SEASONAL AND SPATIAL FLUCTUATIONS OF THE PHYTOPLANKTON IN MONTEREY BAY
A Thesis Presented to the Graduate Faculty of California State University, Hayward
In Partial Fulfillment of the Requirements for the Degree Master of Arts in Biological Science
by Russ Waidelich January, 1976 ABSTRACT
The seasonal and spatial fluctuations of phyto
plankton in Monterey Bay were described for an 18-month
period. Low winter levels were mainly due to instabi
lity of the water column, while the spring bloom was
brought a·bout by the commencement of upwelling and the
subsequent stabilization of the water column, with resi
dence time of water regulating chlorophyll concentra
tions. The summer minimum shortly followed the cessa
tion of upwelling, when it appeared that nitrates were
limiting, although zooplankton was a major factor in re
ducing the algal standing stock at this time.
Adequate illumination and relaxation of grazing
pressure rather than specificity of water type appeared
to be the major factors regulating the occurrence of the
fall bloom, indicating that the seasonal pattern of
Monterey Bay conforms generally to that of the mid
latitude marine ecosystems.
An examination of the spatial distribution o.f phytoplankton revealed lower values over the canyon during upwelling months due to advection of water to the
shelf areas, and lower values in the same area during non-upwelling months due to subsidence or downwelling and tur·bul enc e .
ii ACENOWLEDGlVJENTS
Many people were responsible for helping to make this thesis possible. I would first like to thank my major advisors, Dr. Mary Silver and Dr. John Martin, for their valuable advice and support. In addition, the other memters of my committee, Dr. William Broenkow and Dr. James Nybakken, gave their time freely. I am indebted to the staff and students of Moss Landing Marine
La-boratories for making this work possible. A special thanks goes to Jane Kinsley for her help and encouragement during the final stages of this work.
iv TABLE OF CONTENTS
Page
ACKNOWLEDGMENTS iv LIST OF FIGURES vii
INTRODUCTION . . 1 MONTEREY BAY AS AN ENVIRONIVIENT 8 PHYSIOGRAPHY 8 CLIMATE 12 CALIFORNIA CURRENT SYSTEM 17 SEASONAL OCEANOGRAPHIC CHANGES IN MONTEREY BAY ...... 18 CIRCULATION OF MONTEREY BAY 19 METHODS 21 STATIONS SAMPLED 21 BIOLOGICAL AND PHYSICAL DATA 23 STATISTICAL TREATMENT OF DATA 26 RESULTS 27 SEASONAL PHYTOPLANKTON CYCLES OF MONTEREY BAY . . . . 39 STATISTICAL DIFFERENCES OF THE SEASONAL PHYTOPLANKTON CYCLES 52 SEASONAL NITRATE CYCLES FOR MONTEREY BAY 53 DISCUSSION . . . . 58 SEASONAL VARIATIONS OF PHYTOPLANKTON STANDING STOCK ...... 58
v vi
Page
FALL BLOOM OF 1971 . . . ' • t • • • • ' • • • • .. 90 THE TIMING OF THE SPRING PHYTOPLANKTON MAXIMUM 98 FACTORS CONTROLLING THE OCCURRENCE AND COMPOSITION OF THE FALL BLOOM 101 PHYTOPLANKTON-ZOOPLANKTON RELATIONSHIPS 104 SPATIAL DISTRIBUTION OF PHYTOPLANKTON IN MONTEREY BAY ...... 111 FACTORS INFLUENCING LOCATION OF UPWELLING 113 SUMMARY 116 LITERATURE CITED . 120 LIST OF FIGURES
Figure Page 1. Hydrografic sampling stations in Monterey Bay, California. 4
2. Numbers 1 through 6: Calcofi hydro grafic sampling stations, 1954-1960. Arrows represent surface movement in Broenkow's drogue study, 1972. 6 J. Monterey Bay, California. 9 4. Location of sewage outfal1s in Monterey Bay. . . . 11 5. Seasonal light curve for Monterey Bay. . . · · · · · · · · · · 16 6. Vertical distribution of temperature ( °C) in Monterey Submarine Canyon (Stations 1115 and 1157)...... 29 7· Distribution of surface temperature (°C) of Monterey Bay, April 197J. JO 8. Distribution of surface temperature (°C) of Monterey Bay, June 1972. Jl 9. Distribution of surface temperature (°C) of Monterey Bay, July 1972. J2
10. Distribution of surface temperature (°C) of Monterey Bay, August 1972 .. JJ
11. Distribution of surface temperature (°C) of Monterey Bay, October 1972. J4
12. Distribution of surface temperature (°C) of Monterey Bay, November 1972. J5
1J. Distribution of surface temperature (°C) of Monterey Bay, January 197J. J6
vii viii
Figure Page 14. Stability diagram for station 1115 in Monterey Bay for 1972. Individual figures combine a depth versus temperature profile and its mirrored image. .38
15. Monthly variation of average chlorophyll a (mg/m2) in upper 10 m water in Monterey Bay. 41
16. Division of Monterey Bay into regions for the purpose of comparing monthly variation of parameters. 42
17. Alternative division of Monterey Bay into regions for the purpose of comparing monthly variation of parameters.
18. Monthly varia~ion of average chloro phyll a (mg/m ) in upper 10 m water of inshore and offshore regions of Monterey Bay. 46
19. Monthly variation of nitrate concen tration (ug-at/liter) in surface water of inshore and offshore regions of Monterey Bay. 48
20. Monthly varia~ion of average chloro phyll a (mg/m ) in upper 10 m water for north, south, and central regions of Monterey Bay. 50
21. Monthly variation of nitrate concen tration (ug-at/liter) in surface water for north, south, and central regions of Monterey Bay. 57
22. Distribution of surface salinity of Monterey Bay, February 197.3. 60
2J. Distribution of nitrate concentration (ug-at/liter) in surface water of Monterey Bay, March 1972. ix
Figure Page 24. Distribution of surface temperature (OC) of Monterey Bay, March 1972. 65 25. Distribution of average chlorophyll a (mg/m2) in upper 19 m water in - Monterey Bay, March 1972...... 66 26. Distribution of average chlorophyll a (mg/m2) in Monterey Bay, April 1972.-. 67
27. Distribution of surface temperature (oc) of Monterey Bay, April 1972.
28. Northwest wind stress for Monterey Bay, October 1971 to April l97J. 71
29. Distribution of average chlorophyll a (mg/m2) in upper 10 m water in - Monterey Bay, May 1972...... 7J
JO. Distribution of surface temperature (°C) of Monterey Bay, May 1972. 74 Jl. Distribution of nitrate concentration (vg-at/liter) in surface water of Monterey Bay, May 1972...... 75 J2. Distribution of apparent bxygen utilization (vg-at/liter) in surface water of Monterey Bay, May 1972. 76
JJ. Distribution of average chlorophyll a (mg/m2) in Monterey Bay, June 1972. 78 J4. Distribution of surface salinity in Monterey Bay, June 1972...... 79 J5. Distribution of nitrate concentration (vg-at/liter) in surface water of Monterey Bay, June 1972...... 80
J6. Distribution of nitrate concentration (ug-at/liter) in surface water of Monterey Bay, July 1972...... 82 X
Figure Page .37· Monthly variation of average chlorophyll a (mg/m2) in upper 10 m water and zooplankton bio mass (ml/m3) in upper 15 m water of Monterey Bay. (Zooplankton volumes were not determined after August 1972.) ...... 85
J8. Distri·b~tion of average chlorophyll a (mg/m ) in upper 10 m water in Monterey Bay, September 1972. . ... 87
.39· Distribution of surface temperature (°C) of Monterey Bay, September 1972...... 88 L!-0 • Distribution of nitrate concentration (ug-at/liter) in surface water of Monterey Bay, September 1972. . ... 89 41. Distribution of average chlorophyll a (mgjm2) in upper 10 m water in Monterey Bay, October 1971. . ... 92
42. Distribution of surface temperature (°C) of Monterey Bay, October 1971. 93 4J. Distribution of nitrate concentration (ug-at/liter) in surface water of· Monterey Bay, October 1971. . .. 94
44. Distribution of apparent oxygen utilization (ug-at/liter) in surface water of Monterey Bay, October 1971. 95 45. Distribution of ammonia concentration (ug-at/liter) in surface water of Monterey Bay, October 1971...... 96 INTRODUCTION
The importance of phytoplankton as the founda
tion of marine food chains has been long established
(Haeckel 1890; Johnstone 1908; Lohmann 1911). Seasonal
fluctuations of phytoplankton standing stock and the general relationship of these primary producers to other trophic levels, while sometimes complex, is also well established. Brandt (1902) was the first to propose that
Liebig's law of the minimum, with respect to nutrients, be used to partially explain the seasonal differences in phytoplankton standing stocks. Nathansohn (1906)
sted that vertical currents were involved in the circulation of phytoplankton nutrients. Since the turn of the century, a great number of investigations have been completed on seasonal phytoplankton cycles and the
tors that regulate these systems. Most work has been done in the mid-latitudes, although extensive research
s also been undertaken in both the tropical and polar
ceions of the sea.
Most early investigations of upwelling areas were primarily concerned with the physical and chemical
pects of the systems. Moberg (1928) was one of the
irst to relate increases in the diatom standing stock
1 2
to the chemical environment of upwelled water. In the
last decade there has been a tremendous increase in the
studies of the upwelling phenomena and its subsequent
biological ramifications (Forsberg 1963; Curl and Small
1965; Smayda 1966; and others). This increased interest
may be related to the quest for more food from the sea
to support the ever-increasing hu'man population. Ryther
(1969) has estimated that about 50% of the world's fish
supply is produced from upwelling areas.
Monterey Bay is a known region of upwelling.
The bay was one of the major sardine fishing grounds on
the west coast of the United States until this fish population declined in the early l950's (Wolf 1958), yet only a few major plankton-related studies have been completed in the bay. Bigalow and Leslie (1930) concluded that upwelling was the major source of nutrients for near surface waters here. Bolin and Abbott (1963) recorded bi-weekly changes ln phytoplankton abundance and composi tion from 1954 to 1960 and established an average annual cycle. This cycle revealed the yearly minimum during the months of November, December and January and the maximum in June. Following the spring maximum, the population constantly declined to the winter minimum, and no fall bloom was recorded. The investigation was incomplete ln that no nutrient data were collected. The conclusion that 3
nitrate was a limiting factor was inferred rather than
proven, although it was probably correct. Nitrate has been shown to be critical in.other areas off the coast of
California (Moberg 1928; Redfield et al. 1963) and in
~onterey Bay (Smethie 1973). Malone (1971) studied the seasonal fluctuations of netplankton and nannoplankton, at Calcofi Station 3, near the mouth of Monterey Bay (Fig. l). He suggested that zooplankton limited nannoplankton year round and
ed the netplankton spring bloom. The apparent con- flict between Malone's and Bolin and Abbott's findings can be partially reconciled by the fact that these two investigations took place in different years, and used different sampling techniques and different spatial coverage.
