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

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I)PETERSEN ET AL.: NEARSHORE HYDROGRAPHIC CONDITIONS AND ZOOPLANKTON BIOMASSCalCOFI Rep., Vol. XXVII, 1986(approx. 0.1 pg-atoms1 -I; Eppley et al. 1979b), wealso averaged 15-m deep ammonium means across theshelf.Ammonium is an important source of nitrogen forplant growth in the Southern California Bight,representing about 40% of the sum of ammonium,nitrate, and urea-N assimilation by phytoplankton(McCarthy 1972; Eppley et al. 1979b). Ammoniumconcentration is often low relative to other nitrogenousforms because of its rapid recycling by phytoplankton.Ammonium concentrations and patterns that weobserved were similar to results of other studies in thenearshore zone of southern California (Eppley et al.1979b; Barnett and Jahn, in press). Mean concentrationof ammonium between mid- 1982 and December1984 was 0.39 pg-atoms-1-I, and there was noobvious seasonal pattern (Figure 5). Highest concentrations(2-3 pg-atoms-1- ') were observed withinSanta Monica Bay in December 1982 and February1983, although concentrations up to 5-20 pgatoms-1-'were observed by Eppley et al. (1979b) inthis area in 1974-77. These differences were probablythe result of station location within Santa Monica Bayand local current patterns, since sewage and refinerywastes were suggested sources of high ammoniumconcentrations in 1974-77 (Eppley et al. 1979b).Lowest ammonium levels were observed at three of thefour transects in October 1983, during a period of unseasonablywarm water (Figures 2 and 3).Nitrate concentrations were often less than 1 .O pgatoms-1- ' at the surface (Figure 6), except duringperiods of strong upwelling. Such low concentrationsin surface waters have been noted previously (e.g.,Eppley et al. 1979a; Zimmerman and Kremer 1984)and may be a result of phytoplankton's rapid use ofnitrate within the euphotic zone. Macroalgae, such asthe giant kelp (Macrocystis pyrifera), are also a largesink for nutrients in the littoral zone (Jackson 1977).Profiles of nitrate at Seal Beach (Figure 6) were typicalof profiles at the other three transects and will thereforebe used to describe the general onshore-offshore andseasonal trends. Between August 1982 and February1983, nitrate profiles along the Seal Beach transectwere similar, with low concentrations throughout thewater column at the inshore stations (8-36 m). Profilesat 75 m had low levels down to about 40 m and a significantnitrate pool (bottom cencentrations to 15.5 pgatoms1~ below 40 m (Figure 6). During the Apriland June 1983 cruises, the nitracline becameshallower, and high nitrate concentrations were thusobserved near the bottom at the 75, 36-, and 22-misobaths.Nitrate levels were low from the surface down to atleast 75 m during late summer to early winter of 1983.2.5 1Ormond BeachPlaya del ReyA Seal Beacho San Onofre1982 1983 1984Figure 5. Average ammonium in the upper 15 m at major transects. Each pointis a cross-shelf average of 15-m, 22-11?, 36-m, and 75-m stations.The nitracline observed during August to December1983 was deeper, and therefore the nutrient pool wasfarther offshore than during the same period of 1982.Sampling at Seal Beach during February 1984 followeda strong wind event that produced intense nearshoreupwelling of cold water. Finally, nitrate profilestaken in April 1984 were fairly similar to those of theprevious year, with high concentrations near thebottom at 75-m and 36-m stations and moderately lowlevels at shallower stations. We discarded nutrient datafrom June to December 1984 because of equipmentmalfunction.The temporal pattern of average nitrate suggests twomajor periods or events of increased nitrate concentration(Figure 7). First, three of the four transects showedincreased nitrate levels during the early summer of1983. At the Playa del Rey