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A revised sediment trap splitting procedure for samples collected in the Antarctic sea

A revised sediment trap splitting procedure for samples collected in the Antarctic sea
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  Methods in Oceanography 8 (2013) 13–22 Contents lists available at ScienceDirect Methods in Oceanography  journal homepage: Full length article A revised sediment trap splitting procedure forsamples collected in the Antarctic sea Francesca Chiarini a,b , Lucilla Capotondi a, ∗ , Robert B. Dunbar c ,Federico Giglio a , Irene Mammì a , David A. Mucciarone c ,Mariangela Ravaioli a , Tommaso Tesi a , Leonardo Langone a a ISMAR-CNR Bologna, Via Gobetti 101, 40121, Bologna, Italy b University of Bologna, Dipartimento di Scienze Biologiche, Geologiche e Ambientali, Bologna, Italy c Geological and Environmental Sciences, Stanford University, Stanford, CA, USA a r t i c l e i n f o  Article history: Received 22 April 2014Accepted 31 May 2014Available online 28 June 2014 Keywords: Sediment trapSplitterSwimmerAntarctic region a b s t r a c t In order to correctly understand the rates and mechanisms of biogeochemical cycling along the water column, special attentionmust be paid to data analysis techniques.Weproposearevisedprocedurecombiningprecisionandprac-ticality to minimize sample handling errors that would affect thedeterminationofbothmassfluxesandthecompositionofmaterialcollected by sediment traps in the Antarctic region. The key pointsto take in account are: (i) the mesh size used for removing ‘‘large’’particlesoraggregates(from150micronto1mm);(ii)theabsenceof filters; and (iii) the use of a microscope to pick out ‘‘swimmers’’.We also recommend: removal of all swimmers using a650-micron mesh; analysis using a stereomicroscope; and quan-titative subdividing using a peristaltic pump. © 2014 Elsevier B.V. All rights reserved.  Abbreviations:  MA, mooring A; MB, mooring B. ∗ Corresponding author. Tel.: +39 051 6398876. E-mail addresses: (F. Chiarini), (L. Capotondi), (R.B. Dunbar), (F. Giglio), (I. Mammì), (D.A. Mucciarone), (M. Ravaioli), (T. Tesi), (L. Langone). © 2014 Elsevier B.V. All rights reserved.  14  F. Chiarini et al. / Methods in Oceanography 8 (2013) 13–22 1. Introduction Particle fluxes are an important means of evaluating the transfer of organic carbon and biogenicparticles(carbonateandopal)throughthewatercolumn.TheconversionofdissolvedCO 2  tobiologicalmaterialsfollowedbyparticleexportliesattheheartofthebiologicalpump.Understandingtheroleof particle fluxes is therefore crucial for evaluating C exchange between the atmosphere and the ocean.Biogenic material accumulating on the sea floor is controlled by the balance between the export of particulatematterfromsurfacewaters,andlosses–throughdissolutionorremineralizationprocesses– occurring as material sinks through the water column (Dunbar et al., 1998; Langone et al., 2003). As a result, the particulate export is subjected to seasonal and inter-annual variability as well as toshort-term climatic events (Buesseler et al., 2007). Sediment traps are one of the most important tools used to determine this temporal variabilityin particle fluxes and to investigate the mechanisms regulating biogeochemical fluxes in theoceans.Many studies have been carried out on the efficiency of sediment traps (Heussner et al., 1990; Buesseleretal.,2007).Cylindricaltrapshavebeenfoundbestforlong-termsamplinginareaswithlow sedimentfluxesliketheSouthernOcean,whileconicaltrapsareconsideredmoreefficientonaccountof their large collection surface (0 . 5–1 . 0 m 2 ) (GOFS, 1989). In all cases, the sediment trap aperture is coveredbyahoneycomb-shapebaffleconsistingofmanycellsshapedlikesmallcylindricalsedimenttraps.Theparticlecompositionofthematerialscollectedbysedimenttrapsallowsestimationofavarietyof vertical fluxes (e.g. organic C, radiolarians, biogenic silica, calcium carbonate) through the watercolumn as well as their seasonal variability. The export of POC produced by phytoplankton in theeuphotic layer gives an estimation of the efficiency of the biological pump (Ducklow et al., 2001). Inordertostudysamplesfromsedimenttraps,aseriesofsamplehandlingproceduresaregenerallyfollowed. Sediment trap samples have to be processed in order to measure the mass flux and splitinto quantities suitable for laboratory analyses. These procedures are subjected to different biasessuch as equipment accuracy and operator skill. While an instrumental error may be quantified,the operator error is difficult to measure. The main problem connected with sample treatment isobtaining incorrect mass flux values in excess or default. It is imperative therefore to establish adefinedprocedureinordertoobtaincomparableresults.Greatattentionshouldbegiventoswimmerremoval, sample preservation, the splitting instruments used, mesh size of any sieves, and the loss