7-Analysis of zooplankton samples

Transcription

1 7-nalysis of zooplankton samples The basic analysis consists of measurements of biomass (standing stock), enumeration of common taxa and species iomass The term biomass denotes the live weight or the amount of living matter present in the zooplankton sample. The value obtained is used to evaluate the secondary productivity and fishery potentials of the study area. The biomass is estimated by the following methods. 1. Volumetric (displacement volume and settling volume) method 2. Gravimetric (wet weight, dry weight and ash free dry weight)method 3. hemical method Prior to determination of biomass, larger zooplankters such as medusae, ctenophores, salps, siphonophores and fish larvae should be separated from the zooplankton sample and their biomass taken separately. The total biomass would be the biomass of bigger forms plus the biomass of the rest of the zooplankton. It should be indicated under remark as given on the analysis sheet. Volumetric method The volume measurements are easy to make in the field or laboratory. The total zooplankton volume is determined by the displacement volume method. In this method the zooplankton sample is filtered through a piece of clean, dried netting material. The mesh size of netting material should be the same or smaller than the mesh size of the net used for collecting the samples. The interstitial water between the organisms is removed with the blotting paper. The filtered zooplankton is then transferred with a spatula to a measuring cylinder with a known volume of 4 % buffered formalin. The displacement volume is obtained by recording the volume of fixative in the measuring jar displaced by the zooplankton. The settled volume is obtained by making the sample to a known volume in the measuring jar. The plankton is allowed to settle for at least 24 hours before recording the settled volume. Gravimetric method The weight measurement should be done preferably in laboratory. It is carried out by filtering the zooplankton. The interstitial water is usually removed by blotting paper. While blotting, due care should be taken not to exert too much pressure as to damage the delicate organisms or specimens. The zooplankton weight is taken on predetermined or weighed filter paper or aluminum foil. The wet weight is expressed in grams. The dry weight method is dependable as the values indicate the organic content of the plankton. nalysis such as the dry weight is determined by drying an aliquot of the zooplankton sample in an electric oven at a constant temperature of 60º. The whole or total sample shouldn t be dried because the subsequent analysis such as enumeration of common taxa and identification of their species wouldn t be possible after drying the sample. The dried aliquot is kept in a desiccator until weighing. The values are expressed in milligram. 1

2 iomass (standing stock) fter estimation of zooplankton biomass the standing stock values are converted into per cubic meter and is calculated as follows: a. Volume of zooplankton (ml/m 3 volume of zooplankton ) = Volume of water filtered (V) b. Wet weight of zooplankton (g/m 3 wet weight of zooplankton ) = Volume of water filtered (V) c. ry weight of zooplankton (g/m 3 weight of zooplankton ) = Volume of water filtered (V) hemical method: In this method, the live zooplankton samples are dry frozen. efore analysis, the samples are rinsed with distillated water. Measurement of constituent elements such as carbon, nitrogen, phosphorus and biochemical elements viz. protein, lipid and carbohydrates are made. Sometimes the biochemical values of a particular taxon and species are undertaken to evaluate food energy transfer at higher trophic levels. The calorific content of the plankton can be used as an index of zooplankton biomass. aunal enumeration Information on the faunal composition and the relative abundance of different zoplankton taxa and their species is obtained by counting the plankters present in the samples. The enumeration of specimens in the total sample is laborious, time consuming and mostly impractical. The number of common zooplankton groups and their species observed in the samples may vary from tens to thousands. or enumeration it is recommended that the subsample or an aliquot is taken for the common taxa. However, for the rare groups, the total counts of the specimens in the samples should be made. or enumeration of zooplankton the subsample or aliquot of 10 to 25% is usually examined. However, the percentage of aliquot can be increased or decreased depending on the abundance of zooplankton in the sample. Subsample (aliquot) Instruments are available for splitting the sample into the fractions (ig. 1 and 2). These are generally made of plastic with internal partitions. olsom plankton splitter and Motodo plankton splitter are widely used. The zooplankton sample to be subsampled is poured into the drum and the drum is rotated slowly back and forth. Internal partitions divide the samples into equal fractions. The fraction may be poured again into the drum for further splitting. The process is repeated until a suitable subsample is obtained for counting. The splitter is thoroughly rinsed to recover the organisms, which may be sticking onto the wall of the drum. The sample is usually splitted into 4 subsamples. One of the subsamples is used for estimation of dry weight, the second for counting the specimens of common taxa, the third for relative abundance of species and the fourth fraction is kept as reference collection. Plastic or glass pipettes are also used to take the subsample for counting. The stempel pipette is used to obtain a certain volume (0.1 to 10 ml). The zooplankton sample in a glass container is diluted to a known volume and is stirred gently. The stampel pipette is then used to remove the subsample or aliquot for counting. 2

