Winter Mixing Period Research Articles

Seasonal and interannual variability of the pelagic ecosystem and of the organic carbon budget in the Rhodes Gyre (eastern Mediterranean): influence of winter mixing

Abstract. The Rhodes Gyre is a cyclonic persistent feature of the general circulation of the Levantine Basin in the eastern Mediterranean Sea. Although it is located in the most oligotrophic basin of the Mediterranean Sea, it is a relatively high primary production area due to strong winter nutrient supply associated with the formation of Levantine Intermediate Water. In this study, a 3D coupled hydrodynamic–biogeochemical model (SYMPHONIE/Eco3M-S) was used to characterize the seasonal and interannual variability of the Rhodes Gyre's ecosystem and to estimate an annual organic carbon budget over the 2013–2020 period. Comparisons of model outputs with satellite data and compiled in situ data from cruises and Biogeochemical-Argo floats revealed the ability of the model to reconstruct the main seasonal and spatial biogeochemical dynamics of the Levantine Basin. The model results indicated that during the winter mixing period, phytoplankton first progressively grow sustained by nutrient supply. Then, short episodes of convection driven by heat loss and wind events, favoring nutrient injections, organic carbon export, and inducing light limitation on primary production, alternate with short episodes of phytoplankton growth. The estimate of the annual organic carbon budget indicated that the Rhodes Gyre is an autotrophic area, with a positive net community production in the upper layer (0–150 m) amounting to 31.2 ± 6.9 gCm-2yr-1. Net community production in the upper layer is almost balanced over the 7-year period by physical transfers, (1) via downward export (16.8 ± 6.2 gCm-2yr-1) and (2) through lateral transport towards the surrounding regions (14.1 ± 2.1 gCm-2yr-1). The intermediate layer (150–400 m) also appears to be a source of organic carbon for the surrounding Levantine Sea (7.5 ± 2.8 gCm-2yr-1) mostly through the subduction of Levantine Intermediate Water following winter mixing. The Rhodes Gyre shows high interannual variability with enhanced primary production, net community production, and exports during years marked by intense heat losses and deep mixed layers. However, annual primary production appears to be only partially driven by winter vertical mixing. Based on our results, we can speculate that future increase of temperature and stratification could strongly impact the carbon fluxes in this region.

Sources and sinks of bottom water oxygen in a seasonally hypoxic fjord

Deoxygenation of the ocean has been occurring over the last half century, particularly in poorly ventilated coastal waters. In coastal and estuarine environments, both the water column and sediments play key roles in controlling oxygen variability. In this study, we focus on controls of oxygen concentration in Bedford Basin (BB), a 70 m deep, seasonally hypoxic semi-enclosed fjord on the West Atlantic coast in Nova Scotia. The basin is connected to the Scotian Shelf via a narrow 20 m deep sill that restricts the resupply of bottom water. Hypoxia was recorded seasonally in 2018, 2019 and 2021 with minimum oxygen concentration of 5, 6.7 and 2.7 μM, respectively. Using a 1-D benthic-pelagic coupled model we investigate oxygen consumption and resupply processes during these years. The model was constrained with weekly water column measurements of temperature, salinity, chlorophyll-a fluorescence and dissolved oxygen from a monitoring station in the central basin together with seasonal measurements of benthic diffusive oxygen uptake. Our model suggested that 29-81%, and up to 36% of bottom water re-oxygenation occurred during the winter mixing period and through summer/fall intrusions of Scotian Shelf water, respectively. Occasional shelf water intrusions occurred rapidly, on a timescale of a few hours, and delivered equivalent amounts of oxygen as winter mixing and were sufficient to end bottom water hypoxia. Collectively, these mechanisms supplied the majority of the oxygen delivered to the bottom water. Oxygen supply to bottom waters during periods of water column stratification accounted for 19-36% of the annual flux. The mean benthic uptake was 12 ± 8 mmol m-2 d-1 and contributed ~20% of the total oxygen consumption below the sill depth. In 2021, sea surface temperature (SST) was unusually high and likely resulted in 50% less bottom water oxygenation compared to 2018 and 2019 due to increased stratification; SST in BB was found to be increasing at a rate of 0.11 ± 0.02 °C/year. Climate control on water column stratification are discussed and numerical experiments are used to compare the effects of different water column mixing scenarios on bottom water oxygenation.

