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1.
The dilution technique, combined with identification and enumeration of pico-, nano- and micro-plankton by microscopy, was used to estimate microzooplankton impact on the microbial community in surface waters of a coastal embayment on the NW Iberian upwelling system. Microzooplankton were important consumers of autotrophic and heterotrophic plankton in this system, feeding up to 93% of standing stock and more than 100% of production of several groups. Heterotrophic bacteria and heterotrophic picoflagellates experienced the highest and constant impact, with 75–84% of their standing stocks and 85–102% of their production being channelled through the microbial food web. Pico- and nano-phytoplankton were also consumed, although maximum grazing occurred on diatoms during upwelling events, coinciding with highest primary production. Predation on pico-nano-heterotrophs was especially relevant under downwelling conditions, when consumption of total carbon and particularly autotrophic carbon was considerably lower than during upwelling. The results suggest that the existence of a multivorous food web, extending from the microbial loop to the herbivorous food web, could be a major feature in this coastal upwelling system. The microbial loop, which occurs as a permanent background in the system, would contribute to sustain the microbial food web during downwelling, whereas the herbivorous food web could coexist with a microbial food web based on large diatoms during upwelling. The multivorous food web would partially divert diatoms from sinking and hence favour the retention of organic matter in the water column. This could enhance the energy transfer to higher pelagic trophic levels in coastal upwelling systems.  相似文献   

2.
《Journal of Sea Research》2010,63(4):238-249
From July 2001 to May 2005, at a fixed station located in Lisbon Bay (Cascais: 38° 41′ N, 09° 24′ W), surface seawater samples were collected on a weekly basis. We aimed to describe at different temporal scales, short-term to interannual, the phytoplankton community in relation to hydrographic conditions.Maxima of the main phytoplankton groups varied according to the seasonality of upwelling/downwelling cycles and nutrient availability and were associated with particular hydrological mesoscale structures highlighted by satellite images. Short succession cycles were identified dependent on coastal upwelling events. Intermittent and weak pulses allowed the coexistence of species from different succession stages and groups, although having consecutive maxima. The interannual differences observed in the phytoplankton community, in Lisbon Bay, varied according to both the duration and strength of the upwelling events and to precipitation and Tagus river flow regimes.Diatoms developed and were dominant, during spring–summer under prevailing upwelling conditions and silicon availability. Short upwelling pulses appeared to be unfavourable for diatoms maintenance. When upwelling weakened and SST increased due to onshore advection of warmer waters, coccolithophores dominated. This assemblage was the second most abundant during the study, in particular during the short transition period from upwelling (summer) to downwelling seasons (autumn) distributing in the largest range of hydrographical conditions between diatoms (maximum turbulence) during early spring and dinoflagellates (maximum stratification) during summer to further dominate during autumn and winter. Nitrites and nitrates seemed to favour greater developments of this group. Dinoflagellates peaked mainly during summer and were the less abundant through the four years due to the decrease of lasting convergence periods. Like coccolithophores, a preference for warmer waters emerged but this group seemed to have a narrow tolerance to turbulence and temperature changes.  相似文献   