In 1972 Silver and others in a twenty-six hour study indicated the complexity of. the phytoplankton sam
ing problems in a part of the bay. At a station lo-
ated over the head of a submarine canyon, they noted a fourfold change in standing stock in relation to a tidal cycle (Fig. 1). During the same investigation, Broenkow
McKain related the rise and fall of the subsurface isotherms to the ebbing and flooding tides, respectively.
is sampling problem may not have been an important factor 4
Figure l. Hydrografic sampling stations in Monterey Bay, California. 5
in Bolin and Abbott's study, since their stations were
located well away from the head of the canyon (Fig. 2).
Smethie (1973) demonstrated the spatial and
seasonal heterogeneity of both conservative and non
conservative physical and chemical properties of the
near surface waters in Monterey Bay. The occurrence of
upwelling was related to the northwesterly winds, and
a residence time for bay waters was calculated. He also
discussed nutrient sources and the relationships of
various nutrients with each other and with the biological
community. The earlier studies cited above have outlined
the seasonal fluctuations and local variability in
Monterey Bay waters. The goals of the present investi
gation were to ela.borate on the relationship shown in
these previous studies "by relating the temporal and spa
tial variations in the phytoplankton standing stock to local and regional environmental parameters. A ·brief geographical description of Monterey Bay is presented which reviews the topographic, climatic, and oceanographic features of the bay. Since the spatial distribution of phytoplankton must be interpreted in relation to the sea sonal changes of phytoplankton concentration, the first part of the discussion is devoted to the mechanisms that 6
Figure 2. Numbers l through 6: Calcofi hydrografic sampling stations, 1954-1960. Arrows represent sur face movement in Broenkow's drogue study, 1972. 7
control and regulate the annual cycle. Seasonal changes in standing stock were related to specific controlling parameters. The temporal occurrence of the spring and fall blooms are then discussed and their differences noted in respect to other mid-latitude marine ecosystems.
The relationship of the phytoplankton and zooplankton seasonal cycles was considered, and compared with Cushing's
(1959) and Heinrich's (1963) models of the interaction of these two communities in the mid-latitudes. The final part of this presentation deals with the spatial distrib ution of phytoplankton in the bay during the upwelling and non-upwelling months, and factors that may influence the location of upwelling sites. MONTEREY BAY AS AN ENVIRONIVJENT
Physiography Monterey Bay is a predominantly arcuate inden
tation 1n the central California coastline located between
J6° 59' N latitude (Fig. J). The embayment occupies an area of approximately 5J4 km2 and extends 16 km inland
from a line drawn between its northern and southern
ends, Point Santa Cruz and Point Pinos, respectively
(Smethie 197J). The headlands of Point Santa Cruz lie J7 km from those of Point Pinos, forming conspicuous bights at each end of the bay.
Three principal rivers, a slough and numerous small streams and creeks empty into the bay. The Pajaro
River, Elkhorn Slough and the Salinas River drain areas of intensely fertilized agricultural land and enter near the central region of the bay. During the dry season the Salinas River is blocked 'by a: sand ·bar, and its waters flow northward in its old course and empty through Elkhorn Slough (Smethie 197J). The San Lorenzo River flows into the bay in the vicinity of the northern bight, while small streams enter throughout the northern portion of the bay
(Fig. J). The discharge of these tributaries closely follows the pattern of seasonal precipitation.
8 9
12'2 ° w 37"N en.
. ' .50'
10 _j
Figure J. Monterey Bay, California. 10
In addition to the input by natural drainage sys-
terns, ten sewage outfalls from the surrounding communities
release a total of about thirty-one million gallons a day
into bay waters (Fig. 4). Twenty-three million gallons
are discharged daily directly into the bay, while the
remainder flows to the bay via two tributaries: the
Salinas River and Elkhorn Slough (Wong 1970).
Monterey Bay is primarily a shallow water expanse,
81% of its area being at depths of less than 180 m. How ever, a submarine canyon comprises 19% of the area and bisects the bay into two approximately equal areas: the north 2J8 km2 and the south 195 km 2 . Both shelf areas deepen from the shoreline toward the canyon rim, the north dropping to 90 m, and the south dropping to 180 m
(Smethie 197J) (Fig. 1).
The Monterey submarine canyon is a conspicuous and dominant bathymetric feature _of the ·bay and has been well described (Shephard and Emery 1941; Dill et al. 1954;
Martin 1964). It lS the largest and deepest canyon found on the west coast of the United States. The head is 0.2 km wide, 18 m deep and O.J km from the shoreline near
Elkhorn Slough. This open, V-shaped, slightly meander ing canyon has two main tri-butaries: Soquel canyon,
8 km long, entering from the northeast within the bay; Figure 4. Location of sewage outfalls in Monterey Bay. 12
and Carmel canyon, entering from the southwest just out
side the mouth of the bay. At the entrance of the bay,
the canyon has deepened to 865 m and. widened to 12 km; it
continues to meander, deepen and wid.en in a general south
west direction to a distance of a·bout JO km offshore, where
it descends to a depth of about 3500 m (Shephard and Emery
1941).
Climate
The high pressure cell over the North Pacific and
the low pressure cell over the cont~nent control much of
the weather of the west coast of California. The inten
sification and northerly shift of t~e Pacific high in the
spring and summer and the southerly shift and decreasing
strength in the fall and winter bri::-_g about changes in the wind patterns which influence and determine the character of the regional climatic pattern (Gillium 1962). During the spring and summer, the westerlies are greatly inten sified as the pressure gradient beccmes large between the cells and the distance 'between them becomes small. The dominating winds are northerly· and ::_orthwesterly; the effect of the Coriolis force, and to a lesser extent the presence of the coastal mountain ra~·-ge, turn these winds equatorward and parallel to the coast (Gillium 1962).
The winds are strongest off Baja Ca~ifornia in April and 13
May, off the central coast 1n May and June, and off the
northern coast in June and July, reflecting the northerly
shift of the Pacific high (Smith 1968). By late fall, the northerly winds usually weaken
and disappear, allowing transoceanic winds from the west
and southwest to reach the Pacific coast. These winds are
almost entirely extratropical cyclones of the North Pacif
ic Ocean. It is these winds that bring the cold season
precipitation that is characteristic of the west coast
of North America. Maximum precipitation follows the lati
tude progressing southward with time: October is the wet
test month in southern Alaska, while southern Oregon and
central California have their greatest amount of rainfall
in January, and southern California in February. This
pattern reflects the southerly movement of the mean
Pacific cyclonic track in conjunction with the seasonal
expansion of the Aleutian low pressure center (Pyke 1972). Monterey Bay receives an average of 33 em of rain per year. The local precipitation regime starts in Novem·ber and extends to April (Anonymous 1969). The development of the sea-breeze is the diurnal analog of a seasonal monsoon circulation (Cole 1970).
Locally, it typically starts in early April and intensi fies through the summer months (Williams and DeMandel 14
1966). When the temperature gradient becomes pronounced
·between the Salinas Valley and Monterey Bay waters, the
local winds begin. By late morning, the land is usually
warmer than the adjacent water, and an onshore wind
develops. The wind intensifies through mid-afternoon
and dies out toward nightfall when temperature differences
disappear. At night the reverse circulation is observed
as the land 'becomes colder than the adjacent water, and
the offshore breeze becomes established (Gillium 1962).
