In the Atlantic sector, the percentage of diatoms is much lower, the most frequent genera being Guinardia mainly at station 2 and Pseudonitzschia station 1. Station 3, which is located at the western side of Camarinal sill in the main channel, has the lowest abundance of diatoms. Nevertheless, although less abundant, the diatoms that are present below the interface in station 3 belong to similar taxa to those that predominate in the stations of the Mediterranean side stations 6 and 7.
There are also relevant differences in community structure between the stations located in the central transect stations 4 and 5. The northern station 4 is dominated by the genus Guinardia G. However, at the southern station 5, we found a phytoplankton community that included several diatoms typical of upwelling areas. Thus, in this station, species of the genus Chaetoceros which reached only a negligible abundance at station 4 become very abundant. Then, the taxonomic composition of samples of station 5 is more similar to that of northern Mediterranean stations.
Chaetoceros spp. They are highly abundant in Mediterranean stations, where this genus constitutes the bulk of diatom biomass and it forms a maximum in the vicinity of the interface. This fact will lead to the larger proportion of these diatoms being situated in a slower speed layer, and this may affect the time that they spend travelling into the Mediterranean.
These diatoms are also important in south central station 5, where they reach a maximum at very shallow depths, i. Finally, in the Atlantic side, Chaetoceros appears especially in southern station 3 and, although in lower numbers than in Mediterranean stations, it has a noticeable presence at several depths, including very deep waters below the interface, in the outflowing Mediterranean water.
Microplankton abundance estimated from Chl, cell number and total biovolume shows a tendency to increase parallel to a gradual eastward elevation of the AMI in a southwest—northeast direction. However, one of the main problems in the interpretation of our results is that we have only one estimation of biological variables per station at different moments of the tidal cycle. Therefore, in order to include the effect of tide in the evaluation of our results, we have analysed the average depth of the interface and its range of variation assuming the patterns proposed by Bray et al.
The combination of these average interface depths with the proposed tidal oscillation of the interface at different sectors of the Strait [ Bray et al. The amplitude of the changes in the interface depths oscillates from 50 to 90 m, depending on the station Figure 5. We can consider on average the m surface layer as the potentially productive euphotic zone. We can also take into account that the deep outflowing waters below the interface are nutrient rich, whereas upper Atlantic waters can be considered as nutrient depleted.
These facts can allow our results to be analysed in terms of a coupling between light and nutrients that would lead to an enhancement of primary production.
In this context, Atlantic stations present a marked uncoupling between light and nutrients because AMI is very deep. The enriched deep water will rarely arrive at the euphotic zone, even including the periodic oscillation of the interface activated by tides. The total time that the interface spends in the euphotic zone becomes gradually shorter as we move to the southwest. The dotted area in Figure 5 represents the expected periods in which the interface, probably rich not only in nutrients but also in accumulated materials including viable cells, is in the euphotic zone and a higher primary production can take place.
The consideration of thermocline depths usually shallower and related to microplankton maxima leads to similar conclusions, as its position follows the same pattern, with deeper thermoclines at the southwest and shallower ones towards the northeast Figure 2.
Another important hydrodynamic feature must be taken into account when analysing these results. Thus, the generation of internal waves and the eventual intrusion of deep water in the productive zone could enrich the upper illuminated zone east of the sill. In fact, intermittent mixing events at the sill region are favoured by the special topography of the Strait of Gibraltar, which facilitates the generation of internal bores, an important and perhaps dominant agent of mixing in the Strait Pettigrew and Needell, Also, the interface depth deepens from 55 m to some m on average Bray et al.
This slope forces an upwards component in the direction of the Mediterranean outflowing water which approaches the sill. The Mediterranean outflow cross-section is reduced because of these bathymetric constraints, thus increasing water velocity and the probability of mixing events Wesson and Gregg, Mixing between the two layers is suggested after comparing upper salinity values at both sides of the sill Atlantic and Central stations of this cruise; Figure 2.
