data collected in the quasi-steady state summer/fall conditions were analysed for five areas, i.e. the Bering Basin, Western Subarctic Gyre, the area south of the Aleutians, the Gulf of Alaska, and the Oyashio Region. Average values of stock size of four components of the lower trophic levels showed a clear difference between areas with ranges of 7.5-fold for nitrate, 3.0 for chlorophyll a, 9.9 for microzooplankton, and 2.4 for mesozooplankton. Such differences were more striking when the structure of the lower trophic levels was expressed as a biomass pyramid. In the Gulf of Alaska, Western Subarctic Gyre, and south of the Aleutians, the relative biomass of microzooplankton to phytoplankton is large and large amounts of nitrate remained unused. In addition to possible iron limitation, grazing control by the microzooplankton on small phytoplankton must be substantial in these areas. Conversely, in the Oyashio Region, the nitrate stock is very small indicating higher efficiency of nitrate consumption by phytoplankton. However, since phytoplankton and zooplankton stocks are not particularly large, their products are likely to be transferred, also efficiently, to the higher trophic levels such as planktivorous pelagic fish. The situation in the Bering Basin is intermediate between the Oyashio Region and the other three areas. Inter-annual fluctuations in stock size of the planktivorous fish which migrate into the Oyashio Region in summer/fall were quite large. However, the inter-annual variation of mesozooplankton biomass was small, suggesting the existence of certain mechanisms to stabilize plankton abundance under increasing predation pressure. As a result, the increasing fish stocks likely keep the transfer efficiency from nitrate through to fish higher, at least in the Oyashio Region.  相似文献   

14.
A mesoscale iron fertilization experiment was carried out in the western subarctic Pacific during summer 2004. The iron-patch was traced for 26 days after the enrichment, and the abundance and behavior of meso- and microzooplankton was compared with those outside of the patch. The surface chlorophyll-a concentration in the patch was high between days 10 and 13 (2.5 mg m−3) and decreased to the initial level after day 20. Microzooplankton grazing rates, estimated by a dilution method, was mostly balanced with phytoplankton growth rates throughout the observed period. Dominant mesozooplankton species in the upper 200 m were copepods: dominated by Eucalanus bungii, Neocalanus plumchrus and Metridia pacifica. Species composition did not change in the patch over the observation period. The copepod biomass was 3–5 times higher than in Subarctic Pacific Iron Experiment for Ecosystem Dynamics Study (SEEDS), the previous iron-enrichment experiment in the same area, before the bloom, and exponentially increased both inside and outside the patch, which was mainly brought by the development of N. plumchrus. The development rates of N. plumchrus were not significantly different between inside and outside the patch. Estimated grazing rate suggest that the copepod grazing was main cause of the low accumulation of phytoplankton biomass, and dominance of grazing-resistant organisms such as large ciliates, large diatoms and diatoms with extremely long setae. “Arrested migration” for M. pacifica and upward shift of vertical distribution by E. bungii were observed during the bloom period, even if the accumulation of phytoplankton biomass was very low compared to other iron-enrichment experiments. These results indicate that the copepod grazing shaped the food-web structure of the lower trophic levels (biomass and species composition) in SEEDS II.  相似文献   

