Vertical distribution of the hydrogen sulphide sources in the Black Sea

Data on the diffusion coefficientK _zand the concentration of H₂S in the Black Sea are used to compute the depth distribution of the vertical flux, and the intensity of the H₂S sources and sinks. On average, the total production of H₂S in the Black Sea reaches 37×10⁶ t/year. The main bulk of H₂S is produced not at the bottom, but in the layer of 450–1300 m. Destruction of H₂S prevails above the 400 m layer. Dissolved oxygen penetrating the H₂S zone can oxidize only half of the hydrogen sulphide produced in the sea.Translated by Mikhail M. Trufanov.     

Simulation of the impact of internal waves on the oxygen/hydrogen sulphide co-existence zone in the open part of the Black Sea

This paper focuses on the impact of periodic internal waves on the oxygen/hydrogen sulphide co-existence zone in the open Black Sea. The numerical model is based on a set of transport/diffusion equations governing the evolution of oxygen/hydrogen sulphide concentrations and considering the reaction between them. The wave velocity field is determined by solving a spectrum problem for preinertial period internal waves, using the characteristic vertical density profile for summertime. Via analysis of the model data, the influence of wave characteristics on the parameters of the O₂/H₂S co-existence zone has been assessed.Translated by Vladimir A. Puchkin.     

Indication of thiols in black sea deep water

Potentiometric titrations of deep Black Sea water give reasonably precise values of sulphide in the concentration range 30–300 μmol l⁻¹ and a strong indication of thiols in the concentration range 10–30 μmol l⁻¹. Organic analysis of Black Sea water should therefore include the search for compounds containing SH groups. A simple stoichiometric model indicates that sulphur-containing proteins might be the main source of thiols after hydrolysis and deamination. The alkalinity and total sulphide are simply related by A_t = 3287 ± 30 + (3.84 ± 0.10) [H₂ S]_t μmol kg⁻¹. The slope of 3.84 instead of the stoichiometric slope of 2.31 indicates a lack of reduced sulphate in the form of hydrogen sulphide.     

Some chemical characteristics of the brines in Bannock and Tyro Basins: salinity, sulphur compounds, Ca, F, pH, At, PO4, SiO2, NH3

Results of the chemical investigation on the Bannock and Tyro Basins are reported.Both basins were found to be hypersaline ( 10 times higher than salinity of normal seawater) and anoxic. In all investigated basins a region of transition, a few meters thick, was identified at depths > 3327 dbar. It is characterized by a sharp gradient of salinity, and all concentrations of analysed species increase strongly except for dissolved oxygen and nitrate, which immediately drop to zero. This region appears as a sharp boundary that prevents mixing. As a result, in the presence of organic matter, an anoxic condition developed with the complete depletion of dissolved oxygen. At the same time, hydrogen sulphide and ammonium accumulated within the brine. Between the Bannock and the Tyro brines differences occur in the measured concentrations of H₂S, SO²⁻₄, Ca²⁺ and NH₃. There are some differences also within the Bannock area sub-basins.The Libeccio sub-basin, in the Bannock area, contains a double-layered brine: the upper layer is 140 dbar thick and the lower layer is 300 dbar thick. A second interface between upper and lower brines develops at a depth of 3500 dbar. Nearly all of the measured concentrations vs. depth show the double layer, with the exception of ammonium, the concentration of which remains nearly constant throughout the anoxic column. Profiles of the other species analyzed show remarkable differences on passing from the upper to the lower brine. Hydrogen sulphide, sulphate and fluoride concentrations appear constant and then increase at the second interface. The calcium concentration is also constant in the upper brine, but decreases at the second interface. Total alkalinity and phosphate concentrations show a maximum peak just below the first interface. However, after passing through the second interface all the chemical parameters exhibit an almost constant behaviour down to the bottom.Hypersaline conditions are attributed to the dissolution of Messinian evaporite, and anoxia is suggested to originate from the oxidation of organic matter present in sediments and from the absence of bottom water circulation in such a deep and enclosed environment.The chemical conditions can be summarized as follows: in the Libeccio Basin the values for the species analysed have the ranges: 39–321 psu for ‘salinity’, 8.2−6.5 for pH, 2.7–4.0 mM for total alkalinity, 0.2-0 mM for dissolved oxygen, 0–1669 μM for hydrogen sulphide, 0–198 μM for thiol, 31–99 mM for sulphate, 11–21 mM for calcium, 7–100 μM for fluoride, 0.2–3080 μM for ammonium, 5.8-0 μM for nitrite, 0.2–12 μM for phosphate and 8–130 μM for silicate.     