3.
This study describes the main seasonal stages in oceanographic conditions and phytoplankton off La Coruña (Galicia, NW Spain), during 1991 and 1992, based mainly on monthly cruises near the coast. Upwelling conditions were studied using an upwelling index calculated from local winds. The Galician coast is affected by a long upwelling season for most of the year. The upwelling pulses interact with the thermal stratification-mixing cycle of surface waters, primarily affecting the dynamics of phytoplankton. In addition, the presence of water masses of different salinity in the subsurface layers changes the stratification of the water column. The less-saline North Atlantic Central Water (NACW) was normally associated with upwelling events during summer. However, on several occasions during the study, the presence of Eastern North Atlantic Water (ENAW) of subtropical origin was observed with salinities up to 36·22 and temperatures between 13 and 14 °C.Observations were grouped into five main stages related to the degree of surface stratification and characteristics of phytoplankton communities. These stages were recognized in both annual cycles, and were termed: winter mixing, spring and autumn blooms, summer upwelling, thermal stratification and special events (red tides and downwelling). A homogeneous water column was the main characteristic of the winter stage, with high nutrient concentrations and low phytoplankton biomass. Eastern North Atlantic Water appeared at the end of this stage, which lasted from November to February. The spring and autumn blooms occurred along with weak thermohaline gradients at the surface, producing high phytoplankton concentrations. Favourable upwelling conditions and the presence of ENAW in a subsurface layer were the factors that most likely induced earlier blooms, while thermal gradients developed at the surface could have been more important for later blooms. Upwelling events during summer were related to a reduction in the depth of the surface mixed layer as the pycnocline moved upwards, and can produce significant phytoplankton accumulations. These summer blooms interrupted the thermal stratification stage, characterized by low nutrient and phytoplankton concentrations at the surface. The dominant phytoplankton in the study was composed mainly of diatoms, especially during blooms. However, a proliferation of red-tide dinoflagellates was observed along with weak upwelling conditions in late summer. Also in late summer, strong downwelling conditions caused the accumulation of warmer shelf waters inshore, inducing the sinking of particulate matter produced at the surface.  相似文献   

4.
During the 1980s, there was a major shift in the understanding of the structure of marine foodwebs. As a result, the microbial loop has been incorporated into the classical concept of the planktonic food chain. Heterotrophic bacteria and Protozoa have been shown to be important components of the plankton biomass in many parts of the world's oceans, and their role in the trophic dynamics of pelagic foodwebs has been studied intensively. In the southern Benguela, field, laboratory and modelling studies have been combined to calculate carbon and nitrogen fluxes through the microheterotrophic portion of the pelagic foodweb. A size-based simulation model incorporating recent hypotheses on the structure and functioning of the pelagic foodweb after upwelling predicts rapid growth of a phytoplankton community dominated by netphytoplankton and chain-forming nanophytoplankton cells. After nitrate-depletion the bloom declines, to be followed by a bloom of single nanophytoplankton cells dependent upon regenerated nitrogen. During conditions of high nitrate availability and a netphytoplankton-dominated community, mesozooplankton ingest 44 per cent of the total primary production through herbivory and 1,4 per cent through carnivory. During periods of low nitrate availability and a nanophytoplankton-dominated community, mesozooplankton ingest 0,6 per cent of the total primary production through herbivory and 5 per cent through carnivory. Food chains are longer and microheterotrophs are an important link between primary producers and the larger heterotrophs. Simulation results show that microheterotrophs are an important component of the pelagic foodweb, primarily as regenerators of nitrogen, which sustains phytoplankton growth, and as a food source for larger heterotrophs of the metazoan foodweb during the nanophytoplankton-dominated bloom.  相似文献   

5.
A recent one-dimensional numerical model, the so-called GJ model, neglected horizontal advection and diffusion terms and used vague physical definition for its boundary conditions. An alternative two-dimensional numerical model, which corrects the GJ model’s deficiencies, is proposed to evaluate the physical effects of upwelling/downwelling on suspended sediment concentration (SSC) on continental shelves. The general SSC patterns derived from the two models are similar. However, in the case where settling velocity is larger than the maximum upwelling/downwelling velocity, the GJ model shows a homogeneous distribution of SSC, whereas the two-dimensional model identifies a distinct maximum SSC in the middle of the water column in combined upwelling and downwelling systems, and a SSC gradient in upwelling settings. Furthermore, when the maximum upwelling velocity is larger than the settling velocity, the GJ model shows a maximum SSC at the water surface, whereas the SSC is lower at the water surface when using the two-dimensional model.  相似文献   