15.
《Journal of Sea Research》2009,61(4):246-254
The aim of this study was to investigate controls on the phytoplankton community composition and biogeochemistry of the estuarine plume zone of the River Thames, U.K. using an instrumented moored buoy for in situ measurements and preserved sample collection, and laboratory-based measurements from samples collected at the same site. Instrumentation on the moored buoy enabled high frequency measurements of a suite of environmental variables including in situ chlorophyll, water-column integrated irradiance, macronutrients throughout an annual cycle for 2001 e.g. nitrate and silicate, and phytoplankton biomass and species composition. The Thames plume region acts as a conduit for fluvial nutrients into the wider southern North Sea with typical winter concentrations of 45 μM nitrate, 17 μM silicate and 2 μM phosphate measured. The spring bloom resulted from water-column integrated irradiance increasing above 60 W h m− 2 d− 1 and was initially dominated by a diatom bloom mainly composed of Nitzschia sp. and Odontella sinesis. The spring bloom then switched after ∼ 30 days to become dominated by the flagellate Phaeocystis reaching a maximum chlorophyll concentration of 37.8 μg L− 1. During the spring bloom there were high numbers of the heterotrophic dinoflagellates Gyrodinium spirale and Katodinium glaucum that potentially grazed the phytoplankton bloom. This diatom–flagellate switch was predicted to be due to a combination of further increasing water-column integrated irradiance > 100 W h m− 2 d− 1 and/or silicate reaching potentially limiting concentrations (< 1 μM). Post spring bloom, diatom dominance of the lower continuous summer phytoplankton biomass occurred despite the low silicate concentrations (Av. 0.7 μM from June–August). Summer diatom dominance, generally due to Guinardia delicatula, was expected to be as a result of microzooplankton grazing, dominated by the heterotrophic dinoflagellate Noctiluca scintillans, controlling 0.7–5.0 μm ‘flagellate’ fraction of the phytoplankton community with grazing rates up to 178% of ‘flagellate’ growth rate. The Thames plume region was therefore shown to be an active region of nutrient and phytoplankton processing and transport to the southern North Sea. The use of a combination of moorings and ship-based sampling was essential in understanding the factors influencing nutrient transport, phytoplankton biomass and species composition in this shelf sea plume region.  相似文献   

16.
Phytoplankton growth rates and mortality rates were experimentally examined at 21 stations during the 2017 spring intermonsoon(April to early May) in the northern and central South China Sea(SCS) using the dilution technique, with emphasis on a comparison between the northern and central SCS areas which had different environmental factors. There had been higher temperature but lower nutrients and chlorophyll a concentrations in the central SCS than those in the northern SCS. The mean rates of phytoplankton growth(μ_0) and microzooplankton grazing(m) were(0.88±0.33) d~(–1) and(0.55±0.22) d~(–1) in the central SCS, and both higher than those in the northern SCS with the values of μ_0((0.81±0.16) d~(–1)) and m((0.30±0.09) d~(–1)), respectively.Phytoplankton growth and microzooplankton grazing rates were significantly coupled in both areas. The microzooplankton grazing impact(m/μ_0) on phytoplankton was also higher in the central SCS(0.63±0.12) than that in the northern SCS(0.37±0.06). The microzooplankton abundance was significantly correlated with temperature in the surface. Temperature might more effectively promote the microzooplankton grazing rate than phytoplankton growth rate, which might contribute to higher m and m/μ_0 in the central SCS. Compared with temperature, nutrients mainly affected the growth rate of phytoplankton. In the nutrient enrichment treatment,the phytoplankton growth rate(μn) was higher than μ_0 in the central SCS, suggesting phytoplankton growth in the central SCS was nutrient limited. The ratio of μ_0/μn was significantly correlated with nutrients concentrations in the both areas, indicating the limitation of nutrients was related to the concentrations of background nutrients in the study stations.  相似文献   

17.
Phytoplankton production was measured at the shelf edge region of the Celtic Sea in April/May 1994 at the beginning of the spring bloom. Size fractionated 14C uptake experiments showed that phytoplankton >2 μm dominated the bloom although, in the period immediately before the increase in phytoplankton biomass, picophytoplankton (<2 μm) was responsible for up to 42% of the production; in these late winter conditions, chlorophyll concentrations were generally <0.7 μg l-1 and primary production was ca. 70 mmol C m-2 d-1. As the spring bloom developed, phytoplankton production rates of 120 mmol C m-2 d-1 were measured. Chlorophyll concentration increased to >2 μg l-1 as a result of growth of larger phytoplankton, including diatoms, with large numbers of Nitzschia, Thalassionema and Chaetoceros dominating the assemblage. Picophytoplankton production declined as the spring bloom progressed. Nutrient concentrations were not depleted during the sampling period, and NO-3 concentrations were >6 μmol l-1. Nutrient assimilation rates were measured at the same time as primary production was estimated. Before the development of any substantial phytoplankton biomass, the uptake rates for ammonium and nitrate were very similar, with f-ratios ranging from 0.5 to 0.6. Assimilation of ammonium remained relatively constant after the onset of stratification and bloom development, but nitrate uptake increased by a factor of 2 or more, resulting in f-ratios >0.8. There was significant phosphate uptake in the dark, which was generally ca. 50% of the rate in the light. The C : N : P assimilation ratios changed as the bloom developed; in the pre-bloom situation, when small phytoplankton cells dominated the assemblage, the C : N assimilation ratio was variable, with some stations having ratios less than (ca 2.5), and some higher than (ca. 9), the Redfield ratio. The most actively growing assemblages had N : P ratios close to the Redfield ratio, but the C : N ratios were consistently lower. New production was found to be closely correlated with the size of the species making up the phytoplankton assemblage, and high f ratios were measured when larger phytoplankton dominated the assemblage.  相似文献   

