The Trouble with Chlorophyll: Cautions Regarding the Collection and Use of Chlorophyll Data:stb No.aqss-10

10 Pages·1996·0.22 MB·English

by  Ontario Ministry of the Environment

Ontario
MINISTRY OF ENVIRONMENT AND ENERGY
SCIENCE AND TECHNOLOGY BRANCH
AQt^T4€ SCIENCE SECTION
STBTechnicalBuUetinNo.AqSS-10
October 1996
THE TROUBLE WITH CHLOROPHYLL:
CAUTIONS REGARDING THE COLLECTION AND USE OF
CHLOROPHYLL DATA
Resource managers and researchers from man> from the onset. chloroph>ll data can present
agencies commonl\ use chloroph\ll as a trophic uncertainties if the samples ha\e not been collected
status mdicator Although \anation m chlorophxll understncth controlledconditions.
concentrationtendstobethemostcaidentconsequence
ofchangesintrophicstatus,thereareproblemsimohed Chlorophyll samples are often collected as euphotic
withusingthistestas abasis foreithersettingtrophic zonecomposites and reported as ice-fi"eemeans. The
status objeclnesordetectinglong-termchange These euphoticzone.usualK approximatedast\\icetheSecchi
problemscanbesummarizedasfollows: disc MSibilitN. ISsometimes wellmixed smcemuchof
• the collection and submission ofchlorophyll this layer is composed ofepilimnion. Howe\er algal
samples require precautions that arecomplex cells will often stratif> dramaticalK below the
comparedtoothertrophicstatusparameters epilimnion and this can occur e\en in mixed la>ers
• changes in anahtical methods ma> disrupt (Fig.1). This means that chloroph\ll concentrations
long-termchlorophylldatasets. based on euphotic zone composite samples may vary
• significantseasonalandmter-aimualvariation basedsimpK onthephvsicalcollectionmethodsie;how
in chloroph\11 requires thecollection oflarge thewateriscombmedinproportionfromgi\endepths.
numbersofsampleso\ermanyyears. Thisis\er\ rele\antinsituationswherethedepthofthe
• man\ different chloroph\ll pigments are euphoticzonerelateetothethermoclmechangesover
commonh measured, le: Chi a. b. c. chl a lime.
correctedetc..concentrationsofthesepigments
ma> not correspond to actual ph>1oplankton
celldensities.
DAT.ACOLLECTION
Chloroph\ll samples must be collected into
opaquebottlesandimmediateK fixedwithmagnesium
carbonate (MgCO-, ensures that the sample remams
"basic" to a\oid con\ersion of priman pigments to
phaeopigmcntsunderacidicconditions). The\ mustthen
be kept cool and filtered as soon as possible. The
filtratemustbetrozaiandtransportedtothelabwithout
being allowed to thaw. This makes the remote D^th(m)
collectionofsamples ditTicultorimpossiblesuch that, Figure 1:VerticaldistnbutionofChlainPlasticLake.

CHANGESINAN/\LVTItAl.METHODS Long-tenn, Ice-fr?e Means
The reported concentrations ofchl a ha\e been
subjectto methodschangesattheMOEElaboratories.
Thelong-termdatabasehasbeenmostnotablybroken GravenhurstBay
duetochangesinthemethodsthatoccurredin 1985. At
thattime,aswitch tonylon filtersincreasedextraction
efficienciesoftheacetone. Thisresultedinanmcrease
in post "85 values. Although it may be possible to
•align" the data before 1985 to match current values,
thtTCisnosinglecorrectioniaclorthatcanbeappliedto
thesedata. Databasemanagerswhohavechlorophyll
vraelfeurersisngpatnontihnegm1e9t8h5osdhsouclhdarnegfeesrtwohithcehdwoacsumpeunbtlaitsihoend 1986 1988 1990 1992 1994
bytheLabServicesBranchm 1985. Year
Figure3: Long-termvariationinice-freeChlafor
SEASONAI.ANDINTER-AMVIIAI,VARIATION GravenhurstBayandSouthBay(LakeMuskoka).
The largest problem with the interpretation of
chlorophyll data is associatedwith seasonal andinter- These problems tend to increase in severity with
annual vanation. Chlorophvll concentrations vary increasing trophic status such that the situations that
ssihgonwifidciafnflelrvenotnseaasseoansaolnpaaltbtaesrinsswbeittwhienenlalkeaskeasn(dFiogf.t2e)n. raelqsuoirteentdhetmoosrteqautitreenttihone,mleo;smtorseamepnlnecshetdo sdyesstcenmbs,e
In addition there is a great amount of long-term, or accuratelv-
between-\ear \anation in the ice-frce means for
mcodlileMcdtunauimelrakoeus.ss(Faimgp.l3e)seTahcihsvmcaarketsodietnnvecceiscsea-rfvreeto Taanbdleth1e;Nnuummbbeerrooffchvleoarrosphovf11dastaamprleeqsuur-eeqduirtoedbeeac\h\ivtehairn
meansthatareclosetotheactual\alue.andman\ years specified percentsofthe acUialmean. Numberofyearsof
ofthis t\pe ofdata are required to estimate the long- dataLsbasedonseasonalmean.sthatareapprox.within 20%.
term mean (Tabii- 1). Thus it is difficult to assess
whetherobserved changes m chlorophvll are actualK #samples/year #years
%ofmean 10 20 50 10 20 50
BlueCHALK 55 14 2 3 1 1
Harp 59 15 2 7 2 1
Dickie 126 32 5 16 4 1
GravenhurstBay
Chub
Dickie
RedChalk CELLDENSITYVSPIGMENTCONCENTRATION
Finally,thewholepictureis furthercomplicated
b> the fact that chlorophvll concentrations arc not
Figure2; SeasonalchangesinchlaforGravenhurst aiwavstiedtophvtoplanktoncelldensities. Theactual
Bav.Chub.DickieandRedChalklakesin 1993. concentrationofchlorophv11inalgalcellsisdetermined
by incident radiation, species composition, nutnent
supply andcertamaspectsofalgalphvsiologv. These
reflectinglong-termchangeorwhetherthe> aresimpK determinantshaveaseasonalcomponentsuchthatthe
noisebasedonthecollection oftoo fewsampleseach correspondence between chlorophvll a and algal cell
year ortoofew vears ofdata bemg usedtodetectthe densitiesisnotconstant.
change. Development objectives for individual lakes
thatarebasedonchlorophyllwillthereforebedifficult Theserelationshipscanfurtherbespecifictodifferent
toassesssinceitwillbeimposibletotelltheditTerence chlorophyllpigments, himostcaseschlorophvllaora
betweentheactualsurpassmgofobjectivesandsimple versionofchlorophv11acorrectedforphaeopigmentsis
variation based on the collection oftoo few samples. used to represent the phvtoplankton community

