HOBSON and CHESS: TROPHIC INTERACTIONS 



RESULTS 



Volumes of Zooplankters, Day and Night 



Our collections with the plankton net were too 

 few, and spaced over too much time, to attach 

 much significance to the differences in volume 

 between various samples. Nevertheless, certain 

 characteristics probably are meaningful. 



The volumes of samples taken day and night at 

 full moon compared with day and night at new 

 moon are as follows: FULL MOON-daytime, 

 middepths {n = 3): 1 to 18 ml, x = 8.3; daytime, 

 near bottom {n = 3): 0.2 to 5 ml, x = 4; nighttime, 

 middepths (n = 3): 2 to 13 ml, x = 4; nighttime, 

 near bottom {n = 3): 5 to 15 ml, x = 10.3. NEW 

 MOON-daytime, middepths (n = 3): 2 to 5 ml, x = 

 4; daytime near bottom (n = 3): 1.2 to 5 ml, x = 3.7; 

 nighttime, middepths {n = 3): 3 to 13 ml, x = 8; 

 nighttime, near bottom (n = 3): 9 to 36 ml, x = 

 19.6. 



Thus, during the day the volumes of collections 

 made at the middepths generally were greater 

 than those made near the bottom, whereas the 

 situation was reversed at night. Furthermore, 

 volumes tended to be greater at night than during 

 the day, with the greatest volumes of all taken 

 near the bottom on dark nights. 



Activity Patterns of 

 the Zooplankters, Day and Night 



The zooplankters are here grouped into a series 

 of categories (Tables 1, 2),^ most of which represent 

 phylogenetic classes or subclasses. 



Radiolahans 



Based on the collections made with the plankton 

 net (Tables 1, 2), radiolarians are consistently 

 present in the water column during both day and 

 night, sometimes in large numbers. 



Polychaetes, Swimming 



We saw polychaetes in the water column only at 

 night. Highly motile epitokous nereids were 



*The data in Tables 1 and 2 are from collections with the 

 plankton net made above the open field of low benthic algae 

 adjacent to the kelp forest. A set of day-night collections was also 

 made within the forest (see Methods), where the fishes discussed 

 below spend part (in some cases most) of their time. Because the 

 data from these collections are essentially like those shown in the 

 tables, they are not presented. 



especially prominent when they swam at mid- 

 depths during reproductive periods. Polychaetes 

 are underrepresented in the plankton collections 

 (Tables 1, 2), however, because their mobility 

 permitted many to evade our net. 



Mollusk Larvae 



Based on specimens taken in the plankton net 

 (Tables 1, 2), mollusk veligers occur in the water 

 column in similar numbers during both day and 

 night. 



Cladocerans 



Cladocerans (Figure 5C) were consistently 

 present in the collections during both day and 

 night (Tables 1, 2), although they were more 

 numerous in the daytime collections. 



Ostracods 



We saw ostracods in the water column at night, 

 but never during the day. Our daytime plankton 

 collections took only a few individuals, these close 

 to the bottom (Table 3). At night, however, several 

 species were consistently numerous in both mid- 

 depth and near-bottom collections (Table 3). The 

 most numerous ostracod, Parasterope sp. A 

 (Figure 5H), was numerous in the surface layers of 

 the sand during the day (Hobson and Chess in 

 prep.), and during the middle of the night we 

 observed and collected it concentrated at the 

 water's surface. 



Calanoid and Cyclopoid Copepods 



Calanoid and cyclopoid copepods were numerous 

 in the water column during both day and night, 

 based on our observations in the water as well as 

 on our collections (Tables 1, 2). Indeed, calanoids 

 were the most numerous of all organisms larger 

 than about 1 mm taken in the net. Calanoids and 

 cyclopoids were collected in greater numbers at 

 night (Table 2), but because the plankton is gen- 

 erally richer after dark, they represented a smaller 

 percentage of the sample volumes at night than 

 during the day (Table 1). 



The vast majority of calanoids and cyclopoids in 

 the collections were subadults, and some species 

 could be recognized only as adults. Of those iden- 

 tified, the major calanoids were Acartia tonsa 

 (Figure 5F) and Calanus pacificus, with others 



571 