The night breeze, because of a smaller temperature gradi
ent, is less pronounced, and therefore the net wind move
ment is onshore (Smethie 1973). The seasonal light curve for Monterey Bay is sinus
oidal ln shape (Fig. 5). The minimum incident solar radi ation occurred in December 1971, November and December 1972, and in January 1973 (Schwanz, personal communication). The maximum values were recorded in June and July of 1972. However, this light curve may not represent the true light regime for Monterey Bay: the ampere-hour meter was located in a relatively fog-free area, and the fog that accompanies upwelling and is usually present from March to September may skew the light curve so that the maximum radiation occurs in October (Malone 1971) . No evidence is presently available to reject or confirm this hypoth esis. 15
Figure 5. Seasonal light curve for Monterey Bay. 16
2
Lt.. r0 )'-.. J (J)
0 z
0
(/) ,...,. (/) "''""<:! Lt.. ~ J (j) 0 _____j__j__J_I.---''---'--~/_L_L--"-L _ _!_UJ: t\J ~~unoH ~-HJ3dVJ\f 17 falifornia Current System Just as the winds are the major influence on climate, they are also the prime factor responsible for the great clockwise ocean circulation of the North Pacific. The southeasterly flow along the west coast of North America is the eastern boundary current of the North Pacific gyral and is known as the California Current System (Smith 1968). The system includes the southeasterly flowing California Current, the deep north- westerly flowing countercurrent and the related vertical circulation currents of upwelling and downwelling. The California Current is a wide (-1000 km), shallow (~500 m), slow (~25 em/sec) current which trans- ports sub-Artie low temperature, low salinity, nutrient- 0 rich water to about 25 N latitude (Wooster and Reid 1963). At this latitude the current turns west to become the North Equatorial Current. The northerly winds, typically present from late February to September, in conjunction with the Coriolis force displace a portion of the upper water- mass offshore, subsequently causing the upwelling of subsurface water along the coast. Sverdrup and Fleming (1941) estimated that the water was upwelled from less than 200 m in depth. The weakening and cessation of 18 the northwesterly winds 1n the fall marks the end of upwelling. It is at this time that the California countercurrent surfaces between Point Conception and British Columbia, either due to the lack of northwesterly winds or in response to the southwesterly winds (Smith 1968). Once the countercurrent surfaces, it is known as the Davidson Current, and it lies east of the California Current, extending out as far as 80 km from the coast (Reid and Schwartzlose 1962). The combined influence of the southerly winds on the surface water and the Coriolis effect result in a convergence of water along the coast and the subsequent downwelling of surface water (Bolin and Abbott 196 3) . Seasonal Oceanographic Changes in Monterey Bay The seasonal hydrographic changes in Monterey Bay correspond to the seasonal changes in the California Current System. The typical annual cycle of the bay has been well documented. The original work was done by Skogsberg (1936) and confirmed in later studies (Skogsberg and Phelps 1946; Bolin and Abbott 1963; Ahbott and Albee 1967; and Smethie 1973). Skogs'berg divided the annual cycles into three hydrographic periods: (1) an upwelling period from March to September, (2) an oceanic period from September to November, and (3) the Davidson Current period 19 from November to March. Since the annual cycle displays some variability from year to year and unseasonal winds can cause upwelling or dovmwelling during any month, it was thought best not to divide the year into particular oceanographic periods in this investigation. Rather, the bay will be charac terized by the water type that is present at a particular time. Upwelled water is typically high in nutrients and salinity, and low in temperature and oxygen. Oceanic water is typically hrgh in temperature (depending on the position of the sun), high in salinity, and low in nutrients. Circulation of Monterey Bay The horizontal circulation pattern of Monterey Bay is not definitely known. Past drogue studies (Broenkow, personal communication) and the horizontal distribution of physical and chemical parameters (Smethie 1973) suggest that during upwelling, water flows shore ward in the central portion of the bay, diverging into clockwise and counter-clockwise branches that flow toward the southern and northern bights, respectively. Average current speeds measured during drogue studies varied between 10 and 15 em/sec (Broenkow, personal communication). The presence of closed isopleths of various parameters 20 suggest the occurrence of localized eddies near the ends of the bay (Smethie 1973). Little is known about the driving force of the circulation of the bay. Broenkow (personal communication) has suggested that local winds may be a major factor in the movement of surface waters. Garcia (1971) demon strated that the circulation could be driven by oceanic currents outside the bay. METHODS Stations sampled Phytoplankton standing stock and hydrographic samples were simultaneously collected on monthly crulses by personnel at Moss Landing Marine Laboratories in Monterey Bay. Sampling of Monterey Bay waters originally began in February 1971, but the present study only inclu des data from October 1971 to April 1973, during which time a sampling pattern with nineteen stations was used. (Fig. 1 and Table 1). Before September 1972, all samples were collected within three days; from September onward all samples were collected within two days. Radar was used as the principal means of naviga tion. Horizontal triangulation, range and bearings were used to compute positions, and these were confirmed by the location over a known depth with an echo-sounder. Stations are assumed to have been occupied within 0.13 km radius of their nominal positions. Water samples were collected with 5-liter Niskin plastic bottles at standard depths of 0, 10, 20, JO, 50, 75, 100, 150, 200, 250, JOO, 400m. At shallower inshore stations of less than 20 m, an additional sample was taken at 5 m. Samples were collected from all hydrographic 21 22 TABLE 1 Hydrographic Stations in Monterey Bay Station Latitude N. Longitude W. Depth (m) Number 1101 36° 44.7' 121° 49.3' 15 1105 36° 50. 8' 121° 49.6' 15 1108 36° 4?.4' 121° 50.0' 240 1110 36° 45.6' 121° 52.0' 70 1112 36° 48.0' 121° 52.2' 240 1115 36° 46.6' 121° 57.2' 718 1116 36° 43.3' 121° 55. 6' 97 - 1120 36° 43.1' 121° '51. 5' 57 1121 36° 37. 6' 121° 51.2' 18 1122 36° 36.6 121° 52.9' 16 1123 36° 30.2' 121° 53.1' 68 1124 36° 38.8' 121° 56.3' 40 1150 36° 52.9' 121° 51.3' 18 1152 36° 55. 3' 122° oLJ .• 4• 35 1153 36° 56.8' 122° 00.1' 15 1154 36° 55. 2' 121° 52.7' 15 1157 36° 50.2' 121° 58 .1' 366 1158 36° 55.1' 121° 56.7' 26 1159 36° 57.1' 121° 56.2' 15 23 stations and depths on crulses ML 18 (October 1971) through ML 35 (March 1973). During the last cruise ML 36 (April 1973), five stations were not sampled (stations 1115, 1157, 1124, 1116, . and 1110) due to severe weather conditions. Chlorophyll §-phaeophytin a concentrations were determined for all depths, but only the values from the upper 10 m were used in this study. Cumulative chlorophyll a for the upper 10 m of the water column was calculated by integration of the discrete chlorophyll data and expressed as chlorophyll a ln. mg I m2 . Biological and Physical Data The determination of the phytoplankton standing stock was achieved by measurement of chlorophyll a con- centrations. The presence of chlorophyll§ was determined by the use of a fluorometer, and its characteristic pro portional fluorescence was related- to changes in pigment concentration. This method for the determination of phytoplankton chlorophyll § and phaeophytin a has ·been described by Yentsch and Menzel (1963) and Holm-Hansen (1965). Phytoplankton pigment samples were obtained from filtration of 500 ml seawater samples that were drawn for nutrient analysis. The samples- were filtered through a 24 Gelman Type A filter with an effective pore size of O.J urn. Before filtration was complete, l-2 ml of MgCOJ solution was introduced to prevent possible acidifica tion of the sample. Once the filter was removed from the filter apparatus, it was folded in half with the pigmented side innermost, placed in a disposable petri dish, and frozen until pigment extraction in the la'bora tory could be undertaken. Phytoplankton pigments were extracted in 90% aqueous acetone. The filter was placed in a "Potter" type tissue grinder and ground for one minute in approx imately 2 ml of acetone. The contents were then trans ferred to a centrifuge tu·be and diluted to 15 ml and placed in a refrigerator for a period of between two and twenty hours to complete extraction. The material was then centrifuged at approximately 5000 rpm for two 10-minute periods, the tubes being. slightly agitated 'between centrifugations. The supernatent was decanted into two 5 ml cuvettes to provide replicates. Fluores cence of chlorophyll ~ was read on a Turner Model III Fluorometer fitted with a CS 5-60 primary filter, a CS 2-64 secondary filter, a blue lamp (G.E. No. F4T4/Bl), and an R-446 photomultiplier. The samples were then acidified with 2 drops of 5% HCL and agitated, and the 25 fluorescence of phaeophytin a was read. Precision of the method is about 5% (2SD). The calibration methods described by Strickland and Parsons (1968) were used to determine the equations that specified the relationship between fluorescence and pigment concentration. A Beckman DU 2 spectrophotometer and Parsons' and Strickland's formulae were used to relate extinction values to chlorophyll a and phaeophytin ~ pigment concentrations. Light data were made available by Russell Schwanz of the Monterey Naval Postgraduate School. The data were collected at the site of the school, near the city of Monterey, about l km from Monterey Bay. The radiation measurements were recorded with an ampere-hour meter and expressed as ampere-hours/day. The physical and chemical data presented in this paper were obtained from the publications of Smethie (1973) and Broenkow (1972, 1973) which describe the hydrographic conditions of Monterey Bay from February 1971 to April 1973 The data were collected simultaneously with chlorophyll a data. Chlorophyll a and zooplankton data from October 1971 to September 1972 were very generously given for use in this paper by Dr. Mary Silver of the University of California at Santa Cruz. 26 Statistical Treatment of Data The chlorophyll~ and nitrate data from all previously mentioned sources were both paired by month for different sectors of the bay. The Wilcoxon Signed Rank test was used to compare population concentrations. (It was felt this test would be sensitive to small monthly differences without assuming a normally distributed popu lation). The Signed Rank-test is about 95% efficient on normally distributed data (Peatman 1963). In most cases the data were fairly continuous, thus satisfying a prlme requirement of this test. 27 RESULTS Upwelling was in progress in Monterey Bay during December 1971, February through June 1972, September 1972, and March and April 1973. The process is denoted by the ascending isotherms over the deep water stations 1115 and 1157 (Fig. 6). In April 1973, stations 1115 and 1157 were not sampled, but upwelled water was evidenced by the 110 to 120 C surface water in the bay. Upwelled water was present in the bay in June, again indicated by the 11° to 12° C surface temperatures (Figs. 7 and 8). The slightly descending isotherms show that the upwelling rate was decreasing in June. Oceanic water was advected into the bay in July, August, and October 1972. Figures 9, 10, and ll show the characteristic high surface temperatures found in oceanic waters, and figure 6 reveals the descending isotherms due to subsidence in the water column. In November 1971 and January 1972, and from November 1972, to February 1973, the Davidson Current waters flowed shoreward and piled up water .along the coast, causing downwelling to occur in Monterey Bay. This is indicated by the decreasing but uniform surface temperatures (Figs. 12 and 13) and the uniformity in the temperature gradient between 0 and 100 m (Fig. 14). 