TABLE 6-3 Interrelations between depth, blomass of plankto 

 and blomass of benthos in five Canadian lakes (from Raws. 

 1955). 



Average dry Average dry 



Average weight of weight of Ratio total 



depth, net plankton, benthos,' biomass, 



meters g/m^ g/m^ plankton/benthc 



'Minus weight of shells in mollusks 



fish to benthos was in the ratio of 2.7:1 (Ball 1948). 

 The measurement of productivity is difficult, and 

 methods presently in use require several assumptions. 

 If, throughout the year, the plankton population of 

 Lake Mendota, Wisconsin, should replace itself 

 every two weeks, then the annual productivity is 624 

 grams ash-free, dry, organic matter per square meter 

 of water surface. Of this amoimt, 585 g would come 

 from phytoplankton and 39 g from zooplankton. The 

 benthos reproduces less rapidly, nekton, still less so. 

 The annual productions of bottom fauna and fish m 

 Lake Mendota is estimated at 4.5 and 0.5 g/m- re- 

 spectively, and the large aquatic vegetation at 51.2 

 g/m2 (Juday 1940). Disregarding the large aquatic 

 plants, the ratio of productivity between plankton and 

 benthos is approximately 139:1: between benthos 

 and nekton, 9:1 ; and between plankton, benthos, and 

 nekton taken together, 1248:9:1. The ratio of annual 

 productivity between plankton and benthos is much 

 higher, therefore, than is the ratio of their biomasses 

 or standing crops. No attempt was made in this study 

 to determine the standing crop of fish. 



■h' 



1 



fly (fr. 



2 (a) Larva, (b) pupa, 

 Shelford 1913 after Johar 



(c) adult of a midge 



In a detailed study of the net productivity of the 

 benthos in the Russian Lake Beloie (Borutsky 1939), 

 it was found that the standing crop increased during 

 the year by 125 per cent. Of this total biomass, 55 

 per cent died without being eaten by other organisms 

 or was replaced by the small biomass of new eggs be- 

 ing laid: 14 per cent was consumed by other organ- 

 isms, chiefly fish : and 6 per cent emerged as adults 

 that subsequently left the lake ecosystem. The re- 

 maining 25 per cent constituted the standing crop of 

 the following year. However, this standing crop was 

 only 56 per cent of what it was the year before, so 

 these percentages are not representative of stabilized 

 populations. 



It was estimated that in Costello Lake, Ontario, 

 the standing population of midge fly larvae was re- 

 placed during the 135 days of summer eight or nine 

 times in the epilimnion, and two or three times in 

 the hypolimnion. Consumption of larvae by fish was 

 small in shallow waters but amounted to 50 per cent 

 of the standing crop in deep water (Miller 1941). 



LIFE HISTORIES 



Although most midge fly larvae. Chirono- 

 midae, are aquatic, some forms live in decaying or- 

 ganic matter, under bark, or in the ground. The 

 earliest larval stage is a wiggler, which may be car- 

 ried by the current or may squirm about from place 

 to place. Later, this wiggler larva becomes sluggish 

 and builds a case or tube, open at both ends, by ce- 

 menting particles of sand, debris, or silt about itself 

 with mucous from its salivary glands. Construction 

 is accomplished in about three hours. The larvae 

 e.xtend themselves from these cases for feeding, and 

 in some species may even move the cases to better 

 feeding areas. The larval period is the longest part of 

 the life cycle: it lasts at least two months (Mac- 

 donald 1956). Most of the pupation period, which 

 is probably less than a week, is spent in the larval 

 case, but towards the end the pupa swims to the sur- 

 face of the water. At this time it is preyed upon ex- 

 tensively bv fish. The adult imago struggles out of 

 the case and flies off. The adult lifespan is probably 

 short, as there is no evidence that they feed. They 

 may occur in immense swarms in the evening. Eggs 

 are laid in masses of several hundred or in sticky 

 gelatinous strings that float at the surface attached 

 to some object or sink to the bottom. The eggs hatch 

 in a few days, and the cycle is repeated (Cavanaugh 

 and Tilden 1930, Johnson and Munger 1930). Some 

 larvae (e.g. Procladius. Tanypus) are carnivorous 

 rather than herbivorous or saprophagous. They do 

 not build cases, but roam over the bottom. The num- 

 ber of generations varies in different species from 

 two per year, to one per year, or one in two years. 



76 Habitats, communities, succession 



