CalCOFI Reports, Vol. 27, 1986 - California Cooperative Oceanic ...

PETERSEN ET AL.: NEARSHORE HYDROGRAPHIC CONDITIONS AND ZOOPLANKTON BIOMASSCalCOFI Rep., Vol. XXVII, 1986prior to the storm, a long plume of cold water stretchedsoutheast of the Palos Verdes Peninsula, but othernearshore areas of the bight had surface temperaturessimilar to areas farther offshore, or only slightly cooler(Figure lla). Five days after the storm, strands of coldsurface water were observed off Point Dume, in thenorthern part of Santa Monica Bay, south of PalosVerdes Peninsula, and in the Point Loma area (Figurellb). Upwelling occurred along much of the coastfrom Point Dume to San Diego during the windstorm.In agreement with transect data, no well-definedpatches of cold water were present along the coast onMarch 5, indicating that upwelling had relaxed andsurface waters had warmed by solar heating or horizontaladvection (Figure llc). Satellite images from otherdays during this period could not be used because ofcloud cover.Zooplankton BiomassZooplankton volumes were first averaged over eachtransect to examine seasonal, annual , and geographicvariability (Figure 12). Nearshore zooplankton biomassshowed strong seasonality, with highest volumesduring April-June and minima during December-February.Analyses of the variation of zooplankton volumesfor each month (Feb, Apr, June, Aug, Oct, Dec)indicated a significant transect-by-year interaction forall months except February (Table 2), suggesting thatzooplankton communities at different transects respondedasynchronously during 1982-84. The seasonalpattern of biomass observed at Seal Beach seems tohave differed from the other three transects during this3-year period (Figure 12). Ormond Beach, Playa delRey, and San Onofre zooplankton volumes followedsimilar patterns through the 36 months of sampling,having been somewhat lower during 1983 and 1984than during 1982. Biomass levels appeared to be aboutthe same during 1983 and 1984 at these three transects.In contrast, Seal Beach zooplankton volumes weresimilar during 1982 and 1983 but dropped sharplyduring the spring and summer of 1984. June 1984 zooplanktonvolume at Seal Beach was lowest of the fourllOOr - 0 Ormond Beach01 ...........' 8 . 1 . . . I 1 . 1 1 . 1 1 1 . ~ 1 ~ d . . f i 1F A J A O O F A J A 0 0, F A ;98A4 0 D,I 1982 I 1983Figure 12. Average zooplankton displacement volume along the majortransects.transects, whereas estimates were generally higher atSeal Beach than the other three transects during 1982and 1983.Cross-shelf zooplankton biomass averages for allnearshore transects were higher very near the coastthan at the 75-m depth contour, particularly fromAugust through December (Figure 13; Table 2).Zooplankton biomass at the 75-m stations was alwayslower than at stations closer to the shore, being only38.5% of the nearshore maximum in October. FromFebruary to August, peak zooplankton biomass wasfound at stations from the 15-36-m depth contours,although the trend for February, April, and June wasnot statistically significant (Table 2). During the periodof increasing zooplankton abundance-Februarythrough June-the mode of maximum zooplanktonbiomass shifted progressively from inshore ( 15-misobath) to mid-transect (36-m isobath), suggestingthat the zone of highest zooplankton production and/orsurvivorship near the coast gradually shifts fartheroffshore as the season develops (Figure 13). Thisseasonal movement of the zooplankton biomass modewas observed during 1982 and 1983, but zooplanktonvolumes, within a sample period, were similar fromthe 8-m depth to the 36-m station in 1984. BetweenJune and August, zooplankton volumes declined ateach depth across the shelf. Minimum zooplanktonbiomass was observed in December.The source of nutrients, particularly nitrate, thatTABLE 2Monthly Means and Effects of Transect, Year, and Station Depth for Zooplankton Displacement Volume (1982-1984)Month Mean (S.D.) 3-way ANOVA results(cc/1000m3)Transect (T) Year (Y) Station (S) TxY TxS YxSFebruary 278.8 (140.1) NS ** NS NS NS NSApril 518.7 (319.5) - - NS *** NS NSJune 520.4 (273.0) - - NS * NS NSAugust 370.8 (147.5) - - *** ** NS NSOctober 264.5 (234.1) - - ** ** NS NSDecember 189.1 ( 95.4) - - *** NS NSNS = not significant; *P