Effect of mixing on microbial communities in the Rhone River plume

The biological processes involved during mixing of a river plume with the marine underlying water were studied off the Rhone River outlet. Samples of suspended and dissolved matter were collected while tracking a drifting buoy. Three trajectories were performed, at 2-day intervals, under different hydrological and meteorological situations. A biological uptake was evidenced from ammonium (NH₄) and phosphate (PO₄) shortage, indicating an early “NH₄-dependent” functioning occurring before the well-known “NO₃-based” cycle. The different ratios between NH₄, NO₃ and PO₄, as a function of salinity, were discussed to detail the preferential use in PO₄ and NH₄. Salinity zones with enhanced bacterial production, high chlorophyll a concentration, as well as DOC, NH₄ and PO₄ consumption were evidenced from 20 to 35 in salinity. It was shown that the successive abundance of bacteria and phytoplankton during transfer reflected the competition for PO₄ of both communities. On the Rhone River plume, the role played by temperature, light conditions and suspended matter upon biological activity seems relatively minor compared to salinity distribution and its related parameter: nutrient availability. It can be concluded that biological uptake in the Rhone River plume was closely related to the dilution mechanism, controlled itself by the dynamics of the plume. In windless conditions and close to the river mouth, the density gradient between marine and river water induced limited exchanges between the nutrient-rich freshwater and the potential consumers in the underlying marine water. Consequently, little biological activity is observed close to the river mouth. Offshore, mixing is enhanced and a balance is reached between salinity tolerance and nutrient availability to form a favourable zone for marine phytoplankton development. This can be quite far from the river mouth in case of a widely spread plume, corresponding to high river discharge. Under windy and wavy conditions, the plume freshwater is early and rapidly mixed, so that the extension of the “enhanced production zone” is drastically reduced and even bacteria could not benefit from the fast mixing regime induced.     

Carbon and nutrient dynamics in a hypertrophic lagoon in southwestern Taiwan

Dissolved and particulate phases of carbon (DIC, DOC, POC) and nutrients (DIN, DIP, DSi, DON, DOP, PN) were investigated bimonthly from August 1999 to August 2000 to study biogeochemical dynamics of carbon and nutrients in Tapong Bay, a small semi-enclosed and hypertrophic lagoon in southwestern Taiwan. The lagoon has only a tidal inlet for exchanging water between Tapong Bay and Taiwan Strait, which may result in low water exchange rates and various oxygen-deficient conditions in bottom water of the inner bay during warm seasons. The water exchange time of Tapong Bay ranges from 7 days (summer) to 13 days (winter) with a mean of 10 days. Nutrient dynamics were largely ascribed to allochthonous inputs, biological and exported removals in the lagoon. Diffusion fluxes from sediments to overlying water accounted for only about 7.6% of annual DIN inputs and 1.0% of annual DIP inputs. High primary productivity (89 mol C m⁻² year⁻¹) supported by abundant nutrients primarily drove the lagoon into a hypertrophic condition as particulate organic matter was derived mainly from biological production. Excess of DIP appeared to occur throughout the study period in the lagoon. Temperature, solar radiation and turbidity, rather than nutrients, perhaps controlled seasonal variations of primary productivity. The net ecosystem production (NEP) derived from daily changes of DOC and POC inventories was about 6.3 mmol C m⁻² day⁻¹ that was close to 6.7 mmol C m⁻² day⁻¹ simulated from the biogeochemical modeling. Therefore, the net ecosystem production (NEP) rate of organic carbon estimated from the biogeochemical model was reliable, and the NEP was temporally variable with an annual mean of 5.8 mol C m⁻² year⁻¹, implying that Tapong Bay was an autotrophic system. Although calcification proceeded pronouncedly in warm seasons, an invasion of CO₂ was significant in this system. In terms of nitrogen budget, the annual nitrogen fixation exceeded the annual denitrification with a magnitude of 1.30 mol N m⁻² year⁻¹, which may be supported by the abundance of nitrogen fixation microplanktons in the lagoon.     

