ZOOPLANKTON IN .METABOLISM OF LAKES 



87 



terminations of total plankton are not avail- 

 able. 



By far the best and must acenrate index 

 of plankton prodnctivity of a lake is the 

 gravimetric determination of total plankton 

 as organic matter, and it is astonishing that 

 this important tool has not been more 

 widely nsed by linniologists; most of onr 

 data from this source comes from the work 

 on Wisconsin lakes. Thns Birge and Juday 

 (1934) reported an examination of 529 

 lakes. They found that the centrifuged or 

 particulate organic matter of the surface 

 waters ranged from 280 to 12,000 nig per 

 cubic meter, with a mean of 1,360. Some 

 lakes showed little or no seasonal varia- 

 tion, others as much as threefold. The an- 

 nual variation ranged from zero to eight 

 times, but most of the lakes showed an- 

 nual variations within the seasonal limits. 

 Drainage lakes were 45 per cent more pro- 

 ductive than seepage lakes, due to tlie in- 

 coming waters carrying minerals and other 

 constituents. Greater productivity was ob- 

 served in shallower lakes, the greatest being 

 from lakes 2 to 3 meters deep. Other fac- 

 tors were the size and the shape of the lake 

 basin. The complete circulation of water 

 and repeated use of raw materials seemed 

 to explain the larger yield of shallower 

 lakes. The vertical distribution showed 

 considerable variation; the largest amount 

 occurring at the surface in 10 per cent of 

 the lakes over 18 meters depth, at the bot- 

 tom in 34 per cent and at intermediate 

 depths in 56 per cent. In 43 lakes the maxi- 

 mum was between 3 and 10 meters; in 20 

 lakes below 10 meters but not at the bottom. 

 The organic matter was found to consist of 

 37 ])er cent crude protein, 4 per cent ether 

 extract, and 59 per cent carbohydrate. 

 Soft-water lakes were found to give smaller 

 amounts of organic matter than hard-water 

 lakes (2-4 v). Trout Lake showed 128 kg 

 per hectare, Nebish 61.3, and Mendota 240. 

 (Tlie summer average for Chautauqua Lake 

 in 1937 was 118 kg per hectare.) The dis- 

 solved organic matter, as contrasted with 

 the particulate matter removable by the cen- 

 trifuge, was found to range between 16,000 

 and 17,000 mg per cubic meter in the aver- 



age lake, of wliich 8 to 9 per cent was made 

 up by tlie plankton. The extreme figure ob- 

 tained was over 50,000 mg per- cubic meter. 

 The main source of the organic matter was 

 the plankton plus mud and higher plants. 

 External sources were Inniuis, marsli, and 

 peat extractives. Seepage lakes had much 

 less organic matter and averaged 3,000 to 

 6,000 mg per cubic meter, of which 15 to 18 

 per cent was plankton. 



The organic matter and its distribution 

 has been studied in a few New York lakes 

 (Tressler and Bere 1936, 1937, 1938). In 

 the 19 lakes in which determinations were 

 made the maximum amount occurred at the 

 surface in 31 per cent, at the bottom in 31 

 per cent and at intermediate depths in 37 

 per cent. Twenty-one per cent of the lakes 

 showed a maximum amount at 5 meters. 

 Values ranged between 420 mg per cubic 

 meter and 3,690. Seasonal variations are 

 considerable in most lakes and follow rather 

 closely the periodicity of plankton abun- 

 dance. Chautauqua Lake, for example, 

 showed three peaks, one in the fall, one in 

 the spring, and a third and by far the 

 largest in late August. The actual values 

 varied from 1,037 to 2,343 mg per cubic 

 meter with 1,728 the average. As a rule 

 there is a distinct correlation of the organic 

 matter with the plankton counts, although 

 in some cases there seems to be no relation- 

 ship between them. Meehan (1937) in some 

 Avork on bass ponds found no correlation 

 between the dissolved organic content and 

 the average pliyto- and zooplankton counts 

 and suggested that this indicated that a 

 large part of the source of food supply for 

 zooplankton is closely associated with bac- 

 teria rather than phytoplankton. Hori- 

 zontal variation of organic matter in lakes 

 has been rather infrequently reported. In 

 New York State two linear series of 10 

 samples taken from one end of the lake to 

 the other have sliown substantially the same 

 results, viz.. a marked increase at the outlet 

 end. This increase is a gradual cumulative 

 one as one proceeds down the lake. In 

 Otsego Lake (Tressler and Bere 1936) or- 

 ganic matter at 3 meters' depth showed a 

 112 per cent increase, while in Chautauqua 



