Primary Production in Lakes

The mean photosynthetic rate of lake phytoplankton under optimal light (natural conditions) is about 0.5 micromoles of 02 evolved or CO2 consumed per microliter of organisms per hour; l-2 micromoles per milligram ash-free dry Ljceight per hour; and 0.2 micromoles per microgram of chlorophyll per hour. Simple equations are presented for computing photosynthetic yield per square meter of water surface. Computations for several lakes yielded values mostly lying between 150-200 millimolcs per square meter per day. A yearround study under completely natural conditions in western Lake Erie showed writer yields of about 40 millimoles per square meter per day and summer maxima of about 300. The annual curve followed the solar radiation curve closely. It is estimated that 28 days are required for a gram of phytoplankton (ash-free dry weight) to produce an additional gram of glucose available for reproduction; 80 per centJ of the photosynthate produced is required to support phytoplankton respiration. Three quantities, related to primary production in lakes, are measured by aquatic* biologists: (1) the volume of autotrophic organisms, (2) the ash-free dry weight of suspended particles (organic seston), and (3) the concentration of chlorophyll-eac*h of these expressed per unit volume of water. These are measures of standing (arop, and they represent roughly quantitative estimates of the populat,ion of autotrophic organisms upon whi& primary producation is based. Some biologists have studied photosynthcsis under approximately natural cwndit,ions and have related the ohscrvcd Oz-prodwtioll or COZ-consumption lo one or mow of thcl quantities named above. l“ron1 sucah data it is possible to compute ~ncan photos?;nt)hetic yields per unit of standing crop. ‘I‘abl~ 1 present’s a summary of suc*h yields w~mputed from the literat,urc. These data rrlprcsent total or true photosynthesis, i.e. the experiments incaluded darkened hot tles to take account of the C‘C )s cwntrihutccl by rwpiration, or of the 02 so (Y~IISUII~C~. Riley’s (I 941) data arc inc~ludcd lwur~~~ t holy permit computation iI all t hrw c*atcgoriw. and because they shop\I hat the rates for marine phytoplankton NO in gc~~ral agrwrnent with those for f’rwh IIX~W phytoplankt,on. The data in t)his tahlrl suggest that t!hc mean photosynthctich ratct of a nlic~rolitcl of aut atrophic organisms, under optimal light’ (natural conditions) amounts to about 0.5 mic*romoles of 02 evolved or CO2 consumed per hour (6 fig C). The rate per milligram of ash-free dry weight is about 1 or %, ( 12-24 pg C) and the rate per microgrant of chlorophyll is about 0.2 micromoles of ( J3 (2.4 pg C). These values represent ~~un~crous t,ests and can he regarded as fairly reliable means, but results from individllal test,s vary widely. The data reprewit rlpilimnion collections. Manning and .Juday ( 1941) state that hypolimnion samples exhibit yields which are less than one-third so high. A’I smvey of the literature she\\-s that there is a rough inverse correlation between the dr@h of the epilimnion (or total depth ill llnstratified lakes) and the standing crop of auf obrophic plants (,Manning and Juday 19-l 1, Rwnak 1919, Verduin 1954). Thus lakw \vith an epilimnion layer of the order of CHH~ meter are likely t,o have standing cbropx of t,hc order of 50 microlitcrs of plant volu~nc: per liter of water, 15 milligrams asht’rcc dry w4ght, and :30 micrograms of chlorophyll. Ilakes having an epilimnion depth of’ thrb order of 10 meters are likely to have st atldillg (*raps about one-tenth so high. rI’hc%~~ values arc intended to indicatNc ordcr~ of magnitude only. Large \-:triations iii stallding crop arc obsci*\-cd, ot’ ~~oi~rsc,