Seasonal variation of riverine nutrient inputs in the northern Bay of Biscay (France), and patterns of marine phytoplankton response

Seasonal variations in nutrient inputs are described for the main rivers (Loire and Vilaine) flowing into the northern Bay of Biscay. The river plumes are high in N/P ratio in late winter and spring, but not in the inner plume during the summer. Conservative behavior results in most nutrients entering the estuary and eventually reaching the coastal zone. Temporal and spatial aspects of phytoplankton growth and nutrient uptake in the northern Bay of Biscay distinguish the central area of salinity 34 from the plume area. The first diatom bloom appears offshore in late winter, at the edge of the river plumes, taking advantage of haline stratification and anticyclonic “weather windows.” In spring, when the central area of the northern shelf is phosphorus-limited, small cells predominate in the phytoplankton community and compete with bacteria for both mineral and organic phosphorus. At that period, river plumes are less extensive than in winter, but local nutrient enrichment at the river mouth allows diatom growth. In summer, phytoplankton become nitrogen-limited in the river plumes; the central area of the shelf is occupied by small forms of phytoplankton, which are located on the thermocline and use predominantly regenerated nutrients.     

Large-scale physical controls on phytoplankton growth in the Irminger Sea, Part II: Model study of the physical and meteorological preconditioning

The upper water column in the Irminger Sea is characterized by cold fresh arctic and subarctic waters and warm saline North Atlantic waters. In this study the local physical and meteorological preconditioning of the phytoplankton development over an annual cycle in the upper water column in four physical zones of the Irminger Sea is investigated. Data from four cruises of the UK's Marine Productivity programme are combined with results from a coupled biological–physical nitrogen–phytoplankton–zooplankton–detritus model run using realistic forcing. The observations and model predictions are compared and analyzed to identify the key parameters and processes which determine the observed heterogeneity in biological production in the Irminger Sea. The simulations show differences in the onset of the bloom, in the time of the occurrence of the maximum phytoplankton biomass and in the length of the bloom between the zones. The longest phytoplankton bloom of 90 days duration was predicted for the East Greenland Current of Atlantic origin zone. In contrast, for the Central Irminger Sea zone a phytoplankton bloom with a start at the beginning of May and the shortest duration of only 70 days was simulated. The latest onset of the phytoplankton bloom in mid May and the latest occurrence of the maximum biomass (end of July) were predicted for the Northern Irminger Current zone. Here the bloom lasted for 80 days. In contrast the phytoplankton bloom in the Southern Irminger Current zone started at the same time as in Central Irminger Sea, but peaked end of June and lasted for 80 days. For all four zones relatively low daily (0.3–0.5 g C m^− 2d^− 1) and annual primary production was simulated, ranging between 35.6 g C m^− 2y^− 1 in the East Greenland Current of Atlantic origin zone and 45.6 g C m^− 2y^− 1 in the Northern Irminger Current zone. The model successfully simulated the observed regional and spatial differences in terms of the maximum depth of winter mixing, the onset of stratification and the development of the seasonal thermocline, and the differences in biological characteristics between the zones. The initial properties of the water column and the seasonal cycle of physical and meteorological forcing in each of the zones are responsible for the observed differences during the Marine Productivity cruises. The timing of the transition from mixing to stratification regime, and the different prevailing light levels in each zone are identified as the crucial processes/parameters for the understanding of the dynamics of the pelagic ecosystem in the Irminger Sea.     

Spatial and temporal variability of size-fractionated biomass and primary production in the Ross Sea (Antarctica) during austral spring and summer

The results of a study on the spatial and temporal dynamics of size-fractionated biomass and production of phytoplankton in the Ross Sea during the austral spring and summer are reported. The spring cruise took place in the offshore Ross Sea from 14 November to 14 December 1994. Sampling was carried out on a transect of 27 stations distributed from 76.5 to 72.0°S along 175°E, and covered the three main Antarctic environments of the polynya open waters, the marginal ice zone and the pack ice area. Three subsystems were identified. The subsystem of the polynya was characterised by the predominance of the micro- and nano-planktonic fractions, chlorophyll (Chl a) concentrations from 69.6 to 164.7 mg m⁻² and production rates from 0.68 to 1.14 g C m⁻² day⁻¹. The second subsystem, the marginal ice zone, showed a relative increase of the micro-planktonic fraction, high biomass levels (from 99.64 to 220 mg Chl m⁻²) and production rates from 0.99 to 2.7 g C m⁻² day⁻¹. The subsystem of the pack ice area had a phytoplankton community dominated by the pico-planktonic fraction and showed low biomasses (from 19.4 to 37.7 mg Chl m⁻²) and production rates (0.28 to 0.60 g C m⁻² day⁻¹). Selective grazing by krill is considered an important factor in determining the size structure of the phytoplankton communities. The summer study consisted of a time series carried out in inshore waters of Terra Nova Bay from 12 January to 8 February 1990. In a well stabilised water column and with high levels of PAR always available, the primary production rates of a community dominated by micro-plankton varied from 0.52 to 1.2 g C m⁻² day⁻¹ (average 0.84). A high P/B ratio, up to 3, and a remarkably elevated mean phaeopigment (Phaeo)/Chl a ratio of 2.4 indicated an active removal of biomass by grazing, confirmed by the presence of faecal pellets in quantities reaching 6000 m⁻³ in the upper 50 m. The peculiarities of the inshore versus offshore environments in terms of community size structure, production processes and their implications as regards the food web are discussed.     

