Physical and biological characteristics of an upwelling at a station off La Jolla, California during 19711

Estuarine

and Coastal Marine

Science

(1974)

2,

425-432

Notes and Discussions Physical and Biological Characteristics an Upwelling at a Station off La Jolla, California during 1971'

of

Daniel Kamykowski Department of Oceanography, Dalhousie University, Halifax, Canada Received 3 October I973 and in revised form 13 May 1974

Nova Scotia,

A station located I km west of Scripps Institution of Oceanographypier La Jolla, California was occupied 15 times in the ST-day period from 9 February to 7 April, 197x. The condition of the environment and of the phytoplankton community was observedbefore, during and after a major upwelling (26 February to I March). Classicalinteraction between the environment and the phytoplankton was followed for three weeksafter the strong WNW winds declined. The biological successioninitiated by the upwelling ended on 22 March when a new water massmarked by a high silicate concentration apparently entered the area by surfacetransport. Introduction The major biological studies of upwelling off the Pacific coast of North America were historically performed on semi-enclosedwater masses(Bolin & Abbott, 1963; Smayda, 1966). After the upwelling the currents of the embaymentsmaintainedthe samewater mass over extended periods (months), imparting a permanency of water massthat allowed the study of a biological successionin the phytoplankton. The location chosenfor the present study is representative of an open coast. In addition to irregular occurrencesof upwelling, the station is subject to a net southerly advection. This condition permits water masseswith different historiesto occupy the samelocation on successivedays. The present paper examinesthe length of time that a biological succession exists at the La Jolla station. Methods The water column was sampledat a station on the 50-m contour located I km west of the Scripps Institution of Oceanography (SIO) pi er, La Jolla, California (32°50’~5”N, 117~1o’~“W). The sampleswere taken about every 4 days midway between the daytime high and low tide in an attempt to minimize the aliasingof internal tides (Tabata, 1965). Data presentedby Cairns& La Fond (1966)and Kamykowski (1973) suggestthat the surface tide and the internal tide are sufficiently coherent for the sampleprogram to benefit from

426

D. Kamykowski

these procedures. The data collection consistedof a bathythermogram cast to 40 m, a hydrocast with five z*5-liter Van Dorn bottles placed at 10-m intervals from the surface to 40 m and a Secchi depth determination. The water from the hydrocast was divided into four aliquots for the determination of salinity, nitrate, phosphate, silicate, chlorophyll a, phaeo-pigments,cell numbers and phytoplankton speciescomposition. Salinities were determined using an Autolab salinometerwithin three weeksof sampling. The nutrient sampleswere frozen unfiltered and were stored for a maximum of two weeks before analyses. Nitrate was determined by copper-cadmium reduction and subsequent analysis for nitrite (Woods et al., 1967), phosphateby molybdate complexing (Murphy & Riley, 1962)and silicate by molybdate complexing (Mullin & Riley, 1955).Acetone extracts of the particulate matter were analysedfor chlorophyll a and phaeo-pigmentsby fluorometry (Holm-Hansen et al., 1955). The samplesfor phytoplankton cell counts and speciesidentification were preserved with a 4% formalin solution buffered to pH 8. Cell counts on the greater than IO pm fraction were made by the Utermijhl procedure (1931) used in conjunction with a Unitron Inverted-Phase Contrast microscope at either IOO x or 200 x . Specieswere determined according to Cupp (1943) and Schiller (in Rabenhorst)(1933,1937). Incident light was recorded with a bimetallic actinograph maintained on the roof of Sverdrup Hall (SIO). Wind direction and speedwere obtained from the National Weather Service station at Lindbergh Field (about 25 kilometers south of SIO). Since Cairns & LaFond (1966) found the WNW vector to be most effective in causing thermocline movements in the water column off southern California, the averagedaily WNW wind vector was calculated. Results and discussion The time course of the physical generation and of the classicalbiological responseof a major upwelling during early 1971 was followed at one station by 15 sampling days within the 57-day interval extending from 9 February to 7 April. Figure I is a record of the surface conditions on the sampling days before the upwelling (9-24 February), after the upwelling (2-16 March) and after the classicalbiological response(22 March-7 April). The pre-upwelling conditions are representedby the samplestaken from 9-24 February. The daily solar radiation penetrating to ground level varied becauseof cloud cover. A wellmixed water column was evident with only a 2 “C temperature change in the upper 20 m. The average surfacetemperature (13.6 “C) and the averagesurface salinity (33*35x0) were both somewhat less than the IO-year means for the month (14.2 “C, 33*55%J. Nitrate, phosphateand silicate were generally at levels typical for a February with dinoflagellates dominating the phytoplankton. This domination wasmostevident on the 16th and the 19th of February, when surfacechlorophyll valuesreached 8.96 and 7.85 pg Chl a l-l, respectively. Gonyuulux polyedu was the most numerous phytoplankter observed. On 24 February the surface chlorophyll a values dropped to 0.71 pg Chl a 1-l. The Secchi depth, however, decreasedfrom II m on 16 February to 6 m on 24 February. The particulate phaeopigment/chlorophyll ratio (P/C) increased on 24 February indicating increased grazing pressure(Lorenzen, 1967)on the plant crop observedon the previous sampling days. The winds turned to the WNW on 26 February. The speedgradually increasedon the 27th until an average wind of greater than 20 km h-l with gusts up to 35 km h-1 blew continuously through I March. The winds continued from the WNW until the 7th of March but with gradually decreasingspeed.The high winds were associatedwith clear skies; the incident radiation was near the maximum possiblefor early March.

