Abstract: As part of the SHEBA/JOIS drift experiment, we continually analysed abundance and biomass of autotrophic and heterotrophic microbes in the upper 120 m of the water column of the ice-covered Central Arctic Ocean from November 1997 through August 1998. Microbial biomass was concentrated in the upper 60 m of the water column. There were low but persistent stocks of heterotrophic and autotrophic microbes during the winter months. Phytoplankton biomass began increasing when winter snow melted from the ice-pack in early June, after which there was a progressive decline of nitrate and silicate in the euphotic zone. We observed three distinct blooms over the summer. The initial bloom consisted of diatoms and phytoflagellates, mainly 2 μm-sized Micromonas sp.; the two subsequent blooms were dominated by the flagellated (non-colonial) Phaeocystis sp. The carbon:chlorophyll ratio of the phytoplankton was 31±11. Stocks of bacteria and heterotrophic protists approximately doubled during the growing season, increasing in tandem with increase in phytoplankton biomass. Increase in cell abundances of bacteria and of the phytoflagellate Micromonas over 40–50 d periods during the initial bloom period yielded estimates of realised growth rate of 0.025 d −1 for bacteria and of 0.11 d −1 for Micromonas . Heterotrophic protists included flagellates, ciliates, and dinoflagellates, with biomass divided nearly evenly between nanoplankton (Hnano, 0–20 μm) and microplankton (Hmicro, 20–200 μm) size classes.

Abstract: The oligotrophic waters of the Subtropical Gyres cover >60% of the total ocean surface and contribute >30% of the global marine carbon fixation. Despite apparently uniform growth conditions over broad areas, primary production in these regions exhibits a remarkable degree of variability. In this study of 34 stations in the North and South Atlantic Subtropical Gyres, we found a 20 fold variation (from 18 to 362 mgC m-2 d-1) in water-column-integrated primary production rate (UPP), while chlorophyll biomass only varied by a factor of 3. The changes in productivity were not associated with variations in incident surface irradiance, chlorophyll concentration, phytoplankton C biomass or phytoplankton size structure. The rate of nutrient supply to the euphotic layer, as estimated from variations in the depth of nitracline, appeared as the most relevant environmental factor in explaining the observed variability in UPP. We found significant changes in the composition of the picophytoplankton community across the range of measured productivities. The relative biomass contribution of Synechococcus spp. and the picoeukaryotes tended to increase with increasing UPP, whereas the opposite was true for Prochlorococcus spp. Across the wide range of measured primary productivity rates, the persistent dominance of picophytoplankton indicates that the microbial loop and the microbial food web continued to be the most important trophic pathways. Our observations of the oligotrophic ocean reflect a dynamic ecosystem where the microbial community responds to environmental forcing with significant changes in biological rates rather than trophic organization.

Abstract: An investigation of vertical distribution, rhythms of vertical migration and division of epipelic free-living diatom algae was carried out at different sediment depths on intertidal sandflat in the Chernaya River Estuary (Kandalaksha Gulf, White Sea, Russia) during the summers of 1994-2001. The depth of penetration of viable microalgae cells in the sediment did not exceed 4.2 cm in the areas with clay sublayers, where zones with high negative Eh values were located closer to the surface. Diatom habitats in the areas with coarse sand sublayers extended to more than 8 cm depth. Approximately 40% of diatoms were present in the topmost 2 mm layer. The subsurface portion of diatom cells on average was concentrated within the layer with Eh values ranging from 0 to -80 mV. Eh values below -100 mV limited diatom penetration in deep anoxic sediment layers. The deep portion of diatom populations consisted basically of dividing cells. A cytological analysis of diatom cell distribution showed that the proportion of cells in the different phases of mitosis increased with increasing depth, and reached 80 to 90% at a depth of 2 to 3 cm. The regular presence of dividing diatom cells almost exclusively in aphotic anoxic sediments was noted first. The study of the dynamics of microphytobenthic vertical distribution revealed active diurnal vertical migration depending on temperature, light and tide regimes. The frequency of doublet cells (FDC) in epipelic diatom populations was in inverse proportion to cell concentrations in the photic zone. Diatom divi- sion was registered only during incubation in the porewater, which was rich in inorganic nutrients and dissolved organic matter. We suppose that a more stable habitat with higher concentrations of nutrients (especially nitrogen) in their more reduced forms (NH4 + instead of NO3 - ) provides more favourable conditions for cell growth and division at the depth of several cm within the sediment compared, to the surface layer. Migration activity of diatom algae is an adaptation which permits cells to consume energy and to undergo fission under optimum conditions in spatially disconnected zones.