Contribution of photic and aphotic N2 fixation to production in an oligotrophic sea

AbstractDinitrogen (N2) fixation was investigated at a pelagic station in the oligotrophic waters of the northern Gulf of Aqaba, Red Sea, between February 2016 and December 2018. In situ 15N2 and 13C incubations were used to evaluate photic and aphotic N2 fixation rates and diazotrophic contribution to water column productivity. N2 fixation rates were typically low (below detection to <0.5 nmol N L−1 d−1). Maximal rates of 3.1 nmol L−1 d−1, measured at 100 m when conspicuous slicks of the cyanobacterium Trichodesmium appeared on the surface water. Amplicon sequencing of nifH demonstrated that non‐cyanobacterial diazotrophs, mostly α‐ and γ‐proteobacteria comprised the majority (82–100%) of amplicon sequence variants retrieved from photic and aphotic depths when low N2 fixation rates were measured, while amplicons representing cyanobacteria were nearly absent, but appeared in low abundance (~ 2%) when maximal rates were measured. During the stratified summer‐period, water‐column N2 fixation rates (10–75 μmol m−2 d−1) comprised ~ 1–40% of new production (NP). During the winter mixing period N2 fixation rates were considerably higher (11–242 μmol m−2 d−1) but made up only <1% of NP. This is because, during this period, nitrate supplied to the photic zone by the vertical mixing becomes the major N source for NP. We conclude that on an annual average, diazotrophy plays a minor role in the NP of the Gulf. The major “new” nitrogen sources are cross‐thermocline turbulent diffusion of nitrate during summer stratification and vertical mixing during the fall–winter.

Evaluation of Biological Carbon Pump Metrics in the Subtropical Gulf of Aqaba, Northern Red Sea

AbstractThe oceanic biological carbon pump modulates atmospheric CO2 concentrations by transporting carbon from the sunlit surface to greater depths. The efficiency of the biological pump and its response to warming temperatures are of great importance to future projections of global change. Here, we investigate a time series of organic carbon fluxes from a monthly resolved sediment trap mooring in the Gulf of Aqaba (GOA), northern Red Sea, between 2014 and 2016. We evaluate the attenuation of sinking organic carbon in the context of the seasonally changing euphotic zone and provide the first estimates of biological pump efficiency in this region. The base of the euphotic zone changed seasonally as the system transitioned from oligotrophic and stratified conditions in summer to mesotrophic conditions during the winter mixing period. Carbon attenuation assessed using a power law fit yields an average coefficient of b = 0.80 ± 0.37, lower than expected based on the warm temperatures in the GOA. Estimates of export efficiency decreased from 40% in summer to 20% in winter, and show the opposite seasonal pattern as transfer efficiencies, which increased from 50% in summer to ∼95% in winter. Overall, the efficiency of the carbon pump was close to ∼20% in both seasons. These observations challenge the notion of a globally uniform positive correlation between increasing temperature and increasing carbon attenuation in the ocean and imply that warm subtropical ecosystems can support moderately enhanced carbon pump efficiencies, possibly also related to increased, dust‐driven, mineral ballasting in low latitude regions such as the GOA.

Anomalously low diatom fluxes during 2009–2010 at Alfonso Basin, Gulf of California

A long-term information baseline is necessary to identify the seasonal and interannual variability of diatom production driven by environmental dynamics. This motivated our compiling hydrographic data and concurrent diatom fluxes at the Alfonso Basin sub-zone. The diatom data time series spanned from February 2008 to September 2012. These data were analyzed along with primary productivity and chlorophyll-a concentration estimates derived from satellite imagery from 2002 to 2012. Planktonic diatom flux (range: 1.62 × 106–3.55 × 107 valves·m-2d-1) was significantly correlated (r2 = 0.37) with chlorophyll-a concentration. The bimodal production season (December–July) exhibited a winter mixing period influenced by water from the Gulf of California. The diatom flux was dominated by fast-growing surface species such as Chaetoceros spp. and Thalassiosira spp. The second peak in May occurred under the influence of southwest winds concurrent with the entry of tropical surface water and a cyclonic eddy that injected nutrients into the base of the euphotic zone, eliciting a bloom of Pseudo-nitzschia spp. The summer stratification resulted in the deepening of Subtropical Subsurface Water, and the diatom flux included large species of oceanic affinity such as Chaetoceros (Phaeoceros), Rhizosolenia acuminata, and Rh. imbricata. The environment at Alfonso Basin promotes diatom sinking; thus, material from sediment traps accurately reflects the primary production cycle associated with hydrographic processes. Interannual variability was characterized by anomalously low diatom fluxes between August 2009 and July 2010. A significant negative association between diatom flux anomalies and El Niño Modoki Index values suggests that oceanographic changes are related to this event. Additional reductions in diatom fluxes were coeval with SST positive anomalies recorded from July to February 2008–2009 and 2011–2012, which showed a significant relationship with the tropical Pacific Meridional Mode. The temporal trends like the additive signals of interannual (CP-El Niño) and decadal (Pacific Meridional Mode) time scales indicate that climatic disturbances affected the local ecosystem in the basin, reducing the vertical flux of diatoms initially dominated by Pseudo-nitzschia spp., and boosting the presence of oceanic warm-water species and small highly silicified species such as Thalassionema nitzschioides var. parva.