A box model approach for a long-term assessment of estuarine eutrophication, Szczecin Lagoon, southern Baltic

We develop a layered “box model” to evaluate the major effects of estuarine eutrophication of the Szczecin lagoon which can be compared with integrating measures (chlorophyll a (Chl a), sediment burial, sediment oxygen consumption (SOC), input and output of total nutrient loads) and use it to hindcast the period 1950–1996 (the years when major increase in nutrient discharges by the Oder River took place). The following state variables are used to describe the cycling of the limiting nutrients (nitrogen and phosphorus): phytoplankton (Phy), labile and refractory detritus (D_N, D_Nref, D_P, D_Pref), dissolved inorganic nitrogen (DIN), dissolved inorganic phosphorus (DIP), and oxygen (O₂). The three layers of the model include two water layers and one sediment layer. Decrease of the carrying capacity with respect to the increased supply of organic matter of the system with advancing eutrophication over the period studied is parameterized by an exponential decrease of the sediment nitrogen fluxes with increasing burial, simulating changing properties from moderate to high accumulating sediments. The seasonal variation as well as the order of magnitude of nutrient concentrations and phytoplankton stocks in the water column remains in agreement with recent observations. Calculated annual mean values of nutrient burial of 193 mmol N m⁻² a⁻¹ and 23 mmol P m⁻² a⁻¹ are supported by observed values from geological sediment records. Estimated DIN remineralization in the sediments between 100 and 550 mmol N m⁻² a⁻¹ corresponds to SOC measurements. Simulated DIP release up to 60 mmol P m⁻² a⁻¹ corresponds to recent measurements. The conceptual framework presented here can be used for a sequential box model approach connecting small estuaries like the Szczecin lagoon and the open sea, and might also be connected with river box models.     

Analysis of the hydrophysical structure of the Sea of Azov in the period of the bottom anoxia development

The hydrophysical and hydrochemical structure of the Sea of Azov, with developed bottom anoxia, was studied during the RV “Akvanavt” cruise from July 31 to August 03, 2001. The anoxic zone with a thickness from 0.5 to 4 m above the bottom was found in all deep regions of the Sea. Concentrations of hydrochemical parameters were similar to the pronounced anoxic conditions (about 90 mmol m^− 3 of hydrogen sulfide, 17 mmol m^− 3 of ammonia, 6 mmol m^− 3 of phosphate, 7 mmol m^− 3 of total manganese). The hydrophysical structure was characterized by the uniform distribution of temperature in the upper 6–7 m mixed layer (UML). Below this a thin (0.4–0.8 m) thermocline layer was observed, just above the anoxic waters. Formation of this phenomenon was connected with that summer weather conditions. Intensive rains led to increased influx of river waters in June. That resulted in large input of allochtonous organic matter (OM) and inorganic nutrients; the latter were consumed on the additional autochthonous organic matter production. In July the weather was characterized by a significant rise in the daily averaged air temperature and large oscillations of temperature during the day. In this period a wind of constant direction was absent, but wind bursts were observed. The completed analyses showed that the formation of such a structure could be connected with the following factors: (i) positive growth trends of the daily averaged temperature and the daily oscillations of temperature, (ii) presence of wind bursts. The joint action of these factors resulted in the formation of the UML. The amplitude of wind bursts determined the depth of UML, and the value of trend determined the value of the temperature change in the thermocline. An initial presence of bottom halocline (caused by the Black Sea water influx to the bottom of the Sea of Azov) prevented the heating of the bottom layer and therefore led to an increase of vertical gradient of temperature in the thermocline. The spatial distribution of the turbulent exchange coefficient confirmed the existence of a “stagnation” area located above the anoxia zone, which is also, apparently, the reason for its occurrence.     