Abstract: Long-term trends in the Secchi depth of the Baltic Sea have been interpreted in terms of eutrophication (1, 2). The spectral attenuation coefficient Kd (490) can be estimated from remote sensing data (3). Given the empirical and theoretical relationships between diffuse attenuation and Secchi depth, it is therefore possible to estimate the trophic state from remote sensing data. This paper considers relationships among remotely sensed and in-water measured Kd (490), and Secchi depth data obtained during dedicated sea-truthing campaigns in the eastern Baltic Proper in 1999 (4) and in the western Baltic Proper/Himmerfjarden area during 2001 and 2002. In-water measurements are used to establish the relationship between the PAR and the spectral attenuation coefficient in the Baltic Sea via regression analysis. The analysis showed that in the area of investigation Kd (490) is about 1.48 times higher than Kd (PAR). This relationship is then used to define the link between the photic zone depth and the ...

Abstract: subtropical latitude, with a general high species diversity and a well-defined depth distribution. It is in fact possible to recognize an upper photic zone assemblage, dominated by E. huxleyi and characterized by higher concentration and species diversity and a lower photic zone where typically deep-living species (i.e., F. profunda, G. flabellatus )a re present. These two zones are separated by a transition layer, where species of both zones are represented and new ones appear. Such vertical distribution appears to be strictly related to the local hydrology, with the zone boundaries rising and falling as a function of the location of the isotherms. In particular the first significant occurrence of F. profunda from surface to the deep photic zone corresponds with the start of the thermocline. Comparison of present plankton data with the surface sediment record, although displaying a consistent pattern of species assemblage, shows some differences in the presence and relative abundance of some species (G. oceanica): this can be related to seasonal as well as interannual variations in the pattern and intensity of surface circulation in the investigated area. INDEX TERMS: 4855 Oceanography: Biological and Chemical: Plankton; 4815 Oceanography: Biological and Chemical: Ecosystems, structure and dynamics; KEYWORDS: eastern Mediterranean Sea, coccolithophorids, ecology, phytoplankton Citation: Malinverno, E., P. Ziveri, and C. Corselli, Coccolithophorid distribution in the Ionian Sea and its relationship to eastern Mediterranean circulation during late fall to early winter 1997, J. Geophys. Res., 108(C9), 8115, doi:10.1029/2002JC001346, 2003.

Abstract: A rise in global temperatures could potentially lead to less ice in the Arctic, including a reduction in the ice-covered period. The consequence of a changing ice cover on the food web structure and production in Disko Bay, Western Greenland, is analysed through application of a dynamical model for the planktonic food web. The model is successfully calibrated and tested for sensitivity, using a detailed data set for 1996–1997. Model scenarios are (1) extended ice cover and (2) no ice. These scenarios are compared to model runs with measured ice cover in two normal years. In the extended ice scenario, assuming unchanged copepod behaviour, copepods are starving or feeding in the ice/water interface from the time they ascend to the surface layer from over-wintering depths until the ice break-up in June. The total annual primary production reaches the same level as it does in the average year, but copepod ingestion and, as a consequence, vertical carbon export is reduced by app. 40%. In the ice-free situation, an early diatom bloom is initiated by stratification of the water in March, before the copepods ascend. The diatom bloom is grazed upon by protozooplankton, which reach a high biomass before the copepods ascend in April. Annual primary production increases by 52% while copepod ingestion and vertical loss of carbon is reduced by 57%. This study illustrates how a change in the ice cover in Arctic areas can potentially create a mismatch between spring primary production and copepod grazers. The result may be a planktonic food web dominated by protozooplankton, resulting in lower export of organic material out of the photic zone despite increased primary productivity, or alternatively lead to changes in species composition or behaviour.