transect, nitrate levels increasedsteadily from August 1982 to a high of over 3.0pg-atoms-1-' in early June 1983, whereas nitrate atOrmond Beach and San Onofre showed significant increasesbetween April and June 1983. Nitrate at SealBeach was relatively constant during spring-summerof 1983. Closer examination of the data indicates thatincreased nitrate levels at Ormond Beach, Playa delRey, and San Onofre between April and June 1983primarily resulted from increased concentrations in thelower part of the 15-m water column. Nitrate increasednear the bottom at almost all stations of these threetransects, while near-surface (surface-to-6-m) levelsremained low and similar from April to June.42
PETERSEN ET AL.: NEARSHORE HYDROGRAPHIC CONDITIONS AND ZOOPLANKTON BIOMASSCalCOFI Rep., Vol. XXVII, 1986Nitrate (ugatoms I‘ )Dec80EYThe second period of increased nitrate levelsoccurred during February 1984 (Figure 7). Nitrate increasedslightly at Ormond Beach and San Onofrebetween December 1983 and February 1984 butshowed a dramatic rise at Seal Beach-from 0.3 pgatoms-1-’to 5.5 pg-atoms-1-’ at the 15-m isobath.Bightwide sampling during February 1984 was interruptedby a strong, brief windstorm that caused nearshoreupwelling and significant cross-shelf gradients intemperature, salinity, and nutrients. Normal samplingprotocol called for the ship to begin on the northernmosttransect (Ormond Beach) and proceed south tosuccessive transects over a period not to exceed 10days. Ormond Beach was sampled on February 15,1984, but strong winds and heavy seas forced the shipoff the scheduled Playa del Rey transect, which wasfinally sampled on March 7. Average daily wind speedat Los Angeles International Airport (LAX) increasedfrom 2.8 m/sec on February 15, 1984, to 7.9 dsec onFebruary 16 (NOAA 1984). Winds remained strong onFebruary 17, averaging 6.8 dsec. During the storm,the wind blew steadily from the northwest. Winds fromthis direction were parallel to the shoreline of theSouthern California Bight and produced strongaFigure6. Vertical profiles of nitrate atSeal Beach between August 1982and April 1984.offshore transport of nearshore surface waters, assuggested by the upwelling index calculated for thisperiod (Figure 8). Seal Beach and San Onofre transectswere sampled soon after the wind event, on February20 and 22, respectively.Specific density (sigma-t) and nitrate profiles at SealBeach (Figure 9) for the February 1984 cruise indicateintense movement of surface water away from theshore and replacement of this water from depth. Waterfrom the 8-m station was significantly more dense than0 Ormond Beache Playa del ReyA Seal Beach1982 I 1983 ’ 1984Figure 7. Average nitrate in the upper 15 rn at major transects. Each point is across-shelf average of 15-rn,22-m, 36-rn, and 75-rn stations.43
Page 1: REPORTSVOLUMECICTCIBERXXVll1986
Page 4 and 5: EDITOR Julie OlfeSPANISH EDITOR Pat
Page 7: COMMITTEE REPORTCalCOFI Rep., Vol.
Page 10 and 11: FISHERIES REVIEW: 1985CalCOFl Rep.,
Page 12 and 13: FISHERIES REVIEW: 1985CalCOFI Rep.,
Page 18 and 19: BINDMAN: 1985 NORTHERN ANCHOVY SPAW
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Page 28 and 29: WOLF AND SMITH: MAGNITUDE OF SARDIN
Page 34 and 35: PUBLICATIONSCalCOFI Rep., Vol. XXVI
Page 37 and 38: Part I1SYMPOSIUM OF THE CALCOFI CON
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Part 111SCIENTIFIC CONTRIBUTIONS
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BAILEY ET AL.: FORECASTING HAKE REC
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ESPINO AND WOSNITZA-MENDO: PERUVIAN
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DuracionQUINONEZ-VELAZQUEZ Y GOMEZ-
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SCHMITT: SELECTIVITY AND FEEDING OF
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BRINTON ET AL.: GULF OF CALIFORNIA
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