6.
In this paper, we use a coupled biological/physical model to synthesize and understand observations taken during the US JGOFS Arabian Sea Process Study (ASPS). Its physical component is a variable-density, -layer model; its biological component consists of a set of advective–diffusive equations in each layer that determine nitrogen concentrations in four compartments, namely, nutrients, phytoplankton, zooplankton, and detritus. Solutions are compared to time series and cruise sections from the ASPS data set, including observations of mixed-layer thickness, chlorophyll concentrations, inorganic nitrogen concentrations, particulate nitrogen export flux, zooplankton biomass, and primary production. Through these comparisons, we adjust model parameters to obtain a “best-fit” main-run solution, identify key biological and physical processes, and assess model strengths and weaknesses.Substantial improvements in the model/data comparison are obtained by: (1) adjusting the turbulence-production coefficients in the mixed-layer model to thin the mixed layer; (2) increasing the detrital sinking and remineralization rates to improve the timing and amplitude of the model's export flux; and (3) introducing a parameterization of particle aggregation to lower phytoplankton concentrations in coastal upwelling regions.With these adjustments, the model captures many key aspects of the observed physical and biogeochemical variability in offshore waters, including the near-surface DIN and phytoplankton P concentrations, mesozooplankton biomass, and primary production. Nevertheless, there are still significant model/data discrepancies of P for most of the cruises. Most of them can be attributed to forcing or process errors in the physical model: inaccurate mixed-layer thicknesses, lack of mesoscale eddies and filaments, and differences in the timing and spatial extent of coastal upwelling. Relatively few are clearly related to the simplicity of the biological model, the model's overestimation of coastal P being the most obvious example. Overall, we conclude that future efforts to improve biogeochemical models of the Arabian Sea should focus on improving their physical component, ensuring that it represents the ocean's physical state as closely as possible. We believe that this conclusion applies to coupled biogeochemical modeling efforts in other regions as well.  相似文献   

7.
We construct a one-dimensional ecosystem model (nitrate, ammonium, phytoplankton, zooplnakton and detritus) with simple physics and biology in order to focus on the structural relations and intrinsic properties of the food web that characterizes the biological regime in the central equatorial Pacific at 140°W. When possible, data collected during the EgPac and other cruises were used to calibrate model parameters for two simulations that differ in the limiting nutrient, i.e. nitrogen or iron. Both simulations show annual results in good agreement with the data, but phytoplankton biomass and primary production show a more pronounced annual variability when iron is used as the limiting nutrient. This more realistically reproduces the variability of biological production and illustrates the greater coupling between vertical physical processes and biological production when the limiting nutrient is iron rather than nitrogen. The iron simulation also illustrates how iron supply controls primary production variability, how grazingbalances primary production and controls phytoplankton biomass, and how both iron supply and grazingcontrol primary production. These results suggest that it is not possible to capture primary production variability in the central equatorial Pacific with biological models using nitrogen as the limiting nutrient. Other indirect results of this modeling study were: (1) partitioning of export production between dissolved and particulate matter is almost equal, suggesting that the importance of DOC export may have been previously overestimated; (2) lateral export of live biomass has to be taken into account in order to balance the nitrogen budget on the equator at 140°W; and (3) preferential uptake of ammonium (i.e. nitrate uptake inhibition by ammonium) associated with high regeneration of nitrogen (low f ratio as a consequence of the food web structure imposed by iron limitation) largely accounts for the surface build-up of upwelled nitrate.  相似文献   

8.
龟足（Capitulum mitella Linnaeus）的食物组成可分为4类,动物性食物、植物性食物、有机碎屑和无机颗粒。动物性食物主要以桡足类为主,植物性食物主要以硅藻类为主,有机碎屑在龟足的食物组成中占据重要地位。龟足食物的食物多样性指数在夏季最高,冬季最低。小规格龟足主要摄食植物性食物和有机碎屑,未见摄食动物...  相似文献   

9.
This study aimed at establishing the trophic significance of the kelp Laminaria digitata for consumers inhabiting two rocky shores of Northern Brittany (France), displaying contrasted ecological conditions. The general trophic structure did not vary between these two sites, with a wide diversity of filter-feeders and predators, and only 14% of the species sampled belonging to the grazers' trophic group. The diversity of food sources fueling the food web appeared also similar. The food webs comprised four trophic levels and the prevalence of omnivory appeared relatively low compared to previous studies in the same area. Conversely, to the food web structure, which did not differ, the biochemical composition of L. digitata differed between the two sites, and was correlated to a larger diversity of grazers feeding on this kelp in sheltered conditions. This indicated that the spatial variability occurring in the nutritive value of L. digitata is likely to deeply affect the functioning of kelp-associated food webs. The contribution of L. digitata-derived organic matter to the diet of filter-feeders inhabiting these two environments was assessed using the mixing model Isosource, which showed the higher contribution of kelp matter in sheltered conditions. These results highlight the spatial variability that may occur in the functioning of kelp-associated food webs. Moreover, this suggests that hydrodynamics is likely to control the availability of kelp-derived organic matter to local filter-feeders, probably through an increase of detritus export in exposed areas.  相似文献   