Dynamics of inorganic nutrient species in the Bohai seawaters

Within the frame of a Sino-German Joint Research Program, two cruises of “R/V Dong Fang Hong 2” were carried out in September–October 1998 and April–May 1999, respectively, to understand the dynamics of nutrients in the Bohai. Nutrient species (NO₃⁻, NO₂⁻, NH₄⁺, PO₄³⁻ and SiO₃²⁻) are determined colorimetrically on board for five anchor and 30 grid stations. In situ incubation experiments are performed to determine planktonic nutrient uptake and benthic exchange flux. Nutrient concentrations display short-term variability and seasonal change in the Bohai, with higher levels in shallow coastal waters than in the Central Bohai. The influence of riverine discharge on nutrient levels can be seen from salinity isopleths, nutrient distribution and species ratios. Near-bottom (nb) waters have similar nutrient concentrations as to the surface waters in the Central Bohai, whereas stratification takes place in the Bohai Strait and North Yellow Sea. In situ incubation experiments provide evidence that the uptake ratio (i.e. N, P) by phytoplankton is proportional to the ratios among nutrient species in ambient waters. Based on the data of this study and previously publications, a preliminary estimate of nutrient budgets via riverine input and atmospheric deposition is established. The results indicate that atmospheric deposition gains importance over rivers in delivering nutrients into the Bohai and sustain the new production, following recent decrease in riverine inflow caused by drought periods in North China and damming practices. A historical review of nutrient data indicates that concentrations of nitrogen increase and phosphorus and silica decrease in the Central Bohai over last 40 years. This potentially has an important influence on the health of ecosystem in Bohai (e.g. food web and community structure), though further study is needed to examine the scenario in more detail.     

Bacterial production and microbial food web structure in a large arctic river and the coastal Arctic Ocean

Globally significant quantities of organic carbon are stored in northern permafrost soils, but little is known about how this carbon is processed by microbial communities once it enters rivers and is transported to the coastal Arctic Ocean. As part of the Arctic River-Delta Experiment (ARDEX), we measured environmental and microbiological variables along a 300 km transect in the Mackenzie River and coastal Beaufort Sea, in July–August 2004. Surface bacterial concentrations averaged 6.7 × 10⁵ cells mL^− 1 with no significant differences between sampling zones. Picocyanobacteria were abundant in the river, and mostly observed as cell colonies. Their concentrations in the surface waters decreased across the salinity gradient, dropping from 51,000 (river) to 30 (sea) cells mL^− 1. There were accompanying shifts in protist community structure, from diatoms, cryptophytes, heterotrophic protists and chrysophytes in the river, to dinoflagellates, prymnesiophytes, chrysophytes, prasinophytes, diatoms and heterotrophic protists in the Beaufort Sea.Size-fractionated bacterial production, as measured by ³H–leucine uptake, varied from 76 to 416 ng C L^− 1 h^− 1. The contribution of particle-attached bacteria (> 3 µm fraction) to total bacterial production decreased from > 90% at the Mackenzie River stations to < 20% at an offshore marine site, and the relative importance of this particle-based fraction was inversely correlated with salinity and positively correlated with particulate organic carbon concentrations. Glucose enrichment experiments indicated that bacterial metabolism was carbon limited in the Mackenzie River but not in the coastal ocean. Prior exposure of water samples to full sunlight increased the biolability of dissolved organic carbon (DOC) in the Mackenzie River but decreased it in the Beaufort Sea.Estimated depth-integrated bacterial respiration rates in the Mackenzie River were higher than depth-integrated primary production rates, while at the marine stations bacterial respiration rates were near or below the integrated primary production rates. Consistent with these results, P_CO2 measurements showed surface water supersaturation in the river (mean of 146% of air equilibrium values) and subsaturation or near-saturation in the coastal sea. These results show a well-developed microbial food web in the Mackenzie River system that will likely convert tundra carbon to atmospheric CO₂ at increasing rates as the arctic climate continues to warm.     

Estimates of Southern Ocean primary production—constraints from predator carbon demand and nutrient drawdown

In view of the wide range of estimates for the total primary production for the Southern Ocean south of the Subantarctic Front—current estimates range from 1.2 to 3.5 Gtonne C year⁻¹—we have examined two indirect methods for assessing primary production. First, we have estimated the primary production needed to sustain the carbon requirements of the endotherm top predators in the ecosystem. Estimation of the carbon requirements for crabeater seals of about 7 Mtonne C year⁻¹ is extrapolated to a value for all endotherm predators of 15–30 Mtonne C year⁻¹. Current data indicate that 70–80% of the diet of this suite of predators is zooplankton (predominantly the euphausiid krill), making for highly efficient transfer from primary production to top predators. Our best estimate of Southern Ocean primary production by this method is of the order of 1.7 Gtonne C year⁻¹, or an averaged areal primary production of about 30–40 g C m⁻² year⁻¹. Our second approach is to estimate primary production from the drawdown of inorganic nutrients, based on the limited suite of studies from which an annual nutrient deficit can be calculated. Again, this indicates annual primary production of the order of 1.5 Gtonne. Although both methods have inherent uncertainties, taken together they provide a relatively robust constraint on annual primary production. For both methods to underestimate primary production by the 1–1.5 Gtonne C implied by the higher current estimates, carbon export from the Southern Ocean pelagic ecosystem would need to be much higher than is normally found in other oceans.     