Abstract: Mesoscale eddies have been shown to be a common feature of open ocean regions such as the Sargasso Sea. By lifting nutrients up to the lower portions of the euphotic zone, these eddies can cause episodic phytoplankton blooms that can lead to substantial new production. In the Sargasso Sea, it has been estimated that such blooms can account for 35%‐50% of annual new production. In the present study, it was shown that a common large diatom Stephanopyxis palmeriana is capable of growing in a low-light environment typical of the bottom 50 m of the euphotic zone at rates sufficiently high to sustain the contemporary estimates of new production that have been attributed to mesoscale eddies. The diatom was grown in laboratory batch experiments at irradiance levels from 11 to 79 mmol photons m 22 s 21 , equivalent to irradiance levels occurring in the Sargasso Sea during the summer at depths from ;100 m (the 1% light level) up to 50 m. Resulting growth rates were compared with growth rates from the literature for similar large diatoms, and a simple model of new production was developed to show the dependency of new production on specific growth rate. Included in the model were other important parameters such as the depth of nutrient incursion resulting from lifting of the thermocline when an eddy passes, the duration of the bloom, and the size of the prebloom diatom population. Using realistic ranges for these parameters, it was evident from the model that there is no physiological constraint on these large diatoms from growing fast enough at very low light levels to meet the new production estimates resulting from eddies.

Abstract: The vertical distribution and temporal changes in aggregate abundance and sizes were measured in the Ross Sea, Antarctica, during two field seasons, one in austral spring 1994 and one in early summer, 1995/96. Aggregate abundance, size and potential sinking rates were determined by photographic techniques. Measurements ofwater column parameters, including particulate organic carbon concentrations, were assessed simultaneously, as was the flux oforganic matter with floating sediment traps. The numbers ofaggregates (and to a lesser extent their size) increased with time, although there was substantial spatial variability in these variables at any point in time. Some aggregates appeared to sink extremely rapidly, and for these, our photographic measurements were able to estimate only a minimum sinking rate, which equaled 288 m d � 1 . Estimates ofaggregate organic carbon flux were compared to those determined by floating sediment traps. From these results, aggregate fluxes appear to have dominated the vertical export oforganic matter f the euphotic zone. The genesis and flux ofaggregates in the Ross Sea are the critical processes controlling the export of biogenic material from the surface layer. r 2003 Elsevier Science Ltd. All rights reserved.

Abstract: A recently published evaluation of bacterioplankton abundance and productivity in the bathypelagic North Pacific suggests that these properties are generally coupled with particulate organic carbon (POC) fluxes. In that analysis, bacterial biomass and productivity were several-fold greater in subarctic than subtropical waters, consistent with the basin-scale distribution of POC flux and suggestive of a sinking POC -e DOC -X bacteria transformation of the carbon. To test this hypothesis, we sought to determine whether the very strong spatial and temporal gradients in POC flux in the Arabian Sea would force similar deep-ocean gradients in bacterial variables. On both a withinand between-cruise basis, there was variability in bacterial abundance and thymidine incorporation in the deep Arabian Sea, but correspondence was equivocal between these variables and several correlates to export: flux of biogenic carbon from the euphotic zone, state of the monsoon, and proximity to productive coastal upwelling zones. However, when annual mean bacterial abundance at 2,000 m was compared with annual POC flux at that depth, a strong correspondence emerged: high annual flux supported high bacterial abundance (such a correspondence was not found for bacterial productivity). This finding suggests that bathypelagic bacterial abundance responds to the long-term mean input of organic matter and less to episodic inputs. A comparative evaluation of the North Pacific revealed that although the bathypelagic bacteria there showed correspondence to deep POC flux, that variable alone would not account for the wide meridional variations in bacterial abundance that have been reported.

