Modelling the response of the planktonic food web to iron fertilization and warming in the NE subarctic Pacific

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 (L_Fe-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.     

Doubled CO2 impact on subarctic marine carbon cycle: a case study at Ocean Station P

Along with meteorological observations, complementary and systematic oceanographic observations of various physical, biological and chemical parameters have been made at Ocean Station P (OSP) (50°N, 145°W) since the early 1950s. These decadal time scale data have contributed to a better understanding of the physical, biological and chemical processes in the surface layer of the northeastern subarctic region of the Pacific Ocean. These data have demonstrated the importance of the North Pacific in the global carbon cycle and, in particular, the role of biological/chemical processes in the net exchange of CO₂ across the air–sea interface. Although we do not fully comprehend how climatic variations influence marine communities or marine biogeochemistry, previous studies have provided some basic understanding of the mechanisms controlling the seasonal and inter-annual variations of biological and chemical parameters (such as phytoplankton, bacteria, nitrate/ammonium concentration) at OSP, and how they affect the carbon cycling in the subarctic North Pacific. In this study, we investigate how these mechanisms might alter the seasonal variations of these parameters at OSP under a 2XCO₂ condition. We examine these influences using a new biological model calibrated by the climatological data from OSP. For the 2XCO₂ simulation, the biological model is driven off line (i.e., no feedback to the ocean/atmospheric model components) by the climatology plus 2XCO₂−1XCO₂ outputs from a global surface ocean model and the Canadian GCM. Under the 2XCO₂ condition, the upper layer ocean shows an increase in the entrainment rate at the bottom of the mixed layer for OSP during the late autumn and winter seasons, resulting in an increase in the f-ratio. Although there is an overall increase in the primary production (PP) by 3–18%, a decrease in the biomass of small phytoplankton and microzooplankton (due to mesozooplankton grazing) lowers the concentration of dissolved organic matter (DOM) by 4–25%. The model also predicts a significant increase in the concentrations of nitrate and ammonium, and in bacterial production during July and August. Doubling of the atmospheric CO₂ from 330 to 660 ppm forces the marine pCO₂ to increase by about 63%, much of which is driven by an increased flux of CO₂ from the atmosphere to the oceans.     

A physical–biochemical model of plankton productivity and nitrogen cycling in the Black Sea

A one-dimensional, vertically resolved, physical–biochemical upper ocean model is utilized to study plankton productivity and nitrogen cycling in the central Black Sea region characterized by cyclonic gyral circulation. The model is an extension of the one given by Oguz et al. (1996, J. Geophys. Res. 101, 16585–16599) with identical physical characteristics but incorporating a multi-component plankton structure in its biological module. Phytoplankton are represented by two groups, typifying diatoms and flagellates. Zooplankton are also separated into two groups: microzooplankton (nominally <200 μm) and mesozooplankton (0.2–2 mm). The other components of the biochemical model are detritus and nitrogen in the forms of nitrate and ammonium. The model incorporates, in addition to plankton productivity and organic matter generation, nitrogen remineralization (ammonification) and ammonium oxidation (nitrification) in the water column. Numerical simulations are described and compared with the available data from the central Black Sea. The main seasonal and vertical characteristics of phytoplankton and nutrient dynamics inferred from observations appear to be reasonably well represented by the model. Fractionation of the biotic community structure is shown to lead to increased plankton productivity during the summer period following the diatom-based early spring (March) bloom. The annual nitrogen budget for the euphotic zone reveals the substantial role of recycled nitrogen in the surface waters of the Black Sea.     

Bottom-up control of phytoplankton growth in spring blooms in Central Yellow Sea,China

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 (K_I=0.062–1.730). The external nutrient pools (K_E=<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 (K_R=<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.     