18.
Distribution of larger protozoans (armoured dinoflagellates, tintinnids, heliozoans, radiolarians and foraminiferans >64 μm) is presented for three major water masses of the Southern Ocean: the Polar Front region (PFr), the southern Antarctic Circumpolar Current (southern ACC) and the northern Weddell Gyre. Sampling took place during the SO-JGOFS cruise ANT X/6 of R/V Polarstern (October–November 1992) along a meridional transect at 6°W between 48°00′S and 59°30′S. Multinet samples (64 μm mesh size) were taken at six stations from the surface down to 500 m depth at five different depth intervals. In the upper 100 m of the water column abundances of larger protozoans varied between 94 and 10,930 ind. m–3, with highest abundances in the PFr, where phytoplankton blooms occurred, and lowest values in the Antarctic Circumpolar Current–Weddell Gyre Boundary (AWB). Foraminiferans and polycystine and smaller (<300 μm) phaeodarian radiolarians dominated larger protozoan assemblages in the PFr. In open water of the southern ACC, tintinnids, armoured dinoflagellates, foraminiferans and smaller (<300 μm) phaeodarian radiolarians were equally important. The heliozoans Sticholonche spp. and nassellarian radiolarians dominated assemblages in the Weddell Gyre and AWB. Larger protozoan biomasses ranged between 2 and 674 μg C m−3 and were always dominated by larger (>300 μm) phaeodarians. Highest biomasses were found in the AWB between 200 and 500 m depth. Standing stocks of larger protozoans constituted a negligible fraction of zooplankton biomass in the upper 200 m of the water column. In deeper layers of the ice-covered Weddell Gyre and AWB their biomasses, dominated by larger (>300 μm) phaeodarians, was significant contributing up to 45% to total larger protozoan and metazoan biomass. Analysis of correlation between distribution patterns and environmental conditions at the stations sampled indicate that spring distribution patterns of heterotrophic armoured dinoflagellates, polycystine radiolarians and foraminiferans follow productivity in the water column. Of the protozoan groups studied the smaller (<300 μm) phaeodarian radiolarians also showed a significant correlation with productivity during spring, however, results from previous studies do not suggest a consistent pattern. Spring distribution patterns of other larger protozoans were not related to differences in productivity in the water column, and effects such as ice-cover, grazing or silica limitation might be determining. Dead radiolarian skeletons constituted on average 27, 8 and 11% of the population of nassellarians, spumellarians and smaller (<300 μm) phaeodarians, respectively. The contribution of dead radiolarian skeletons to total radiolarian stocks varied with depth and water mass. Differences between live and skeleton assemblages composition were observed. These differences should be taken into consideration when interpreting the geological record.  相似文献   