Modelling the silica pump in the Permanently Open Ocean Zone of the Southern Ocean

A coupled 1D physical–biogeochemical model has been built to simulate the cycles of silicon and of nitrogen in the Indian sector of the Permanently Open Ocean Zone of the Southern Ocean. Based on a simplified trophic network, that includes two size classes of phytoplankton and of zooplankton, and a microbial loop, it has been calibrated by reference to surface physical, chemical and biological data sets collected at the KERFIX time-series station (50°40′S–68°25′E). The model correctly reproduces the high nutrient low chlorophyll features typical of the studied area. In a region where the spring–summer mixed layer depth is usually deeper than 60 m, the maximum of chlorophyll never exceeds 1.5 mg m⁻³, and the annual primary production is only 68 g C m⁻² year⁻¹. In the surface layer nitrate is never exhausted (range 27–23.5 mmoles m⁻³) while silicic acid shows strong seasonal variations (range 5–20 mmoles m⁻³). On an annual basis 71% of the primary production sustained by nanophytoplankton is grazed by microzooplankton. Compared to North Atlantic, siliceous microphytoplankton is mainly prevented from blooming because of an unfavourable spring–summer light-mixing regime. Silicic acid limitation (high half saturation constant for Si uptake: 8 mmoles m⁻³) also plays a major role on diatom growth. Mesozooplankton grazing pressure excerpts its influence especially in late spring. The model illustrates the efficiency of the silica pump in the Southern Ocean: up to 63% of the biogenic silica that has been synthetized in the photic layer is exported towards the deep ocean, while only 11% of the particulate organic nitrogen escapes recycling in the surface layer.     

Nutrient cycling and primary production in the marine systems of the Arctic and Antarctic

Primary production events in both the Arctic and the Antarctic are highly localized. Carbon-14 incubations that did not account for this caused antarctic primary production estimates to be revised too far downwards from the historic view of high productivity. The primary production regime in the Arctic is even more heterogeneous than in the Antarctic. Arctic primary production rates are in the process of being revised upwards because of a better spatial and temporal distribution of incubation experiments and a re-awakening of interest in estimating new production from the distribution of chemical variables. Similarly, recent examination of temporal changes in nitrate concentrations and recognition of the importance of ice-edge blooms has caused antarctic primary productivity to be revised upwards. In both the Arctic and the Antarctic, the ratio of “new” to total primary production is high, and neglect of this fact can lead to an underestimation of the potential that these regions have for influencing global cycles of bioactive chemicals. Some recent data on temporal changes in nitrate from Fram Strait emphasize the poor state of our knowledge by suggesting an unexpectedly high “new” production rate of 1 g C m⁻² d⁻¹ for a 35 day experiment that encountered an early Phaeocystis bloom. Chemical distributions suggest that new production over the shelf seas that border the Polar Basin is about 50 g Cm⁻² yr⁻¹.The shelves in the Arctic Ocean's marginal and adjacent seas comprise 25% of the total global continental shelf. These extensive shallow regions have much higher rates of primary production than the Polar Basin and may be globally significant sites of denitrification. Globally significant silica deposition could occur on these shelves or on the adjacent slopes.Because of the differences in geomorphology and stratification, global warming is likely to increase primary production in the Arctic and will probably decrease antarctic primary production.In addition to sharing high ratios of “new” to total primary production, high ammonium concentrations occur in the Arctic and Antarctic. It is possible that these accumulations arise from a strong repression of nitrification at low temperatures.     

Influence of the Mackenzie River plume on the sinking export of particulate material on the shelf