Phytoplankton distribution in the Agulhas system from a coupled physical–biological model

The greater Agulhas Current system has several components with high mesoscale turbulence. The phytoplankton distribution in the southwest Indian Ocean reflects this activity. We have used a regional eddy-permitting, coupled physical–biological model to study the physical–biological interactions and to address the main processes responsible for phytoplankton distribution in three different biogeochemical provinces: the southwest Subtropical Indian Gyre (SWSIG), the subtropical convergence zone (SCZ) and the subantarctic waters (SAW) south of South Africa. The biological model with four compartments (Nitrate–Phytoplankton–Zooplankton–Detritus) adequately reproduces the observed field of chlorophyll a. The phase of the strong modelled seasonality in the SWSIG is opposite to that of the SCZ that forms the southern boundary of the subtropical gyre. Phytoplankton concentrations are governed by the source-minus-sink terms, which are one order of magnitude greater than the dynamical diffusion and advection terms.North of 35°S, in the SWSIG, phytoplankton growth is limited by nutrients supply throughout the year. However, deeper stratification, enhanced cross-frontal transport and higher detritus remineralization explain the simulated higher concentrations of phytoplankton found in winter in the SWSIG. The region between 35° and 40°S constitutes a transition zone between the SCZ and the oligotrophic subtropical province. Horizontal advection is the main process bringing nutrients for phytoplankton growth. The front at 34°S represents a dynamical barrier to an extension further to the north of this advection of nutrients.Within the SCZ, primary production is high during spring and summer. This high productivity depletes the nutrient standing stock built up during winter time. In winter, nutrients supply in the convergence zone is indeed large, but the deep mixing removes phytoplankton from the euphotic zone and inhibits photosynthesis, yielding lower surface chlorophyll a concentrations.Waters south of the Subantarctic Front have a summer biomass close to that of frontal waters and higher than for subtropical waters. However, these simulated concentrations are slightly higher than the observed ones suggesting that limitation by iron and/or silica may play a role.     

The downward flux of biogenic material in the NE subarctic Pacific: importance of algal sinking and mesozooplankton herbivory

In the present study we examine factors that affect the downward flux of biogenic carbon in the NE subarctic Pacific, one of the important high-nutrient-low-chlorophyll (HNLC) regions in the open ocean. We focus on the role of mesozooplankton, since their seasonal peaks in biomass and growth are in phase with the seasonal variations in the downward POC fluxes, whereas phytoplankton biomass is more-or-less uniform year-round. The relative importance of mesozooplankton and algal sinking was examined using the pigment composition of material accumulated in short-term free-drifting sediment traps positioned just below the upper stratified surface layer (ca. 100–200 m). This was compared with the phytoplankton composition in the surface waters, and with the grazing activity (gut pigments and fecal pellet production rates) of the most abundant large copepods. We also examined whether the relationships between the downward flux of carbon and pelagic processes were similar in the coastal, continental margin and offshore HNLC regions of the NE subarctic Pacific, the latter represented by Ocean Station Papa (OSP).Our results show that grazing had a variable impact on the downward flux of biogenic carbon. Carbon-transformed pheopigments (particularly pyropheophorbide a, frequently associated with copepod grazing) represented up to 13% of the total downward POC flux inshore (in May 1996) and 8–9% at OSP in May and February 1996, respectively. This flux of pheopigments was accompanied by a large potential input of fecal pellets from large copepods (as estimated from defecation rates of freshly collected animals) only in May 1996 at OSP, suggesting that pheopigments came from other sources (other herbivores, senescing algae) in February. The larger flux of pheopigments in May was probably related to the abundance of mesozooplankton at that time of the year. During summer (August 1996), both the flux of pheopigments and the potential input of fecal pellets from large copepods were negligible at OSP, consistent with more intense pelagic recycling reported in other studies. Inshore, the flux of carbon-transformed pheopigments was slightly higher than at OSP, and its contribution to the downward POC flux in May 1996 was twice that in August 1996. In contrast, the potential input of feces carbon was higher in August than in May 1996, again suggesting other sources for pheopigments found in the traps. The contribution of sinking phytoplankton to the downward biogenic flux was negligible in summer, when prymnesiophytes (indicated by the presence of 19′-hexanoyloxyfucoxanthin) and pelagophytes (19′-butanoyloxyfucoxanthin-containing) dominated in surface offshore waters. The contribution of sinking algae was maximal (9%) in winter (February 1996) at OSP, when fucoxanthin (mainly a diatom marker) dominated the carotenoid composition in the traps and when the abundance of diatoms in surface waters showed its seasonal maximum for this station. Inshore, the low contribution of diatoms (fucoxanthin) to the sinking fluxes may have resulted from inadequate sampling (i.e. the spring bloom may have been missed).Overall, we conclude that: (a) large copepods significantly influenced the downward POC flux only during spring at OSP; (b) unidentified herbivores (e.g. salps, pteropods) producing pigmented, fast-sinking fecal material likely had an important impact during winter; (c) algal sinking made a small contribution to the downward POC flux (maximum in winter); and (d) neither algal sinking nor mesozooplankton grazing had a significant influence on the downward flux of biogenic material in summer at OSP.     

