Zooplankton 289 



zooplankton nutrition is, of course, an old one and dates back at least to 

 Naumann (1921, quoted in Saunders 1969). More recently Nauwerck 

 (1963) and Saunders (1969, 1972) have championed this idea. It has been 

 happily ignored by most aquatic scientists, in part because of the 

 difficulties of quantifying the nannoplankton and bacteria. As techniques 

 are now available for counting both of these, perhaps the relationships 

 among zooplankton, algae, bacteria and detritus will receive detailed 

 study. 



In this presentation, the word seston is synonymous with particulate 

 organic carbon (POC). Seston is made up of algae, bacteria, and detritus 

 (which is defined as non-living particulate matter). Operationally, it is 

 impossible to separate attached bacteria from the detritus. In this case it 

 does not matter because less than 20% of the total bacteria were attached 

 to particles (Rublee personal communication). 



The available evidence pointing to the importance of detritus and 

 bacteria for zooplankton nutrition comes from three different approaches. 

 The first is an indirect approach that examines the amount of 

 phytoplankton available as food. Juday (1942) and Nauwerck (1963) 

 found inverted biomass pyramids in lakes where zooplankton biomass is 

 much greater than phytoplankton. For example, in Lake Erken the 

 phytoplankton to zooplankton ratio averaged 1 :7 (Nauwerck 1963). Even 

 when Nauwerck measured the algal production, it was not enough to 

 produce the zooplankton. Accordingly, he believed that detritus and 

 bacteria were important as food. 



The second approach, also an indirect one, stresses the abundance of 

 detritus. In Lake Erken, the ratio of detritus to living matter is about 10: 1 

 (Hobbie personal communication) but in other eutrophic lakes it may be 

 1:1. In Frains Lake, Michigan, Saunders (1969) found that the ratio 

 ranged from 2.7 to 14.2 (no bacteria were included). In the Barrow ponds, 

 the size distribution of the seston, measured with a Coulter Counter, is 

 similar to that of oceanic seston (Sheldon and Parsons 1967, Sheldon 

 1972). Most of the particles are less than 25 fim in diameter and are 

 therefore in the size range of particles taken by zooplankton. Given that 

 most of the particles of seston are detritus, the zooplankton must collect 

 more detritus than anything else. In small lakes and reservoirs the detritus 

 can be so important that it is the base of most of the food chain (Wright 

 1959). 



The third approach investigates the actual assimilation rates of algae, 

 bacteria, and detritus by zooplankton. Saunders (1969, 1972) found in one 

 series of experiments that their assimilation of natural algae 

 {Rhodomonas and Cryptomonas) was 52 to 88%, bacteria was 13 to 52%, 

 and detritus was 3 to 14% of the amount ingested. These experiments have 

 the basic problem that the detritus used was an artificial one that may not 

 be similar to natural detritus. It was, if anything, likely to be more 

 nutritious so the assimilation value above may be too high. 



