SEASONAL CYCLE OF ZOOPLANKTON ABUNDANCE AND 

 SPECIES COMPOSITION ALONG THE CENTRAL OREGON COAST 



William T. Peterson and Charles B. Miller 1 



ABSTRACT 



Species composition of zooplankton collected during 3 yr of sampling close to the coast at Newport, 

 Oreg., varied with season. In all seasons the most abundant plankters were copepods. Dominant species 

 in summer were Pseudocalanus sp., Acartia clausii, A . longiremis, Calanus marshallae, and Oithona 

 similis. These are primarily coastal forms with northern affinities, and they were present all year. 

 Dominant species in winter were Paracalanus parvus and Ctenocalanus vanus, forms of southern 

 affinities. They tended to disappear completely in summer. These geographic affinities are in corre- 

 spondence with the source regions for surface waters that are implied by the direction of flow in the 

 different seasons. Abundances are about one order of magnitude higher in summer than in winter. 

 Copepod diversity is greater in winter than summer: the winter checklist contains 5 1 species, while the 

 summer list contains only 38 species. 



An analysis of differences in the zooplankton of the three winter periods of the study shows 1969-70 

 to have had much greater dominance by southern forms and a larger variety of them than 1970-71 or 

 1971-72. This corresponds with differences in the wind patterns between the years. Winds in the 

 winter of 1969-70 were gentle and directly from the south, while the other winters had the more usual 

 southwesterly storms. Gentle winds directly from the south were more effective at moving sur- 

 face water northward alongshore than southwesterly storms, despite their lesser overall northerly 

 component. 



The hydrography and pelagic ecology of the Pacific 

 Ocean very close to the Oregon coast are strongly 

 seasonal. Winter winds from the southwest, which 

 produce surface flow from the south and toward 

 shore, alternate with summer winds from the 

 north, which produce flow from the north and 

 away from shore, generating coastal upwelling. 

 These seasonal changes in the source of currents 

 flowing through the area cause changes in the 

 species of zooplankton that are present. In this 

 paper we describe this cycle of change in species 

 composition from a series of samples collected 

 along a transect normal to the coast at Newport, 

 Oreg., approximately every 2 wk from June 1969 

 through July 1972. In a previous paper (Peterson 

 and Miller 1975) we have used these data to make 

 a detailed comparison of the upwelling seasons of 

 the years 1969, 1970, and 1971 with emphasis 

 upon the differences between years. Here we con- 

 sider the entire annual cycle with emphasis upon 

 consistent aspects of the differences between sea- 

 sons. The discussion includes a consideration of 

 the differences between the three winters of the 

 study. 



'School of Oceanography, Oregon State University, Corvallis, 

 OR 97331. 



MATERIALS AND METHODS 



Detailed description of collection and laboratory 

 procedures are given in Peterson and Miller ( 1975, 

 1976). Plankton samples were collected with a 

 240-/xm mesh net hauled obliquely from near the 

 bottom to the surface at stations 2, 5, 9, and 18 km 

 from the Oregon coast along a transect at lat. 

 44°40'N. The stations will be referred to as NH 1, 

 NH 3, NH 5, and NH 10, respectively, which stand 

 for Newport Hydrographic stations at 1, 3, 5, and 

 10 n.mi. from the shore. Water depths for the four 

 stations were 20, 46, 55, and 80 m. Surface tem- 

 perature and salinity measurements were made at 

 most stations, and a bathythermograph was usu- 

 ally lowered. A total of 213 samples from 56 dates 

 are included in the present analyses. Distribution 

 of samples among stations, exact dates, and com- 

 plete data for all samples can be found in Peterson 

 and Miller (1976). 



There are important limitations on the zoo- 

 plankton data. We chose to express numerical 

 abundance as numbers of individuals per cubic 

 meter (no. m~ 3 ). Because our nets were towed 

 obliquely through the entire water column, the 

 quantitative abundance estimates are actually 

 abundances averaged over the water column. If an 



Manuscript accepted April 1977. 



FISHERY BULLETIN: VOL. 75, NO. 4. 1977. 



717- 