10.
为提高单体“海大1号”长牡蛎(Crassostrea gigas)的培育效率,本研究以3种培育模式(下降流、上升流和静水)、6个水流量梯度(20、35、50、65、80和95 mL/s)和6个培育密度梯度(2.5、5、7.5、10、12.5和15个/mL)对单体“海大1号”长牡蛎苗种中间培育生长情况进行了研究。研究表明,单体“海大1号”长牡蛎苗种中间培育时,上升流和下降流均比静水培育效果好,培育前5天采用下降流、第6~29天采用上升流时,稚贝生长快、死亡率低;水流量小于65 mL/s时,随水流量的增加,稚贝生长加快,死亡率降低,水流量在65~80 mL/s时,单体牡蛎稚贝生长存活最优,第29天时最大壳高达(1534.81±65.23)μm。单体“海大1号”长牡蛎稚贝苗种中间培育时,随着培育密度的增大,挤迫效应逐渐显现,密度为7.5个/mL时,死亡率相对较低,且稚贝生长速度快,在第29天时稚贝壳高最大可达(1520.63±51.72)μm。基于以上研究结果,建议在规模化单体“海大1号”长牡蛎稚贝中间培育时,采用上升流和下降流相结合的流水培育方式,适当增加水流量,及时疏散密度。本研究优化了单体“海大1号”长牡蛎室内培育模式,为单体牡蛎规模化培育提供了参考方法和理论依据。  相似文献   

11.
Numerical experiments were carried out to evaluate the physical effects of upwelling/downwelling on the suspended sediment concentration (SSC) on continental shelves. The results show that (1) because the upwelling or downwelling velocity tends to be of the same order of magnitude as the settling velocity of suspended sediment, various SSC distribution patterns are possible, depending on the relative importance of the velocities; (2) the presence of upwelling can enhance the SSC; and (3) in an upwelling/downwelling system, maximum concentrations may be situated in the middle of the water column, if the settling velocity is larger than the upwelling velocity. Hence, the physical processes associated with upwelling/downwelling are relevant to the SSC distribution observed over the mud patch near Cheju Island, East China Sea. Furthermore, a qualitative analysis indicates that the presence of upwelling/downwelling in the overlying water column increases the accumulation rate at the seabed.  相似文献   

12.
Numerical experiments were carried out to evaluate the physical effects of upwelling/downwelling on the suspended sediment concentration (SSC) on continental shelves. The results show that (1) because the upwelling or downwelling velocity tends to be of the same order of magnitude as the settling velocity of suspended sediment, various SSC distribution patterns are possible, depending on the relative importance of the velocities; (2) the presence of upwelling can enhance the SSC; and (3) in an upwelling/downwelling system, maximum concentrations may be situated in the middle of the water column, if the settling velocity is larger than the upwelling velocity. Hence, the physical processes associated with upwelling/downwelling are relevant to the SSC distribution observed over the mud patch near Cheju Island, East China Sea. Furthermore, a qualitative analysis indicates that the presence of upwelling/downwelling in the overlying water column increases the accumulation rate at the seabed.  相似文献   

13.
Distributions of salinity, dissolved total nitrogen (DTN) and dissolved oxygen (DO) were observed once a month throughout a year in Mikawa Bay, one of the most eutrophic bays in Japan. Supply of freshwater, DTN and DO from the land and precipitation was estimated. Hydrography, circulation and transformation of DTN and DO during the stratified period were investigated simultaneously by a two-layered box model analysis dividing the bay into five boxes. The two-layered circulation was confirmed. In the upper layer of the river mouth regions, a high production of particulate organic nitrogen (PON) due to the strong upwelling together with the river inflow was found. On the other hand, in the lower layer of the bay mouth region, a high deposition of PON due to weak upwelling and sometimes downwelling and a high decomposition of PON due to the inflow of DO abundant water were also found. The reason why the eutrophication is more severe in the eastern part than in the north-western part of the bay is discussed.  相似文献   