On the sulphur chemistry of a super-anoxic fjord, Framvaren, South Norway

The concentrations of total carbonate (C_t), sulphate, sulphide, thiols and oxygen, the ratio between the stable sulphur isotopes ³⁴S and ³²S in sulphate and sulphide, and the density (used to calculate salinity) were determined on samples from the water column of Framvaren, a superanoxic fjord in southern Norway. From a depth of 18m (the oxic-anoxic boundary) the initial sulphate concentration, ([SO₄]_init), as calculated from salinity, is significantly higher than the sum of the measured sulphur species. This is attributed to a loss of sulphur from the water column. The amount of total carbonate produced, corrected for the initial concentration (C_t - 2.4 Sal/35) is found to be proportional to the amount of sulphate consumed, ([SO₄]_init - [SO₄]), according to the following relation C_t- 2.4 Sal/35 = 1.84 ([SO₄]_init - [SO₄]). Isotopic fractionation caused by bacterial sulphate reduction in the anoxic part of the water column produces sulphide with a δ³⁴S 40‰ lower than the δ³⁴S for sulphate at corresponding depths. The isotopic fractionation also results in δ³⁴S value for the remaining sulphate at depths below 80 m being considerably higher than the mean value for ocean water, which is close to + 20‰. The δ³⁴S values for sulphate at depths between 10 and 50 m were lower than + 20‰ which indicates oxidation of sulphide, which follows upon diffusion of sulphide from deeper parts of the water column and inflow of oxygenated seawater over the sill into the anoxic water of the fjord. A conclusive scenario of the Framvaren sulphur chemistry is presented.     

两种光强条件下亚心形扁藻各生长阶段的产氢能力

海洋亚心形扁藻(Platymonas subcordiformis)在光照生长后,经过暗培养诱导其可逆产氢酶,然后转移到光照下可产生氢气。在300 mL产氢反应器中对海洋亚心形扁藻产氢情况的考察表明,不同的光强条件可导致扁藻产生不同的生长曲线,而在不同的培养阶段,扁藻的产氢能力也有很大的差别。其他培养条件相同,高光照强度(E=13000 lx)条件下,扁藻生长较快,而且可以获得更大的产氢能力,300 mL密度为3×106个/mL、叶绿素质量浓度为5.8 mg/L的藻液最大产氢浓度可达17.5%,比低光照强度(E=5000 lx)时提高了32%,叶绿素含量降低了61%。根据产氢前后以及暗诱导后扁藻生理状态的考察结果,可以推测出是不同生长阶段扁藻的生理状态差异导致了其产氢能力的变化。     

The hydrogen sulphide boundary in the Black Sea in the 1920s and 1980s

Maps of the boundary topography for anaerobic waters in the Black Sea are plotted for the summers of the 1920s and 1980s. The mean depths of the H₂S boundary for this period are determined. It is shown that the variation in the distribution and average location of the hydrogen sulphide zone has remained constant for the last 60 years.Translated by Mikhail M. Trufanov.     