Seasonal nutrient dynamics in the P depleted Eastern Mediterranean Sea

The Eastern Mediterranean Sea (EMS), is ultra-oligotrophic with unusual anti-estuarine circulation. It is P depleted and the limited enrichment studies which have been carried out, suggest seasonal changes in nutrient limitation. In this study high sensitivity dissolved nutrients (and associated parameters) were determined monthly over an annual cycle at a pelagic location in the SE Levantine basin. Nitrate & Nitrite (Nox) concentrations were high (300–500 nM) during the winter mixing period and enabled a concurrent phytoplankton increase in which larger picoeukaryotes and eukaryotes were dominant. After the thermal stratification of the water column commenced, Nox decreased through early summer transition period to low values (generally 50 nM or less) in the late summer. DIP remained at low nM concentrations the entire year while DOP decreased from 40 nM in winter to 30 nM in summer.. Prochlorococcus, the smallest picocyanobacteria, that does not typically utilize nitrate, dominated during the summer when both Nox and DIP concentrations were lowest. Ammonium concentrations were low (10–100 nM) with no systematic changes with season or depth. As a result, the DIN:DIP ratios were high (20–825) in winter and low (2–66) in summer, showing that these ratios vary seasonally. The dynamics of nutrient availability combined with the temporal changes in total chlorophyll and the altered dominance of the predominant phytoplankton species (i.e. nitrate-metabolizing Synechococcus during winter versus the smaller Prochlorococcus abundant in summer), leads to our hypothesis that the seasonal change in DIN:DIP indicates a switch from P limitation in winter during the annual phytoplankton increase to N&P or even N limitation in summer. Export Production (172 mmol N m−2 y−1) determined from the calculated loss of Nox from the photic zone, was similar to previous estimates in the EMS. Our results in terms of seasonally changing nutrient dynamics and resulting productivity confirm that the EMS has many of the characteristics found in P starved ocean gyre systems.

Bloom of the potentially toxic cyanobacterium P. rubescens: seasonal distribution and possible drivers of its proliferation in the Vrutci reservoir (Serbia)

Abstract Planktothrix rubescens is a filamentous cyanobacterium with a worldwide distribution in lakes and reservoirs of varying size and morphology. The objective of this study was to identify the population dynamics and the major drivers of P. rubescens blooms. Samples for qualitative and quantitative analysis of phytoplankton were collected every month in 2014 and 2015 from stationary depths, while the main physical parameters were measured in situ along the water column. Samples for chemical analysis were collected below the surface, at mid-depth (metalimnion) and near the bottom. In the periods of thermal stratification, P. rubescens formed the maximum biomass in the metalimnion, however, its volume-weighted biomass reached the maximum value in the winter mixing period (4.40 mm3 l−1). Changes in the P. rubescens population in the reservoir were related to changes in the measured environmental parameters. Significant physical, hydrological, meteorological and chemical parameters were distinguished based on the multivariate analysis. The variable associated with water-level fluctuations showed the highest positive correlation with P. rubescens biomass. We conclude that it is necessary to establish water quality monitoring and prevent excessive water-level fluctuations to ensure a high quality of water supplies from the reservoir.