Characteristics

of an upwelling

off La Jolla

427

(9

lLLLl :h)

(il

-LLLL II

a 4 0

lLLl !k)

1000 100 IO

I

i

1

*I

1 Ill

IO00 100 IO 1

I (m

03 02 01 0

L

a 12 FEB

16

20 24

28

4 MAR

a

I2

I6

‘0

I

24

28

1

5

I

428

D. Kamykowski

Smith (1969) discussed the physical theory of upwelling in the northern hemisphere and considered the influence of wind direction, the orientation of the coast, wind stress and the Coriolis effect. The events after 26 February closely followed the suggested patterns of an upwelling. The response of the water column to the WNW winds that occurred from 27 February to 6 March was readily seen in the shoaling of the I I “C isotherm. The average rate of upwelling was at least I-Z m day-l (II “C isotherm shoaled from an observed depth of 20 m to an observed depth of 8 m in IO days). On 2 March, the surface temperature decreased to 11.7 “C and the surface salinity increased to 33*60x,. The gradual increases in nitrate, phosphate and silicate were related to the above changes in the water column indicating that nutrient-rich water was maintained at the surface for about 7 days. This was ample time to allow surface turbulence to mix the upwelled water with some of the previous low-nutrient surface water at some location offshore. By IO March the surface temperature increased to 13.7 “C; the surface salinity decreased, but only to 33.50%~; surface nutrients dropped to very low levels; chlorophyll a values increased greatly; and the Secchi depth decreased to 4 m. The time lag between the maximum nutrients and the maximum chlorophyll a at the surface was six days. Plankton counts were made on aliquots taken each sampling day. Assuming that exponential growth occurred over the q-day period from 6-10 March, a growth constant, k=o*oIz h-l, was calculated for the total phytoplankton community. The doubling time of 25 h corresponding to this k value falls within the range of literature values (3-48 h) observed for natural populations in defined water masses (Eppley & Strickland, 1968). Table I gives the growth constants and the doubling times computed for centric diatoms, pennate diatoms and dinoflagellates separately. The observed rates are in the range found for representative species of the groups in culture (Fogg, 1966). These growth calculations for the phytoplankton population as a whole and for the comparison of the three phytoplankton groups suggest that a single biological water mass was sampled over the 4-day period. TABLE

pennate

I. Growth constants (k) diatoms and dinoflagellates Centric

k d.t.