This fact is more evident in station 5, which presents a weaker halocline. Very recent unpublished data from a cruise performed in November have revealed the presence of singularities just after high water, with high values of vertical velocities using an Acoustic Doppler Current Profiler and a high abundance of particulate material estimated by a transmissometer-CTD at single moments Bruno et al.
The phenomena described above imply consideration of the Strait as a pulsating upwelling area, in which nutrient-rich deep water upwells in short time single episodes of mixing, as previously suggested by Minas et al. Minas et al. This should constitute a fertilization mechanism for the westward region of the Strait, and a mechanism that partially explains the high biomass found to the northeast.
The background idea is highly related with the Gran effect [e. Mann and Lazier, ] involving enhanced production and blooming of phytoplankton when cells remain packaged in a shallower mixed layer with enough light and nutrients. Although persistence of phytoplankton patches in a fixed zone depends on transport, turbulence and mean replication time of cells, the problem will become complicated given that the organisms could control their position relative to the interface depth.
The region close to the interface will present a near zero horizontal velocity and, given the marked velocity gradients, a dramatic change in mean transport could be caused by a relatively small vertical repositioning relative to interface depth.
Moreover, we hypothesize that the high values of Chl can be a general feature of the Mediterranean side near the Strait that can extend farther eastward of the sampled region.
In this case, the phenomena on the sill can be considered as the potential agents of these plankton patterns that could then be considered as a permanent feature in the vicinity of the Strait.
The average tendencies discussed above refer to total biomass or Chl data. When the taxonomic structure of phytoplankton is taken into account, some new features are interesting to point out. Microplanktonic community structure is quite different at both sides of the sill, with similarities between Central and Mediterranean stations and a very different community in the Atlantic sector. Also, the diatoms Chaetoceros spp.
These stations are connected by Strait topography because they are aligned along the main channel parallel to the m isobath; Figure 1. This alignment follows the main velocity direction of the Mediterranean outflowing water Sein et al. We propose a hypothesis based on the advective connection between the stations through the opposite bilayer currents characteristic of the Strait.
Along the main channel, some microplanktonic organisms would travel in a closed cycle using both currents during their life cycle to avoid adverse effects of advection and to maintain themselves in the enriched upwelled water in the vicinity of the Strait. This conveyer belt drawn by sea water in the circulation scheme could be used by diatoms, which could then be able to adjust their life cycles to this pattern of water movement.
In an attempt to explain the empirical data that we have obtained at the different stations analysed, we propose the tentative conceptual model given in Figure 6.
This hypothesis takes into account a circulation scheme that includes intrusion to surface waters at the sill region and the return of part of the expected outflow into the Mediterranean sector. The coupling of diatom behaviour and life cycles to this predictable circulation scheme would allow them to stay in the nutrient-rich area of the eastern side of the sill.
The proposal of this coupling is based on different mechanisms and adaptations described for diatoms by different authors, together with some direct evidence coming from our own results.
A schematic draft of the hypothesis includes four steps. Sinking and aggregation. At the northeastern sector stations 6 and 7 , the phytoplankton community is mainly composed by colonies of diatoms with a high abundance colonies ml —1.
This abundance is higher than the critical concentration predicted by models for floc formation Jackson, The presence of spiked protuberances such as setae can further enhance the aggregation of particles when a critical concentration level is achieved Riebesell, In this case, the phytoplankton community is dominated by colonies of a genus Chaetoceros that displays abundant long setae Figure 4. Disaggregation and seeding.
There is a separation of colonies into smaller fragments or individual cells in deep waters Smetacek, During aggregate sedimentation, microbial exudates and binding material may be sufficiently decomposed by bacteria Chow and Azam, to weaken the attraction between attached clusters within the aggregates.
Colony disintegration and dispersal of the small individual cells will ensure that the seeds are scattered over a wide depth range in nutrient-rich subsurface waters, thus increasing the chance of transport of some of them back to the surface during further upwelling Smetacek, It has also been proposed that diatoms accumulate carbohydrate reserves and increase their sinking velocity after a nutrient depletion Richardson and Cullen, Return to the upper layer.
