California State University, Stanislaus and ... List of Figures vii. Abstract . ..... Monterey Bay (ME) is located on the central California coast about 120 km south of ...
SEASONAL INTRUSIONS OF EQUATORIAL WATERS IN MONTEREY BAY AND THEIR EFFECTS ON MESOPELAGIC ANIMAL DISTRIBUTIONS
A Thesis Presented to the Faculty of California State University, Stanislaus and Moss Landing Marine Laboratories
In Partial Fulfillment Of the Requirements for the Degree Master of Science in Marine Science
By Brian Schlining February 1999
7
ACKNOWLEDGEMENTS
This research was conducted using data collected by the Monterey Bay Aquarium Research Institute (MBARI). Funding for this work was provided by MBARI and the Dr. Earl H. Myers and Ethel M. Myers Oceanographic and Marine Biology Trust. I would like to thank the MBARI midwater group, especially Dr. Bruce Robison, Kim Reisenbichler, Rob Sherlock, Kevin Raskoff, and Dawn Murray, for advice and support. I would also like to thank Dr. Francisco Chavez and Todd Anderson for access to ADCP data and Drs. Pamela Roe and George Matsumoto for help and guidance. I am indebted to Dr. William Broenkow for his advice, encouragement, and tutelage. This research would not have been possible without the love and support of my wife, Kyra.
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TABLE OF CONTENTS PAGE Acknowledgements
1lI
List of Tables. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
VI
List of Figures
vii
Abstract .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
ix
Introduction
1 Monterey Bay California Current System .:.......................... Oceanographic variability and Seasonality in Monterey Bay. . . Nanomia bijuga . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Spiciness Hypotheses. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3
3 8 10 12 16
Methods. ... ... ... . . .. . . .... .. . . .. . . .. . . . . . .. . . .. . .. . ... . . .. . ..
18
ROV Observations Buoy Observations Upwelling Indices. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
18 22
24
Results. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
26
Mean Profiles of Salinity, Temperature and Oxygen . . . . . . . . Density. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Spiciness Nanomia bijuga Distributions. . . . . . . . . . . . . . . . . . . . . . . . . . ADCP Upwelling Indices. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
26 28 28 36 36 45
Discussion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
47
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PAGE Conclusions and Summary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
56
Upwelling. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Oceanic-CUC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Early Davidson. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Late Davidson
57 57 58 58
Literature Cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
60
Appendices. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
65
A. Potential Density. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B. Spiciness..............................................
66 67
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LIST OF TABLES
TABLE 1.
2.
3.
PAGE Dates ofROV Veniana dives at 36 0 44.9'N, 1220 G2'W ± 2 km used for this study. . . . . . . . . . . . . . . . . . . . .
19
Inclusive date of Acoustic Doppler Current Profiler data from the MBARI Ml mooring located at 0 36 44.9'N, 1220 2.3'W ... . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
24
Volume of offshore transport. . . . . . . . . . . . . . . . . . . . . . . . . . . .
45
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...tn
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r LIST OF FIGURES FIGURE 1.
PAGE Map of Monterey Bay and the central and northern California coast. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4
T-S diagram of the four water types that mix in the California Current System. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5
3.
Illustration of the physonect siphonophore, Nanol1lia bijuga . . .
11
4.
The mean eo-s curve bracketed by ± 1 standard deviation, overlaid with a cast from September 3, 1992
15
Top: Mean profiles for salinity, temperature, and spiciness overlaid with a cast from September 3, 1992 for comparison. Bottom: Spiciness anomalies, in both units and standard deviations, of the September 3, 1992 cast from the mean spiciness profile. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
21
Four-year mean profiles for salinity, temperature, oxygen, potential density, and spiciness bracketed by ± 1 standard deviation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
27
7.
Contour of potential density, as . . . . . . . . . . . . . . . . . . . . . . . . . .
29
8.
Comparison ofthe depths of the 26.4 as surface for the years 1991-1995 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
30
9.