Fine metagenomic profile of the Mediterranean stratified and mixed water columns revealed by assembly and recruitment

BackgroundThe photic zone of aquatic habitats is subjected to strong physicochemical gradients. To analyze the fine-scale variations in the marine microbiome, we collected seven samples from a single offshore location in the Mediterranean at 15 m depth intervals during a period of strong stratification, as well as two more samples during the winter when the photic water column was mixed. We were able to recover 94 new metagenome-assembled genomes (MAGs) from these metagenomes and examine the distribution of key marine microbes within the photic zone using metagenomic recruitment.ResultsOur results showed significant differences in the microbial composition of different layers within the stratified photic water column. The majority of microorganisms were confined to discreet horizontal layers of no more than 30 m (stenobathic). Only a few such as members of the SAR11 clade appeared at all depths (eurybathic). During the winter mixing period, only some groups of bloomers such as Pseudomonas were favored. Although most microbes appeared in both seasons, some groups like the SAR116 clade and some Bacteroidetes and Verrucomicrobia seemed to disappear during the mixing period. Furthermore, we found that some microbes previously considered seasonal (e.g., Archaea or Actinobacteria) were living in deeper layers within the photic zone during the stratification period. A strong depth-related specialization was detected, not only at the taxonomic level but also at the functional level, even within the different clades, for the manipulation and uptake of specific polysaccharides. Rhodopsin sequences (green or blue) also showed narrow depth distributions that correlated with the taxonomy of the microbe in which they were found but not with depth.ConclusionsAlthough limited to a single location in the Mediterranean, this study has profound implications for our understanding of how marine microbial communities vary with depth within the photic zone when stratified. Our results highlight the importance of collecting samples at different depths in the water column when comparing seasonal variations and have important ramifications for global marine studies that most often take samples from only one single depth. Furthermore, our perspective and approaches (metagenomic assembly and recruitment) are broadly applicable to other metagenomic studies.

Mechanisms preventing a decrease in phytoplankton biomass after phosphorus reductions in a German drinking water reservoir—results from more than 50 years of observation

Abstract To counteract the severe consequences of eutrophication on water quality and ecosystem health, nutrient inputs have been reduced in many lakes and reservoirs during the last decades. Contrary to expectations, in some lakes phytoplankton biomass did not decrease in response to oligotrophication (nutrient reduction). The underlying mechanisms preventing a decrease in biomass in these lakes are the subject of ongoing discussion. We used a hitherto unpublished long‐term data set ranging from 1961 until 2016 from a German drinking water reservoir (Rappbode Reservoir) to investigate the underlying mechanisms preventing a decrease in biomass. Total phosphorus (TP) concentrations in the Rappbode Reservoir dropped abruptly in 1990 from 0.163 to 0.027 mg/L within three consecutive years, as a result of banning phosphate‐containing detergents. Despite substantial reductions in TP, total annual phytoplankton biomass did not decline in the long‐run, and therefore, the yield of total phytoplankton biomass per unit phosphorus largely increased. Regression analysis revealed a positive association between the yield and potentially phagotrophic mixotrophs (R2 = .465, p < .001). We infer that by ingesting bacteria, mixotrophic species were capable of exploiting additional P sources that are not accessible to obligate autotrophic phytoplankton, eventually preventing a decrease in algal biomass after TP reductions. Long‐term epilimnetic phosphorus concentrations during the winter mixing period decreased to a greater degree than summer phosphorus concentrations. Apparently, TP losses over the season were less intense. Spring diatom biomass also markedly decreased after oligotrophication. In fact, spring diatom biomass was positively related to the TP loss over the season suggesting diatoms play an important role in P reduction. However, this intraannual P processing was not the primary factor when focusing on the average yearly yield, which remained to be fully explained by mixotrophs. Our study demonstrates this ecosystem's ability to compensate for changes in resource availability through changes in phytoplankton community composition and functional strategies. We conclude that an increase in mixotrophy and the ability to make bacterial phosphorus available for phytoplankters were the main factors that allowed the phytoplankton community of the Rappbode Reservoir to adapt to lower nutrient levels without a loss in total biomass.

Distribution of phthalates in Marseille Bay (NW Mediterranean Sea)

Phthalic Acid Esters (PAEs) are a group of emerging organic contaminants that have become a serious issue because of their ubiquitous presence and hazardous impact on the marine environment worldwide. Seawater samples were collected monthly from December 2013 to November 2014 in the northwestern Mediterranean Sea (Marseille Bay). The samples were analyzed for dissolved organic carbon (DOC) as well as the molecular distribution of dissolved PAEs by using solid phase extraction followed by gas chromatography and mass spectrometry (GC/MS) analyses. The results demonstrated the occurrence of six PAEs, including dimethyl phthalate (DMP), diethyl phthalate (DEP), di-isobutyl phthalate (DiBP), di-n-butyl phthalate (DnBP), benzylbutyl phthalate (BzBP) and diethylhexyl phthalate (DEHP), with total concentrations ranging from 130 to 1330ngL−1 (av. 522ngL−1). In Marseille Bay, the highest concentrations were detected in the bottom water from June to November 2014 and in the whole water column during the winter mixing period. This result suggests that resuspension of PAE-rich sediment, in relation to the accumulation of plastic debris above the seabed, or the higher degradation rate in the upper layer of the water column, plays a significant role in the PAE dynamics in coastal water. DEHP was the most abundant PAE in all of the surface samples and the summer bottom samples, followed by DiBP and DnBP, which also represent the largest fractions in the other bottom samples.