The possible movement of part of the particulate material, including settled cells, back to the surface waters would be partially related to the topography of the Strait of Gibraltar and the episodic water mixing processes that have been described above for the sill region. Other biological mechanisms such as positive buoyancy described in some diatoms Villareal et al. Horizontal advection in the upper layer.
Once in the upper illuminated Atlantic water, the cells coming from the deep outflowing layer could activate their photosynthetic mechanisms. Diatoms are able to maintain their photosynthetic capacity after a long time in dark conditions Peters, ; Berges and Falkowski, , being able to photosynthesize when they arrive at the euphotic zone.
It is likely that the cells arriving from deep waters had incorporated nutrients in their internal reserves Probyn et al. The sum of light and nutrients will allow these cells to perform active growth during the transport into the Mediterranean, in the surface current, with several divisions in a day Malone, However, an appropriate positioning in the water column is decisive.
During the displacement, the cells must accumulate preferentially at the vicinity of the interface where the velocity is lower. Then, the numerical abundance would be higher in this zone Figures 3 and 4. This is the structure that we have found at the Mediterranean stations mainly 6 and 7 where aggregation processes would take place, closing the cycle. In coastal upwelling regions, usually Ekman transport drags the surface waters outwards with a deep current moving towards the coast.
Phytoplankton cells settle to deeper layers when reaching poor surface waters far from the coast and are then recycled with the upwelling circulation Barber and Smith, ; Bodungen et al. In these areas, diatoms exploit the opposite bilayer current system to maintain their position in the vicinity of the upwelling Brown and Field, These coastal upwelling areas depend mainly on wind, a much more unpredictable phenomenon than tides.
A similar feature of coupling is proposed in this paper for the pulsating upwelling in the sill of the Strait of Gibraltar, but in this case the forcing agent of the upwelling is the tide, a much more predictable event. The coupling of life cycles with a more regular physical event would be easier and the fluctuations could be better internalized. Further investigation at a wider spatial and temporal scale and including more accurate estimations in the framework of a quantitative model will be necessary in the zone to confirm this hypothesis.
Bathymetry of the Strait of Gibraltar, showing the position of the sill and the sampling stations. Sampling was carried out spending a day in each station from 2 station 1 to 9 station 8 September Continuous and dashed arrows indicate the interface AMI and thermocline depths, respectively.
Note the different scale used at each station. Continuous and dashed arrows indicate the AMI and thermocline depth, respectively.
Interface depth along a semidiurnal tide cycle HW, high water; LW, low water. The euphotic zone is considered down to m depth.
The dotted area indicates the period during which nutrient-rich waters below the interface are illuminated. Microphytoplankton biovolume is expressed as mm 3 per m 2 , showing the relationship between the dotted area and microphytoplankton biomass at each station. Scheme of the proposed circulation in the Strait of Gibraltar, and the coupling with some microphytoplankton species. See the text for details. MdS helped with plankton identification and calculations.
LC analyzed the toxins. JY helped with data interpretation. All authors revised and made contributions to methodology, results and discussion in this manuscript. JY was funded by a productivity grant from CNPq. The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. Agawin, N. Variability in the abundance of Trichodesmium and nitrogen fixation activities in the subtropical NE Atlantic.
Plankton Res. Anagnostidis, K. Modern approach to the classification system of cyanophytes. Google Scholar. Arun Kumar, M. Occurrence of Trichodesmium erythraeum bloom in the coastal waters of South Andaman. Barbosa, F. Bif, M. Distribution of the marine cyanobacteria Trichodesmium and association with iron-rich particles in the Southwest Atlantic Ocean.
Aqua Microbial Ecol. Observations on the development and the biology of the Miraciidae Dana Copepoda: Crustacea. Bray, J. An ordination of the upland forest communities of Southern Wisconsin. Capone, D. Trichodesmium, a globally significant marine cyanobacterium. Science , — Carpenter, E. Carpenter and D. Carvalho, M. Trichodesmium erythraeum bloom on the continental shelf off Santos, Southeast Brazil.