14.
The inner part of the Ariake Sea is one of the most productive estuarine systems in Japan. To examine potential food items for estuarine organisms, we conducted monthly observations of the dynamics of particulate organic matter along the macrotidal Chikugo River estuary in 2005 and 2006. In the neighboring macrotidal Midori and Kuma River estuaries, comparative observations were made. High turbidity and strong vertical mixing were observed only at low salinities (<10) in the Chikugo River estuary. In contrast, the Midori and Kuma River estuaries were characterized by less turbid and less mixed waters. Concentrations of particulate organic carbon often exceeded 5?mg?l?1 in or close to the estuarine turbidity maximum (ETM) of the Chikugo River estuary. However, such high concentrations were rarely observed in the other two estuaries. The observed differences could be attributable to different hydrodynamic processes related to the different lengths of tidal reaches: 23, 8, and 6?km in the Chikugo, Midori, and Kuma Rivers, respectively. In the Chikugo River estuary, spatiotemporal changes of chlorophyll a suggested that phytoplankton occurred abundantly up- and/or downstream from the ETM especially during the warm season. In contrast, pheophytin (i.e., plant detritus) always accumulated in or close to the ETM. Carbon stable isotope ratios and carbon to nitrogen ratios indicated that the plant detritus was derived from phytoplankton and terrestrial plants. The Chikugo River estuary has a high potential to support the production of estuarine organisms through abundant plant detritus in the well-developed ETM all the year round.  相似文献   

15.
The Huizache–Caimanero coastal lagoon complex on the Pacific coast of Mexico supports an important shrimp fishery and is one of the most productive systems in catch per unit area of this resource. Four other less important fish groups are also exploited. In this study, we integrated the available information of the system into a mass-balance trophic model to describe the ecosystem structure and flows of energy using the E approach. The model includes 26 functional groups consisting of 15 fish groups, seven invertebrate groups, macrophytes, phytoplankton, and a detritus group. The resulting model was consistent as indicated by the output parameters. According to the overall pedigree index (0.75), which measures the quality of the input data on a scale from 0 to 1, it is a high quality model. Results indicate that zooplankton, microcrustaceans, and polychaetes are the principal link between trophic level (TL) one (primary producers and detritus) and consumers of higher TLs. Most production from macrophytes flows to detritus, and phytoplankton production is incorporated into the food web by zooplankton. Half of the flow from TL one to the next level come from detritus, which is an important energy source not only for several groups in the ecosystem but also for fisheries, as shown by mixed trophic impacts. The Huizache–Caimanero complex has the typical structure of tropical coastal lagoons and estuaries. The TL of consumers ranges from 2.0 to 3.6 because most groups are composed of juveniles, which use the lagoons as a nursery or protection area. Most energy flows were found in the lower part of the trophic web.  相似文献   