29 "~~'"'--.'-.. '"\ \\j\ '·-"'--, ~ ~~-)'·\· .< r--~) --(l) y 0 w d ~ ~ ; / . . ~ .§ ~ -rJ ~ ti.J I ,.:-- "'/ j """' c- ~~ ~ l,; b lrJ I ~- . !- <( ...,... ""'-·,);.~ ll. - N I ~~ !-? ~ ~<:::· J~ ~ -1~ f \ . . - _ __j_____! __..L. __ ,J~ .. -~L-~-=='"'.1 o 0 0 0 0 0 0 0 ~- 0J t0 ·uJ Hl,d.3.CJ JO / 0 .,,_ 12° 0 0 0 0 0 0 12-13° 0 0 Q Plf-!·0}- S---'------'___t__ l_.t_._t_ ...L ....L..__,__ .... _l ...... L-!__L...... J•---'--...... L-- ...... J_~L...... !- 0 1 /220 oo'· fj 12/ 50 Figure 7. Distribution of surface temperature (°C) of Monterey Bay, April 1973. Jl . -t·~-i-r---.--c---,-r-1 0 0 0 0 ~ 13-14~ -l 0 Fi=::-...:::::-.:::: 8. Distribution of surface temperature (°C) of ===~terey Bay, June 1972. J2 0 0 40 t Figure 9. Distribution of surface temperature (°C) of Monterey Bay, July 1972. 33 0 0 0 0 (1fELKHORN I i SLOUGH- 0 I ' I 0 0 ol (?/.. 0 ~~~ 0 ~~ 1> 0 0 Figure 10. Distribution of surface temperature (°C) of Monterey Bay, August 1972. J4 0 () 0 0 0 r------p 0 0 0 0 Figure 11. Distribution of surface temperature (°C) of Monterey Bay, October 1972. 35 0 0 0 13-14 '' 0 0 0 ''r-,) :.Jro'r·' \; 0 j~ ~ ELKHORN 0 ( SLOUGH 0 0 0 0 0 0 - 40' 0 0 Figure 12. Distribution of surface temperature (°C) of Monterey Bay, November 1972. 0 0 0 <(~ ~ 0 \")..-::,'V 12-!3° ~' _/ .. 0 _,6o,-Q;.I 0 .5 I 0 i\ ~ n(Elf:HORN 0 i. SLOUGH - 0 0 J 0 t 0 J'~ "1- 0 0 . ~<~-1.5' 1 ,__., ,, 10C.-- i'-"l Figure 13. Distribution of surface temperature (°C) of Monterey Bay, January 1973. 2:r AJANUARY FEBRUARY ool- ~ I I 75r 1--=1, lOG~t_l or /~ "\7 I /-"" 25["i ~-( MAY ~~J: E I -·-- :: 5 c 1 ~--'------; t I I ---"\ ~ 75~I ~~ I ------~ 100 l ' __j I or 6 SEPTEMBER DECEMBER 25 ~ ~ NOVEMBER I 50f 75 L r-----_j I ·~g I IOOl \...0 OJ 39 Seasonal Phytoplankton Cycles of Monterey Bay During the midwinter months of November, December and January the phytoplankton populations of the bay were at their annual minimum; this was true for both years (Fig. 15). The average minimum value was 5.5 mg/m2 of chlorophyll_§:, and it fluctuated from 3.3 to 6.9 mg/m2 Both years presented the same growth pattern; the maximum occurred 1n April 1972, and the highest level in 1973 was recorded 1n March, the last month of complete sampling. In May 1972, a marked drop in the standing stock to a level of about two-thirds the April value was noted. The standing stock increased in June, approaching the April level, then decreased sharply in July and reached a summer minimum in August of 7.4 mg/m2 chl a. The fall flowering took place in September and reached a maximum of 26.2 mg/m2 chl a. The previous year the fall flowering occurred in October and reached a concentration of 33.6 mg/m2 chl a The bay was divided into smaller areas, and chloro phyll and nitrate cycles for these were developed. Two schemes wer.e used to subdivide the bay; one divided the bay into offshore and inshore areas, and the other divided the bay into north, south, and central areas. Figures 16 and 17 show how the bay was divided and what stations con stitute a specific region. The following paragraphs will outline the phytoplankton cycle for each of the regions. 40 Figure 15. Monthly variation of average chlorophyll a (mg/m2) in upper 10 m water in Monterey Bay. 41 42 \ \ \ 50 ' \9 ' \ ...... 57 0 ' OFFSHORE \ REGION \ \ I 15 0 J 1 / 4o' Figure 16. Division of Monterey Bay into regions for the purpose of comparing monthly variation of para meters. 4J NORTH--- - ..._ 0 57' ...... _ - C"ENTRAL REGION ~ ~15 .,.,.. --- / 40' Figure 17. Alternative division of Monterey Bay into regions for the purpose of comparing monthly variation of parameters. 44 The annual standing stock cycle for the inshore stations displayed the same general pattern that was exhibited throughout the entire bay (Fig. 18). The most obvious difference was the occurrence of a spring maximum two months later in June for the inshore stations. Although a strong peak in the biomass was seen in April, it did not approach the magnitude of the June maximum. In December of 1972 a slight increase 1n biomass was noted in contrast to the preceding month of November and the succeeding January; the increase was more pronounced for the inshore. stations than for the bay as a whole. The winter and summer minima, the beginnings of the spring increase, and the fall blooms reflected the trends of the whole ·bay. The standing stock profile for the offshore area 1s unique when compared to the plots of the bay as a whole and the inshore stations (Figs. 15 and 19). The offshore winter minima and apparent inception of the spring flower ing coincided with the biomass changes in the other parts of the bay (Fig. 20). The phytoplankton biomass increased fairly consistently from January 1972, to the maximum in April (25 mg/m2 chl ~), then gradually decreased to the yearly minimum value (2.1 mg/m2 chl .§) in November. No 45 Figure 18. Monthly variation of average chloro phyll a (mg/m2) in upper 10 m water of inshore and offshore regions of Monterey Bay. 46 0 I \ """ ,..,.---.... ) / ...... I \ // / ( l I I I - I o wI z{0-, _\ \ ' ' ~~ ~-- 48 w \ 0::: w \ 0 0::: I I 0 I en I I LL U) I I 6 z I 0 I . . L z I !JJ 50 <-£> d: >- >- ro <.t:.<..J I!- m m <1: I") - ,. ::r: rr: /'-. ~ !-·· t-~ -:> (j) o:::JZ: - 0 0 liJ z(J)O a I l z /'/ • 0 ~---c·. (;') -2-·,. I. ./, ..., ,------~-)) ~ . ' ·- ··-· - -- ..., - -...... ' ) ~ :2' /f/ •• . 1-· / . ' .....-:' \: . . ~,---- .. :;s 1/'\.. •.. • tr_ . ·-~-- -...!..•\ l ~ . /. ..., ffi (: l 0 . . . )~l2 ____)__L___L ------I !__ t__j_t_j__t_./ .· 0 0 0 0 0 0 0 0 N 0 (() (J) v (\j 51 secondary peak was noted in June, as shown in different areas of the bay, although the standing stock did remain at the level of the previous month. The decline in the standing stock continued, and no response or decrease ln the descent occurred in September, the month when the fall bloom occurred in other parts of the bay. In October of the previous year little response was seen in the offshore region, while the fall bloom was displayed in other parts of the bay. In October 1971, the north bay sector reached a level of 101.4 mg/m2 chl ~' the highest value for an area recorded during the investigation (Fig. 20). The concen- tration dropped rapidly in November and continued to decline to the Dec em·ber level of 5.7 mg/m2 chl a. A slight increase occurred in January; then, the level fell to the annual minimum in February of 2.J mgjm2 chl a. The spring maximum was 38.6 mg/m2. chl a in April, with a secondary peak in June, a summer minimum of 7.8 mgjm2 chl ~ in August, and a fall bloom of 27.5 mgjm2 chl a in September. By November, the chlorophyll concentration had approached the winter minimum for the north bay. The following spring increase in phytoplankton began ln February and reached a maximum level by March. Three distinct peaks of increasing magnitude 52 occurred in the phytoplankton population in the south bay during the first six months of 1972: February (40.7 mg/m2 chl _9:), April (45.J mg/m2 chl _9:), and June (75.3 mg/m2 chl _9:) . The February peak occurred when the north bay was beginning its spring increase (Fig. 20). The phytoplankton maximum in June was the highest in the bay at this time and was followed in July by the south ·bay summer minimum of 5.4 mg/m2 chl £· Since two of the four central bay stations are included among stations considered offshore, the curves describing the variations in standing stock in the central and offshore areas should be similar (Figs. 19 and 20). The major difference in the two areas was the display of a fall bloom in the central area, but not in the offshore area in 1972. Neither area displayed an increase in bio- mass in October 1971. Statistical Differences of the Seasonal Phytoplankton _gycles The differences noted between the inshore and off- shore phytoplankton standing stock was found to be highly significant: (P=.Ol) for a period spanning the length of the investigation and for a period of one year (October 1971 to September 1972), with the inshore values being higher. During the periods of both upwelling and non- 53 upwelling, the difference was significant at the P=.05 level. The chlorophyll~ concentration found in the northern and southern areas of the bay over an eighteen month period and a one year period were found to be signif icantly higher (P=.05) than the concentration found in the central bay. The result of tests for differences in chlorophyll for shorter periods of time were not as con sistent. During the upwelling period no difference was noted ·between the north and central portions of the bay. The high chlorophyll values at stations 1108 and 1112 were responsible for this uniformity. At this time, the southern area was significantly higher (P=.05) in chloro phyll than either the northern or central region. In non upwelling periods the northern sector was higher in phyto plankton standing stock than the central bay area, while no difference was noted between the southern and central regions. Seasonal Nitrate Cycles for Monterey Bay The nitrate concentration varied with time 1n Monterey Bay. The major changes in nitrate levels were correlated with changes in the hydrographic seasons of the bay. Increases in nitrate were related to upwelling, while decreases were associated with advection of depleted oceanic water into the bay and photosynthetic activity. 54 Figure 19 shows the change in the nitrate concen tration-for the offshore area. This was the cycle thought to be least altered by photosynthesis and mainly the result of the seasonal change of water masses entering the bay. In Figures 6, 19, and 21 the ascending and descending isotherms reflect the increases and decreases in the nitrate concentrations. The cycle for the inshore area follows the same general pattern as that for the offshore area. During upwelling months, when light levels were sufficient for net photosynthesis, a highly significant difference (P=.Ol) in nitrate concentration existed between the inshore and offshore crea (Fig. 20). Nitrate levels were greater offshore. Upwelling occurred in December 1971, when the incoming solar radiation was at its minimum (Fig. 5), and no difference was noted between the .inshore and offshore nitrate levels. In non-upwelling periods the two areas generally had the same amount of nitrates. Both the north and south ·bay show a significantly lower amount (P=.05) of nitrate than the central bay during non-winter upwelling periods. The difference is greater between the central bay and the north bay; the south bay tends to be higher in nitrates than the north bay. In non-upwelling months the bay tends towards a 55 homogeneous nitrate concentration (Fig. 21). The central and the offshore areas developed almost identical nitrate cycles for the duration of the investigation due to both regions having two common stations (Figs. 19 and 21). Nitrates were depleted in parts of Monterey Bay during the summer months. In June two stations exhibited a total lack of nitrates in the upper 10 m of the water column. By July, seven stations were completely devoid of nitrates. Again, in August the nitrates of seven stations were exhausted, and surface values of four addi tional stations were reduced below detectable levels. Nitrates were consistently absent at stations 1101 and 1128 during the three summer months. An upwelling pulse 1n September replenished the nitrate concentrations, but by October nitrates were again depleted in several stations. 57 > ~ .. <( >- >- OJ / \ . ~ <{ / \ . co _j // 1,: .J_ <{ I .•... ; I I 0::: I (.· J 1- f- ~- CL:JZ '·.' ..... · ,.,. .... I 9 0 l.LJ .... ' .z (f) () \'·,. "~\.- (-:.::-:~ .