We examined the influence of the Mackenzie River plume on sinking fluxes of particulate organic and inorganic material on the Mackenzie Shelf, Canadian Arctic. Short-term particle interceptor traps were deployed under the halocline at 3 stations across the shelf during fall 2002 and at 3 stations along the shelf edge during summer 2004. During the two sampling periods, the horizontal patterns in sinking fluxes of particulate organic carbon (POC) and chlorophyll a (chl a) paralleled those in chl a biomass within the plume. Highest sinking fluxes of particulate organic material occurred at stations strongly influenced by the river plume (maximum POC sinking fluxes at 25 m of 98 mg C m^− 2 d^− 1 and 197 mg C m^− 2 d^− 1 in 2002 and 2004, respectively). The biogeochemical composition of the sinking material varied seasonally with phytoplankton and fecal pellets contributing considerably to the sinking flux in summer, while amorphous detritus dominated in the fall. Also, the sinking phytoplankton assemblage showed a seasonal succession from a dominance of diatoms in summer to flagellates and dinoflagellates in the fall. The presence of the freshwater diatom Eunotia sp. in the sinking assemblage directly underneath the river plume indicates the contribution of a phytoplankton community carried by the plume to the sinking export of organic material. Yet, increasing chl a and BioSi sinking fluxes with depth indicated an export of phytoplankton from the water column below the river plume during summer and fall. Grazing activity, mostly by copepods, and to a lesser extent by appendicularians, appeared to occur in a well-defined stratum underneath the river plume, particularly during summer. These results show that the Mackenzie River influences the magnitude and composition of the sinking material on the shelf in summer and fall, but does not constitute the only source of material sinking to depth at stations influenced by the river plume.     

Nitrogenous discharges to the eastern Gulf of Finland, the Baltic Sea: Elemental flows, stable isotope signatures, and their estuarine modification

We studied the nutrient input to the Gulf of Finland via River Neva, the largest river discharging freshwater to the Baltic Sea, and characterised the isotopic signatures (¹⁵N, ¹⁸O, ¹³C) in dissolved and particulate substances (NO₃⁻, PON, POC, DIC) in the River Neva over two seasonal cycles, as well as in samples from St. Petersburg wastewater treatment plants (NO₃⁻, NH₄⁺, PON, POC). These riverine and municipal discharges account for 40% of terrestrial inorganic N loading to the Gulf of Finland, representing annually 7% of the total nitrogen pool in the water mass of the whole Gulf. To describe and evaluate the modification of these isotopic signals along a Gulf of Finland transect towards the Baltic Proper, two cruises were arranged, one in late spring after the annual maximum in River Neva runoff, and one in autumn, in the late phase of the annual growth season.River Neva nitrate signatures of ¹⁵N and ¹⁸O indicated major agricultural fertilizer origin of nitrogen, and the isotopic composition was clearly lighter (δ¹⁵N-NO₃⁻ mean of 2.4‰ air) than previously measured from more southern rivers discharging into the Baltic Sea. Because of the light composition of the River Neva N source, close to the ¹⁵N signatures of the open Gulf, as well as of the efficient depletion of the inorganic load already in the innermost estuary, straightforward end-member tracer analysis of the transport of N in the basin is problematic. St. Petersburg wastewater ammonium showed, however, high δ¹⁵N values (ca. 13‰), which gives a first estimate of 5.8‰ for δ¹⁵N of the easternmost estuarine total inorganic N source. The available sediment data from the basin (δ¹⁵N 6 to 8‰) somewhat exceeds the average source signature. This emphasizes the significance of biological transformation processes, most importantly assimilation of inorganic nitrogen, food web interactions and denitrification, which all involve isotopic fractionation, for the mass balance models describing the dynamics of the sources and sinks of the N cycle of the basin.     

Production of Acartia omorii (Copepoda: Calanoida) in Ilkwang Bay, southeastern coast of Korea

Production of the marine calanoid copepod Acartia omorii was measured from 2 October 1991 to 8 October 1992 at a station in Ilkwang Bay on the southeastern coast of Korea. A. omorii (nauplii + copepodites + adults) were present in the plankton throughout the year, with seasonal variation in abundance. Biomass of A. omorii was averaged at 0.44 mgC m^− 3, with peaks in February and July, and relatively low biomass in late summer and fall. Egg production rate ranged from 2.4 to 151.9 μgC m^− 3 day^− 1, which was equivalent to 95–6075 eggs m^− 3 day^− 1. Fecundity of an adult female was averaged at 38 eggs female^− 1 day^− 1. Instantaneous growth rates of copepodites were higher than those of nauplii stages. Annual production of A. omorii ranged from 33.5 mgC m^− 3 year^− 1 to 221 mgC m^− 2 year^− 1, showing a seasonal variation of daily production rate with peaks in February and July. The daily production rate of A. omorii was significantly correlated with chlorophyll a concentration. These results suggest that standing stocks and/or productivity of phytoplankton are the major influencing factors, rather than water temperature for the seasonal variation of production of A. omorii in Ilkwang Bay.     