Clarke, K. Non-parametric multivariate analyses of changes in community structure. Costa, P. A potential vector of domoic acid: the swimming crab Polybius henslowii Leach Decapoda brachyura. Toxicon 42, — Detoni, A. Trichodesmium latitudinal distribution on the shelf break in the southwestern Atlantic Ocean during spring and autumn.
Global Biogeochem. Toxic Trichodesmium bloom occurrence in the southwestern South Atlantic Ocean. Toxicon , 51— D'Silva, M. Algal blooms: a perspective from the coasts of India.
Hazard 63, — Ehrenberg, C. Evans, G. Phytoplankton accumulation in Langmuir cells. Fraenkel, G. The raison d'Etre of secondary plant substances. F Costa, M. F, and Kutner, M. Hammer, O. Brief Notes. Hasle, G. Tomas San Diego: Academic Press , 5— Hawser, S. Carpenter, D. Capone, and J. Rueter Dordrecht: Springer. Toxicity of blooms of the cyanobacterium Trichodesmium to zooplankton.
Hynes, A. Comparison of cultured Trichodesmium Cyanophyceae with species characterized from the field. Karl, D. Dinitrogen fixation in the world's oceans. Biogeochemistry 57, 47— Modern taxonomic revision of planktic nostocacean cyanobacteria: a short review of genera.
Hydrobiologia , — Krienitz, G. Schagerl Munchen: Elsevier Gmbh Taxonomic classification of cyanoprokaryotes Cyanobacterial genera , using a polyphasic approach. Preslia 86, — LaRoche, J. Importance of the diazotrophs as a source of new nitrogen in the ocean. J Sea Res. Madhu, N. Occurrence of cyanobacteria Richelia intracellularis —diatom Rhizosolenia hebetata consortium in the Palk Bay, southeast coast of India. Indian J.
Miguez, A. Yasumoto, Y. Oshima, and Y. The effects of river discharge and seasonal winds on the shelf off southeastern South America. Shelf Res. Monteiro, J. New record of Trichodesmium thiebautii Gomont ex Gomont Oscillatoriales-Cyanophyta for the continental shelf of northeast Brazil. Acta Bot. Distribution and annual variation of Trichodesmium thiebautii Gomont Ex Gomont Oscillatoriales - cyanophyta in tropical waters of northeastern Brazil Western Atlantic.
O'Neil, J. Ingestion of 15 N 2 -1abelled Trichodesmium spp. Screening the toxicity and toxin content of blooms of the cyanobacterium Trichodesmium erythraeum Ehrenberg in northeast Brasil. Venomous Anim. Toxins Trop. Reguera, J. Blanco, M. Wyatt Vigo. Rourke, W. Rapid postcolumn methodology for determination of paralytic shellfish toxins in shellfish tissue.
AOAC Int. PubMed Abstract Google Scholar. Sherr, E. Heterotrophic dinoflagellates: a significant component of microzooplankton biomass and major grazers of diatoms in the sea.
Shunmugam, S. Unraveling the presence of multi-class toxins from Trichodesmium bloom in the Gulf of Mannar region of the Bay of Bengal. Toxicon , 43— Silva, L. Master Dissertation. Baptista Neto, M. Wallner-Kersanach, S. Siqueira, A.
Smayda, T. Harmful algal blooms: their ecophysiology and general relevance to phytoplankton blooms in the sea. Srinivas, B. Total Environ. Stamp, N. Out of the quagmire of plant defense hypotheses. Steindinger, T. TerBraak, C. A theory of gradient analysis. Zurvervollkommnung der quantitativen phytoplankton-methodik. Mitt Int. Villareal, T. Buoyancy regulation and the potential for vertical migration in the oceanic cyanobacterium Trichodesmium.
Microbial Ecol.
0コメント