-r·l/ \~·· ..... " ·· .. ,_ ': Seasonal Variations of Phytoplankton Standing Stock The relationship of phytoplankton production to the environment is complex, and control of production cannot ·be attributed to one factor, but must result from a combination of factors. Although at times one factor may exert more influence and control than others, such controls may be transitory, with the proximal limitation shifting from factor to factor during the course of a season. The winter months will be discussed first because it is felt that the factors contributing to the low stand ing stock during these months are the least conplex. A chronological monthly description will follow, with the complicated fall bloom of 1971 considered in a separate section. In the winter months of November through January, solar radiation is at its minimum (Fig. 5), while storms and precipitation are at their maxlmum. Typically, the water in the bay is oceanic in origin and is converging on the coast, causing downwelling (Skogsberg 1936). These processes reduce the stability of the water column, so that the winds during this time effectively mix the oceanic water with the deeper California Current water, in effect 58 59 increasing the uniformity and decreasing the stability of the water column (Bolin and Abbott 1961). Dilution due to run-off might be expected to help stabilize the water mass. In this study lower salinities were observed adjacent to the shoreline along the central and northern parts of the bay (Fig. 22), but the effect of the small volume of fresh water on the large volume of the bay is not sufficient to override the vertical mixing caused by the seasonal storms. The unseasonal upwelling pulse that occurred in December 1971 carried nutrients into the euphotic zone (Figs. 19 and 21), yet the phytoplankton standing stock was at its annual minimum (Figs. 15, 18 and 20). Accord ing to Strickland (1965), light levels in December, in mid-latitudes, are normally high enough to stimulate phytoplankton growth; therefore, it appears likely that the upwelling pulse was not of sufficient duration to stabilize the water column and thus maintain the phyto plankton cells in the euphotic zone and allow for suffi cient growth. The processes that carry plant cells out of the relatively thin euphotic zone (vertical mixing, downwelling, subsidence and sinking of cells) were ulti mately responsible for the suppression of growth in the phytoplankton in the winter months. 60 I 40 6 0 0 Figure 22. Distribution of surface salinity of Mon terey Bay, February 1973. 61 In the early months of both years, while solar radiation was increasing (Fig. 5), the commencement of upvvelling marked the beginning of the spring flowering (Fig. 6). The s~bsequent stabilization of the water column -n• ( l' lg. 14) was the prime factor that initiated phytoplankton growth, augmented by increased solar radiation. A~bient nutrient levels were high enough following the winter months (Fig. 19 and 21) to support the initial growth in the phytoplankton standing stock (Fig. 15). The physical process of upwelling resulted in the establishment of a thermocline. This is a dynamic one- way thermocline which permits nutrient addition to the water above it, yet retards vertical mixing between the upper and lower water masses. During upwelling, dense, deep, nutrient-rich water is gradually warmed and diluted as it ascends to the surface, resulting in a stratified water column (Smayda 1966). From February to April 1972, the upwelling inten- sith remained close to the same level, as indicated by the fairly stable 10° C isotherm (Fig. 6). Yet during this period the phytoplankton standing stock increased to its annual maximum (Fig. 15). The low phytoplankton levels in February can be attributed to the moderate amount of vertical turbulence, the low level of solar radiation, 62 and short day length. The standing stock increase 1n March reflects water column stability (Figs. 5 and 14), increased light levels, and longer day length. While the moderate to low light levels may still have been a limit ing factor in March, the greatest inhibition of production was probably due to the transitory movements of the bay water with its accompanying phytoplankton. The influence of upwelling on bay circulation as indicated by Smethie (1973) is thought to be a shoreward near horizontal flow from over the canyon area, dividing and flowing northward and southward along the bay's per imeter, and then offshore. While the circulation may have been in effect in earlier upwelling months, it was not thought to be limiting to phytoplankton production because the other determining factors of stabilization of the water column, low light levels and short day length were suppressing growth. But as these factors ceased to limit primary production, residence time became an important consideration. If the proposed circulation of bay waters is correct, the nitrate concentration in March at stations 1152 and 1153 would indicate that water is leaving the bay before its nutrients are being fully utilized by the devel oping phytoplankton population (Fig. 23). The general trend, in the northern sector of the bay, of higher - -r-,--i----r-r-J-~-r--.-----r-~-- r-----. S f..IH/\ cnuz 0 01-2. 0\ . / -----·------~2-4 ------\ () ~J ------~ 0 'ij?\>-~/ ------~-~ . ~ 4-5 0 /ELKHORN 0 / SLOUGH - >B o (} I ~ -·~· I I ( -1 0 J 0 0 0 Figure 2J. Distribution of nitrate concentration (ug-at/liter) in surface water of Monterey Bay, March 1972. 64 nitrates over the canyon decreasing shoreward and then northward would tend to substantiate the circulation pattern. The higher temperatures (Fig. 24) and lower nitrate values observed in the northern bight indicate that this water is older than the surrounding water (Smethie 197J) and suggests the presence of a localized eddy. It is significant that it was in this area that the highest chlorophyll level for March was observed (Fig. 25). This suggests that a longer residence time in the 'bay would yield higher phytoplankton standing stocks. By April, the standing stock for the bay had increased to the annual maximum (Fig. 15). Some increase in phytoplankton biomass would have been expected, simply because of increased sunlight in a stable, nutrient-rich environment. However, the increase was so great t~at it is apparent that other factors were involved. Substantial increases in chlorophyll were noted at the inshore stations, while the greatest increases were seen in the central portion of the bay (Figs. 20 and 26). During other upwell ing months the central portion of the bay had consistently displayed a much lower phytoplankton stock than the other areas (Fig. 20) . The maintenance of a phytoplankton population in 65 -~----1--·r- ~~-~-~- ---r---1---,- ~-.-----.-1--1--1-1--I-T-! ::J~~~~ 0 0 0 0 Figure 24. Distribution of surface temperature (°C) of Monterey Bay, March 1972. 66 30-90 0 0 ~-- I \>- "({J 0 \ s 2 6~50'N U 3-10 0 t ~ ~~~~'('1-- 0 ~J' -'? ) 30-90 0 Figur~ 25. Distribution of average chlorophyll a (mg/m ) in upper 10 m water in Monterey Bay, March 1972. 67 r--,-1___...--~--r-·...--t----,-----.---r--r~--l-T-r--.-y-- Ls'"nP. CRUZ . 0~ 0 0 0 30-90 10-30 0 0 0 0 0 Figure 26. Distribution of average chlorophyll a (mg/m2) in Monterey Bay, April 1972. 68 an area of upwelling requires either an upwelling rate which is slow enough to allow the rate of phytoplankton increase to exceed losses due to advection, or effective turbulent mixing (Smayda 1966). April lS unique in that it is the only month in 1972 in which the surface water temperature over the Monterey canyon fell to 10° to 110 C. (Fig. 27). These low temperatures would suggest that upwelling had inten sified, but Figure 6 shows that the 100 C isotherm actu ally moved slightly downward. The occurrence of strong winds during the time of sampling (Fig. 28), and the descending 10° C isotherm indicate that vertical mixing to a moderately shallow depth of about 40 m was occurring (Fig. 14), while the net upward transport of water was maintaining the overall stability of the water column. Vertical mixing which enhanced phytoplankton growth in the central region of the bay caused a rapid and continual inoculation of surface phytoplankton cells into the newly upwelled water. This may have decreased the time generally required to seed upwelled water by normal surface diffusion. A secondary result of the vertical mixing, but not necessarily of minor importance, may be the increased vertical orbit of phytoplankton cells. The consequence of this would be a reduced rate of 0 0 0 0 ______------cJ) •·c / 10t...- II 0 0 0 0 Figure 27. Distribution of surface temperature (°C) of Monterey Bay, April 1972. 70 Figure 28. Northwest wind stress for Monterey Bay, October 1971 to April 1973. 71 1 (/) (i} ;z w ~ n:: w 1- :;: IJ) >- Cl <( ~ 0 >"? J1') 3i 2 .., .c.:.,_ c 0 ,....,. -J ~, J (JJ ~-= J ~-- l- -- «:;::-- ~ N ll. I-- (}) -;:, 0 0 72 shoreward transport of cells and an increased residence time in the central bay. It is possible that an increased vertical orbit would have resulted in plankton being carried below the critical depth; however, since standing stock was at a maximum at this time, it is not likely that the increased orbit had an adverse effect. No mean ingful light data is available for this month to estab lish the depth of the 1% light level. The increased residence time 1n the central bay would subsequently increase the total residence time in the ·bay, and it is for this reason that a corresponding increase in chlor ophyll was noted at the inshore stations. The ascent of the 100 isotherm in May (Fig. 6) indicates a relative increase in the upwelling rate. The observed horizontal distribution of biological, physical and chemical parameters would also suggest a stronger than normal upwelling intensity (Figs. 28 tu J2). Low tern perature, high AOU, nutrient-rich water was found adjacent to the shoreline between the Pajaro and Salinas rivers. The low standing stock of phytoplankton was due to a decrease in residence time in the bay. Massive upwelling results in a decrease of the total path length traveled ·by some phytoplankton cells. The population over those upwelling sites which are adjacent to the shore will 73 ----y--·r--•-j---"I----r----.--,----.---r--r-...... ,.--...... -..,---,---:--..,--r-, Sf\:nA CRUZ s-- !0-30 J ~ 0 0 J ~------30-90 0 ~ J40' Figure 29. Distribution of average chlorophyll~ (mg/m2) in upper 10 m water in Monterey Bay, lv1ay 1972. 0 0 0 I l 0 ,1- 40 1 0 0 Figure JO. Distribution of surface temperature (°C) of Monterey Bay, May 1972. 75 0 J 0 10-15 0 0 5-10 0 0 15-20 0 0 PT.~ PifWS \._:> Figure Jl. Distribution of nitrate concentration (ug-at/liter) in surface water of Monterey Bay, May 1972. 0 6 (>•. o:::c;' ) J\J/';' -:-TO ··SO 0 0 40 I 0 Figure J2. Distribution of apparent oxygen utilization (pg-at/liter) in surface water of Monterey Bay, May 19?2. 77 naturally have a shorter surface circulation than those of the previous sites, which appeared to be further offshore. In June the phytoplankton concentration was at its annual maximum for the inshore stations, and approached the maximum for the bay as a whole. The observed hori zontal contours of the various parameters (Figs. 8, 33, 34 and 35) indicate that upwelling was occurring, while the descent of the 10° isotherm reflects a decrease in the upwelling rate (Fig. 6). The increase in the phyto plankton standing stock and the descent of the isotherm imply that the upwelling intensity had sufficiently slowed to allow a phytoplankton increase for both the inshore area and the bay as a whole. As in most previous upwelling months, the central bay was the site of the lowest phytoplankton concentration. Since little vertical mixing was eyident over the canyon (Fig. 14), the uvwelling rate and surface advection was still excessive for signif icant growth to occur in the area. At first glance, it would appear that the descend ing isotherms and the increased surface temperatures found over much of the shelf areas were due in part to advection of warm oceanic water into the bay. But the general over all high nitrate levels indicate that the observed contours were the result of a lower ll.pwelling rate rather than '78 0 0 0 30-90 0 I r 0 0 0 0 30-90 0 0 Figure JJ. Distribution of average chlorophyll a rng/m2) in Monterey Bay, June 1972. 79 r-----,--,----,.---.-,---l~-.,---r--r !. SMHA CRUZ _s------~ 0 ~' 0 1 0 (536°50'N ELI\HOAN, (jlj SLOUGH 0 I I 0 33.75 -[ i 0 0 j___j__ _l_f_ Figure J4. Distribution of surface salinity 1n Mon terey Bay, June 1972. 80 ------0 0 0 5-10 0 0-5 0 0 0 - 0 Figure 35. Distribution of nitrate concentration (ug-at/liter) in surface water of Monterey Bay, June 19?2. 