530 Thiihi-sirlh Aniiii.il Mrr/iinj 



prdccss t;ikcs |il;i(c ;il iiii^iit in m ici'. liiil ord iii;ii'il\- t(i \it\ 



small (Icpllis. Id pMici'al. wf iiiav >ay thai iliii-iii- cai-ly ami 

 inidsniiiiiici-. I.cl'div the iicriod of -ciicral (•(>(. lin- Ucuiiis. these 

 )iri)ccsscs do iiol cxiriid lo -rcalcr dcplhs ihaii ten or liriccn 

 reel. Al Ihc scas(.n. I hcivroiv. when vital |ii'(ic,.sscs arc iiio>l 

 acti\i' and the need lor oxyp'ii is .iz'rcatcst, coiixcct ion cni'i-ciil^ 

 all'oi'd a niiiiinuini of assistance in distrihut in.i;- it. The main 

 i-cliaiice. thci-erorc, for the disti'ihut ion of ow^'cii is in the third 

 factor, the wind. This, as ;dread\- said, is \-er\- enieiciit when 

 the lake is uniform in temperature: hut dui'iui;- the spi'iiiL;-. as 

 the lake uaians. it hecomes increasiniily inelVecti\c and duiani;- 

 the sumnu'r its action is conlined to the ujtper wai'mcd layei' id' 

 the hike, and the iovwi-. cooler water is wliolly shut otV fi-om tlie 

 direct intlueiiee of the wind cui-i-ents. 



These facts show that an iidand hdvc lias an extrenudy inef- 

 ficient apparatus for al)sorhing and distril)utin(i- oxyi^cn and the 

 net result is that in many lakes tlie amount and cdiaractei' of the 

 hig-lier lif\' whic-h the l;d<e will su|)])ort are conditioncMl hy th.' 

 amount of o.xy^iX'n wliich the lake contains ratlu'r tlian Ijy the 

 amount of food which it can pi'oduce. The o\y,ti\'n in the lower 

 and cooler water of tlie lakes cannot l)e I'euewed hetween sprin.i;' 

 and tall. This amount would ])v iiKh'cd ain]ile to sustain a 

 lai'ii'e amount of aninnd lite in full activity. l)Ut its use can- 

 not l)e confiiu'd to the necessities of ordinai-y life. The pro- 

 cesses of decomiiosition draw u])on it nuudi more hea\ilv tlian 

 does the animal oi- the ordinary ve^u'etahle life. All the i)lants 

 and animals of the u])])ei- water, wliich die and sink into the 

 dee})ei- strata, the leaves hlown into the lake, and the malei'ial 

 waslied in from the shore, dei-ompose in tlie cooler watei' and in 

 the process of (lecom])osition use up a ,yreat amount of oxyueii. 

 This depletion of the stock of oxygen <2,(h.s on with a !'aiiidit\ 

 which x-aries with the amount of decom] losing matter droiiping 

 into the Iomci- watci-; with the tempei'atui'c of the lower water, 

 whicdi to some extent regulates the rapidity (d' decomposition: 

 and with the depth of the water, on whicdi (le|tends the duantiiv 

 of o-xygeii contained in it. I'-atdi of these factors may and do's 

 dilfei- in ditferent lakes. Imt the result is that in a very large 

 proportion of our inland lakes the bottom water loses its stock 

 of oxygen comjiaratixidy early in the season and liecomes un- 