Abstract: On average, the water column of Lake Superior is undersaturated with respect to dissolved oxygen and supersaturated with respect to carbon dioxide during the summer-stratified period. On the basis of temporal changes in water column dissolved oxygen, we calculate rates of oxygen consumption that range from 0.19 to 0.75 mmol m 3 d 1 . These rates are a factor of 5–10 times larger than can be supported by the particulate carbon settling rates and benthic oxygen consumption rates. In addition, on the basis of the limited information available, dissolved allochthonous carbon inputs are insufficient to account for the calculated rates of carbon oxidation. Rates of nitrate and total CO2 (CO2) production are 0.019 0.012 and 0.13 0.06 mmol m 3 d 1 , respectively, and are consistent with the oxidation of a dissolved organic component that is similar in composition (C:N ratio) to the settling particulate material. Previously published estimates of total primary production were smaller but similar in magnitude to our integrated water column respiration rates. We interpret the observed imbalance between particulate carbon delivered to the deep lake and the calculated rate of carbon oxidation to be the result of the decomposition of dissolved organic carbon that appears to have both an autochthonous and an allochthonous component. A simplistic view of aquatic ecosystem carbon cycling is that photosynthetic production, limited by the availability of one or more major nutrients, generates a large pool of fixed (autochthonous) carbon in the euphotic zone. Most of that organic material is ‘‘recycled’’ within the euphotic zone with some, typically small, fraction being ‘‘exported’’ out of the euphotic zone. A portion of this export production is then remineralized within the deep water column or within the sediments, and the balance is permanently buried (e.g., Dymond et al. 1996 and references therein). The record of this buried ‘‘residual’’ production serves as the basis for paleoclimate studies. For these studies, variations in the accumulation rate of biogenic material in lacustrine systems are

Abstract: Under an apparent monotony characterized by low phytoplankton biomass and production, the Pacific equatorial system may hide great latitudinal differences in plankton dynamics. On the basis of 13 experiments conducted along the 180° meridian (8°S–8°N) from upwelled to oligotrophic waters, primary production was strongly correlated to chlorophyll a (chl a), and the productivity index PI (chl a-normalized production rate) varied independently of macronutrient concentrations. Rates of total (14C uptake) and new (15N-NO3 uptake) primary production were measured in situ at 3°S in nutrient-rich advected waters and at 0° where the upwelling velocity was expected to be maximal. Primary production was slightly higher at the equator, but productivity index profiles were identical. Despite similar NO3 concentrations, new production rates were 2.6 times higher at 0° than at 3°S, in agreement with much higher concentrations of biogenic particulate silica and silicic acid uptake rates (32Si method) at the equator. Furthermore, phytoplankton carbon concentrations from flow cytometric and microscopical analyses were used with pigment and production values to assess C:chl a ratios and instantaneous growth rates (μ). Growth rates in the water column were significantly higher, and C:chl a ratios lower at 0° than at 3°S, which is consistent with the more proximate position of the equatorial station to the source of new iron upwelling into the euphotic zone. For the transect as a whole, compensatory (inverse) changes of C:chl a and μ in response to varying growth conditions appear to maintain a high and relatively invariant PI throughout the equatorial region, from high-nutrient to oligotrophic waters.