81 advection (Fig. J5), since ocean1c water is character ized by low nitrate concentrations. The higher temper atures would therefore seem to reflect: ( 1) a longer surface circulation path due to upwelling occurring in the offshore canyon area, and (2) increased solar radi ation (Fig. 5). The cessation of upwelling and the subsequent end of the spring bloom in July demonstrate the impor tance of the upwelling process in regulating the phyto plankton standing stock in Monterey Bay (Figs. 19, 20 and J6). From March to June, upwelling primarily con trolled the system by altering residence time of the phytoplankton within the bay. With the termination of massive nutrient addition as a result of a decrease 1n the northwesterly winds, (which are the driving force of upwelling), the low nitrate level 'became limiting to pr1mary production (Fig. 28). Grazing pressure in July from a near maximum zooplankton population (Fig. J6) severely and effectively cropped the existing phytoplankton standing stock. The continued nitrate depletion and grazing resulted in the summer phytoplankton minimum in August (Fig. 15). The co-occurrence of nitrate depletion and maximum grazing pressure is unfortunate in that it tends to obscure the 82 0 0 1-2 \ 0 0-1 --.,-----~ ELKHOf\N Ill$ SLOUGH 0 ( l-2 0 0 0-1 0 0 /~-1-2 Figure 36. Distribution of nitrate concentration (ug-at/liter) in surface water of Monterey Bay, July 1972. 83 cause of the declining phytoplankton population. Thus it may not be possible to determine which factor initially caused the decline of the population, since a combination of both factors resulted in the summer minimum. It is my belief that nitrate depletion initially slowed production. This is substantiated by the fact that the grazing zooplankton population was greater in May and June than in July (Fig. 37), yet the phytoplank ton population increased to a near annual maximum in June (Fig. 15). The zooplankton species composition is assumed to be the same during these periods, although no data are available to substantiate this. The conclusion that nitrate is limiting is somewhat in contrast to Malone's (1971) findings, but consistent with Anderson's (1964) data for the upwelling area off the Washington and Oregon coast. Malone determined that nutrients in Monterey Bay were not limiting by noting the consta·ncy of assimilation ratios over a wide range of productivity values in spite of large variations in ambient nitrate concentrations. He attributed the phytoplankton decline to grazing pres sure. These conflicting conclusions may be due to sam pling differences in that Malone's data are from station Calcofi 3 (Fig. 1), which by my definition (p. 8) is out side Monterey Bay, whereas my data are the averages from 84 Figure 37. Monthly variation of average chlorophyll a (mg/m2) in upper 10 m water and zooplankton biomass - (ml/m3) in upper 15 m water of Monterey Bay. (Zoo plankton volumes were not determined after August 1972). 85 0 ;«.: ..J 0 ..J }- >- Y. ::z: z ll. cq: 0 -l ,... 0 I)_ 0 0 _j 0 ::c z N 0 0 U) ...... (I) < ..c.... c 0 J E ... ,, ...... ·· ~ .. ··········· .... ········· ..... ······. l.!J ... J d! ...... ······ ...... j:: ... "5 ······ "- ...... ~ '• ······ <( ········~~...... ---=-- '· ... ··· ... 2 (IJ I '•o, ... lL !'- (]) "":.) 0 ...... ··· z, ...... ················ Op..~ ____ _j _ __J_____ ,____ J_ ___ ,,. .•• l~~j___ J ___L___._. 0) U) i'-.. \.0 !.!) ·.;,t f.f') N 0 ( ~ll / JUJ) SS"fV.JO :8 ~-JOJ. )iNVldOOZ 86 19 stations within the bay. The discrepancy may also be due in part to the difference between an oceanic and a neritic environment, or from an unrealistic comparison of the parameters of different years. Following the summer minimum, the fall bloom in September 1972 occurred in response to an upwelling pulse. All contours are characteristic of upwelling (Figs. 38 to 40). The similarity of the horizontal chlorophyll con tours for March 1972 and September is most likely the result of the same processes of upwelling and advection (Figs. 25 and 38). There is another similarity which may be coinci dental, but should be noted. There is a conspicuous paral lel in the near-equal amount of incident solar radiation in these two ~onths (Fig. 5). The occurrence of higher phytoplankton 'biomass over shallow water suggests that a relationship between the depth of mixing and the euphotic zone may occur. However, the establishment of a thermo cline in deeper water as a result of upwelling would generally act to hold phytoplankton in the euphotic zone in the same manner as the 'bottom would hold phytoplankton 1n shallow water. A relationship between light levels and phytoplankton distribution may still be present, but the process by which light cohtrols this is unclear. 87 SMHf~ CRUZ ~-~ 0 0 0 0 \ \ 0 0 0 I ~- 1 [ ;::~~; J Lj__J_L_L_j __ L __l__.! _____t_~--·- _[ __L ___J_l_ _J_L__L__l_ /'/2. '-' tlQ I 1•'1 j') '0 li(...,.J I , ._ "-" l· >· t~ I u t: Figure J8. Distribution of average chlorophyll~ (mg/m2) in upper 10 m water in Monterey Bay, Septem- be!' 19'?2. 88 ~-~--r --.-r-~-·-r-·- r---r------r----. I I l I L ~ ~ ~ ·40 l 0 () Figure 39. Distribution of surface temperature (DC) of Monterey Bay, September 1972. 89 '------70 7 40 I Figure 40. Distribution of nitrate concentration (ug-at/liter) in surface water of Monterey Bay, September 19'?2. 90 There is yet another similarity which may be coincidental, but will be considered later in the section dealing with phytoplankton-zooplankton relationships. Following the autumn flowerings of both years the phytoplankton populations continued to decline to their winter levels. Fall Bloom of 1971 The preceding year the fall bloom occurred 1n October. The 12-14° C surface temperature and the domi nant phytoplankton forms of Gonyaulax and Ceratium indi cate that Davidson Current water was entering the bay. Normally the presence of this water is characterized ·by uniform surface temperatures and descending isotherms. However, during that October the surface temperatures were not uniform and the upper 40 m were stable, although deeper waters were sinking· (Smethie 1973). This unusual configuration of the bay waters indicates a state of flux, and probably represents a transition of input from northern to southern oceanic water. The phytoplankton 'bloom in the fall of 1971 was a red tide assemblage dominated by dinoflagellates of the genus Gonyaulax and Ceratium (M. Silver 1972). 91 The Davidson Current brings southern oceanic water into Monterey Bay (Bolin and Abbott 1963), and it is not an unreasonable assumption that this water would first enter the southern portion of the bay and then flow northward. Red water was observed near Monterey a week prior to sampling (M. Silver, personal communication). The occurrence of the bloom at the time of sampling, predominantly in the northern portion of the bay, implies advection from the south (Fig. 41). Horizontal contours of other parameters also suggest a northward movement of water. The use of hori zontal contours to trace water movement is hazardous at best, and the use of non-conservative properties com pounds the problem. It is with this knowledge that the following evidence is presented. The distribution of each parameter considered separately supplies little positive evidence for north ward flow of water in the bay, but considering these parameters together and in combination with the above observations, the following argument may be presented. Figures 42, 43, 44 and 45 display a tongue of water extending from Point Pinos toward the Salinas River and support a flow in this direction. Some decrease in nitrates and ammonia toward the Salinas River would 92 Sf-l.NT.O. CRUZ 0 10-30 0 0 3-10 0 0 0 0 PT. ....---- ) ?!NOS 0 ____t_ ___t_ _,___j,_J.___L(.,____,___,_____L_.I __ .-LI----''---''-· _L__ _L _L-J. _....J___l_ 122(; ool ~~~ /2(~50 1 Figure 41. Distribution of average chlorophyll~ (mg/m2) in upper 10 m water in Monterey Bay, October 1971. 93 12-13° 0 0 0 ·o ~--~ORN n SLOUGH ( 0 0 0 -40' Figure 42. Distribution of surface temperature (°C) of Monterey Bay, October 1971. ·----y--~--~---y--....--.------r---.-----,r--.---,--r .-,------y----,·-,----.-----, - S/\fHA CRUZ 0 0 2 - -;?.6 0 50 I N 2 0 0 Figure 4). Distribution of nitrate concentration (vg-at/1iter) in surface water of Monterey Bay, Oc.tober 1971. 95 ------.- .-·-r-r--r---.- - S/H·:·rr~ cnuz 0 0 "2soro'"'. 0 liJ >o ~0 -1 TO -25 0 ( P~/ 0 Pli•WS \o Figure 44. Distribution of apparent ~xygen utiliza tion (ug-at/liter) in surface water of Monterey Bay, October 1971. Figure 45. Distribution bf ammonia concnetration (ug-at/liter) in surface water of Monterey Bay, October 1971 97 be expected simply due to dilution. Figure 42 displays two parcels of ll-12° C water in the northern portion of the bay. The low temperatures exclude sewage and run-off as a source for this water. Broenkow (1971) demonstrated that tidal oscillation over the head of the submarine canyon can bring water from a depth of 75 m to the sur face. The average depth of the 12° C isotherm for all the canyon stations in October was 40 m (Smethie 197J). There fore, pockets formed by the strong tidal oscillations and moved northward by the flow of water could result in the observed temperature distribution in the northern half of the bay. The above data suggest that the bloom was advected into the bay, but nitrate depletion and low AOU values in the north bay indicate continued production (Figs. 4J and 44). Little is known about the initiating factors of a red tide bloom, but the advection-and growth would indicate a positive effect of transportation of red tide species to a neritic environment. The mixture of nutrient depleted oceanic water with richer subsurface and inshore waters and high temperatures may tend to favor production of southern forms. An extremely high concentration of phytoplankton would be favored if grazing pressure were small. Figure J7 shows a low zooplankton standing stock 98 ln October which probably reflects a summer phytoplank ton minimum and the unbalanced relationship of the two populations. Some species of the genus Gonyaulax are known to produce toxic extra-cellular metabolites. If this were the case in Monterey Bay (although there is no evidence that it was), inhibition of grazing and compet ing phytoplankton species would favor the development of a red tide bloom. The Timing of the Spring Phytoplankton Maximum Normally in mid-latitudes the spring peak in algal densities occurs in March or April (Raymont 1967). The 1972 spring maximum for Monterey Bay was recorded ln April, ·but a strong secondary peak in June approached the earlier level. Bolin and Ahbott (1963) in their seven year study of the bay consistently recorded the peak in algal numbers in June. The two month delay of a near maximum suggested by this study and a maximum recorded by Bolin and Abbott probably reflects the influence of the upwelling process. While the regional winds in June are generally considered to be at their maximum, this was not the case in Monterey Bay for 1971 and 1972 (Fig. 28). The winds in June were notably less strong than those during the pre ceding months in both years. Two years is not sufficient 99 time to establish an annual pattern. If, however, this is the seasonal trend, the decreased winds which slow the upwelling rate and the subsequent increased residence time would be a reasonable explanation for the consistent occurence of the phytoplankton maximum in June (Fig. 6). Bolin and Abbott stressed the point that the northerly winds become sporadic about this time, which may result in an alternating offshore and onshore move ment of water. During offshore movement of surface water, upwelled water provides nutrients to the euphotic zone, and the ensuing onshore movement returns the rapidly dividing plankton cells into the recently upwelled water. These sporadic winds would therefore increase total resi dence time of phytoplankton cells in the bay and favor higher production. Another possibility favoring the phytoplankton ~ax1mum in June may be the interaction-between local and regional winds. The regional winds drive the upwelling process, which brings cold water to the surface of the :ay. Incoming solar radiation is at its maximum, heating ~he water and to a greater extent the land (Fig. 5). It ~ay be at this time, with the cold surface water in the ~ay and maximal heating of the land, that the air temper a~ure differential is greatest betweeri land and sea and 100 therefore the intensity of the