Requirement of endogenous iron for cytotoxicity caused by hydrogen peroxide in Euglena gracilis

It is widely thought that redox-active metals in cells such as iron or copper catalyze the reduction of hydrogen peroxide to toxic hydroxyl radicals or their equivalent. However, this has not been directly demonstrated in vivo. To probe this requirement, the freshwater microorganism Euglena gracilis was used. Its intracellular iron content can be modulated by the concentration of iron in the defined growth medium without effect on the proliferation rate of the cells. E. gracilis contains two large storage pools of cytosolic iron which can be monitored to assess cellular iron status. The toxicity of H₂O₂ in E. gracilis was inversely related to the amount of iron in the extracellular medium. At high levels of external iron, the metal carried out the Fenton reaction with H₂O₂ outside the cell, producing hydroxyl radicals as detected by electron-spin resonance spin-trapping experiments. This reaction reduced the amount of H₂O₂ that could diffuse into cells to cause toxicity. When cells with different intracellular iron content were placed in iron-deficient media, the the toxicity of H₂O₂, measured by inhibition of proliferation, was directly related to the concentration of internal iron. Using cells deficient in iron, this oxidant did not inhibit proliferation at low concentration but was somewhat effective at higher concentrations. Although the iron chelating agents, 1,10-phenanthroline and desferrioxamine, also depressed cytosolic iron, they were cytotoxic to E. gracilis and so could not be used unambiguously to examine the role of intracellular iron in the toxic effects of hydrogen peroxide.     

Modes of sapropel formation in the eastern Mediterranean: some constraints based on pyrite properties

Pyrite formation within and directly below sapropels in the eastern Mediterranean was governed by the relative rates of sulphide production and Fe liberation and supply to the organic-rich layers. At times of relatively high SO²⁻₄ reduction, sulphide could diffuse downward from the sapropel and formed pyrite in underlying sediments. The sources of Fe for pyrite formation comprised detrital Fe and diagenetically liberated Fe(II) from sapropel-underlying sediments. In organic-rich sapropels, input of Fe from the water column via Fe sulphide formation in the water may have been important as well. Rapid pyrite formation at high saturation levels resulted in the formation of framboidal pyrite within the sapropels, whereas below the sapropels slow euhedral pyrite formation at low saturation levels occurred. δ³⁴S values of pyrite are −33‰ to −50‰. Below the sapropels δ³⁴S is lower than within the sapropels, as a result of increased sulphide re-oxidation at times of relatively high sulphide production and concentration when sulphide could escape from the sediment. The percentage of initially formed sulphide that was re-oxidized was estimated from organic carbon fluxes and burial efficiencies in the sediment. It ranges from 34% to 80%, varying significantly between sapropels. Increased palaeoproductivity as well as enhanced preservation contributed to magnified accumulation of organic matter in sapropels.     

Inorganic nitrogen acquisition by the tropical seagrass Halophila stipulacea

The tropical seagrass Halophila stipulacea is dominant in most regions of the Indo‐Pacific and the Red Sea and was introduced into the Mediterranean Sea after the opening of the Suez canal. The species is considered invasive in the Mediterranean Sea and has been progressively colonizing new areas westward. Growth and photosynthetic responses of H. stipulacea have been described but no information is yet available on the nitrogen nutrition of the species. Here we simultaneously investigated the uptake kinetics of ammonium and nitrate and the internal translocation of incorporated nitrogen in H. stipulacea using ¹⁵N‐labelled substrates across a range of N_i levels (5, 25, 50 and 100 μm ). The ammonium uptake rates exceeded the nitrate uptake rates 100‐fold, revealing a limited capacity of H. stipulacea to use nitrate as an alternative nitrogen source. The uptake rates of ammonium by leaves and roots were comparable up to 100 μm ¹⁵NH₄Cl. At this concentration, the leaf uptake rate was 1.4‐fold higher (6.22 ± 0.70 μmol·g^?1 DW h^?1) than the root uptake rate (4.54 ± 0.28 μmol·g^?1 DW h^?1). The uptake of ammonium followed Michaelis–Menten kinetics, whereas nitrate uptake rates were relatively constant at all nutrient concentrations. The maximum ammonium uptake rate (V_max) and the half‐saturation constant (K_m) of leaves (9.79 μmol·g^?1 DW h^?1 and 57.95 μm , respectively) were slightly higher than that of roots (6.09 μmol·g^?1DW h^?1 and 30.85 μm , respectively), whereas the affinity coefficients (α = V_max/K_m) for ammonium of leaves (0.17) and roots (0.20) were comparable, a characteristic that is unique among seagrass species. No substantial translocation (<2.5%) of ¹⁵N incorporated as ammonium was detected between plant parts, whereas the translocation of ¹⁵N incorporated as nitrate was higher (40–100%). We conclude that the N_i acquisition strategy of H. stipulacea, characterized by a similar uptake capacity and efficiency of leaves and roots, favors the geographical expansion potential of the species into areas with variable water‐sediment N levels throughout the Mediterranean.     