Nitrogen isotopic fractionation of particulate organic matter production and remineralization in the Prydz Bay and its adjacent areas

During the 29th Chinese National Antarctic Research Expedition, spatial variations in nitrogen isotopic composition of particulate nitrogen (δ15N_PN) and their controlling factors were examined in detail with regard to nitrate drawdown by phytoplankton and particulate nitrogen (PN) remineralization in the Prydz Bay and its adjacent areas. To better constrain the nitrogen transformations, the physical and chemical parameters, including temperature, salinity, nutrients, PN and δ15N_PN in seawater column were measured from surface to bottom. In addition, the nitrogen isotopic fractionation factor of nitrate assimilation by phytoplankton in the mixed layer, and the nitrogen isotopic fractionation factor of PN remineralization below the mixed layer were estimated using Rayleigh model and Steady State model, respectively. Our results showed that suspended particles had its lowest δ15N_PN in the surface layer, which was due to the preferential assimilation of 14N in nitrate by phytoplankton. The δ15N_PN in the mixed layer of the Prydz Bay and its adjacent areas decreased from the inner shelf to the outer basin, ascribing to the effect of isotope fractionation during phytoplankton assimilation. In mixed layer, the spatial distribution of δ15N_PN associated with particulate organic matter (POM) production can be well interpreted according to Rayleigh model and Steady State model. The nitrogen isotope fractionation factor during phytoplankton assimilating nitrate was estimated as 10.0‰ by Steady State model, which was more reasonable than that calculated by Rayleigh model. These results validate the previous reports of fractionation factor during nitrate assimilation by phytoplankton. Increasing δ15N_PN with depth below the euphotic zone correlated with the decreasing PN contents, and it was attributed to preferential remineralization of 14N in PN by bacteria. In subsurface and deep layer, the δ15N_PN distributions also conformed to Rayleigh model and Steady State model during PN remineralization, with a fractionation factor of about 3.6‰ and 3.2‰, respectively. It is the first time to estimate the fractionation factor during POM production and remineralization in the Prydz Bay and its adjacent areas. Such fractionation may provide a useful tool for the follow-up study of the nitrogen dynamics in the Southern Ocean.     

A biophysical NPZ model with iron for the Gulf of Alaska: Reproducing the differences between an oceanic HNLC ecosystem and a classical northern temperate shelf ecosystem

Modeling the coastal Gulf of Alaska (CGOA) is complicated by the highly diverse physical and biological features influencing productivity and energy flow through the region. The GOA consists of the offshore oceanic environment, characterized by iron limitation, high-nutrients and low-chlorophyll. The coastal environment is consistently downwelling, with high iron levels from glacial melt water and runoff, but lower concentrations of macronutrients, and with a spring bloom, nutrient depletion cycle (low-nutrient, high-chlorophyll). Cross-shelf movement of water masses mixes coastal and oceanic ecosystem elements.Simulations and field data indicate that the minimum model complexity necessary to characterize lower trophic-level production and biomass in the offshore and coastal regions includes 10 boxes: iron, nitrate, ammonium, small phytoplankton, large phytoplankton, small microzooplankton, large microzooplankton, small copepods, large oceanic copepods and detritus, with copepod mortality as a model closure term. We present the model structure, equations required (and initial parameters used) to simulate onshore and offshore lower trophic-level production in the Gulf of Alaska, along with the information from field data and simulations used to construct the model. We show the results of simulations with and without iron, and with and without two size classes of phytoplankton. These simulations indicate that our method of inclusion of iron works well to distinguish the coastal and the oceanic ecosystems, and that the inclusion of two size categories of phytoplankton is also necessary to generate the differences between these two ecosystems.     