diatoms

0.024 h-l 13 h

and doubling from 6-10

Pennate

diatoms

0.029 IO

h-l h

times March

(d.t.)

for

centric

diatoms,

Dinoflagellates 0.006 h-’ 50 h

The change in the phytoplankton species composition that took place after the upwelling demonstrated a pattern commonly observed in the early spring off La Jolla. The diatoms and the dinoflagellates alternate in abundance. The diatoms achieve dominance subsequent to nutrient enrichment of the surface waters, presumably as a result of their faster growth rates. The dinoflagellates usually reach their maximum concentrations when surface waters are low in nutrients. The community biomass and the species composition show that the diatom bloom peaked on the 12 March and was declining by the 16th. The sample on the 10th was dominated by the pennate diatoms Nitzschia delicatissima and N. pungens, but the centric diatoms were well represented by Chaetoceros species (mainly curviietus) and Cerataulina bergonii. The abundance of the two pennate diatoms had increased by the 12th and Chaetoceros curvisetus remained the most abundant centric. By 16 March the diatoms had begun to decline; Nitzschia delicatissima and Leptocylindrus danicus became the most abundant forms. The

Characteristics

of

an upwelling

off La Jolla

429

dinoflagellateswere gaining influence on the 16th; Peridinium spp. dominated the water mass.The sequencein the changesof speciesfrom 2-16 March followed a classicalphytoplankton succession(Margalef, 1967). The role of grazing in the decline of the diatom bloom cannot be determined from the present data. The particulate P/C ratio, however, was decreasingat the sametime as the decreasein chlorophyll a. Nutrient deficiency probably limited diatom growth rates toward the end of the bloom. On zz March, the next sampling date, an abrupt large increasein surface silicate concentration (266-13.66 pg-at Si 1-r) occurred with smaller proportional increasesin the nitrate (02o.41 pg-at N 1-r) and in the phosphate (0.41-0.62 pg-at P 1-l) concentrations. Figure 2 showsthe relationship betweennitrate and silicate for 9-24 February, 2-16 March and 22 March-7 April. Nutrient concentrations on all datespreceding 22 March follow the samelinear trend, while the five datesfollowing 22 March are all distinctive becauseof the relatively high silicate levels coincident with low nitrate levels (
12 16

Nitrate

concn

20

(pg-otom

24

28

32

N liter-‘)

Figure 2. The silicate concentration plotted against the nitrate concentration the same water sample. The symbols designate the three time periods before upwelling [~24 February (A)] after the upwelling [2-16 March ( X )] and after classical biological response [22 March-7 April (a)].

in the the

Dugdale (1972) presented a relationship between nitrate and silicate for the upwelling zone off Peru. The surfacewaters containeda significant nitrate concentration while depleted in silicate. Off La Jolla, California, the nitrate-silicate relationship is opposite to that off Peru. The coastalwaters contain significant silicate concentrationswhile they are depleted in nitrate. As suggestedby Dugdale, the rate of ammonia regenerationmay influence this relationship. The ratio of nitrate to silicate in the upwelled water must also be important. The phytoplankton population after 22 March was dominated by dinoflagellates(Peridinium spp., Gymnodinium spp., Gonyaulax polyedra and Prorocentrum micans) while the diatoms were only r/zoth asabundant asthe previous sampling date. Two explanations for the altered characteristicsafter 22 March are either that a new water massentered the area or that a rapid in situ regeneration of silicate occurred during the dinoflagellatebloom.