19.
We examined bacterioplankton biomass and heterotrophic production (BHP) during summer stratification in the northwestern Mediterranean in four successive stratification seasons (June–July of 1993–1996). Values of phytoplankton biomass and primary production were determined simultaneously so that the data sets for autotrophic and heterotrophic microbial plankton could be compared. Three standard stations were set along a transect from Barcelona to the channel between Mallorca and Menorca, representing coastally influenced shelf waters, frontal waters over the slope front, and open sea waters. Conversion factors from 3H-leucine incorporation to BHP were empirically determined and varied between 0.29 and 3.25 kg C mol-1. Bacterial biomass values were among the lowest found in any marine environment. BHP values (between 0.02 and 2.5 μg C L-1 d-1) were larger than those of low nutrient low chlorophyll areas such as the Sargasso Sea and lower than those from high nutrient low chlorophyll areas such as the equatorial Pacific. Growth rates of bacterioplankton were highest at the slope front (0.20 d-1) and lowest at the open sea station (0.04 d-1). Phytoplankton growth rates were similar at the three stations (∼0.50 d-1). Integrated values of bacterioplankton biomass, BHP and bacterial growth rates did not show significant differences among years, but differences between the three stations were clearly significant. Phytoplankton biomass, primary production, and phytoplankton growth rates did not show significant differences either with year or with station. As a consequence the bacterioplankton to phytoplankton biomass (BB/BPHY) and production (BHP/PP) ratios varied from the coastal to the open sea stations. The BB/BPHY ratio was 0.98 at the coast and ∼0.70 at the other two stations. These ratios are similar to those found in other oligotrophic marine environments. The BHP/PP ratio was 0.83 at the coast, 0.36 at the slope and 0.09 at the open sea station. The last value is also similar to values found in other oligotrophic marine environments. Vertical distribution of these ratios was also examined.The comparison of microbial parameters at the three stations indicates a different kind of relationship between bacterioplankton and phytoplankton in oligotrophic open sea waters and in coastal, nutrient-richer waters. According to such parameters and to the values of the BB/BPHY and BHP/PP ratios, open waters in the northwestern Mediterranean (despite their relatively short distance from the shore) were intermediate between the extremely oligotrophic waters of the eastern Mediterranean or the Sargasso Sea and the more productive waters of the equatorial Pacific.  相似文献   

20.
Using the seawater dilution technique, we measured phytoplankton growth and microzooplankton grazing rates within and outside of the 1999 Bering Sea coccolithophorid bloom. We found that reduced microzooplankton grazing mortality is a key component in the formation and temporal persistence of the Emiliania huxleyi bloom that continues to proliferate in the southeast Bering Sea. Total chlorophyll a (Chl a) at the study sites ranged from 0.40 to 4.45 μg C l−1. Highest phytoplankton biomass was found within the bloom, which was a mixed assemblage of diatoms and E. huxleyi. Here, 75% of the Chl a came from cells >10 μm and was attributed primarily to the high abundance of the diatom Nitzschia spp. Nutrient-enhanced total phytoplankton growth rates averaged 0.53 d−1 across all experimental stations. Average growth rates for >10 μm and <10 μm cells were nearly equal, while microzooplankton grazing varied among stations and size fractions. Grazing on phytoplankton cells >10 μm ranged from 0.19 to 1.14 d−1. Grazing on cells <10 μm ranged from 0.02 to 1.07 d−1, and was significantly higher at non-bloom (avg. 0.71 d−1) than at bloom (avg. 0.14 d−1) stations. Averaged across all stations, grazing by microzooplankton accounted for 110% and 81% of phytoplankton growth for >10 and <10 μm cells, respectively. These findings contradict the paradigm that microzooplankton are constrained to diets of nanophytoplankton and strongly suggests that their grazing capability extends beyond boundaries assumed by size-based models. Dinoflagellates and oligotrich ciliates dominated the microzooplankton community. Estimates of abundance and biomass for microzooplankton >10 μm were higher than previously reported for the region, ranging from 22,000 to 227,430 cells l−1 and 18 to 164 μg C l−1. Highest abundance and biomass occurred in the bloom and corresponded with increased abundance of the large ciliate Laboea, and the heterotrophic dinoflagellates Protoperidinium and Gyrodinium spp. Despite low grazing rates on phytoplankton <10 μm within the bloom, the abundance and biomass of small microzooplankton (<20 μm) capable of grazing E. huxleyi was relatively high at bloom stations. This body of evidence, coupled with observed high grazing rates on large phytoplankton cells, suggests the phytoplankton community composition was strongly regulated by herbivorous activity of microzooplankton. Because grazing behavior deviated from size-based model predictions and was not proportional to microzooplankton biomass, alternate mechanisms that dictate levels of grazing activity were in effect in the southeastern Bering Sea. We hypothesize that these mechanisms included morphological or chemical signaling between phytoplankton and micrograzers, which led to selective grazing pressure.  相似文献   

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