Abstract: A coupled physical–biological model was developed to simulate the low-silicate, high-nitrate, and low-chlorophyll (LSHNLC) conditions in the equatorial Pacific Ocean and used to compute a detailed budget in the Wyrtki box (5°N–5°S, 180–90°W) for the major sources and cycling of nitrogen and silicon in the equatorial Pacific. With the incorporation of biogenic silicon dissolution, NH 4 regeneration from organic nitrogen and nitrification of ammonia in the model, we show that silicon recycling in the upper ocean is less efficient than nitrogen. As the major source of nutrients to the equatorial Pacific, the Equatorial Undercurrent provides slightly less Si(OH) 4 than NO 3 to the upwelling zone, which is defined as 2.5°N–2.5°S. As a result, the equatorial upwelling supplies less Si(OH) 4 than NO 3 into the euphotic zone in the Wyrtki box, having a Si/N supply ratio of about 0.85 (2.5 vs. 2.96 mmol m −2 day −1 ). More Si(OH) 4 than NO 3 is taken up with a Si/N ratio of 1.17 (2.72 vs. 2.33 mmol m −2 day −1 ) within the euphotic zone. The difference between upwelling supply and biological uptake is balanced by nutrient regeneration and horizontal advection. Excluding regeneration, the net silicate and nitrate uptakes are nearly equal (1.76 vs. 1.84 mmol m −2 day −1 ). However, biogenic silica export production is slightly higher than organic nitrogen (1.74 vs. 1.59 mmol m −2 day −1 ) following a 1.1 Si/N ratio. In the central equatorial Pacific, low silicate concentrations limit diatom growth; therefore non-diatom new production accounts for most of the new production. Higher silicate supply in the east maintains elevated diatom growth rates and new production associated with diatoms dominate upwelling zone. In contrast, the new production associated with small phytoplankton is nearly constant or decreases eastward along the equator. The total new production has a higher rate in the east than in the west, following the pattern of surface silicate. This suggests that silicate regulates the diatom production, total new production, and thereby carbon cycle in this area. The modeled mean primary production is 48.4 mmol C m −2 day −1 , representing the lower end of direct field measurements, while new production is 15.0 mmol C m −2 day −1 , which compares well with previous estimates.

Abstract: Surveys of bacterial biomass (38–673 mg C m−2), bacterial production (6–179 mg C m−2 d−1), primary production (17–2079 mg C m−2 d−1) as well as other hydrographic variables within the mixed layer or euphotic zone were conducted in the East China Sea (ECS) shelf in December 1997 and March 1998 (cold seasons; SST, 10–26°C) as well as June and October 1998 (warm seasons; SST, 21–30°C). The results showed that bacterial rate parameters and biomass were regulated independently. Substrate supply and temperature might be the two most important factors interactively affecting the spatial and seasonal patterns of bacterial rate parameters. However, their relative importance shifted with season and location. During warm seasons, when SST was high (>20°C), integrated bacterial productivity (IBP; 9–179 mg C m−2 d−1) and turnover rate (Bμ; 0.03–0.37 d−1) over the entire shelf were dominated by substrate supply, as judged from their positive relationships with integrated primary productivity (IPP; 18–2079 mg C m−2 d−1). Multiple regression analysis indicated that during cold seasons, spatial variations of IBP (6–59 mg C m−2 d−1) and Bμ (0.06–0.23 d−1) were explained better by temperature inside, and substrate supply outside, the middle-shelf. Annual average of IBP:IPP ratio was 17±13%, which is close to the global average of 25%. Bacterial carbon demand (BCD) estimated by two independent approaches yielded similar values of 430 and 482 mg C m−2 d−1. This implies that bacteria might consume organic carbon equivalent to seasonal averaged IPP (370 mg C m−2 d−1; Gong et al., 2003). Substrate sources from non-algal components plays an important role in supporting BCD in the ECS shelf. For all four seasons, IBP:IPP ratios (6–86%) were negatively correlated with IPP, suggesting a greater response of phytoplankton to inorganic nutrient inputs than that of bacteria to organic substrate supply. Possible mechanisms and implications are considered.

Abstract: A full understanding of the biogeochemical cycling of silica in the North Atlantic is hampered by a lack of estimates of silica uptake by phytoplankton. We applied the 32 Si radiotracer incubation technique to determine silica uptake rates at 10 sites during the UK-(Natural Environment Research Council) Faroes‐Iceland‐Scotland hydrographic and environmental survey (FISHES) cruise in the Northeast Atlantic, May 2001. Column silica uptake rates ranged between 6 and 166 mmol Si m 22 d 21 ; this data set was integrated with concurrent hydrographic, chemical, and primary productivity data to explain these changes in silica uptake in terms of the progress of the spring bloom. In order to interpret data covering a relatively large spatial and temporal scale, we used mean photic zone silica concentration as a proxy time-series measure of diatom bloom progression. Both absolute and specific silica uptake rates were highest at dissolved silica concentrations .2 mmol L 21 . Si and C uptake were vertically decoupled at those stations where surface silica was strongly depleted. Absolute primary productivity was not strongly correlated with dissolved silica concentrations, owing to either exhaustion of silica at diatom-dominated stations or to dominance of the community by other phytoplankton. Silica uptake as a function of increased substrate concentration was linear up to 25 mmol L 21 ; we consider some possible reasons for the nonhyperbolic response.