Influence of the environmental factors on the intensity of the oxygen,ammonium, and phosphate metabolism in the agar-containing seaweed <Emphasis Type="Italic">Ahnfeltia tobuchiensis</Emphasis> (Ahnfeltiales,Rhodophyta)

A complex study of the influence of various environmental factors on the rate of the oxygen (M_{O 2}), ammonium (M_{NH 4}), and phosphate (M_{PO 4}) metabolism in Ahnfeltia tobuchiensis has been carried out in situ in the Izmena Bay of Kunashir Island. The following environmental factors have been included into the investigation: the photosynthetically active radiation (PAR); the ammonium (NH₄); the phosphate (PO₄); and the tissue content of carbon (C), nitrogen (N), phosphorus (P), and chlorophyll a (Chl). The population of agar-containing seaweed A. tobuchiensis forms a layer with a thickness up to 0.5 m, which occupies about 23.3 km²; the population’s biomass is equal to 125000 tons. The quantitative assessment of the organic matter production and nutrient consumption during the oxygen metabolism (M_{O 2}) has been carried out for the whole population. It has been shown that the daily rate depends on the PAR intensity, the seawater concentrations of PO₄ and NH₄, and the tissue content of N and P (r ² = 0.78, p < 0.001). The daily NH₄ consumption averages 0.21 μmol/(g_DW h) and depends on the NH₄ and O₂ concentrations in the seawater and on the C and Chl a content in the algal tissues (r ² = 0.64, p < 0.001). The daily PO₄ consumption averages 0.01 μmol/(g_DW h) and depends on the NH₄ concentration in the seawater and on the P content in the algal tissues (r ² = 0.40, p < 0.001).     

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.     

Ammonium input to the sea via large sewage outfalls-Part 2: Effects of ammonium on growth and photosynthesis of southern California phytoplankton cultures

Cultures of six marine phytoplankton were grown at ammonium concentrations ranging up to 200 μg-atom NH₄---N litre⁻¹. Only the growth of dinoflagellates, Gymnodinium splendens and Gonyaulax polyedra was inhibited at the two highest concentrations used. In 3-h photosynthetic ¹⁴CO₂ uptake experiments, only Gymnodinium was inhibited at concentrations of NH₄---N greater than 100 μg-atom litre⁻¹. We conclude that the increased ammonium concentrations found near Southern California sewage outfalls would not be inhibiting to phytoplankton in the vicinity of such outfalls.     

Ammonium uptake kinetics and interactions between nitrate and ammonium uptake inChattonella antiqua

Ammonium uptake kinetics and interactions between nitrate and ammonium uptake were examined inChattonella antiqua. After the addition of ammonium to the culture ofC. antiqua, the ammonium concentration decreased linearly with time. The ammonium uptake rate as a function of ammonium concentration followed the Michaelis-Menten equation; the maximal uptake rate was 2.0 pmol cell^–1hr^–1 and the half saturation constant, 2.2M. Although the ammonium uptake was not affected by nitrate, uptake of nitrate was rapidly (15min) suppressed by ammonium and a 50% reduction in nitrate uptake was observed at an ammonium concentration ofca. 2M.     