Contour of spiciness,
31
10.
Values of spiciness,
..
32
11.
Contour of spiciness anomalies, in terms of standard deviations, from the mean profile. . . . . . . . . . . . . . . . . . . . . . . ..
34
12.
Seasonal occurrences offour positive spiciness anomalies .....
35
13.
Top: Vertical distribution and abnndance of Nanol1lia bijuga in Monterey Bay. Bottom: Mean encounter rates of Nanol1lia bijuga. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
37
2.
5.
6.
1t . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1t,
on the 26.4 as surface
vii
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FIGURE 14.
15.
16.
PAGE
Median encounters of Nanomia bijuga per minute compared to the mean vertical profile of spiciness . . . . . . . . . . . . . . . . . . ..
38
Hodographs of ADCP data taken from the MBARI Ml mooring
40
a. b. c. d. e. f.
40 40 41 41 42 42
Vector-average of year-round data. . . . . . . . . . . . . . . . . . . . . Vector-average of data between days of year 335 and 89 . . . Vector-average of data between days of year 90 and 182. .. Vector-average of data between days of year 183 and 227.. Vector-average of data between days of year 228 and 274 . . Vector-average of data between days of year 275 and 334 . .
Left: Magnitude of vector-averaged velocities and mean speeds for different periods in Monterey Bay. Right: Directionality indices
43
17a.
Feather plot of wind velocity. .. . . .. . .. . . .. . .. .. .. . ... ...
46
17b.
ADCP current velocities at 17.5 m . . . . . . . . . . . . . . . . . . . . . . .
46
17c.
ADCP current velocities at 201.5 m . . . . . . . . . . . . . . . . . . . . . .
46
17d.
Bakun's upwelling indices. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
46
18.
Intrusions of warm, salty waters in Monterey Bay. . . . . . . . . . .
48
19.
Contours of spiciness anomalies (in standard deviations) for the years 1991-1994 overlaid with boxes delineating the different intrusion periods. . . . . . . . . . . . . . . . . . . . . . . . . . .
49
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r ABSTRACT Between January 30, 1991 and July 31,1995, the Monterey Bay Aquarium Research Institute's (MBARI) remotely-operated vehicle (ROV) Ventana made 104 dives to observe hydrography and abundances of mesopelagic animals. The locations of the dives were 36 0 42' N, 1220 02' W, near the center of the mouth of Monterey Bay and over the axis of the Monterey Submarine Canyon. Analysis of hydrographic data between the surface and 1000 m showed annual intrusions of three water masses into Monterey Bay. Using characteristics of these intrusions along with information from Acoustic Doppler Current Profiler (ADCP) data, the year was divided into four hydrographic periods, Upwelling (April-June, days 90-182), Oceanic-CUC (JulySeptember, days 183-274), Early Davidson (October-November, days 275-334), and Late Davidson (December-March, days 335-89). Evidence collected by the ROV supports the hypothesis that the California Undercurrent (CUC), which occurs during the Oceanic-CUC Period, transports the mesopelagic zooplankter, Nanomia bijuga, into Monterey Bay.
ix
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r .~..
INTRODUCTION
Although it is an interesting realm, both in terms of its physics and ecology, the deep sea is one of the last frontiers on earth still to be explored (Gage & Tyler, 1991). In terms of hydrography, the deep sea is considered to be the region of the oceans below the permanent thermocline (Gage & Tyler, 1991). Historically, the study of the deep-sea has been limited because it is both time-consuming and expensive. However, advances in remotely operated vehicle (ROV) technology, along with close proximity to deep waters, have enabled the Monterey Bay Aquarium Research Institute (MBARI) to collect a unique data set in the Monterey Submarine Canyon. Using this data set, it is possible to examine abundances and vertical distributions of mesopelagic animals in relation to physical mixing and transport processes occurring near the mouth of the Monterey Submarine Canyon. The mesopelagic zone extends from 100 - 1000 m and represents the most populated habitat on earth (Moyle & Cech, 1996). Despite its vast size, much of the biology and ecology of the animals that inhabit this zone is unknown (Robison, 1995). Many mesopelagic animals are planktonic and drift horizontally with the prevailing currents (Gage & Tyler, 1991). However, oceanographic variability and the currents associated with this variability are poorly understood below the mixed layer. Because of this, it is difficult to determine if changes in abundance of an animal population are due to either resident population growth or to advective transpOit of new recruits into an 1
1IIIIIIIIbr
........