Beneath the surface: Characteristics of oceanic ecosystems under weak mixing conditions – A theoretical investigation

This study considers an important biome in aquatic environments, the subsurface ecosystem that evolves under low mixing conditions, from a theoretical point of view. Employing a conceptual model that involves phytoplankton, a limiting nutrient and sinking detritus, we use a set of key characteristics (thickness, depth, biomass amplitude/productivity) to qualitatively and quantitatively describe subsurface biomass maximum layers (SBMLs) of phytoplankton. These SBMLs are defined by the existence of two community compensation depths in the water column, which confine the layer of net community production; their depth coincides with the upper nutricline. Analysing the results of a large ensemble of simulations with a one-dimensional numerical model, we explore the parameter dependencies to obtain fundamental steady-state relationships that connect primary production, mortality and grazing, remineralization, vertical diffusion and detrital sinking. As a main result, we find that we can distinguish between factors that determine the vertically integrated primary production and others that affect only depth and shape (thickness and biomass amplitude) of this subsurface production layer. A simple relationship is derived analytically, which can be used to estimate the steady-state primary productivity in the subsurface oligotrophic ocean. The fundamental nature of the results provides further insight into the dynamics of these “hidden” ecosystems and their role in marine nutrient cycling.     

Phytoplankton dynamics in the NE subarctic Pacific

Ocean Station Papa (OSP, 50°N 145°W) in the NE subarctic Pacific is characterised as high nitrate low chlorophyll (HNLC). However, little is known about the spatial extent of these HNLC waters or the phytoplankton dynamics on the basin scale. Algal biomass, production and size-structure data are presented from winter, spring and summer between 1992 and 1997 for five stations ranging from coastal to open-ocean conditions. The inshore stations (P04–P16) are characterised by the classical seasonal cycle of spring and late summer blooms (production >3 g C m⁻² d⁻¹), diatoms are not Fe-stressed, and growth rate is probably controlled by macronutrient supply. The fate of the phytoplankton is likely sedimentation by diatom-dominated spring blooms, with a pelagic recycling system predominating at other times. The offshore stations (P20/OSP) display low seasonality in biomass and production (OSP, mean winter production 0.3 g C m⁻² d⁻¹, mean spring/summer production 0.85 g C m⁻² d⁻¹), and are dominated by small algal cells. Low Fe availability prevents the occurrence of diatom blooms observed inshore. The main fate of phytoplankton is probably recycling through the microbial food web, with relatively low sedimentation compared to inshore. However, the supply of macro- and micro-nutrients to the coastal and open ocean, respectively, may vary between years. Variability in macro-nutrient supply to the coastal ocean may result in decreased winter reserve nitrate, summer nitrate limitation, subsequent floristic shifts towards small cells, and reduced primary production. Offshore, higher diatom abundances are occasionally observed, perhaps indicating episodic Fe supply. The two distinct oceanic regimes have different phytoplankton dynamics resulting in different seasonality, community structure and fate of algal carbon. These differences will strongly influence the biogeochemical signatures of the coastal and open-oceanic NE subarctic Pacific.     

Microzooplankton and mesozooplankton in an upwelling filament off Galicia: modelling and sensitivity analysis of the linkages and their impact on the carbon dynamics

This paper reports estimates of trophic flows of carbon off the Galician coast from a 1D ecological model, which are compared with field data from a two week Lagrangian drift experiment. The model consists of 9 biological components: nitrate, ammonium, >5μm phytoplankton, <5μm phytoplankton, heterotrophic nanoflagellates/dinoflagellates (5–20 μm), heterotrophic dinoflagellates (>20 μm), ciliates, fast sinking detritus and slow sinking detritus. Calculations were made for the fluxes of carbon between biological components within the upper 45m of the water column. The temporal development of primary production during the simulation period of two weeks was in good agreement with field estimates, which varied between 248 and 436mgC.m⁻².d⁻¹. Heterotrophic nanoflagellates had the greatest impact on carbon flux, with a grazing rate of 168mgC.m⁻².d⁻¹. Herbivorous grazing by microzooplankton amounted to 215mgC.m⁻².d⁻¹, whereas grazing by copepods on phytoplankton was 35mgC.m⁻² d⁻¹. Copepods grazing on microzooplankton was minor (0.47mgC.m⁻².d⁻¹) and the export flux from the upper 45m was 302mgC.m⁻².d⁻¹. Sensitivity analyses, in which the grazing parameters (i.e the functional relationship between ingestion and food concentration) were changed, were carried out on the heterotrophic dinoflagellate, ciliate and heterotrophic nanoflagellates/dinoflagellate components of the model. These changes did not alter the temporal development of heterotrophic nanoflagellates/dinoflagellates biomass significantly, but ciliates and heterotrophic dinoflagellates were more sensitive to variations in the grazing parameters. The overall conclusion from this modelling study is that the coupling between small phytoplankton and heterotrophic nanoflagellates was the quantitatively most important process controlling carbon flow in this region.     