16.
Idealized numerical experiments with a depth level coordinate ocean circulation model (GFDL MOM3) have been conducted to investigate the structure of interdecadal variability from thermally driven circulations. The model oceans are driven by steady surface heat fluxes in the absence of surface wind stresses. Interdecadal variability is observed, with characteristics similar to those reported in many previous studies. To explain the nature of the variability we propose a new mechanism based on two local horizontal advective processes. This overcomes the limitations in previous theories based on the interplay between global properties such as zonal and meridional temperature gradients and overturning. One of the two advective processes is a zonal flow anomaly induced by a temperature anomaly along the northern wall through geostrophy southward of the temperature anomaly. A cold (warm) anomaly along the northern wall produces a positive (negative) zonal flow anomaly that induces a warm (cold) temperature anomaly by enhancing (weakening) warm advection from the western boundary along the path of the zonal flow anomaly. The temperature and flow anomalies are transported toward the eastern boundary by the mean eastward zonal flow. When the positive (negative) zonal flow anomaly that accompanies the warm (cold) temperature anomaly encounters the eastern wall, a downwelling (upwelling) anomaly is produced. To dissipate the vorticity due to this downwelling (upwelling) anomaly, a northward (southward) flow anomaly, which is another advective process governing the variability, is generated within a frictional boundary layer next to the eastern wall. The northward (southward) flow anomaly circulates cyclonically along the perimeter of the basin while enhancing (reducing) warm advection. So does the warm (cold) temperature anomaly carried to the eastern wall by the mean zonal flow while pushing the cold (warm) anomaly that produced the positive (negative) zonal flow anomaly westward and initiating the other half cycle of the variability. During the anomalous downwelling or upwelling, the available potential energy stored in the anomalous density field is released to maintain the variability. Thus, neither barotropic nor baroclinic instability supplies energy for the variability. The anomalous vertical velocity is stronger along the northern boundary and the northern part of the eastern boundary. A shallow continental slope added along those boundaries prohibits the anomalous vertical motion and weakens variability very effectively, while one along the western boundary does not.  相似文献   

17.
The recent NE subarctic Pacific study of the Canadian JGOFS project was designed primarily to address why phytoplankton biomass and production at Ocean Station Papa (OSP: 50°N, 145°W) are not as high as the nitrate concentrations could potentially support. To examine the possible role of iron (Fe) limitation in concert with microzooplankton grazing and physical supply of nitrate, we have coupled a four-compartment Nitrogen–Phytoplankton–Zooplankton–Detritus planktonic ecosystem model with a 60-layer (each 2 m thick) one-dimensional mixed-layer model (Mellor–Yamada level 2.5), driven by annual forcing characteristic of OSP. Both the physical and ecological models are forced with the same annual heat budget, mean phytoplankton concentration was tuned with the equilibrium solution of the model, and the zooplankton parameter values were chosen to be representative of microzooplankton. Modelled sea surface temperature ranged between 6 (fixed – late winter) and 13–14°C, depending on the distribution and amount of phytoplankton and detritus calculated by the model. Simulations with Fe limitation reducing the maximum specific growth rate of phytoplankton (for Fe-replete conditions) by a factor of ∼3 best reproduced the annual cycle of surface layer nitrate, although the resulting annual f-ratio calculated from the fluxes into and out of the nitrogen compartment was marginally higher than recent estimates of f-ratio based on observations at OSP. The best simulations with Fe limitation agreed with observations of the annual cycle of surface nitrate concentration, the f-ratio, particulate nitrogen concentration in the euphotic layer, the export production, and the remineralization depth scale for sinking detritus, to within ∼50%, probably within the range of observational uncertainty and/or seasonal and interannual variability. Possible modifications include separating the detrital pool into suspended and sinking organic matter, decreasing the rate of remineralization with increasing depth, and examining the supply of nitrate to the surface layer by means of horizontal advection. The observational basis required to formulate these processes is marginal at present.  相似文献   

18.
A theoretical framework for the time-dependent processes leading to the high rates of new production in eastern boundary upwelling systems has been assembled from a series of past upwelling studies. As part of the CoOP WEST (Wind Events and Shelf Transport) study, new production in the Bodega Bay upwelling area and it's control by ambient nitrate and ammonium concentrations and the advective wind regime are described. Data and analyses are focused primarily on the WEST 2001 cruise (May–June 2001) when the two legs differed greatly in wind regimes but not nutrient concentrations. Elevated concentrations of ammonium in upwelled water with high nitrate were observed in both legs. Nitrate uptake by phytoplankton as a function of nitrate concentration was linear rather than Michaelis–Menten-like, modulated by inhibitory levels of ammonium, yielding coefficients that enable the specific nitrate uptake element of new production to be estimated from nutrient concentrations. The range of specific nitrate uptake rates for the two legs of WEST 2001 were similar, essentially a physiological response to nutrient conditions. However, the low “realization” of new production i.e. incorporation of biomass as particulate nitrogen that occurred in this system compared to the theoretical maximum possible was determined by the strong advective and turbulent conditions that dominated the second leg of the WEST 2001 study. These data are compared with other upwelling areas using a physiological shift-up model [Dugdale, R.C., Wilkerson, F.P., Morel, A. 1990. Realization of new production in coastal upwelling areas: a means to compare relative performance. Limnology and Oceanography 35, 822–829].  相似文献   