Phytoplankton composition in a neritic area of the Balearic Sea (Western Mediterranean)

From September 2000 to September 2001 the concentration of chlorophylla, and the abundance and composition of the phytoplanktonic community was studied in a neritic station of the Mallorca Channel (Western Mediterranean). Sampling was performed approximately every 12 days. Chlorophyllaconcentration and phytoplankton abundance reached maxima of 1.79 μg L−land 352 cells mL−1, respectively. It was a relatively productive period, as a result of the high convective mixing in winter and the prevalence of northern waters during most of the cycle. Phytoplankton proliferations (chlorophyllaconcentration >1 mg L−1) were detected in January, February, March and June. Those blooms mainly happened under the influence of northern waters, with the exception of the February proliferation, when mixing conditions were found. During bloom conditions it highlights the presence of coccolithophores as proliferation precursors. During no-bloom situations the phytoplankton community was mainly constituted by nanoplanktonic flagellated forms. The Winter Mixing period was dominated by different groups of nanoflagellates, including coccolithophores, undetermined flagellates and dinoflagellates. However, in the most oligotrophic conditions (from April until November) dinoflagellates were clearly dominant, except in the DCM in summer where diatoms prevailed.

Composition and structure of the larval fish community in the NW Iberian upwelling system during the winter mixing period

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 533:245-260 (2015) - DOI: https://doi.org/10.3354/meps11348 Composition and structure of the larval fish community in the NW Iberian upwelling system during the winter mixing period J. M. Rodriguez1,*, A. Cabrero2, J. Gago2, A. Garcia3, R. Laiz-Carrion3, C. Piñeiro2, F. Saborido-Rey4 1Instituto Español de Oceanografía, Centro Oceanográfico da Gijón 33212, Gijón, Spain 2Instituto Español de Oceanografía, Centro Oceanográfico de Vigo 36390, Vigo, Spain 3Instituto Español de Oceanografía, Centro Oceanográfico de Málaga, 29640 Fuengirola, Málaga, Spain 4Instituto de Investigaciones Marinas (CSIC), Eduardo Cabello 6, 36208 Vigo, Spain *Corresponding author: j.m.rodriguez@gi.ieo.es ABSTRACT: The composition and structure of the larval fish community (LFC) in the NW Iberian upwelling system in late winter 2012 were examined, and the influence of abiotic and biotic factors on the horizontal distribution of fish larvae and on the horizontal structure of the LFC was investigated. The LFC, composed of 62 taxa (59 species and 3 genera) in 28 families, was dominated by Micromesistius poutassou followed by Scomber scombrus, Merluccius merluccius and Maurolicus muelleri. Multivariate analyses identified depth, sea surface temperature, geostrophic velocity and mesozooplankton biomass as the environmental factors involved in explaining variability in the LFC parameters (larval fish abundance, species richness and diversity) and revealed that the LFC was structured into coastal and slope larval fish assemblages. The main factor defining these assemblages was depth. However, the unusual coastal downwelling that occurred during the study would have participated in creating them and in maintaining assemblage boundaries, so that the importance of depth to the cross-shelf structuring of the LFC may have been overestimated. Our results suggest that larval fish assemblages in regions of coastal upwelling are driven by coastal downwelling during the winter mixing period and support the idea that fish larvae act as short-term (~10 d) biological tracers of the hydrography of the region during that period. KEY WORDS: NW Iberian upwelling system · Hydrography · Winter mixing period · Downwelling · Ichthyoplankton · Larval fish community Full text in pdf format PreviousNextCite this article as: Rodriguez JM, Cabrero A, Gago J, Garcia A, Laiz-Carrion R, Piñeiro C, Saborido-Rey F (2015) Composition and structure of the larval fish community in the NW Iberian upwelling system during the winter mixing period. Mar Ecol Prog Ser 533:245-260. https://doi.org/10.3354/meps11348 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 533. Online publication date: August 06, 2015 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2015 Inter-Research.