3 olsom Plankton Splitter General Procedures: level table with adjusting screws pour sample into console and mix thoroughly rotate sample drum on rocker arm sample is split into two compartments that drain into separate holding trays when rotation is complete Motodo Plankton Splitter General Procedures: level table with adjusting screws pour sample into mixing area rotate splitter on rocker arm sample is split by cutting edge and contained into two compartments one of the compartments is easily drained while the other holds the remaining half sample for further fractionation (when splitting a single sample more than once, this feature conserves one step as compared to the olsom method (See olsom Plankton Splitter) 3

4 Quantitative analysis of zooplankton samples fter splitting, the next step in the analysis is to sort and count the specimens. Use the plastic Pasteur pipette to fill the bottom of a plastic Petri dish with sample and study your sample under the stereomicroscope at different magnifications. fter you have studied your specimens under the compound microscope in detail, you should be able to recognize the different taxa even under low magnification on the stereomicroscope. ll the specimens present in the subsample are counted. The total number of specimens are later calculated for the whole sample depending on the percentage of subsamples examined. The Steps To ollow 1. Identify the number of different organisms present in your sample. 2. ount the total number of each organism. 3. Use the numbers to estimate various measurements Of the taxa you can differentiate and recognize LULT 1. Organism ensity (org.m -3 )- The total number of each target taxon in a cubic meter of seawater Three steps used for calculation. Step One: alculating the volume of water sampled by the net (You calculated this in lab two, V =.d, where = π.r 2, and d = haul distance) Step Two: alculating the average numbers of each species per ml sampled under the microscope You must know: 1- the total number of each species counted (a sum taken from all the milliliters you analyzed) 2- the total number of milliliters you analyzed alculation: simply divide the total number of each species by the total number of milliliters you analyzed; use two decimal places of accuracy. ormula: #/ml = total number of each taxon you observed/ total number of milliliters you analyzed Step Three: alculating the number of each species in a cubic meter of seawater. (#/m 3 ) You must know: 1- The volume of filtered seawater by the plankton net in cubic meters (step one). 2- The average number of each species per milliliter. (You calculated this in step two.) 3- The total number of milliliters in the sample bottle. alculation: Multiply the average number of each taxon by the total volume of the entire plankton sample. This gives the total number of each target organism per sample. ivide this by the volume (m 3 ) of seawater sampled. ormula: The density of each target taxon (Number of the target taxon/m 3 ) (n)(v s) = = No. of organisms/m 3 Vm Where: n = verage number of organisms in 1 ml subsample V s = Volume of plankton sample (ml) V m = Volume of seawater sampled by the net (m 3 ) 4