430

D. Kamykowski

According to the first hypothesis, the water masssampledfrom 2-16 March had a significant growth of diatomswhich utilized the silicate in proportion to the nitrate and phosphate. Soon after 16 March a new water masswastransported into the areaby advection. This water masshad been supporting the growth of phytoplankton which had a low requirement for silicate, leaving an excessof silicate in the surfacewaters relative to nitrate and phosphate. The different kinds of organismsin these two water massesmay be attributed to the available seedpopulationsand to the degreeof new surfacewater mixed in by the upwelling process. As mentioned previously, the water massat the sampling station prior to the upwelling was dominated by dinoflagellates.This community moved offshore in response to the currents accompanyingupwelling, but a degreeof mixing occurred betweenthe cold, nutrient-rich, upwelled water and the warm, nutrient-poor surface water. Various proportions of each water masscontributed to the final volume in different areas.The water mass sampledfrom 2-16 March wassufficiently diluted for the introduced diatomsto overwhelm the initial dinoflagellatepopulation. The water masssampledafter 16 March was a blend of upwelled water and surfacewater that possessed a relatively high concentration of nutrients, but the initial dinoflagellate population had not been overwhelmedby a growth of diatoms. This dinoflagellatecommunity utilized nitrate and phosphatebut left silicate. The precedent for this kind of relationship was discussedby Pratt (1966). He attributed the unpredictable annual dominanceof Skeletonemucostatumor Olithodiscuslateusin Narragansett Bay to the relative abundancesof these specieswhen conditions becamesuitable for growth. The one with the larger incipient population grew to dominance. The secondhypothesis relieson a rapid dissolution(IO pg-at Si l-1 in 6 days) of silica in the surfacewaters. The literature discussingsilicate regenerationgives a wide range of rates. This is understandable since the rate of diatom frustule dissolution depends on species composition, growth conditions, acid treatment and crushing of frustules after ingestion by grazers, sinking rate and other factors (Lewin, 1961). Generally, the rate of regenerationof silicate is thought slower than that observedin Figure r(h) (Grill & Richards, 1964). The first explanation, advection, is supported by the similarities between the species compositionsbefore the upwelling and after the silicate increase.The phytoplankton community is viewed as a seriesof distinct biological successionseach present off La Jolla, California, for a short time interval. After the silicate increasethe water masscharacteristicsoff La Jolla were similar for the next five sampling days (16 March-5 April). Increasing solar radiation contributed to an increase in stratification. The surface temperature continued to rise while the surface salinity remained at the upwelling levels. Nitrate and phosphateremained low except for a minor increaseon 2 April; silicate remained high throughout the interval. Surface chlorophyll a values rangedaround 3 pg Ch a 1-i except for a large increaseon 2 April to 10.56pg Chl a 1-l. The Secchi depth determination suggestedthat the crop wasbelow the surfaceon 7 April. Dinoflagellates remained dominant over the diatoms. The particulate P/C ratio decreasedfrom the upwelling period but wasgenerally higher than that prior to the upwelling. Concluding remarks The observationssuggestthat distinct biological successionsexist for short time intervals in the water off La Jolla, California despite advection. The time interval during which the classicalsequenceof physical, chemical and biological events can be followed during the upwelling seasonis determined partly by the varying intensity of the WNW winds and partly by the regular southerly advection. In the present study the events initiated by an

Characteristics

of an upwelling

off La Jolla

431

upwelling were followed for three weeks. Advection was probably responsible for the large silicate increase which ended the biological response of the classical upwelling sequence. The results have implications for other aspects of marine ecology. The benthos is probably affected by the relationship between the timing and intensity of upwelling and the biomass and condition of the phytoplankton. Nutritional variability contributes to the degree of growth and reproductive success demonstrated by benthic organisms in a particular year. Coastal pollution from sewer and power plant outfalls is more difficult to monitor at certain times of the year because of the changes in planktonic composition and biomass caused by upwelling and advection. Environmental surveys can detect only large instantaneous disturbances in the plankton resulting from outfall pollutants. More subtle effects must accumulate through time in order to be seen above the natural planktonic variability. Acknowledgements I would like to thank E. W. Fager and R. W. Holmes for helpful discussions of the data. C. M. Boyd and S. J. Zentara kindly edited the manuscript. Support for this project was received from the Office of Naval Research Contract N 00014-69-AO~OO-6006 while completing a Ph.D. degree at the Universitv of California, San Diego. References Bolin,