Vertical distribution and migration of fish larvae in the NW Iberian upwelling system during the winter mixing period: implications for cross‐shelf distribution

AbstractThe vertical distribution and vertical migrations of fish larvae and implications for their cross‐shelf distribution were investigated in the northern limit of the NE Atlantic upwelling region during the late winter mixing period of 2012. The average positive values of the upwelling index for February and March of this year were far from normal, although the average hydrographic conditions during the period of study were of downwelling and the water column was completely mixed. Fish larvae, most in the preflexion stage, were concentrated in the upper layers of the water column and their distribution was depth stratified, both day and night. However, the larval fish community was not structured in the vertical plane and fish larvae did not show significant diel vertical migration (DVM), although five species showed ontogenetic vertical migration. In regions of coastal upwelling and in the absence of DVM, the location of fish larvae in the water column is crucial for their cross‐shelf distribution. Thus, the cross‐shelf distribution of the six most abundant species collected in this study can be explained by the surface onshore flow associated with coastal downwelling, retaining larvae of the coastal spawning species with a relatively shallow distribution in the shelf region and transporting larvae of slope spawning species onto the shelf. The wide vertical distribution shown by larvae of the offshore spawning species could be an adaptation of these species to ensure that some larvae reach the inshore nursery areas.

On the relationship between satellite‐estimated bio‐optical and thermal properties in the Gulf of Mexico

Three years of Sea‐viewing Wide Field‐of‐view Sensor (SeaWiFS) ocean color data were combined with three‐dimensional thermal fields generated by the U.S. Navy's Modular Ocean Data Assimilation System (MODAS) in order to examine the interdependencies between bio‐optical fields and their relationship to seasonal and mesoscale changes in upper ocean thermal structure. The combined data set suggests that the oceanic boundary layer within the Gulf of Mexico may be broadly defined by two seasonally occurring bio‐thermal periods. A winter mixing period, characterized by net heat losses to the atmosphere, deepening of the isothermal layer depth, and annual maxima of satellite‐estimated colored detrital matter (CDM) absorption coefficients and surface pigment concentration, was followed by a thermally stratified period characterized by net surface ocean heating, reduced isothermal layer depths, and annual minima in surface bio‐optical fields. Variability in the interdependencies of ocean color products was used to diagnose an attendant shift in the size‐structure of surface phytoplankton communities as well as identify CDM as the constituent responsible for the majority of blue‐light absorption in Gulf of Mexico surface waters. The mesoscale circulation, as resolved by MODAS thermal fields into cold and warm‐core eddies, appears to significantly modulate the seasonal bio‐optical cycle of CDM absorption and surface pigment concentration. An empirical model was developed to describe CDM absorption as a function of upper ocean thermal energy. The model accounted for nearly half the variance in the satellite‐estimate of this bio‐optical variable. Large mismatches between the model and satellite data implied episodes of shelf water export to the deep Gulf of Mexico.

FCO2sw variability in the Bay of Biscay during ECO cruises

The Bay of Biscay is part of the North Atlantic Ocean, the most important sink of CO2, and a subduction zone of mode waters that favours the entry of carbon to the ocean interior. To investigate the seasonal and interannual variability of CO2 uptake, continuous underway measurements of the partial pressure of CO2 at sea surface were performed along a commercial route between Vigo (Spain) and St. Nazaire (France). An unattended measuring system of CO2 fugacity (fCO2), with meteorological station, and temperature, salinity, oxygen and fluorescence sensors, was installed on board of ships of opportunity (RO-RO L’Audace and RO-RO Surprise).The dataset collected between December 2002 and December 2004 reported a significant interannual variability; mainly for the winter season. The noticeable increase of fCO2sw during the winter mixing period of 2004 was associated to biogeochemical differences related to nutrient ratios, phytoplankton activity and atmospheric CO2 uptake. Dividing the seasonal cycle into three periods, prebloom (October–February), bloom (March–May) and postbloom (June–September), the fCO2sw for the entire seasonal cycle was correctly predicted by empirical relationships with an error lower than 10μatm in spite of the high interannual variability. The fCO2sw variability at seasonal scale was mainly controlled by processes of synthesis and remineralization of organic matter during prebloom and bloom periods whereas SST was the key parameter during postbloom period. The surface waters of the Bay of Biscay showed a clear role as atmospheric CO2 sink ranging from −2.7±2.8 (±0.3) to −0.08±0.41 (±0.04)molCm−2yr−1 (mean±standard deviation (±error)) throughout each complete period although CO2 release to atmosphere was also observed during short episodes of summer. Using the regular wind speed sources of CO2 fluxes estimation ranged from −1.3±1.7 (±0.1) to −2.4±2.7 (±0.1)molCm−2yr−1 at annual scale, exceeding the sink capacity of the nearby regions of the North Atlantic Ocean.