of material during handling.There is no single, standardized handling procedure in use today. Different protocols are adopteddepending on the composition and srcin of material. Methods differ mainly in the way swimmersare removed (picking vs sieving). For example, a larger sieve mesh may be used for samples withlarge swimmers while the removal of smaller organisms will require a smaller mesh. Since differentswimmer removal methods may affect biogenic flux values, it is important to adopt a suitableprocedure for the area under investigation.Another important step is the way the sample is handled once the swimmers are removed(wet splitting and bulk drying, Table 1). Different splitters modify measurement precision of thesample mass. Some authors (Conte et al., 2001; Honjo and Manganini, 1993; Miquel et al., 1994; Karl et al., 1996) use one or more sieves with different meshes (125, 500, 600, 1000, 1500 micron and 1 mm) before splitting to remove the larger swimmers or flocculated material. This procedureis followed on occasion by swimmer picking under a microscope (Miquel et al., 1994; Conte et al., 2001). In some cases, swimmer removal is by sieving alone (Honjo and Manganini, 1993; Karl et al., 1996). Others (Steinberg et al., 2001; Antia et al., 1999) remove swimmers only by picking under a microscope. Splitting and drying methods vary widely depending on the different analyses to beperformed.In addition, there are differences also in the splitting schemes (number of fractions) (Honjo andManganini, 1993; Miquel et al., 1994; Karl et al., 1996; Antia et al., 1999; Steinberg et al., 2001; Conte et al., 2001). In Table 1 we report these methods and, where available, the devices used.  F. Chiarini et al. / Methods in Oceanography 8 (2013) 13–22  15        T     a       b       l     e       1     D     i     f     f   e   r   e   n    t   s   p     l     i    t    t     i   n   g   m   e    t     h   o     d   s     (     f   r   o   m    B   u   e   s   s   e     l   e   r   e    t   a     l . ,    2    0    0    7 ,   m   o     d     i     f     i   e     d     ) .    I   n   v   e   s    t     i   g   a    t   e     d   a   r   e   a      *     T   r   a   p     d   e   p    t     h     (   m     )    T     i   m   e     f   o   r   p     i   c     k     i   n   g     (     h     )    M   e   s     h   s     i   z   e     (   m     i   c   r   o   n     )    T   y   p   e   o     f   p     i   c     k     i   n   g    S   p     l     i    t    t     i   n   g   m   e    t     h   o     d    R   e     f   e   r   e   n   c   e    B   e   r   m   u     d   a    A    t     l   a   n    t     i   c    T     i   m   e  -   s   e   r     i   e   s    S    t   u     d   y     (    B    A    T    S     )    1    5    0    1  –    2   n   o   n   e    2    5    0      ×     V    E    R    T    E    X   m   e    t     h   o     d    S    t   e     i   n     b   e   r   g   e    t   a     l .     (    2    0    0    1     )    2    0    0   o   r    3    0    0    5    0    0      ×    m   a   g   n     i     f     i   c   a    t     i   o   n    H   a   w   a     i     i    O   c   e   a   n    T     i   m   e  -   s   e   r     i   e   s     (    H    O    T     )    8    0    0    0    1    0    0    0    N   o   n   e    S   a   m   p     l   e   s   p     l     i    t     i   n    t   o    4     f   r   a   c    t     i   o   n   s   w     i    t     h   a   r   o    t   a    t     i   n   g   s   p     l     i    t    t   e   r     d   e   v     i   c   e    K   a   r     l   e    t   a     l .     (    1    9    9    6     )    1    5    0    0    2    8    0    0    4    0    0    0    M   e     d     i    t   e   r   r   a   n   e   a   n    S   e   a    8    0    0 .    5  –    1    I   n   s   o     l   u    t     i   o   n   a   n     d   w     i    t     h    5    0      ×    m   a   g   n     i     f     i   c   a    t     i   o   n    A     f    t   e   r   s     i   e   v     i   n   g    t     h   r   o   u   g     h    1    5    0    0   m     i   c   r   o   n   s     i   e   v   e ,    1       /     4   o   r    1       /     8   o     f    t     h   e     l     i   q   u     i     d   s   a   m   p     l   e     i   s   s   e    t   a   s     i     d   e .    O   n   c   e   s     i   e   v   e     d   a   n     d   p     i   c     k   e     d ,    t     h   e   r   e   m   a     i   n     i   n   g   s   a   m   p     l   e     i   s     d   e   s   a     l    t   e     d   a   n     d     f   r   e   e   z   e  -     d   r     i   e     d    M     i   q   u   e     l   e    t   a     l .     (    1    9    9    4     )    2    0    0    1    5    0    0    1    0    0    0    6    0    0    E   u   r   o   p   e   a   n   c   o   n    t     i   n   e   n    t   a     l   m   a   r   g     i   n    6    0    0    3  –    5    9     f   o   r     h     i   g     h     f     l   u   x   n   o   n   e    1    2    0      ×    m   a   g   n     i     f     i   c   a    t     i   o   n    S   a   m   p     l   e   s   s   p     l     i    t     i   n   a   r   a   n   g   e     b   e    t   w   e   e   n    1       /     8   a   n     d    1       /     1    2    8    A   n    t     i   a   e    t   a     l .     (    1    9    9    9     )    1    0    5    0   o   r    6    0    0    1    4    4    0    3    2    2    0    N   o   r    t     h    A    t     l   a   n    t     i   c    B     l   o   o   m    E   x   p   e   r     i   m   e   n    t     (    N    A    B    E     )    1    0    0    0    0    1    0    0    0    N   o   n   e     >     1    0    0    0   m     i   c   r   o   n     f   r   a   c    t     i   o   n   s   p     l     i    t     i   n    t   o    4   s   u     b   s   a   m   p     l   e   s   ; 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Chiarini et al. / Methods in Oceanography 8 (2013) 13–22 Fig. 1.  Moorings A and B location in the Ross Sea (Antarctica). This work intends to highlight some issues concerning different trap split procedures in order toprovide the scientific community an efficient and standard method. This will be a great advance interms of being able to compare data from different laboratories. In detail, we propose a processingmethodology for samples collected by a sediment trap in the Antarctic region that combines thestrengths of two currently employed procedures (Dunbar and Mucciarone, 2003; Heussner et al., 1990).We use material collected in the Antarctic from two moorings (A and B) deployed in the Ross Sea(Fig. 1). (The analyses results are reported in Chiarini et al. (2014a,b).) These kinds of investigations allow more accurate quantification of particle fluxes that control theocean’s biological pump through the water column. 2. Sample processing procedures in the Antarctic region TwoproceduresaregenerallyusedtoprocessAntarcticsedimenttrapsamples:thefirst,developedby Dunbar and Mucciarone (2003) is mainly used at Stanford University; the second was initially describedbyHeussneretal.(1990).AsreportedbyLangoneetal.(2003)andDunbaretal.(1998),the results obtained by both procedures are comparable. The main differences between the two methodsare the way the sample is arranged for analysis and the timeframes involved.The Heussner protocol requires a preliminary planning stage to establish the most appropriatesample split depending on the sample mass and the type of analyses desired. The sample is then splitinto different fractions (typically between 1 / 8 and 1 / 128) using a precision peristaltic pump untilthe required number of subsamples is achieved. These preliminary procedures are time-consuming.Moreover, having to establish the analyses to be carried out beforehand makes the procedure lessflexible. In the Dunbar and Mucciarone method, on the other hand, splitting consists in dividing thesampleinhalfseveraltimesusingaFolsomPlanktonSplitter(McEwenetal.,1954),dependingonthe amount of sample available and the numbers and sizes of subsamples desired (Fig. 2). The Heussner method involves the use of filters, which requires sample pre-treatment to removethe organic component and carbonatic fraction. The Stanford method, on the other hand, produces a  F. Chiarini et al. / Methods in Oceanography 8 (2013) 13–22  17 Fig. 2.  Example of splitting scheme using a Folsom Plankton Splitter. An entire sample is split in half several times.  Table 2 Pros and cons of the two sample processing methods.Method Pros ConsSwimmer removal by picking under amicroscopeAll organisms can be catalogued Longer time.Requires more operator experienceSwimmer removal by screening Shorter time Organisms cannot be classifiedSome microscopic organisms likeforaminifera and radiolarians are notidentifiedSubjectivitySplitting with a peristaltic pump Good precision regardless of operator skill.Longer timeHigher maintenanceRequires more accurate picking beforesplittingSplitting with a Folsom PlanktonSplitterShorter time.Accurate picking before splittingnot required.No maintenanceAccuracy also operator-dependentFiltering Less material dispersion A splitting scheme necessary to establishhow many and what analyses are to beperformedSamples to be pre-treated beforeanalysisFreeze-dry method An aliquot of material obtainedthat can be used like any sediment sample fraction via freeze drying. A further difference between the two procedures is the removal ornot of swimmers only from subsamples to be subsequently analyzed.Table 2 summarizes the strengths and uncertainties of each procedure. 3. The proposed method We propose a revised protocol for the analysis of samples collected by a sediment trap in theAntarctic region. It consists in combining some of the steps of both procedures described above(paragraph 2) to allow more rapid sample processing without jeopardizing precision.
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