Abstract: In this study, algal assemblages, phytoplankton biomass, chlorophyll-a and selected physico-chemical parameters were investigated in a drinking water reservoir in Omerli, Istanbul from October 1999 to November 2000. Water samples were collected from three sampling stations at the surface, a depth of 1 m and at 5 m depth intervals to the bottom (approximately 20 m). Particular attention was given to the ecology of steady state phytoplankton assemblages. When the euphotic depth (zeu) was less then 1.52 m and the ratio of the euphotic to mixing zone (zeu/zmix) was 0.2 m (during October 1999 and September and October 2000), Microcystis aeruginosa Kiltz. dominanted, accounting for 85 – 95% of the total phytoplankton biomass. A shift toFragilaria crotonensis Kitton andAsterionella formosa Hass. occurred during April and early May 2000 when zeu > 4.1 m. Other dominant species were Coelastrum microporum Nag. Pediatrum boryanum (Turp.) Menegh. and P. duplex (May to mid June 2000);C. microporum, Staurastrum spp. and Cryptomonas spp. (late June to mid July 2000); and Scenedesmus spp., S. gracile and Nitzschia holsatica Hust. (late July to late August 2000). Below 10 meters, a high biomass of Mougeotia sp. was recorded at all sites. However, in October and early November 2000, this species appeared in the surface water. During late May to late August 2000, the zeu depth changed between 2.7 and 3.38 m. pH and total organic carbon (TOC) concentrations were found to be relatively high in the surface waters, whereas total phosphorus (TP) and Soluble Reactive Silicate (SRSi) concentrations were high below the 5 meters. The total nitrogen (TN) concentrations were high either in the surface waters or in deep layers. It is suggested that the observed increase in cell biomass of some species was controlled by the nutrient concentrations. The changes in the phytoplankton assemblages resulted in a reduction of the euphotic depth.

Abstract: [1] Under an apparent monotony characterized by low phytoplankton biomass and production, the Pacific equatorial system may hide great latitudinal differences in plankton dynamics. On the basis of 13 experiments conducted along the 180° meridian (8°S-8°N) from upwelled to oligotrophic waters, primary production was strongly correlated to chlorophyll a (chl a), and the productivity index PI (chl a-normalized production rate) varied independently of macronutrient concentrations. Rates of total ( 14 C uptake) and new ( 15 N-NO 3 uptake) primary production were measured in situ at 3°S in nutrient-rich advected waters and at 0° where the upwelling velocity was expected to be maximal. Primary production was slightly higher at the equator, but productivity index profiles were identical. Despite similar NO 3 concentrations, new production rates were 2.6 times higher at 0° than at 3°S, in agreement with much higher concentrations of biogenic particulate silica and silicic acid uptake rates ( 32 Si method) at the equator. Furthermore, phytoplankton carbon concentrations from flow cytometric and microscopical analyses were used with pigment and production values to assess C:chl a ratios and instantaneous growth rates (μ). Growth rates in the water column were significantly higher, and C:chl a ratios lower at 0° than at 3°S, which is consistent with the more proximate position ofthe equatorial station to the source of new iron upwelling into the euphotic zone. For the transect as a whole, compensatory (inverse) changes of C:chl a and μ in response to varying growth conditions appear to maintain a high and relatively invariant PI throughout the equatorial region, from high-nutrient to oligotrophic waters.