2
area. However, an examination of a time-series of hydrographic data from a single station can yield understanding of cycles in local hydrography and the physical processes that cause these cycles. Linking water mass intrusions, which are physical processes, to changes in abundances of planktonic animals would suggest advective transport of animals into or out of an area. One of the most pervasive adaptations by animal life in the midwater is a flimsy, gelatinous body (Gage and Tyler, 1991). Accurately assessing the abundance of these fragile organisms using conventional sampling methods of nets and acoustics is problematic. Nets and trawls often sample inadequately (Barham, 1966), become clogged when animals are abundant, or mangle gelatinous animals beyond recognition (Hamner et aI., 1975). Since their tissues are watery and lack hard structures they usually do not reflect sound well enough to use acoustical methods for surveying abundances (Harbison, 1983). Because of these sampling difficulties, in situ observations appear to be more accurate and effective for quantifying abundance (Hamner et al., 1975; Robison et aI., 1998). Cun'ent ROV technology allows simultaneous measurements of hydrographic parameters and animal observations. This makes it possible to compare animal abundances with hydrographic characteIistics to a high degree of spatial and temporal accuracy. There are two main foci for this thesis. The fIrst focus is to characterize cycles of water mass intrusions occurring in the Monterey Submarine Canyon. The second is to show that these intrusions are correlated with the abundances of mesopelagic zooplankton. The abundances of the physonect siphonophore, Nanomia bijuga, were used for this thesis because Nanomia is a relatively common midwater animal in
IIIIIIIIIII?
_
3
Monterey Bay and previous studies in Monterey Bay have shown that Nallom;a exhibits annual cycles of abundance (Robison et ai, 1998), Correlations of abundance with water mass intrusions have not been shown in the mesopelagic zone for Nallom;a or other midwater animals. Moreover, understanding midwater physical processes and their effects on animal abundances will be valuable in examining the ecology of both mesopelagic animals and the animals that depend on them for survival. Monterey Bay Monterey Bay (ME) is located on the central California coast about 120 km south of San Francisco Bay (Fig. 1). It is nearly symmetrical, extending 30 km from the center of the mouth to Moss Landing, with a width of 59 km. Located between 36.5 and 37° N, it is semi-enclosed and open to the ocean. It is tlOt significantly diluted by fresh water from river and stream discharge except dming short, infrequent periods of heavy rains. A distinct and major geographical feature of Monterey Bay is the Monterey Submarine Canyon (MSC). Meandering along the ocean floor through the Bay until it reaches Moss Landing, it divides the Bay roughly into northern and southern halves. Although MSC reaches depths in excess of 3000 m, based on a smface projection of area, only 5% of the Bay is deeper than 400 m, with the majority (80%) being less than 100 m (Breaker and Broenkow, 1994). California Current System The California Current System is composed of several water masses and currents mixing along the eastern edge of the NOlth Pacific Basin (Reid et al., 1958). Four water masses (Fig. 2) are represented in the coastal waters from 0 - 500 m in depth:
IIIIIIIIIIIt
_
411
1
I i i
I
i
I
I
1
1
Contour Map of Monterey Canyon ..... ';;;;:"'" \. I
371
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40
39 36.7
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B
38
36.6
.",
j
36i~~.3
37
122.2 122.;
~~;)J)121.9
121; 121.7 dl.6
•
36
• 35
Line 70
125
124
123
122
121
120
119
118
117
116
Longitude
Figure I: Map of Monterey Bay and the central and northern California coast. The coast and the 20, 50, 100, 200, 500, 1000, 2000, and 3000 ill depth contours are shown on the inset map of Monterey Bay. The positions of the MBARI MI mooring and the midwater station are also indicated. The stations along CaICOFl's line 70 are also shown on the coastal map.