Sometimes chl b or chl c arc quoted but often the
relationship between the concentrations of specific
pigmentsandtheconcentrationsofalgalcellinthelake
do not correspond because the cells in greatest
abundance do not contain pigments that are being
measured. Also, algal communities are changing
seasonallybackandforthbetwt>enthosethatcontainthe
investigator'spigmentofchoiceandthosethatdonot.
CONCLISIONSANDRECOM.MENDATIONS
When all of these problems are considered it 20 40 60 80 20 40 60 80
makesitdifliculttorecommendchlorophyllasatrophic tctalphosphorus(ug/L) tctalphosphorus(ug/L)
status indicator in situations where small amounts of
dataarecolkx'ted. Mostoftheproblemsoutlinedabove
areamplifiedbythecollectionoftoolittledata.Thisis
nottosaythatchlorophylldatashouldnotbecollected. Figure4: Relationshipbetweentotalphosphorusand
AgreatdealofusefulldataexiststhatshowtheelTects chlorophylla(lefi)andphytoplanktoncellvolume
ofphosphorus loadreductions, zebra mussels, etc. on (right).
chloroph\IIconcentrations Thesearcgcneralh based
on large data sets that are not plaguedbyseasonal or Details about estimating the trophic status ofa lake
inter-annual\anation. basedon totalphosphorus are available in STB Tech.
Bull. AqSS4
SincevirtualK noneofthesameproblemsoutlinedfor
thecollectionofchlorophylldataappK tothecollection
oftotalphosphorusdataitisprobabK bettertousetotal
phosphorus as an mdicator of trophic potential in Forfurtherinformation, contact:
situationswhereminimaldatasetsarebeingcollected.
B. Clark
Lastl>.thecostofmonitoringthetrophicstatusofalake Phone: [phone]
basedonspnngtumo\erTPwouldbeafractionofthat Fax: [phone]
involvedwithusingchlorophyll. Springturnovertotal Email: clarkhedepo.gov.on.ca
phosphorusbasedtrophicstatusestimatesv\ouldrequire
onlyone\isiltoeachlakeper\ear. Sinceice-freemean
chloroph)IIestimatesrequireatleast 6or7visitsper
yearandconsideringthatthechloroph\IItestisapprox
4timesasexpensiveasaTPtest,therelati\edifference
intestcostsalonewouldbeintheneighbourhoodof25
times. When staff and transportation costs are
consideredthenumbersbecomesignificantlydifferent.
Cost aside, the results would be much more reliable
whenbasedontotalphosphorussuchthatitwouldbe
recommendedinalmoste\er\ casetobasetrophicstatus
estimatesontotalphosphorus Ifmformationaboutthe
ph\toplankl.oncommunity mustbecollected,managers
should consider collectmg seasonal composite
ph\toplankton enumeration samples. Generalh.
weekl>. bi-weekl\ ormonthh phxtoplankton samples
arecollectedandfixedwithLugols fixati\e. Thesema\
be combined attheennumeration Lab m Re.xdaleand
coimtedtopro\ideseasonal,mean,phytoplanktoncell
densities. These numbers will relate betterto trophic
status than wall chlorophyll estimates (Fig.4) andthe
costswillstillbeapproximatelyhalf