Abstract: In marine ecosystems, gradients of light, temperature, and nutrients occur horizontally (coastal to offshore) and vertically. The extent to which microorganisms acclimate or speciate in response to these gradients is under active investigation. Strain isolation data (e.g., site or depth), environmental DNA clone libraries, and preliminary physiology experiments have indicated that marine Synechococcus strain CC9605 might be adapted to the surface oligotrophic ocean. In the present work, we used an immunofluorescent approach to detect the CC9605 serotype in the California Current during September 1998. At two offshore stations, samples were collected along vertical profiles. The relative abundance of the CC9605 serotype was significantly higher in shallow depths within the mixed layer than in deeper depths at the two stations, with maximum values (6 standard deviation) of 10.3% 6 6.4 and 28.7% 6 9.5. Surface samples along an offshore‐inshore transect showed higher abundance in the most oligotrophic site (8% 6 3), compared with almost 1% inshore, but one coastal site also had high relative abundance of the CC9605 serotype (7% 6 0.5). These data indicate that Synechococcus strains are not uniformly distributed and that some strains, such as CC9605, are more abundant in the mixed layer of the euphotic zone than below the mixed layer. In marine ecosystems, environmental clines exist in temperature, light intensity, spectral composition, and nutrient availability. One striking example of these clines and how they affect marine organisms can be observed in comparing the coastal, highly productive regions with the oligotrophic open ocean. In a transect away from the California coast, for example, in situ surface chlorophyll a concentrations have been observed to begin at 2.39 m gL 21 and decrease to values of 0.13 m gL 21 , an 18-fold difference (Scripps Institution of Oceanography 1999) that is likely driven by nutrient availability. Vertical gradients are also significant. In depth profiles at oligotrophic sites off the coast of California, temperature drops by ;58C between the surface and the bottom of the euphotic zone at 150 m. Silicate and phosphate increase almost fourfold, whereas nitrate and nitrite are undetectable at the surface and reach concentrations of 8 and 0.02 mmol L 21 , respectively, at 150 m (Scripps Institution of Oceanography 1999). Light intensity was also shown to decrease

Abstract: The current lack of adequate investigations of the vertical export above the depth of 200-500 m where the majority of long-term sediment traps have been deployed, results in difficulties to understand and model the carbon flux. There exists a black box of several hundred metres between the surface layers where measurements and algorithms of primary production exists and where data on the carbon export to the ocean interior are available. In this black box, the twilight zone, we face a lack of basic understanding on how vertical export of biogenic matter into the oceans interior is regulated. Essential for this regulation are planktonic key organisms and the structure and dynamics of the pelagic food web. To better comprehend the pelagic carbon cycle and sequestration of CO2, it is instrumental to obtain a basic understanding how the biota determines and transforms the export production in the twilight zone. Here we discuss some of the key organisms involved in vertical flux regulation, present an idealised, conceptual model of vertical carbon export and focus upon the “pelagic mill” and vertical flux regulation in the upper 200 m. An adequate understanding of carbon cycling demands not only adequate investigations of primary production, but also concomitant research on the functional biodiversity of the pelagic zone, plankton dynamics, vertical flux and its regulation in the twilight zone.