sea-breeze is greatest. If, as Smethie (1973) has suggested, local winds oppose the movement of water offshore and decrease the upwelling intensity in the bay, this would again explain the higher phytoplankton values found in June. Even if the upwell ing intensity was not reduced, the residence time could still be increased by the alternating diurnal winds which move surface water in horizontal eliptical orbits within the established circulation pattern (Fig. 2) (Broenkow, unpublished). Vertical wind-induced turbulence above an established thermocline would also be a positive factor for increased algal biomass in June. Realistically, these various events cannot be separated. The occurrence of the spring maximum in June should 'be attributed to the combined action of these processes. However, the assumption on the part of Bolin and Ab'bott that the spring maximum occurs in June may in itself be incorrect. There is some evidence that the Calcofi sampling pattern may have missed areas of higher concentrations of phytoplankton in the bay (Fig. 2). In addition the Calcofi investigation used net drawn samples to develop the annual cycle, while in the present inves tigation and in Malone's (1971) study, chlorophyll~ 101 concentrations from water samples indicated that the maximum occurred in April. The different methods of establishing the seasonal trends somehow may have obscured the seasonal cycle. Possible errors due to methodology will be elaborated upon in the following section. Factors Controlling the Occurrence and Composition of the Fall Bloom The presence of a fall bloom in 1971 and 1972 and the existence of a fairly typical seasonal phytoplank- ton cycle for the bay suggest that the development of a fall bloom is a regular event. The display of a bloom in two different water types (oceanic and upwelled) indicates that water origin is not an important factor in initiating the bloom (p. 90). The factors that are thought to control the occurrence of the bloom are the relaxation of grazing pressure (Fig. 37).and adequate illumination. The factors controlling the adequac~ of illumination are related to the extent of vertical sta- bility of the water column: if mixing occurs to depths greater than the critical light level, phytoplankton will not increase, as indicated in December 1971 (Figs. 6 and 15). Whatever oceanographic event occurs within the fall will determine what type of bloom develops. 102 If upwelling resumes, as happened in 1972, an increase 1n cold water forms will develop. If, during this time, southern oceanic water advects into the bay, an increase in warm water forms will be noted. The occurrence of one type of bloom generally precludes the development of the other, simply because of the approach of the limiting winter conditions of decreasing light levels and water column instability. It is not known which type of bloom is typical for the bay, since Bolin and Abbott (196J) never recorded a fall increase in phytoplankton, but according to their research, dinoflagellate forms are generally dominant at this time. An intuitive guess is that an increase in warm water forms is more common, since the occurrence of warm water in the bay seems to be typical of the fall months. The fact that no phytoplankton increase was reported in the fall for the Bolin and Abbott investiga tion is not entirely inconsistent with the present data. This inconsistency is probably due to differences in sampling locations and techniques. The Calcofi sampling pattern would have generally missed the areas of high phytoplankton concentration (Figs. 2, )8, and 41). Bolin and Abbott's samples were collected at six stations, in cluding Calcofi J, in Monterey Bay (Fig. 2). At best, lOJ three stations may have been located in the areas of higher concentrations. Calcofi seasonal trends were developed by computing the means for all six stations. The high inshore concentrations would be averaged out by the low offshore stations, with the subsequent result that no fall increase was recorded. A second factor which may be responsible for the inconsistency between the two sets of data is that Bolin and Ahbott's (l96J) phytoplankton data were obtained by use of a net with 173 meshes/inch, which would not retain a nannoplankton fraction. Malone (1971), working at Calcofi J, and fractionating water samples according to size through the use of glass fiber filters, concluded that 60% to 99% of the observed phytoplankton standing stock was nannoplankton during the period when oceanic conditions dominated. This would explain the conspicuous absence of a fall bloom in Bolin and Abbott's findings, if the fall bloom normally occurs in oceanic water. Even if the bloom occurred in upwelled water, low standing stocks would also be expected offshore because of advec tion inshore. Chlorophyll data for the netplankton and nanno plankton fractions are not available for the present in vestigation. However, the occurrence of a fall bloom in 104 1971 with a large netplankton standing stock in oceanic water suggests that Malone's findings at the offshore location of Calcofi J should not be considered as repre sentative of Monterey Bay as a whole at this time. Phytoplankton-Zooplankton Relationships In the previous discussion of phytoplankton standing stock, little attention was given to its rela tionship to zooplankton. This was done in order that the relationship between phytoplankton and the physical and chemical parameters could be developed clearly. Zoo plankton samples were collected, but zooplankton volumes probably are not reliable for spring and summer samples; colonial phytoplankton clogged the zooplankton nets at those times and contributed a major, 'but unknown, frac tion to the- total net biomass. The most obvious feature of the -zooplankton cycle is the occurrance of the zooplankton maximum a month before the phytoplankton maximum (Fig. J?). This type of relationship, to my knowledge, has never been reported in the literature. Aside from the probable error in volume determination, due to the retention of phytoplankton, the peak in March and the drop 1n April could be explained by th~ development of a large mero plankton population, which may have settled out by April. 105 While larval forms would not be expected to be adequa tely sampled with a zooplankton net, large numbers were present in the February and March samples with lower numbers present in April. No data are avail~ble to confirm or reject larval settling as the means by which larvae were lost to the zooplankton population. Low zooplankton concentrations in April would have acted to reduce the grazing pressure on the phyto plankton population and may have allowed the phytoplank ton maximum to occur in April. Bolin and Abbott (1963) consistently recorded the algal maxlmum in June. While the zooplankton may have been a factor, the importance of wind mixing on the large algal standing stock in April should not be minimized. The increased turbulence at this time may have had an effect on the zooplankton population by dispers ing them and consequently decreasing their concentration in the upper 12 m, from which they were sampled. Zoo plankton standing stock in fact may not be as low as the values indicate at this time, due to this dispersing by wind mixing. The homogeneous distribution would also influence the sampling of the phytoplankton standing stock, but the positive factors of rapid inoculation and increased residence time would tend to override potential 106 lower values. If the March peak was an artifact or mainly due to meroplanktonic organisms, the phytoplankton and zooplankton cycles would conform to Cushing's (1959) and Heinrich's (l96J) descriptions of an unbalanced re lationship for the mid-latitudes (Fig. J7). With the exception of the expected one month lag time between the onset of the spring phytoplankton bloom and the lncrease in the zooplankton, the lag times and the seQuence of various events are consistent with the published descriptions. The obvious absence of the one month delay period is probably due to the insensitivity of monthly sampling. Cushing's ( 19 59) model of an un·balanced system emphasizes the role of zooplankton populations in regu lating phytoplankton concentrations in the sea. Phyto plankton ·biomass varies seasonally due. to changes in radiant energy, vertical turbulence and zooplankton grazing pressure. Zooplankton concentrations become important during the period when radiant energy and vertical turbulence allow phytoplankton net production to proceed. The termination of the spring bloom and the development of the fall bloom are related to an increase and decrease in grazing pressure, respectively. The 107 nutrient concentrations may determine the amplitude of the phytoplankton bloom, but the above factors will deter mine the pattern (Cushing 1959). Malone (1971), working at Calcofi J, determined that the end of the spring netplankton bloom was due to grazing pressure. He attributed seasonal variations in algal biomass to vertical advection and grazing pres sure alone. Malone suggested that incoming solar radia tion is generally constant in Monterey Bay throughout the year due to the presence of fog in the sum~er and its absence in the winter. High nitrate concentrations were considered a precondition to growth, while low values were excluded as the prime factor in inhibiting growth. The termination of some of the spring blooms 1n Monterey Bay may well have been the result of grazing pressure. The only conditions that would need to be satisfied for continuation of growth ~auld be a constant upwelling rate and a surface circulation rate that allowed the addition of nutrients without excessive advection of cells from the ·bay. If these conditions lasted for an extended period of time (2 to J months), zooplankton could develop to a level that could effectively crop the phytoplankton population and slow production. Of course, it would be the rate of nutrient addition and the rate 108 of export from the bay that ultimately determined the maximum phytoplankton population. Clearly any variation of such factors as upwell ing rate and locations, wind-induced mixing, import and export of water masses, and advection of plankton cells cannot be ignored in the present investigation of the phytoplankton and zooplankton population. The bay changes too rapidly and the differences ln generation time are too great between phytoplankton and zooplankton to allow zooplankton to become the prime factor in limiting production in the spring. Upwelling rates are variable enough alone to prevent a direct relationship from developing between the two populations. Strong upwelling pulses are sufficient to displace phytoplankton, and probably zooplankton, from the ·bay. Although light levels and mixing of the water column are very important consideratio"ns, it is the seasonal change of water masses that is primarily respon sible for the seasonal phytoplankton cycle in Monterey Bay. ·During the termination of the spring bloom, nitrate was the critical factor; the change of water masses was the vehicle. Zooplankton graz1ng was probably irrelevant as a factor bringing about major changes in the algal stock. 