4
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ill
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'-
'-
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'-
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'-
'-
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ill
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32
32.5
33
33.5 34 Salinity (psu)
34.5
35
35.5
Figure 2: T-S diagram of four water masses that mix in tile California Current System. Pacific Subarctic Water (PSAW) from the Vertex 7 cruise, 58° 40.9'N, 147° 57.2'Won August 13, 1987. Monterey Bay Water from the ROV Vemana, 36° 44.9'N, 122° 2.3'W on September, 3, 1992. Equatorial Pacific Water (EPW) from tile Vertex 4 cruise, 18° a.O'N, 107° 34.0'W on AugustS, 1983. North Pacific Central Water (NPCW) from the Vertex 2 ernise, 28° O.O'N, 155° 3.0'W on October 28,1981. Solid contour lines are lines of constmlt aD, i.e. potential density; the dashed lines are lines of constant spiciness, 1t.
j
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6
Pacific Subarctic Water, Eastern North Pacific Central Water, upwelled water and Equatorial Water (Reid et al., 1958; Lynn and Simpson, 1987). The major ocean currents found off central California are the California Current, the California Undercurrent, and the Davidson Current (Reid et al., 1958; Lynn and Simpson, 1987). Water masses are defined by their salinity, temperature and dissolved oxygen at the time they enter the California CUlTent System. Nutrients may also be used to define water masses, but were unavailable for this study. Processes of mixing cause these properties to change as the water moves through the California Current System and encounters water with different origins. Advection of one water mass into another is referred to as an intrusion of a water mass. Intrusions can be detected by changes in TemperatureSalinity (T-S) diagrams over time. An intrusion typically canses an inflection, or bend, in the T-S profile as waters of different temperatures and salinities mix. Pacific Snbarctic Water is formed at high ]atitndes by high levels of precipitation and heat loss across the air-water interface. Characterized by re]atively low salinity, low temperature, and high dissolved oxygen (Reid et al., 1958; NORPAC committee, 1960), Pacific Subarctic Water enters the California Current at 48° N (Pickard, 1964) and is transported equatorward. Found 850 - 900 km off the California coast, Eastern North Pacific Central Water (Sverdrup et al., ]942; Pickard, 1964) is salty, warm, and relatively low in dissolved oxygen. It enters the California Current System from the west. Within 10 - 25 km of the coast, upwelling, occurring from March through May, brings relatively cold, salty, and oxygen-deficient waters to the surface from depth (Reid et al., 1958). Althongh npwelling is confined to a narrow band along the coast its effects influence a much wider region (Huyer, 1983). In Monterey Bay,
...tr_IIIIiIIIIIIIIIIIIIIiIIIIIIIIII
_
7
evidence suggests that upwelling does not actually occur in the Monterey Submarine Canyon, rather upwelled waters are advected in from coastal headlands outside of the Bay (Broenkow & Smethie, 1978; Rosenfeld et al., 1994). Equatorial Pacific Water forms in the Eastern Tropical Pacific and is relatively high in salinity and temperature but low in dissnlved oxygen. It is thought to enter Monterey Bay with the Davidson Current and the deeper-lying California Undercurrent from the South. The most prominent feature of the California Current System is the California Current, the eastern boundary of the anticyclonic North Pacific Gyre. The California Current is a surface current, ranging from 0-300 m deep. Its speeds along the California coast are typically less than 25 cm S·l (Reid and Schwartlose, 1962), although instantaneous speeds may reach 50-70 cm S·l (Huyer et al., 1991). Peak velocities occur in the latter half of the year with the strongest flow occurring 200-300 km offshore (Lynn and Simpson, 1987). The strongest nearshore flow occurs from April to May. Moving equatorward, it transports water whose propelties closely resemble Pacific Subarctic Water (i.e. low temperature, low salinity, and high oxygen) (Tibby, 1941; Tully and Barber, 1960; Reid et aI., 1958). These propelties change abruptly west of the California front (Saur, 1980) at the boundary of the California Current System and Eastern North Pacific Central Water. In winter, the nearshore flow changes direction and flows poleward; this flow reversal is known as the Davidson Cun-ent and it transports equatorial water northward. It has been observed to occur within 150 km of the coast and is thought to be a shoaling or surfacing of the California Undercurrent. The California Undercurrent (CUC) is considered to originate in the eastem equatorial Pacific and flow poleward (Reid et al., 1958). Along central California the
J.
