phosphate in overlying water is marginally associated with the presence of 

 mature tubificid worms, the increase in ammonia is persistently associated 

 with Chironomids, and the reduction in nitrate levels over time appears to 

 be associated with the population of immature tubificids and/or the total 

 macrobenthos population. 



The results for silicon suggest the relationship: 



Flux = 1000 + 2 X Chironomid larvae density, 



where the flux is in micrograms Si/cm^/yr and the density is in numbers m"^. 

 As the mean density of Chironomid larvae at this location is about 500 m"^, 

 roughly half the flux of silicon from the sediments is attributable to the 

 presence of these organisms. This circumstantial evidence for the effect of 

 Chironomids is strengthened by considering Tessenow's experiments with sedi- 

 ments from Lake Heiden, Germany (Tessenow 1964) in which he demonstrated a 

 casual relationship. Addition of Chironomids (Pulmosus group) to his sedi- 

 ments resulted in enhanced silicon release. Converting Tessenow's results 

 to the above form, we find that for his experiments: 



Flux = 1000 + 4 X Chironomid larvae density. 



Graneli (1977) has also observed that Chironomus Pulmosus larvae increase 

 the release of silica as well as phosphorus from sediments of several lakes 

 in Sweden. It would therefore seem likely that at least in shallow waters 

 of the Great Lakes where fine-grained sediments can be found, such as lower 

 Saginaw Bay, and in most of Lake Erie, Chironomid larvae may play a major 

 role in the regeneration of silicon from sediments. In Lake Erie, average 

 Chironomid densities may be as high as 1000 m-2 (p. McCall, pers. comm.). 

 That these organisms may enhance silicon fluxes does not necessarily mean 

 that their removal or inhibition through exposure to aquatic pollutants will 

 result in a long-term reduction in the capacity of the sediments to return 

 silicon to overlying waters. It is always possible that the ecological 

 niche represented by diatom detritus processing can be filled by another 

 biotic or abiotic component. In other words, the role of Chironomid larvae 

 may be mainly a kinetic one. 



Several preliminary experiments have been undertaken to determine the 

 effect of removing the influence of macrobenthos on release of silicon. A 

 method must be chosen which results in minimal alteration of the structure 

 or composition of sediments. In one experiment, a core incubated at in situ 

 temperatures was exposed to 5 megaRads of cobalt-60 gamma radiation, enough 

 exposure to completely sterilize the sediment core and overlying water. The 

 results of this experiment are shown in Figure 13. Prior to irradiation, 

 the silicon flux was 2000 micrograms Si/cmVyr. After irradiation, the flux 

 dropped to 900 micrograms Si/cm^/yr. It is interesting to note that the 

 factor of two reduction in flux is consistent with the relation given above 

 for the flux as a function of Chironomid larvae density. In this particular 

 core, the density of benthos was not measured. A major reduction in the 

 silicon flux also resulted from addition of Chlordane in amount sufficient 

 to destroy the macrobenthos population (about 1 ml of Chlordane in a disper- 

 sant). Results of this and other treatments are given in Table 3. No 



220 