Numerical effects on organic-matter sedimentation and remineralization in biogeochemical ocean models

We investigate the effects of different vertical grid resolutions and algorithms for the calculation of particle sinking on the sedimentation and remineralization of particulate organic matter. Simulations carried out with an idealized 1D model of detritus sinking show that a coarse vertical resolution, such as used in many global biogeochemical models, tends to enhance the particle flux through numerical mixing within the vertical boxes, and thereby simulates deeper remineralization, compared to a model with a fine vertical resolution. This effect can be ameliorated by assuming a distribution of detritus within the individual grid boxes that corresponds to the prescribed sinking and remineralization parameters. Experiments of the different flux algorithms, carried out with 3D global biogeochemical models of different vertical grid resolution reveal impacts on simulated biogeochemical tracer distributions that are similar to those obtained by substantial variations in biogeochemical model parameters. Our results indicate that numerical schemes have to be considered when comparing biogeochemical parameter values of different models and also when porting biogeochemical models among different circulation models.     

The chemical speciation of iron in the north-east Atlantic Ocean

The distribution of dissolved iron and its chemical speciation (organic complexation and redox speciation) were studied in the northeastern Atlantic Ocean along 23°W between 37 and 42°N at depths between 0 and 2000 m, and in the upper-water column (upper 200 m) at two stations further east at 45°N10°W and 40°N17°W in the early spring of 1998. The iron speciation data are here combined with phytoplankton data to suggest cyanobacteria as a possible source for the iron binding ligands. The organic Fe-binding ligand concentrations were greater than that of dissolved iron by a factor of 1.5–5, thus maintaining iron in solution at levels well above it solubility. The water column distribution of the organic ligand indicates in-situ production of organic ligands by the plankton (consisting mainly of the cyanobacteria Synechococcus sp.) in the euphotic layer and a remineralisation from sinking biogenic particles in deeper waters. Fe(II) concentrations varied from below the detection limit (<0.1 nM) up to 0.55 nM but represented only a minor fraction of 0% to occasionally 35% of the dissolved iron throughout the water column. The water column distribution of the Fe(II) suggests biologically mediated production in the deep waters and photochemical production in the euphotic layer. Although there was no evidence of iron limitation in these waters, the aeolian iron input probably contributed to a shift in the phytoplankton assemblage towards increased Synechococcus growth.     

Microzooplankton composition,biomass and grazing rates along the WOCE SR3 line between Tasmania and Antarctica

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.     

High feeding rates on large particles by Neocalanus flemingeri and N. plumchrus,and consequences for phytoplankton community structure in the subarctic Pacific Ocean

The open subarctic Pacific Ocean is a high nitrate low chlorophyll (HNLC) system characterized by low concentrations of phytoplankton, a community dominated by small cells, and iron-limited growth of, especially, the larger phytoplankton. In such systems the main energy and material flow is through the microbial web, with large copepods considered primarily to be grazers on the larger microzooplankton occupying the top of this web. Consistent with this is the recognition that much of the nutrition of the dominant copepods in this system, Neocalanus flemingeri, N. plumchrus and N. cristatus, is derived from microzooplankton. Also, these copepods consume only a small fraction of the total phytoplankton production. In this paper, we show that the contribution made by N. flemingeri and N. plumchrus to establishing and maintaining the community structure of this ecosystem should be re-evaluated. Our experiments indicate these grazers have high clearance rates on large particles, including both large phytoplankton and microzooplankton, and this selective removal contributes to establishment and maintenance of the observed foodweb structure in the Gulf of Alaska. These high feeding rates combined with large populations of these two Neocalanus species concentrated in the upper layer of the ocean, result in population-based feeding rates approximately equal to the growth rates of large phytoplankton under iron-limited conditions. We conclude that N. flemingeri and N. plumchrus populations (a) directly prevent the accumulation of large phytoplankton cells by selectively feeding on them at high rates, and (b) indirectly stimulate the accumulation of the smaller phytoplankton by consumption of their major grazers, the microzooplankton.     