5 2. requency of Occurrence () - It is calculated taking into account the number of samples in which the organism was found, relative to the total number of samples collected, in percent. = Ts. / TS where "Ts" is the number of samples in which the taxon (species) is present, and "TS" is the total number of samples. The results are presented in percentage (%), being used the following approach: > 70% - Much requent 70% 40% - requent 40% 10% - Less requent < 10% - Infrequent/Sporadic 3. Richness (S) richness is a measure of the number of species found in a sample. Procedure: ount the number of species found in a community. xample: If you find 6 different species, the species richness S= Menhinick's species index () Since the larger the sample, the more species we would expect to find, the number of species is divided by the square root of the number of individuals in the sample. This particular measure of species richness is known as, the Menhinick's index. s = N where s equals the number of different species represented in your sample, and N equals the total number of individual organisms in your sample. xample onsider the following data from sample of organisms from a biological community irst ommunity # of individuals alculate the Menhinick's index Number of different species = 6, number of individuals = = s/ N = 6/ = 6/10 =0.6 iversity The species richness index and Menhinick's index, discussed above gives no indication of population distribution or species diversity. If for example we calculate the species richness index and Menhinick's index for this second community, we will find that the resulting value is as the same as the first 5

6 community, of 6 and 0.6 respectively, because both communities have the same total number of individuals (), and the same number of species (6). Second ommunity # of individuals The distribution of the number of individuals among the six species of the two communities however are different. In first community one species ( species ), numerically dominates the other five species. In second community the six species are more evenly represented. ecause of this difference, second community would be considered to be more diverse than the first one, despite both communities having the same total number of individuals and the same number of species. Thus, when measuring species diversity the relative abundance of each species must be taken into account. or this reason, other indices such as species diversity index (e.g. Simpsons Index of diversity or Shannon-Wiener diversity index) are necessary to fully understand the populations. diversity index differs from species richness in that it takes into account both the numbers of species present and the dominance or evenness of species in relation to one another. The values of species diversity can be used to assess the health of the environments. The members of species are less in the polluted areas. 5. Relative abundance (Ra) - with the formula: Ra = N. / Ns where "N" is the number of organisms of each taxon (species) in the sample. "Ns" is the total number of all organisms in the sample. The results are presented in percentage (%), being used to prepare a semi-quantitative list (dominant, abundant, occasionally found (or less abundant), rarely found ) using the following approach: > 70% - ominant 70% - 40% - bundant 40% - 10% - Less bundant < 10% - Rare xample: alculate the relative abundance for each species in first community number of organisms in the sample # of individuals Ra = N. / Ns 59% 12% 11% 10% 5% 3% bundant Less bundant Less bundant Less bundant Rare Rare 6

7 6. Simpson s Indices a. Simpsons Index of diversity This index accounts for the species richness (the number of species) and the proportion of each species (Pi). This index gives a better idea for which species is most abundant. How to calculate the Simpsons Index : numberof individuals of a particularspecies 1) alculate P i for each species: P i = totalnumberof individuals (organisms) 2) alculate P 2 i for each species 3) alculate Simpsons Index =Σ(P i ) 2 irst ommunity # of individuals P i =Σ (P i ) 2 Second ommunity # of individuals P i =Σ (P i ) 2 (P i ) (P i ) Simpson s Index indicates the probability that two randomly selected individuals will belong to the same species. The closer to one (high value of ), the more likely two randomly picked individuals will be the same species, indicating low biodiversity. lower value of indicates higher biodiversity. In our example, the probability that two randomly selected individuals will belong to the same species in the first community ( = 0.388) is higher than second community ( = 0.174) indicating lower biodiversity in the first as compared to second one. will decrease (as in second community) if the percentage of individuals in each species (species proportions) within a community is nearly equal (0.21, 0., 0.19, 0.14, 0.13, 0.13). In first community however, most individuals are belong to species, 59/, so the probability a randomly selected individual will be is 59%, and biodiversity is low. 7

