PETERSON and MILLER: SEASONAL CYCLE OF ZOOPLANKTON ABUNDANCE 



always uncommon or rare in our samples and 

 probably have unimportant roles in the commun- 

 ity. However, taxonomic study of these rare or 

 uncommon species is important because in many 

 cases their presence indicates the presence of a 

 particular water type or mixture of types. Most of 

 the species that are found off Newport only during 

 winter months have southern affinities (Central 

 Pacific waters). They are transported north along 

 the continental shelf by the Davidson Current and 

 are probably very near the extreme northerly 

 limit of their range. These species were Mecyno- 

 cera clausii, Calocalanus styliremis, C. tenuis, 

 Calocalanus sp., Clausocalanus mastigophorus , C. 

 furcatus, C. jobei, C. parapergens, C. paululus, 

 Acartia danae, Corycaeus amazonicus, Oncaea 

 dentipes, and O. subtilis. Other species that were 

 found only during winter months have northern 

 affinities and are usually found in deep water over 

 the continental slope. They were probably trans- 

 ported shoreward as a result of onshore winds. 

 These species were Calanus cristatus, Gaidius 

 brevispinus, Gaetanus simplex, Candacia colum- 

 biae, Heterorhabdus immatures, Pleuromamma 

 borealis, and P. abdominalis . The 16 species that 

 were rare or uncommon in both summer and 

 winter include representatives of both northern 

 and Central Pacific faunal groups. 



Seasonal Cycle of 

 Total Zooplankton Abundance 



The annual cycles of total zooplankton abun- 

 dance for stations NH 1, NH 3, NH 5, and NH 10 

 are shown in Figures 1, 2, and 3. Abundance is 

 high during the upwelling season and often re- 

 mains high during the autumn period of hydro- 

 graphic transition. Abundance is low during the 

 period from November through April. All four sta- 

 tions have this basic pattern, but there are impor- 

 tant changes with distance offshore. Table 2 gives 

 several indices of cycle amplitude. The amplitude 



TABLE 2. — Some indices of the amplitude of the seasonal cycle of 

 zooplankton density off the Oregon coast. Median density esti- 

 mates for summer and winter seasons at the four stations on the 

 Newport, Oreg., transect, the ratio of median densities between 

 seasons, and the number of dates with densities >5,000 m . 



I0 : 



UJ 



o 

 < 



io- 



CD 

 < 



CD 



O 



_l io 2 



JFMAMJJASOND 



MONTHS 



FIGURE 1. — Annual cycle of totaled zooplankton abundance 2 

 km from the Oregon coast at Newport (NH 1) during the 3-yr 

 study period. 



10' 

 UJ 



O 



< 

 a 



2 io 3 



m 



< 



CD 

 O io 2 



,'969' ' r ~ T ~ 



1970 



1971 



1972 



NH 3 



. 4 . • • •, 



\ 



J I I I I I I I I I L 



_l JFMAMJJAS0ND 



MONTHS 



FIGURE 2. — Annual cycle of totaled zooplankton abundance 5 

 km from the Oregon coast at Newport (NH 3) during the 3-yr 

 study period. 



of the cycle is greater inshore. First, there are 

 more dates at NH 1 and NH 3 with densities in 

 excess of 5,000 m 3 (an arbitrary value). Second, 

 the absolute difference between summer and 

 winter density decreases with distance from shore. 

 All of the decrease in the ratio of the densities in 

 the two seasons occurs between 2 and 5 km from 

 shore (NH 1 to NH 3). 



There is a suggestion in the data for NH 1 (Fig- 

 ure 1) that the annual cycle of zooplankton abun- 

 dance is more complex than just a summer high 



719 