Contrasted sulphide chemistries in the environment of 13°N EPR vent fauna

To improve our understanding of the environmental constraints exerted on vent fauna, we investigated sulphide chemistry in the habitats of Riftia pachyptila and Alvinella pompejana, at the Genesis and Elsa EPR 13°N sites. Temperature, pH and sulphide measurement series were acquired in situ, around the organisms, from the submersible Nautile. Hot fluid samples were also collected to evaluate end-member composition at these sites. Under the assumption of conservative mixing, pH, total sulphide concentration and sulphide speciation gradients in relation to temperature were modelled. From the comparison of measured and calculated concentrations, deviation from conservative behaviour was highlighted for total sulphide versus temperature. While the observed sulphide depletion around tubeworms suggests significant subsurface removal or biological consumption, the apparent sulphide enrichment in the alvinellid environment may reveal either conductive cooling of diffusing fluids or a secondary sulphide source. The calculated sulphide speciation appears to be contrasted at the two sites studied. Because of the low iron content in Genesis fluid, iron sulphide would not constitute a dominant sulphide species and the toxic H₂S form would be predominant in the mixing zone. By contrast, iron is expected to play a dominant role in sulphide speciation at the Elsa site where the end-member is iron rich. With respect to sulphide, the conditions encountered in the different habitats considered in this study are strongly contrasted. A low fluid flux was observed in the R. pachyptila habitat, contrasting with previous ideas, and suggests that sulphide availability could be a major limiting factor. Particularly, the bioavailable HS⁻ form is expected to vary weakly along the mixing gradient. In contrast, sulphide in the A. pompejana environment is shown to be particularly high, about one order of magnitude higher than observed for other Eastern Pacific alvinellids. At Genesis, because of the acidic pH and low iron conditions encountered, exposure to high levels of toxic sulphide is expected. A. pompejana thus appears to be particularly tolerant to such toxic conditions, but, as previously suggested, less severe conditions may also be found when iron is rich enough in the medium to dominate sulphide chemistry.     

The effect of a tidal cycle on the dynamics of nutrients in a tidal estuary in the Seto Inland Sea, Japan

A 24 hour time series survey was carried out during a spring tide (tidal range ca.2 m) of May 1995 on a tidal estuary in the Seto Inland Sea, Japan, in the context of an integrated program planned to quantify the dynamics of biophilic elements (carbon, nitrogen and phosphorus) and the roles played by the macrobenthos on the processes. Three stations were set along a transect line of about 1.4 km, which linked the river to the rear to the innermost part of the subtidal zone. Every hour, at each station, measurements were made of surface water temperature, salinity and dissolved oxygen concentration, and surface water was collected for the determination of nutrients [NH₄ ⁺−N, (NO₃ ⁻+NO₂ ⁻)−N, PO₄ ³⁻−P and Si (OH)₄−Si]. During the ebb flow, riverine input of silicate and nitrate+nitrite significantly increased the concentrations of both the intertidal and the subtidal stations. Conversely, during the high tide, river nutrient concentrations were lowered by the mixing of fresh water with sea water. As a result, best (inverse) correlations were found at the river station for salinity against silicate (y=-2.9 Sal.+110.7,r ²=0.879) and nitrate+nitrite (y=-1.3 Sal.+48.4,r ²=0.796). In contrast, ammonium nitrogen concentrations were higher at intermediate salinities. Indeed, no significant correlation was found between salinity and ammonium. The effect of the macrobenthos, which is abundant on the intertidal flat, is discussed as a biological component that influences the processes of nutrient regeneration within the estuary. The effect of the tidal amplitude is an important one in determining the extent of the variations in nutrient concentrations at all three stations, which were stronger between the lower low tide and the higher high tide.     