Abstract: The vertical distribution pattern of algal species, chlorophylls and specific carotenoids present in the dimictic pre-alpine Ammersee (Bavaria, Germany) are given for the year 2001. A detailed taxonomic list of the phytoplankton species is recorded, along with light micrographs and detailed descriptions of the flagellates. A deep chorophyll maximum, mainly built by Planktothrix rubescens, was common in this deep mesotrophic lake. The three most dominant species among 83 identified taxa alternated seasonally and reached significant biovolumes in both the epiand the metalimnion (Planktothrix rubescens > Ceratium hirundinella > unicellular centric diatoms > Asterionella formosa > Fragilaria spp. > Anabaena lemmermannii > Phacotus lenticularis and less frequent dominant was Rhodomonas minuta). We define a steady state phytoplankton assemblage in Ammersee as a stable community in terms of species composition and standing crop. The stability of species composition was measured by Bray-Curtis similarity between monthly samples and indicate the change of individual biovolumes of species from month to month. The stability of standing crop was evaluated by the net change of total biovolume for the same time intervals. Focussing on steady state phytoplankton assemblages we compared three spatially heterogeneous environments for vertical niche separation within the top 12 m: the euphotic epilimnion (2 and 5 m), the euphotic metalimnion (7 m) and the metalimnion below the euphotic zone with dim-light less than 1% (10 and 12 m). The definition of a steady state assemblage on both ends hold true only for metalimnetic layers at dim-light levels below 1% in Ammersee. At this metalimnetic layer more than 80% similarity in phytoplankton composition between successive monthly samples was reached, associated with almost zero net changes of total biovolume only. The greater the contribution of the three most dominant taxa to biovolume, the higher were the Bray-Curtis similarities at metalimnetic depths below the euphotic zone. Zooplankton biomass had very little effect on species assemblages in the metalimnion, while parameters related to stratification (Schmidt stability) as well as those of trophy (TP, Chl) correlated with species changes. The similarity values between successive monthly samples from all the euphotic layers never reached more than 60% and were usually significantly lower, even if biovolume net changes were around zero. Both the high fluctuations of the ratio of photosynthetic versus photo-protective carotenoids (PSC:PPC) and the statistical significance of correlations between the change of species and environmental-biotic parameters separate the euphotic layer of the top 7 m as a homogenous community from deeper strata. At all sampling depths within this euphotic zone the increase of sunshine duration was associated with an increase of the carotenoid ratio PSC:PPC, but no relationship was found for the deeper layers. The change of species in the euphotic layer was not significantly related to thermal stability, TP or the dominance structure of phytoplankton, but linked with the zooplankton biomass and therefore seemed to be top-down controlled. From our observations, we can conclude that only during stratification and only in the metalimnion below the euphotic zone steady state assemblages can be expected in the deep mesotrophic Ammersee.

Abstract: The uptake of atmospheric CO 2 by the oceans via the biological pump and the sustainability of fish catch are driven by new production of organic matter and its export into deep waters or its consumption by organisms of higher tropic level. The f -ratio based on 15 N measurements of discrete samples for continental shelf waters is around 0.4. However, the system-level constraints imposed by mass balance indicate that this f -ratio may be too high. Specifically, it is more than twice as the value determined on the basis of net export of organic carbon. Such a discrepancy is caused by the fact that, unlike the open-ocean system where remineralized nutrients below the euphotic zone do not return easily, the remineralized nutrients on the shelf reenter the euphotic zone quickly. Since new production based on direct 15 N measurements on the shelf actually utilizes nutrients remineralized on the shelf as well, the value is higher than the net export of organic carbon sustained on external sources of nutrients. It is the net export of carbon, however, that sequesters CO 2 and sustains productivity on the shelf.

Abstract: The trophic state of Lake Lugano is still too high to be acceptable, despite extensive recovery measures undertaken in recent decades which have resulted in a reduction of the external phosphorus load to the deepest of the lake's basins (northern basin; Zmax=286 m) to fairly acceptable values. Since meromixis was established in the middle of last century, the deep hypolimnion of the northern basin (the layer between ca 100 m and the bottom) has contained high quantities of nutrients (especially phosphorus) which are a major potential source of internal load. When there are particularly strong winter mixing events, a portion of this phosphorus reserve is redistributed along the upper water column (0-100 m). The impact of meteo-climatic conditions on the plankton biocenosis were analysed using data collected in the northern basin (Gandria station) during the three-year period 1998-2000. The phytoplankton composition, which is typical of eutrophicated waters, shows marked interannual variations, also depending on the degree of mixing of the waters at the start of the vegetative period. Though there is no steady pattern of typical dominant species / master species in the lake, there is a seasonal succession characterised by a marked development of diatoms in spring, and a predominance of chlorophyceans and cyanobacteria in summer and autumn. Under present conditions, the mechanisms of internal replenishment of nutrients towards the euphotic layer, due to the phenomena of late winter and spring mixing, constitute a significant source of nutrients for the spring and summer growth of phytoplankton. On the other hand, pronounced mixing phenomena, like those occurring in the two-year period 1999-2000, can reduce the hypolimnetic nutrient reserves and cause a decrease in the trophic potential of the basin, contrasting with an increase in algal biomass in the euphotic zone.