Dynamics of the planktonic food web in Colgada Lake (Lagunas de Ruidera Natural Park)

Dynamics of the planktonic food web in Colgada Lake (Lagunas de Ruidera Natural Park) In Colgada Lake, one of the 15 lakes belonging to Lagunas de Ruidera Natural Park, the components of the lineal food chain (phytoplankton and metazooplankton) and the microbial loop phytoplankton, metazooplankton, ciliates, autotrophic picoplankton, and bacterioplankton) were studied from June 2003 to December 2004 with a monthly sampling frequency. This lake has monomictic and mesotrophic characteristics and a mean depth of 8 m. Sixty-two species of the phytoplankton community, 27 species of metazooplankton community, and 12 species of ciliates were identified. Phytoplankton and metazooplankton integrated biomass followed seasonal patterns with higher values in summer, up to 10 5 mg WW/m 2 and 2 × 10 4 mg WW/m 2 , respectively. Autotrophic picoplankton did not exceed 500 mg WW/m 2 . Microbial loop components did not show seasonality, and its biomass concentration fluctuated between 500-2500 mg WW/m 2 for ciliates and 100- 2000 mg WW/m 2 for bacterioplankton. Centric Bacillariophyceae, Cryptophyceae, and Dinophyceae represented most of the phytoplanktonic biomass throughout the studied period, although an increase of cyanobacteria was also observed. Cladocerans and calanoid copepods dominated during summer stratification, rotifers and cyclopoid copepods were present during the winter mixing period. The microbial loop biomass in relation to total planktonic biomass was higher in winter and spring 2004. Inter- annual changes in the presence of planktonic groups were observed in the different periods of the year: i) the algal composition included larger and non-edible species (Cyanobacteria, Dinophyceae), ii) a reduction in the clearance function by cladocerans was produced, which were substituted by a cyclopoid predator that iii) can feed on rotifers and ciliates, favouring in this way bacterioplankton and autotrophic pico-nanoplankton. This interannual variation could be related to the ongoing eutrophication process in the Lake. All of this may change the way the lake looks: if spring primary production is not strongly controlled by herbivory, this could threaten the annual recruitment of submerged macrophytes that significantly contribute to improve the water quality of the lake.

Phytoplankton dynamics in the Salton Sea, California, 1997–1999

The dynamics of phytoplankton populations in the Salton Sea were studied over the 3-year period 1997–1999. Dino-flagellates were important components, often contributing over 80% of the total biovolume of cells larger than 5 μm. Gyrodinium uncatenum along with a similar, but rarer species Gyrodinium instriatum, was the most dominant taxon, present year-round, and became especially abundant in spring and summer. Diversity of dinoflagellates was high with three or four species often co-occurring with similar densities. The largest dinoflagellate, Gonyaulax grindleyi (= Protoceratium reticulatum), increased greatly in abundance during 1997–1999. Diatoms co-dominated year round with the colonial araphid Thalassionema sp. also becoming especially abundant in 1999. During the winter mixing period, diatom populations often were dominated by pennate species usually considered benthic, such as Pleurosigma ambrosianum, Ceratoneis closterium and Tryblionella punctata. Cryptomonads, represented by several species, were usually numerous and, although relatively small, constituted as much as 20–30% of total phytoplankton biovolume. A raphidophyte, Chattonella marina, reported to be ichthyotoxic in other locales, was abundant in summer, comprising about a third of total phytoplankton biovolume then and reaching mid-lake densities of nearly 1,500 cells ml−1. Two colonial non-motile green algae were usually present; one of these, Crucigenia rectangularis, increased greatly in density in 1999. A euglenoid, Eutreptia lanowii, was highest in density in summer when it contributed about 5–10% of total biovolume at times after other species were reduced by sulfide events. Filamentous planktonic cyanobacteria were very rare. The increase in large species of phytoplankton over the three-year period coincided with a dramatic decrease in abundance of a planktivorous fish, the Mozambique mouthbrooder (Oreochromis mossambicus x O. urolepis honorum), which likely was responsible for these changes in phytoplankton composition. Phytoplankton biovolume densities and chlorophyll a concentrations each year were highest, 6–16 mm3 l−1 and 30–40 μg 1−1 respectively, in the spring. In summer and early fall, occasional windstorms mixed the water column causing sulfide-laden bottom waters to upwell and strip surface waters of oxygen. Sharp drops in abundance of most phytoplankton species occurred during these events presumably due to poisoning by hydrogen sulfide. Satellite images confirm the presence of large patches of high albedo surface waters at these times, lasting days to over a week, produced by backscattering from abundant crystals of gypsum produced following oxidation of hydrogen sulfide to sulfate.