109 Even if no graz1ng pressure at all occurred, the sea sonal cycle would exhibit the same pattern, but with higher concentrations and less abrupt changes. This suggests that the phytoplankton-zooplankton relationship in Monterey Bay may be similar to those found in Long Island Sound (Riley l956b) and the Gulf of Panama (Smayda 1966) . Phytoplankton is excessive to the need of zooplankton. The unique distribution of chlorophyll for the months of March and September may be due to limited grazing. By considering the large zooplankton standing stock in March as a sampling error or artifact, the following explanation is plausible: little grazing would be expected in March since the spr1ng phytoplank ton growth was just beginning (Fig. 15), and the zoo plankton standing stock would be expected to be near the winter levels (Cushing 1959). The specific phytoplankton distribution in March as well as Septe~ber would be mainly the result of growth and not mortality. During August 1972, the phytoplankton and the zooplankton standing stocks were at or near their su~ner minimum (Figs. 15 and 37). Oceanic water which advected into the bay at this time is known to be nutrient poor and to support a small 110 phytoplankton standing stock (Bolin and Abbott 1963). The zooplankton standing stock associated with this water would also be expected to be small. Therefore, when upwelling occurred in September with the resultant rapid increase in phytoplankton, zooplankton would still be at a low level and would not greatly alter the char acteristic distribution of the phytoplankton. The long delay period between the onset of phytoplankton development and later zooplankton develop ment characteristic of the Artie regions prevents phyto plankton from being controlled by grazing. The excep~ tionally dynamic nature of local waters also prevents this type of control in Monterey Bay. It is not sur prising that the seasonal cycle of a system as complex as the one found in Monterey Bay conforms to the norm for the mid-latitudes 0hen it is remembered that the process of upwelling itself is ultimately controlled by the seasonal shift in the position of the sun. Any variability of the bay phytoplankton as compared with the general mid-latitude pattern is probably due to the added factor of upwelling in the processes of control. 111 .§12atial Distribution of Phytoplankton in Monterey Bay Spatial differences in crops in the bay result from some of the same processes regulating the seasonal ~bundance of phytoplankton. The next several paragraphs will review these processes that also control the areal distribution of phytoplankton in the bay. The most obvious feature of the phytoplankton standing stock is the consistent low values found offshore and in the central area (Figs. 18 and 20). During up welling this is the result of rapid movement of water from· the upwelling site. The process of advection shoreward helps to combine nutrients and the existing phytoplank ton cells. Phytoplankton in the upwelled water have undergone multiple divisions by the time they are trans ported inshore. During most non-upwelling months, greater tur bulence due to the seasonal wind pattern results in sub sidence and downwelling. At this time lower levels 1n the central and offshore areas are normally recorded. During the months when Davidson Current water is present in the bay, a small temperature gradient between 0 m and 50 m indicates a well-mixed water column. This mixed condition increases the time a phytoplanktor spends below the euphotic zone, which in turn slows production. Low 112 light levels and short day length are limiting produc tion both inshore and offshore, but the deep, turbulent mixing in the central and offshore areas results in the difference in chlorophyll concentration between the deep and shallow water regions. Thus the same processes that generally limit production in winter in other mid-lati tude waters were also operating in Monterey Bay. During the 1972 upwelling months, excessive advection of phytoplankton from the upwelling site was also responsible for the lower values in the central area as compared with the north or south bay regions. Wind-induced vertical turbulence above an esta.blished thermocline is felt to be a major factor in the unique spatial distribution of the chlorophyll during April. The chlorophyll content of the central bay at this time 'equalled or exceeded that of the north and south bay. This increased but limited vertical turbulence sped the inoculation of cells into the upwelled water and increased the residence time in the central bay. The fact that the south bay is significantly higher in chlorophyll than the north bay during the upwelling months suggests a positive effect of wind mixing in shallow water. During the upwelling months, winds come from the north or northwest, and the north llJ bay is protected, while the south bay is exposed to these winds. In the non-upwelling months the north bay crops were slightly higher than those in the central and south bay. The difference in the central bay is thought to be the result of the negative effect on production by wind mixing in deep water. During these non-upwelling months, the north bay is directly exposed to the normal south and southwesterly winds, while the south bay is somewhat pro tected; this factor may also have some effect on the spa tial distribution of phytoplankton in the bay. Factors Influencing Location of Upwelling From ·both the literature and the foregoing dis cussion, it is well documented that upwelling can have a great effect on production and location of phytoplankton concentration. Due to this it is felt that some dis cussion of location of upwelling sites is warranted. It is apparent from the horizontal contours that upwelling is not limited to the submarine canyon in Monterey Bay. While an interpretation of horizontal contours can be misleading, and surface currents can alter and shift location of parameters, there is never theless a consistency between parameters which tends to substantiate the presence of other areas of upwelling. 114 During four of the seven upwelling months in 1972, the horizontal contours indicate that upwelling is occurring in a larger area than that of waters direct ly over the canyon. The most consistent pattern suggest lng upwelling is one of high salinity, high nitrate and low temperatures ln the area of the 92 m contour in the southern portion of the bay (Figs. 24 and Jl). Here 183 m to 92 m contours are widely spaced, indicating a gentle rise of the canyon wall. Since the horizontal vector of upwelling waters greatly exceeds the vertical component, the deflection off the southern slope would tend to divert the water upward and promote spillage from the canyon. However, at the northern inshore edge of the canyon, the steep slope would act as a dam and the pre dominantly horizontal flow of water would be deflected shoreward along the canyon axis rather than upward. The headlands of the Monterey Peninsula may also act as a secondary site of upwelling. Frequently the lowest temperatures and the highest nitrate concentra tions have been noted adjacent to Point Pinos (Fig. Jl). Smayda (1966), working in the Gulf of Panama, noted the occurrence of upwelled water near headlands in the path of shoreward moving water. The gentle slope of the southern shelf may spill water against Point Pinos, resulting in 115 the surfacing of this water. This apparent upwelling at Point Pinos could also be the result of upwelled water in Carmel Canyon spilling northward to the headlands, although admittedly little physical data has been con sidered. On occas1on, lower temperatures and higher nitrate values were seen near station 1152 (Figs. 8 and J5). West of the Soquel canyon, the sides of the Monterey canyon become less steep and could spill up welling water into this area. These variations in upwelling loci and the variatio11s of intensity of upwelling rate indicate that perhaps a closer look at the exact direction of the wind in the north to west quarter is warranted. It does not seem improbable that small changes in the prevailing wind would alter the seaward flow of surface water and the resulting inward horizontal flow of upwelling water. A slight change in the deep shoreward flow in conjunction with the irregular topography of the Monterey canyon may affect the upwelling loci and intensity. The wind direc tion in December 1971 may have ·been optimal for upvvelling in the bay, for although the intensity of these winds was low and they were of short duration, this was the only month considered in this study that the l~C isotherm surfaced in Monterey Bay (Fig. 6). SUMMARY The cjcle of phytoplankton varies seasonally in Monterey Bay. The annual pattern observed in this study generally conforms to that of other mid-latitude marine ecosystems, displaying low winter levels, a spr1ng bloom, a summer minimum, and a fall bloom. The factors responsible for the low phytoplankton standing stocks in winter are again similar to those of the general mid-latitude pattern: a well-mixed water column, low light levels, and short day length. The upwelling pulse in December 1971 with little resultant phytoplankton increase shows that nutrients were not a limiting factor. With the commencement of upwelling, the water column stabilizes and marks the onset of the spring bloom. The upwelling process establishes a dynamic thermocline which maintains phytoplankton cells in the euphotic zone, yet permits nutrient addition from deeper waters. The limiting factor in February and March 1972, even though upwelling was taking place, appears to be related to a relatively short residence time of phyto plankton in the 'bay. Turbulent mixing above the thermo cline in April of this year during the time of sampling 116 117 was probably responsible for the max1mum phytoplankton values fourtd in the spring bloom. A near maximum algal density was recorded in June 1972, which is consistent with Bolin and Abbott's findings. Shortly after upwelling stopped, the summer minimum was observed, which was due to lack of nitrates and heavy grazing pressure from the zooplankton. The main factors responsible for the fall blooms of both years are the relaxation of grazing pressure and adequate illumination. Whatever oceanographic event occurs within the fall will determine what type of bloom will develop. Advection of southern oceanic water will 'bring a·bout an increase in warm water forms, whereas an upwelling pulse will result in the development of cold water forms. In this study a fall bloom occurred in oceanic water in 1971; personal observation (Silver) and physical and chemical data suggest that the red tide assemblage of Gonyaulax and £~ratium advected into the bay from the south and flowed northward. The fall 1972 bloom occurred as a response to an upwelling pulse. Species composition and distribution of physical and chemical para meters were similar to those during the spring upwelling. Phytoplankton-zooplankton relationships in Monterey Bay generally conform to the mid-latitude pattern with 118 two exceptions. The occurrence of a zooplankton maximum one month before the phytoplankton maximum is probably due to either an error in technique 1n zooplankton sampling, or to the development and later settling of a large mero planktonic population. The second exception is the ab sence of a one-month lag time between the onset of the spring phytoplankton bloom and the increase in zooplank ton, which is probably due to the insensitivity of monthly sampling. The water in the bay changes too rapidly and the differences in generation time are too great between phytoplankton and zooplankton to allow zooplankton to become the prime factor in limiting phytoplankton produc tion. Spatial distribution of phytoplankton results from the same processes that influence seasonal variations in abundance. Consistent low chlorophyll values are found offshore and in the central area, due_to the rapid move ment of water from upwelling sites. Turbulence and down welling or subsidence in these deep areas during non upwelling results in lower standing stock values. During four of the seven upwelling months 1n 1972, the horizontal contours indicate that upwelling is occur ring in a larger area than that of waters directly over the canyon. The headlands of the Monterey Peninsula, 119 the southern slope of the submarine canyon wall, and the northern slope west of the Soquel canyon may be additional upwelling sites, as indicated by high salinity, high nitrates, and low temperatures. The seasonal cycle of phytoplankton in Monterey Bay conforms to the mid-latitude pattern, since the pro cess of upwelling itself is controlled by the seasonal shift of the sun. LITERATURE CITED Abbott, D. P. and R. Albee. 1967. Summary of thermal conditions and phytoplankton volumes measured in Monterey Bay, California, 1961-1966. Calif. Coop. Oceanic Fish. Invest. Rep. 11: 155-156. Anderson, G. C. 1964. The seasonal and geographic dis tribution of primary productivity off the Wash ington and Oregon coasts. Limnol. Oceanogr. 9: 281-t- J02. Anonymous. 1969. U. S. naval weather service world wide airfield summaries. 8, pt. I: 94-95. Bigelow, H. B. and M. Leslie. l9JO. Reconnaissance of the waters and plankton of Monterey Bay, July, 1928. Bull. 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