Preliminary study on different nutrient pools supplies for the phytoplankton growth in the Jiaozhou Bay in China in the fall of 2004

The source and significance of two nutrients, nitrogen and phosphorous, were investigated by a modified dilution method performed on seawater samples from the Jiaozhou Bay, in autumn 2004. This modified dilution method accounted for the phytoplankton growth rate, microzooplankton grazing mortality rate, the internal and external nutrient pools, as well as nutrient supplied through remineralization by microzooplankton. The results indicated that the phytoplankton net growth rate increased in turn from inside the bay, to outside the bay, to in the Xiaogang Harbor. The phytoplankton maximum growth rates and microzooplankton grazing mortality rates were 1.14 and 0.92 d-1 outside the bay, 0.42 and 0.32 d-1 inside the bay and 0.98 and 0.62 d-1 in the harbor respectively. Outside the bay, the remineralized nitrogen (Kr=24.49) had heavy influence on the growth of the phytoplankton. Inside the bay, the remineralized phosphorus(Kr=3.49) strongly affected the phytoplankton growth. In the harbor, the remineralized phosphorus (Kr=3.73) was in larger demand by phytoplankton growth. The results demonstrated that the different nutrients pools supplied for phytoplankton growth were greatly in accordance with the phytoplankton community structure, microzooplankton grazing mortality rates and environmental conditions. It is revealed that nutrient remineralization is much more important for the phytoplankton growth in the Jiaozhou Bay than previously believed.     

Simulations of Annual Cycle of Phytoplankton Production and the Utilization of Nitrogen in the Yellow Sea

A nutrient dynamic model coupled with a 3D physical model has been developed to study the annual cycle of phytoplankton production in the Yellow Sea. The biological model involves interactions between inorganic nitrogen (nitrate and ammonium), phosphate and phytoplankton biomass. The model successfully reproduces the main features of phytoplankton-nutrient variation and dynamics of production. 1. The well-mixed coastal water is characterized by high primary production, as well as high new production. 2. In summer, the convergence of tidal front is an important hydrodynamic process, which contributes to high biomass at frontal areas. 3. The evolution of phytoplankton blooms and thermocline in the central region demonstrate that mixing is a dominant factor to the production in the Yellow Sea. In this simulation, nitrate- and ammonium-based productions are estimated regionally and temporally. The northern Yellow Sea is one of the highly ranked regions in the Yellow Sea for the capability of fixing carbon and nitrogen. The annual averaged f-ratio of 0.37 indicates that regenerated production prevails over the Yellow Sea. The result also shows that phosphate is the major nutrient, limiting phytoplankton growth throughout the year and it can be an indicator to predict the bloom magnitude. Finally, the relative roles of external nutrient sources have been evaluated, and benthic fluxes might play a significant role in compensating 54.6% of new nitrogen for new production consumption.     

Co-limitation of phytoplankton growth by light and Fe during winter in the NE subarctic Pacific Ocean

Phytoplankton acclimate to low irradiance by increasing their cellular demand for Fe, to allow synthesis of additional light-harvesting pigments and Fe-containing redox proteins involved in photosynthesis. In the open NE subarctic Pacific, Fe concentrations limit primary productivity and irradiances may be suboptimal, particularly during winter. Phytoplankton thus may be unable to fulfill their extra Fe requirements for growth under these low-light conditions and become effectively co-limited. We tested this hypothesis by manipulating Fe and light in in vitro experiments at OSP (Ocean Station PAPA, 50°N 145°W) during winter 1997. The results show that metabolic rates, growth, and photosynthetic parameters of phytoplankton are enhanced in winter by increasing either irradiance or Fe. The greatest response occurs when Fe and light are amended concomitantly, confirming that the community is indeed co-limited by both resources. Analysis of environmental conditions (i.e. incident irradiance, mixed layer depth and Fe concentrations) in winter at OSP reveals that they are similar to those observed in the austral spring and fall at three sites in the Southern Ocean. Extrapolating our experimental field results to the Southern Ocean illustrates that co-limitation by light and Fe also may play an important role in regulating phytoplankton growth in this region.     