8 b. Simpson s index of iversity: 1- Indicates the probability that two randomly selected individuals will be different species. value approaching zero indicates lowering biodiversity. xample or the first community1-= =0.612, or the second community1-= =0.826, So, the first community have lower biodiversity than the second one c. Simpson s Reciprocal Index: 1/ The reciprocal tells us the number species that will produce the observed Simpson's index. larger number indicates evenness among species (absence of dominance). number approaching 1 indicates that only a few or one single species is dominant. In our example, 1/ for the first community = This means that only 2.58 species of the six species will produce our Simpson s Index. This is due to the dominance of the species. or the second community, due to the absence of dominance of certain species, 1/ = This means 5.76 species of the six species will produce our Simpson s Index. 7. Shannon-Wiener diversity index (H) - Similar to the Simpson's index, this measurement takes into account species richness and proportion of each species. The Shannon Index is used to compare diversity between habitat samples y itself, the Shannon Wiener Index H has no meaning, but when used to compare two different habitats (communities) or one habitat (community) at different times, it is a good indicator of change. Similar to the Simpson index, the first step is to calculate P i for each species. You then multiply this number by the log of the number. While you may use any base, the natural log is commonly used (ln). The index is computed from the negative sum of these numbers. In other words, the Shannon-Wiener diversity index is defined as: s i = 1 ( ) H P ln p = The results are presented in bits per individal (bits.ind -1 ), being "1 bit" one information unit. More than 3 bits.ind -1 are considered high diversity, less than 1 bit.ind -1 is considered as low diversity. s H approaches zero, biodiversity decreases. i i 8

9 xample: alculate the Shannon Wiener Index H for each species in both communities Shannon index, H, calculations irst ommunity # of individuals Pi Ln(Pi) H=-Σ Pi*Ln(P i ) Second ommunity # of individuals Pi (Pi) H=-Σ Pi*Ln(P i ) Pi*Ln(Pi) Pi*Ln(Pi) The results indicates that the second community has higher species diversity (H = 1.77) than the first one(h = 1.293) In summary, the species diversity approach is generally a more reliable measure of biodiversity than other indices such as species richness. While mathematically very easy to calculate, the limitations of the species richness concept can be seen when applying it to communities such as and, where it fails to distinguish their quite different community structures. 8. venness Similar to Simpson s Reciprocal Index: 1/, but using species richness (S) and the Shannon-Wiener index (H). venness () is a measure of how similar the abundance of different species are. It uses species richness (S) and the Shannon-Wiener index (H). When there are similar proportions of all species, then evenness value is very close to one, but when the abundance are very dissimilar (some rare and some dominance species) then the value decreases. Using the same log base as with H, evenness is defined as: = H/ln(S) where "S" is the total number of species, and H is the Shannon-Wiener index xample 1 venness () for the first community = 1.293/ln (6) = 1.293/1.792 = 0.72 venness () for the second community = 1.771/ln (6) = 1.771/1.792 =

10 xample 2: In this example we will prove that if the taxons are equally present in a habitat the value will be 1. onsider the following hypothetical sample # of individuals Pi Ln(Pi) H=-Σ Pi*Ln(P i ) Pi*Ln(Pi) = 1.61/ln(5) = 1.61/1.61=1 Thus, the value of 1, indicates that the species are equally present in the habitat. xample 3: calculate the evenness for a community contain 11 species and Shannon- Wiener index (H) of = 0.366/ln(11)=0.153 This indicates poor evenness. Lab report iscuss your observations with the members of your research team to come up with a final assessment of which taxa are more or less abundant in your sample. or the lab report, include the quantitative list of taxa and discuss which taxa were the most abundant in your sample. Which group of zooplankton is the dominant in your sample? Report the most abundant taxa to the classroom blackboard with your station number/location. fter all groups have reported their findings to the blackboard, can we see differences in zooplankton community structure among the sampled stations? iscuss these differences in your lab report. 10

11 Homework onsider the following three samples Sample I Sample II Sample III # # # a. calculate the following 1. Relative abundance (Ra) for each species. Prepare a semi-quantitative list for your results 2. requency of occurrence (). Prepare a semi-quantitative list for your results 3. Richness (S) 4. Menhinick's species index () 5. Simpson s Indices a. Simpsons Index of diversity b. Simpson s index of iversity: 1- c. Simpson s Reciprocal Index: 1/ 6. Shannon-Wiener diversity index (H) 7. venness b. Make a discussion for each result you obtained 8. List two advantages to using a Shannon Index instead of simply a population count to determine diversity. 9. List two disadvantages to using a Shannon Index to determine diversity. 11