Monitoring of a methane-seeping pockmark by cabled benthic observatory (Patras Gulf, Greece)

A new seafloor observatory, the gas monitoring module (GMM), has been developed for continuous and long-term measurements of methane and hydrogen sulphide concentrations in seawater, integrated with temperature (T), pressure (P) and conductivity data at the seafloor. GMM was deployed in April 2004 within an active gas-bearing pockmark in the Gulf of Patras (Greece), at a water depth of 42 m. Through a submarine cable linked to an onshore station, it was possible to remotely check, via direct phone connection, GMM functioning and to receive data in near-real time. Recordings were carried out in two consecutive campaigns over the periods April–July 2004, and September 2004–January 2005, amounting to a combined dataset of ca. 6.5 months. This represents the first long-term monitoring ever done on gas leakage from pockmarks by means of CH₄+H₂S+T+P sensors. The results show frequent T and P drops associated with gas peaks, more than 60 events in 6.5 months, likely due to intermittent, pulsation-like seepage. Decreases in temperature in the order of 0.1–1°C (up to 1.7°C) below an ambient T of ca. 17°C (annual average) were associated with short-lived pulses (10–60 min) of increased CH₄+H₂S concentrations. This seepage “pulsation” can either be an active process driven by pressure build-up in the pockmark sediments, or a passive fluid release due to hydrostatic pressure drops induced by bottom currents cascading into the pockmark depression. Redundancy and comparison of data from different sensors were fundamental to interpret subtle proxy signals of temperature and pressure which would not be understood using only one sensor.     

Ionization of water in seawater

The ion product of water in seawater and the total activity coefficients of hydroxide and hydrogen ions were determined over the temperature range 2° to 35°C, and the salinity range 20‰ to 44‰. At 25°C and 35‰ salinity, the measured values are pK_W^SW = 13.20, f_OH = 0.22, f_H = 0.71 on the molar concentration scale.     

Inorganic nitrogen uptake and related enzymatic activity in the seagrass Zostera noltii

The preferential inorganic nitrogen source for the seagrass Zostera noltii was investigated in plants from Ria Formosa, South Portugal. Rates of ammonium and nitrate uptake were determined at different concentrations of these nutrients (5, 25 and 50 μm ), supplied simultaneously (NH₄NO₃) or separately (KNO₃ and NH₄Cl). The activity of the enzymes nitrate reductase (NR) and glutamine synthetase (GS) was also assessed. The results showed that ammonium is the preferential inorganic nitrogen source for Z. noltii, but, in the absence of ammonium, the species also has a high nitrate uptake capacity. The simultaneous availability of both inorganic nitrogen forms enhanced the uptake rate of ammonium and decreased the uptake rate of nitrate compared to when only one of the nitrogen forms was supplied. The activity of both enzymes was much higher in the leaves than in the roots, highlighting the importance of the leaves as primary reducing sites in the nitrogen assimilation process.     

Ammonium and phosphorus flux through the microplankton community in Agulhas Bank waters

The size-related activities of important heterotrophs and autotrophs were compared at the surface and at the subsurface chlorophyll maximum (Chl_max) in Agulhas Bank waters. The netplankton fraction was dominated by Nitzschia spp. and ciliates, small (diameter c. 3μm) microflagellates being the most abundant nanoplankton group. Uptake ratios of ammonium to phosphate for the total microplankton community were different at the two depths. With reference to the Redfield ratio, it appears that at least 48 per cent of the nitrogen ration at the Chl_max was regenerated even though nitrate was in ample supply. These assimilation ratios also suggest a very large contribution from recycled nitrogen other than ammonium in surface waters. It is unlikely that phosphorus would ever become limiting, except perhaps at the primary production maximum. Microplankton uptake and regeneration of both ammonium and phosphorus were approximately in balance, indicating that variations in assimilation ratios were the result of heterotrophic excretory activity. The size-fractionation studies show that picoplankton were on average the single most important size class in nutrient assimilation. The netplankton size class was, in terms of regeneration, often the most active in the microplankton community especially within the Chl_max. Heterotrophic microflagellates and picoplankton supplied the bulk of ammonium and phosphorus at the surface. The importance of a particular size class to either ammonium or phosphorus uptake/excretion was quantified as a relative assimilation/regeneration index. These calculations demonstrate size-related differences in the relative importance of the microplanktonic groups to the immobilization and recycling of different nutrients.