Diel vertical distribution of fish larvae during the winter-mixing period in the Northwestern Mediterranean

Abstract The vertical distributions of the larvae of shelf and oceanic fish species that spawn during the winter-mixing period in the Mediterranean are described from 22 vertically stratified plankton tows. Diel differences in the vertical distribution patterns in relation to physical data and potential prey abundance throughout the water column were examined. Even in absence of stratification, the larvae of the various fish species showed different patterns of vertical distribution and diel changes. The larvae of shelf-dwelling species were found in the surface layers, mainly above 50-m depth, and with some exceptions, with very little diel variation in depth distribution. Therefore, the vertical distribution of the larvae of these species coincided with the maximum concentrations of their potential food, nauplii and copepodite stages of copepods. The larvae of mesopelagic fishes showed deeper distributions in the water column and most of these species were located closer to the surface during the day than at night. Given the homogeneity of the physical characteristics throughout the water column, except for light, this behaviour may be determined not only by the higher concentration of prey in the surface layers but also by adequate light levels for feeding.

A time-series assessment of the nitrogen cycle at BATS

Abstract The assimilation of nitrate and ammonium in the euphotic zone of the Sargasso Sea at the US JGOFS Bermuda Atlantic Time-Series Study (BATS) site was measured from February, 1992 through April, 1994. Nitrate concentrations varied from 10 nM d−1 during the blooms in 1993 and 1994, and reaching >150 nM d−1 during 1992. Nitrate assimilation by particles passing a GF/F filter was frequently a significant component of total nitrate use regardless of light level. Ammonium assimilation was more constant than nitrate assimilation over the year and small particles rarely contributed more than a few percent to the total assimilation. Nitrate assimilation was correlated with concentration, both at the highest concentrations during the 1992 bloom and at the lower concentrations during the oligotrophic periods. Integrated nitrate assimilation in 1992 was 0.94 mol N m−2 yr−1 compared to 0.14 mol N m−2 yr−1 in 1993, and was completely dominated by assimilation during the winter bloom. New production during the winter mixing period has not been previously assessed and can be a large contribution to the yearly value. Using the traditional definition of the f-ratio, values ranged from 0 to 0.8 with no pattern for nitrate concentrations between 1 and 50 nM. The constant, low nitrate concentrations in the water column, the lack of variation in the vertical profile and the observed regeneration of nitrate during the incubations all suggest that nitrate assimilation during the oligotrophic period does not contribute to “new” production and nitrate should be considered a regenerated nutrient in the oligotrophic ocean.

Environmental Factors Affecting the Occurrence and Production of the Spring Phytoplankton Bloom in Funka Bay, Japan

The concentration of nutrients was measured during the spring phytoplankton bloom in Funka Bay over a 5-year period (1988–92). During the winter mixing period, nutrient concentrations were similar in every year except in 1990 when a high concentration of silicate was observed. There was interannual variation in the onset of the bloom, presumably depending on the stability of the water column. The bloom developed in early March when the Oyashio water (OW), which has a lower density than the existing winter water, flowed into the bay and the pycnocline formed near the bottom of the euphotic zone. In this case, high chl a was found only in the euphotic zone and nutrient utilization was limited to this zone. In the year when the inflow of OW was not observed by April, the bloom took place at the end of March without strong stratification and high chl a was found in the whole water column, accompanied by a decrease in nutrients. Interannual differences were found not only at the beginning of the decrease, but also in the thickness of the layer which showed a decrease in nutrients. Primary production from the beginning to the end of the spring bloom was estimated from the nutrient budget before and after the spring bloom. The integrated production over the spring bloom period ranged from 25 to 73 g C m-2, which accounts for 19–56% of the annual production in this bay. We found that the timing of the bloom was strongly dependent on the inflow of OW, but the amount of production was not clearly related to this timing.