Factors controlling silicon and nitrogen biogeochemical cycles in high nutrient,low chlorophyll systems (the Southern Ocean and the North Pacific): Comparison with a mesotrophic system (the North Atlantic)

A 1-D coupled physical-biogeochemical model is used to study the seasonal cycles of silicon and nitrogen in two High Nutrient Low Chlorophyll (HNLC) systems, the Antarctic Circumpolar Current (ACC) and the North Pacific Ocean, and a mesotrophic system, the North Atlantic Ocean. The biological model consists of nine compartments (diatoms, nano-flagellates, microzooplankton, mesozooplankton, two types of detritus, nitrate, ammonium and silicic acid) forced by irradiance, temperature, mixing and deep nitrate and silicic acid concentrations. At all sites, nanophytoplankton standing crop variations are low, in spite of variations in primary production, because of a “top–down” control by microzooplankton. Although nanophytoplankton sustain more than 60% of the annual primary production in these areas, their contribution to the export production does not exceed 1% of the total. The differences in the seasonal plankton cycle among these regions come mainly from differences in the dynamics of large phytoplankton (here diatoms). In the ACC, the chlorophyll maximum remains <1.5 mg m⁻³, as an unfavourable light/mixing regime and a likely trace-metal limitation keep diatoms from blooming. In the northeast Pacific, trace-metal limitation seems to keep diatoms from blooming throughout the year. In both these systems, light or iron limitations induce high Si/N uptake ratios. Incidentally these high Si/N uptake ratios lead to a net excess of silicic acid utilization over nitrate, and to a subsequent silicic acid limitation during the summertime. In the North Atlantic, under favourable light/mixing regime and nutrient-replete conditions at the onset of the growing period, diatoms outburst and sustain a bloom >3.5 mg Chl-a m⁻³. Thereafter, mesozooplankton grazing pressure and silicic acid limitation induce the collapse of the chlorophyll maximum and the persistence of lower chlorophyll concentrations in summer. Although the ACC and the North Pacific show HNLC features, they support a high biogenic silica production (1.9 and 1.07 mol Si m⁻² yr⁻¹) and export flux (0.79 and 0.61 mol Si m⁻² yr⁻¹), compared to the North Atlantic (production: 0.23 mol Si m⁻² yr⁻¹, export: 0.12 mol Si m⁻² yr⁻¹). The differences in Si production and export between the HNLC systems and the mesotrophic North Atlantic come from both higher Si concentrations and Si/N uptake ratios in the HNLC areas compared to the North Atlantic. Also, the low dissolution rate of biogenic silica compared to nitrogen degradation rate, and the inhibition of nitrate uptake by ammonium, reinforce the net excess of silicic acid utilization over nitrate. As a result, the model also illustrates the efficiency of the silica pump for the three sites: about 50% of the biogenic silica synthesized in the euphotic layer is exported out of the first 100 m, while only 4–11% of the particulate organic nitrogen escapes recycling in the surface layer.     

Distribution of microzooplankton in the Philippine Sea and the Celebes Sea in summer, 1972

Regional and vertical distribution of the microzooplankton in the Philippine and the Celebes Seas is reported in relation to the phytoplankton distribution. The maximum concentration of chlorophylla occurred at the surface in the Celebes Sea and in subsurface layer (50–150 m depth) in the Philippine Sea. On the other hand, the maximum occurrence of the microzooplankton was observed in the subsurface layer (50–150 m) throughout these sea areas; discrepancy in the vertical positions of the chlorophylla and microzooplankton maxima was observed in the former sea area. The higher dominancy of large-sized phytoplankton such as diatoms andTrichodesmium at the surface maximum, probably because most large-sized phytoplankton were uningestible for the microzooplankton, was the main reason why the discrepancy existed in the Celebes Sea. In the Philippine Sea, where the subsurface chlorophylla maximum layer was formed mainly by small-sized phytoplankton such as coccolithophorids and small dinoflagellates, such a discrepancy was not observed. These may indicate the establishment of a close food relationship between the microzooplankton and the small-sized phytoplankton rather than to the large-sized phytoplankton.