R. L. & Abbott, D. P. 1963 Studies on the marine climate and phytoplankton of the central coast area of California 1959-1960. California COOperUtive Fisheries htVeStigUtiOn apOrtS 9, 23-45. Cairns. 1. L. & LaFond, E. C. 1966 Periodic motions of the seasonal thermocline along the southern Cahfornia coast. Journal of Geophysical Research 72, 3903-3915. Cupp, E. E. 1943 Marine plankton diatoms of the west coast of North America. scrspps Institution of Oceanography Bulletin 5, 239 pp. Dugdale, R. C. 1972 Chemical Oceanography and Primary productivity in upwelling regions. Geoforum II, 47-61. Eppley, R. W. & Strickland, J. D. H. 1968 Kinetics of marine phytoplankton growth. In Advances in Microbiology of the Sea Vol. I, pp. 23-62. (Droop, M. & Woods, F. E. J., eds). Academic Press, London. Fogg, G. E. 1966 Algal Cultures and Phytoplankton Ecology. University of Wisconsin Press, Madison. 126 pp. Grill, E. V. & Richards, F. A. 1964 Nutrient regeneration from phytoplankton decomposing in seawater. Journal of Marine Research 22, 51-69. Holm-Hansen, O., Lorenzen, C. J., Holmes, R. W. & Strickland, J. D. H. 1965 Fluorometric determination of chlorophyll. Journal du Conseil, Conseil Permanent International pour 1’Exploration de la Mer 30, 2-15. Kamykowski, D. 1973 Some physical and chemical aspects of the phytoplankton ecology of La Jolla Bay. Ph.D. Thesis, University of California, San Diego. 269 pp. Lewin, J. C. 1961 The dissolution of silica from diatom cell walls. Geochimica et Cosmochimica Acta 21,182-189.

Lorenzen, C. J. 1967 Vertical distribution of chlorophyll and phaeo-pigments: Baja California. Deep Sea Research 24 735-745. Margalef, R. 1967 The food web in the pelagic environment. Helgolander Wissenschaftliche Meersuntersuchungen 15, 548-559. Mullin, J. B. & Riley, J. P. 1955 The calorimetric determination of silicate with specific reference to sea and natural waters. Analytica Chimica Acta 12, 162-176. Murphy, J. & Riley, J. P. 1962 A modified single solution method for the determination of phosphate in natural waters. Analytica Chimica Acta 27, 31-36. Pratt, D. M. 1966 Competition between Skeletonema costatum and Olithodiscus lateus in Narragansett Bay and in culture. Limnology and Oceanography II, 538-547. Schiller, J. 1933 In L. Rabenhorst’s Kryptogamen Flora. Peridineae Band X, Abteilung III, Teil I. 616 pp. Leipzig. Schiller, J. 1937 In L. Rabenhorst’sKryptogamen Flora. Prridineae BandX, Abteilung III, Teil II. 589 pp. Leipzig.

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Smayda, T. J. 1966 A quantitative analysis of the phytoplankton of the Gulf of Panama. III. General ecological conditions and the phytoplankton dynamics at 8”45’N, 79”23’W from November 1954 to May 1957. Inter-American Tropical Tuna Commission Bulletin II, 353-612. Smith, R. L. 1969 Upwelling. In Oceanography and Marine Biology Annual Rtview Vol. 6, pp. 11-46. (Barnes, H., ed.) Tabata, S. 1965 Variability of Oceanographic Conditions of Ocean Station P in the Northeast Pacific Ocean. Transactions Royal Society of Canada Section IV 3, 367-418. Utermohl, H. 1931 Neve. wage in der quantitatiren enfassung des planktors. Internationale Vereinigung

fur Theoretische und angewandte Limnologic. Verhandbtger

5, 567-597.

Woods, E. D., Armstrong, F. A. J. & Richards, F. A. 1967 Determination of nitrate in sea water by cadmium-copper reduction to nitrate. Journal of the Marine Biology Association of the United

Kingdom 47, e3-3

I.