40 Ted R. Angradi 



lent to an average growth rate of about 0.2 mm/d. Assuming the medusa stage 

 originates at about 0.5 mm and attains a maximum size of about 21 mm (Pennak 

 1989), this rate of growth indicates a life cycle for the medusa of about 102 days. 

 The apparently slower growth of medusae away from the main swarm (Fig. 3) 

 may be attributed to disproportionate mortality of the full-growth medusae. 



DISCUSSION 



I observed great variation in abundance of medusae over a scale of a 

 few meters that is not readily explained by available water chemistry data. My 

 finding of larger individuals away from the main concentration of medusae is cir- 

 cumstantial evidence that spatial variation in abundance of the medusae results 

 from dispersion from a highly localized swarm location. 



If the polyps are lotic (Hutchinson 1967), then the distribution of the 

 medusae in Wolf Fork results from events and conditions since delivery of imma- 

 ture medusae from upstream earlier in summer. During most of the year, flow 

 from the stream would induce some current at the head of Wolf Fork, and the 

 location and localization of the swarm at Wolf Fork may have resulted from 

 flow-related concentration (e.g., in an eddy) of polyps or small medusae origi- 

 nally exported from the stream. 



Several authors have also found a highly localized distribution of 

 medusae. Dodson and Cooper (1983) found medusae only in one small sheltered 

 cove of a Wisconsin Lake. Deacon and Haskell (1967) reported medusae from 

 sheltered coves at Lake Mead, Nevada, and not from the open waters of the reser- 

 voir. Garman's (1916) description of the location of a dense swarm in a narrow 

 flooded tributary of an impounded reach of the Kentucky River is similar to Wolf 

 Fork and to other published accounts (e.g., Lytle 1962). 



Few authors have explicitly commented on factors accounting for spa- 

 tial variation in medusae abundance within a reservoir or lake. Acker and Mus- 

 cat (1976) noticed an apparent effect of light on the distribution of medusae 

 which they attributed to a direct light effect or to an indirect effect of light on 

 food concentration. Other factors such as wind (Deevy and Brooks 1943), and 

 ebbulition (Koryak and Stafford 1981) have been proposed. I noticed no appar- 

 ent light effect at Wolf Fork, and the cove is well protected from wind and boat- 

 wake disturbance; these factors are unsatisfactory for explaining the large local 

 variation in abundance of medusae at Wolf Fork. My observations suggest 

 instead that the distribution of medusae may result from the pattern of dispersion 

 from a location that is determined by the habitat requirements of the polyps in 

 the reservoir or in tributary streams. 



My observation of a bi-modal size distribution of medusae (Fig. 2) sug- 

 gest the possibility of more than one period of medusa formation at Wolf Creek, 

 perhaps associated with variation in water temperature, water chemistry, or 

 streamflow during early summer. McClary (1959) reported medusa budding 