Vertical profiles of particulate organic matter and its relationship with chlorophyll-a in the upper layer of the NE Mediterranean Sea

Particulate organic matter (POM), nutrients, chlorophyll-a (CHL) and primary production measurements were performed in the upper layer of three different regions (cyclonic, anticyclonic and frontal+peripherial) of the NE Mediterranean Sea in 1991–1994. Depth profiles of bulk POM exhibited a subsurface maximum, coinciding with the deep chlorophyll maximum (DCM) established near the base of the euphotic zone of the Rhodes cyclone and its periphery, where the nutricline was situated just below the euphotic zone for most of the year. Moreover, the POM peaks were broader and situated at shallower depths in late winter–early spring as compared to its position in the summer–autumn period. Under prolonged winter conditions, as experienced in March 1992, the characteristic POM feature disappeared in the center of the Rhodes cyclone, where the upper layer was entirely occupied by nutrient-rich Levantine deep water. Deep convective processes in the cyclonic gyre led to the formation of vertically uniform POM profiles with low concentrations of particulate organic carbon (POC) (2.1 μM), nitrogen (0.21 μM), total particulate phosphorus (PP) (0.02 μM) and chlorophyll-a (0.5 μg/L) in the euphotic zone. Though the Levantine deep waters ascended up to the surface layer with the nitrate/phosphate molar ratios (28–29) in March 1992, the N/P molar ratio of bulk POM in the upper layer was low as 10–12, indicating luxury consumption of phosphate during algal production. Depth-integrated primary production in the euphotic zone ranged from 38.5 for oligotrophic autumn to 457 mg C m⁻² day⁻¹ for moderately mesotrophic cool winter conditions.     

Remotely-sensed chlorophyll a observations of the northern Red Sea indicate seasonal variability and influence of coastal reefs

The biological dynamics of the open northern Red Sea (21.5°–27.5° N, 33.5°–40° E) have not been studied extensively, due in part to both the inaccessibility of this desert region and political considerations. Remotely-sensed chlorophyll a data therefore provide a framework to investigate the primary patterns of biological activity in this oceanic basin. Monthly chlorophyll a data from the 8-year Sea-viewing Wide Field-of-View sensor (SeaWiFS) mission, and data from the Moderate Resolution Imaging Spectroradiometer (MODIS), were analyzed with the Goddard Earth Sciences Data and Information Services Center (GES DISC) online data analysis system “Giovanni”. The data indicate that despite the normal low chlorophyll a concentrations (0.1–0.2 mg m^− 3) in these oligotrophic waters, there is a characteristic seasonal bloom in March–April in the northernmost open Red Sea (24° to 27.5° N) concurrent with minimum sea surface temperature. The location of the highest chlorophyll concentrations is consistent with a linear box model [Eshel, G., and Naik, N.H., 1997. Climatological coastal jet collision, intermediate water formation, and the general circulation of the Red Sea. J. Phys. Oceanogr. 27(7), 1233–1257.] of Red Sea circulation. Two years in the data set exhibited a different seasonal cycle consisting of a relatively weak northern spring bloom and elevated chlorophyll concentrations to the south (21.5° to 24° N).The data also indicate that large coral reef complexes may be sources of either nutrients or chlorophyll-rich detritus and sediment, enhancing chlorophyll a concentration in waters adjacent to the reefs.     

Turbulence in coastal fronts near the mouths of Block Island and Long Island Sounds

Measurements of turbulence were performed in four frontal locations near the mouths of Block Island Sound (BIS) and Long Island Sound (LIS). These measurements extend from the offshore front associated with BIS and Mid-Atlantic Bight Shelf water, to the onshore fronts near the Montauk Point (MK) headland, and the Connecticut River plume front. The latter feature is closely associated with the major fresh water input to LIS. Turbulent kinetic energy (TKE) dissipation rate, ε, was obtained using shear probes mounted on an autonomous underwater vehicle. Offshore, the BIS estuarine outflow front showed, during spring season and ebb tide, maximum TKE dissipation rate, ε, estimates of order 10^− 5 W/kg, with background values of order 10^− 6 to 10^− 9 W/kg. Edwards et al. [Edwards, C.A., Fake, T.A., and Bogden, P.S., 2004a. Spring–summer frontogenesis at the mouth of Block Island Sound: 1. A numerical investigation into tidal and buoyancy-forced motion. Journal of Geophysical Research 109 (C12021), doi:10.1029/2003JC002132.] model this front as the boundary of a tidally driven, baroclinically adjusted BIS flow around the MK headland eddy. At the entrance to BIS, near MK, two additional fronts are observed, one of which was over sand waves. For the headland site front east of MK, without sand waves, during ebb tide, ε estimates of 10^− 5 to 10^− 6 W/kg were observed. The model shows that this front is at the northern end of an anti-cyclonic headland eddy, and within a region of strong tidal mixing. For the headland site front further northeast over sand waves, maximum ε estimates were of order 10^− 4 W/kg within a background of order 10^− 7–10^− 6 W/kg. From the model, this front is at the northeastern edge of the anti-cyclonic headland eddy and within the tidal mixing zone. For the Connecticut River plume front, a surface trapped plume, during ebb tide, maximum ε estimates of 10^− 5 W/kg were obtained, within a background of 10^− 6 to 10^− 8 W/kg. Of all four fronts, the river plume front has the largest finescale mean-square shear, S² ~ 0.15 s^− 2. All of the frontal locations had local values of the buoyancy Reynolds number indicating strong isotropic turbulence at the dissipation scales. Local values of the Froude number indicated shear instability in all of the fronts.     