19.
Carbon budgets of the mesopelagic zone are poorly constrained, highlighting our lack of understanding of the biota that inhabit this environment and their role in the cycling and sequestering of carbon in the deep ocean. A simple food web model of the mesopelagic zone is presented that traces the turnover of particulate organic carbon (POC), supplied as sinking detritus, through to its respiration by the biota via three pathways: colonization and solubilization of detritus by attached bacteria, production of free-living bacteria following losses of solubilization products during particle degradation, and consumption by detritivorous zooplankton. The relative consumption of detritus by attached bacteria was initially specified as 76%, with the remaining 24% by detritivores. Highlighting an asymmetry between consumption and respiration, the resulting predicted share of total respiration due to bacteria was 84.7%, with detritivores accounting for just 6.6% (with 6.5% and 2.2% by bacterivores and higher zooplankton, respectively). Bacteria thus dominated respiration and thereby acted as the principal sink for POC supplied to the mesopelagic zone, whereas zooplankton mainly recycled carbon back to the base of the food web as detritus or dissolved organic carbon rather than respiring it to CO2. Estimates of respiration are therefore not necessarily a reliable indicator of the relative roles of bacteria and zooplankton in consuming and processing POC in the mesopelagic zone of the ocean. The work highlighted a number of major unknowns, including how little we know in general about the dynamics and metabolic budgets of bacteria and zooplankton that inhabit the mesopelagic zone and, specifically, the degree to which the solubilized products of enzymatic hydrolysis of POC by attached bacteria are lost to the surrounding water, the magnitude and factors responsible for bacterial growth efficiency, the role of microbes in the nutrition of detritivores, and the recycling processes by which zooplankton return what they consume to the food web as detritus and dissolved organic matter.  相似文献   

20.
A one-dimensional ecosystem model with two explicit size classes of phytoplankton was developed for the NE subarctic Pacific to investigate variations in the export of organic particles to the ocean interior due to potential changes in the environment. Specifically, the responses of the planktonic ecosystem to permanent removal of iron limitation and to warming (of 2 and 5 °C) were explored. The ecosystem model consists of five components (small and large phytoplankton, microzooplankton, detritus and nitrogen), and includes grazing by mesozooplankton that varies in time according to long-term observations at Ocean Station Papa (OSP). The model addresses the role of iron limitation on phytoplankton growth and includes temperature dependence of physiological rates. The ecosystem model was forced with annual wind and solar heating from OSP. The model best reproduced the low chlorophyll high nitrate conditions of the NE subarctic Pacific when both small and large phytoplankton were limited by iron such that their maximum specific growth rate was reduced by 10 and 70%, respectively. Sensitivity analysis showed that model results depended on the value of the iron limitation parameter of large phytoplankton (LFe-L) and the grazing parameters of micro- and mesozooplankton. To explore the effect of iron limitation, simulations were carried out varying the iron limitation parameters while maintaining the nitrogen flux at the base of the model constant and the grazing pressure by mesozooplankton unchanged. In the warming case, simulations were carried out increasing ocean temperatures by 2° and 5 °C applied only to the ecological components, the flux of nitrate at the base of the model was increased to obtain a steady annual cycle, and grazing by mesozooplankton remained constant. When compared with the standard case, model simulations indicated that both permanent removal of iron limitation and warming cause changes in food web structure and the carbon cycle. The response was more dramatic in the iron-replete case where the phytoplankton community structure in spring changed from one dominated by pico- and nanoplankton to one dominated by large phytoplankton, and primary production increased until it consumed all the external nutrient (N) supply to the upper layer. However, reducing iron deficiency actually led to lower annual primary production due to a decrease in the regeneration of nitrogen in the euphotic zone. These changes in food web structure influenced the magnitude, composition and seasonal cycle of sinking particles.  相似文献   

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