A conceptual model of the strongly tidal Columbia River plume

The Columbia River plume is typical of large-scale, high discharge, mid-latitude plumes. In the absence of strong upwelling winds, freshwater from the river executes a rightward turn and forms an anticyclonic bulge before moving north along the Washington coast. In addition to the above dynamics, however, the river plume outflow is subject to large tides, which modify the structure of the plume in the region near the river mouth. Observations based on data acquired during a summer 2005 cruise indicate that the plume consists of four distinct water masses; source water at the lift-off point, and the tidal, re-circulating and far-field plumes. In contrast to most plume models that describe the discharge of low-salinity estuary water into ambient high-salinity coastal water, we describe the Columbia plume as the superposition of these four plume types.We focus primarily on a conceptual summary of the dynamics and mutual interaction of the tidal and re-circulating plumes. The new tidal plume flows over top of the re-circulating plume and is typically bounded by strong fronts. Soon after the end of ebb tide, it covers roughly 50–100% of the re-circulating plume surface area. The fronts may penetrate well below the re-circulating plume water and eventually spawn internal waves that mix the re-circulating plume further. The re-circulating plume persists throughout the tidal cycle and corresponds to a freshwater volume equivalent to 3–4 days of river discharge. Finally, the plume water masses are distinguished from one another in term of surface chlorophyll concentration, suggesting that the above classification may also describe different biological growth regimes. The low-salinity re-circulating plume serves as an extension of the estuary into the coastal ocean, or an “estuary at sea”, because residence times during periods of high river flow are greater than those in the estuary.     

On the distribution of dissolved lead in the Loire estuary and the North Biscay continental shelf, France

The dissolved lead was studied in the whole salinity gradient of the system composed of the Loire estuary and the North Biscay continental shelf. About 130 samples were collected in winter 2001 and spring 2002 during Nutrigas and Gasprod campaigns (Programme PNEC-Golfe de Gascogne, RV Thalassa) and metal measurements were conducted on board by Potentiometric Stripping Analysis. In the Loire estuary, levels of dissolved lead ranged from 0.15 to 0.24 nM and from 0.04 to 0.26 nM in winter and spring, respectively. Compared to the concentrations reported in 1987 and 1990 (0.4–1.7 nM; Boutier, B., Chiffoleau, J.F., Auger, D., Truquet, I., 1993. Influence of the Loire river on dissolved lead and cadmium concentrations in coastal waters of Brittany. Estuar. Coast. Shelf S., 36:133–143, Estuarine, Coastal and Shelf Science 36, 133–143) our study indicated much lower values. The fall in concentration in the estuary could be attributed to the stopping of activity of Octel, a big manufacturer of tetra alkyl lead. Discharge in dissolved metal to the continental shelf by the Loire river was assessed as 7.5 and 1.9 kg day^− 1 for winter and spring, respectively. On the continental shelf, levels of dissolved lead varied within 0.06 and 0.27 nM in winter (0.15 ± 0.06 nM, sd = 1.96, n = 49), whereas concentrations measured in spring were in the range 0.06–0.17 nM (0.09 ± 0.03 nM, sd = 1.96, n = 60). This difference in metal concentration was related to the amounts of rainfall that have fallen over the continental shelf: estimations of inputs by this way (74 and 32 kg day^− 1 in winter and spring, respectively) appeared to be significantly higher than inputs from the Loire river (7.5 and 1.9 kg day^− 1 in winter and spring, respectively). The distributions of dissolved metal in the surface waters highlighted the role of suspended particular matter (SPM) for a rapid “trapping” of lead near the mouth of the estuary. The vertical distributions showed, in the stratified area, a biological transfer of lead between winter and spring from surface waters to the halocline.