_
8
core of the California Undercurrent is very narrow «20 km) and t10ws along the continental slope at depths of -250 m. Also along central California, the California Undercurrent weakens and possibly disappears or moves offshore with the onset of upwelling during March through May (Lynn and Simpson, 1987). Distributions of physical characteristics show that the core of the California Undercurrent is found around the 26.4-26.6 as (potential density) surfaces (Lynn and Simpson, 1987; Ramp et
al., 1997; Appendix A). Studies using isobaric drifters between 150 and 600 m depths have clearly shown that t10w is poleward near the coast. There is an absence of equatorial t10w offshore, and a variety of eddy motions exist that propagate westward (Collins et al., 1998). Evidence suggests that the changes in the deep circulation in Monterey Bay may be due to changes in the strength of the California Undercurrent (Koehler, 1990; Ramp et al., 1997b). Oceanographic Variability and Seasonality in Monterey Bay It has been shown that different water masses impinge on Monterey Bay at
different times of the year, causing hydrographic seasons. Using temperature data, Skogsberg (1936) originally proposed a tln'ee-season model for Monterey Bay. These seasons are upwelling (mid-February through late August), oceanic (mid-August to midOctober), and non-upwelling (also called Davidson; mid-November to mid-February). Upwelling occurs when nOlthwest winds predominate along the California coast. South of San Francisco upwelling tends to be strongest in April (Huyer, 1983). The second season, oceanic, occurs when winds favorable to upwelling relax and warm offshore waters enter the Bay. This is thought to occur on an irregular basis, and its occurrence
J.._IIIIIIIIiIIIIII
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9
may be related to a breakdown in a semi-permanent eddy outside the entrance to MB (Barham, 1956; Breaker and Broenkow, 1994). The Davidson season occurs when the predominant northwest winds shift and weaken. The timing of the Davidson Current coincides with the presence of northward geostrophic flow along the central California coast (Lynn and Simpson, 1987). Bolin and Abbott (1963) further refined Skogsberg's seasons using salinity measurements at surface and at 15 m depths, along with temperature measurements between the surface and 50 m. They determined that temperatures during upwelling (March - August, days of the year: 90-220) reach their lowest values for the year, then rise, approaching the annual high. During this time, the thermal gradient between the surface and 50 m is typically 3°C or more with the temperatures at 50 m reaching the lowest for the year. Salinity values also rise to their highest values for the year and start to decline. Oceanic waters spread throughout Monterey Bay between August and December (days 220-330) producing the highest surface temperatures along with a persistent thermal gradient between the surface and 50 m. Temperatures are typically 2-3°C higher than those occuning during upwelling. Salinities also rise slightly. Following the oceanic period, the Davidson season (December - March, days: 330-90) is marked by a sudden decrease in surface temperature followed by a continuous decline. The mixed layer deepens and the temperature difference between the surface and 50 m is usually less than 1°C. The temperature at 50 m is the highest of the year while the surface salinities are the lowest. Geostrophic current velocities along CalCOFlline 70 (Fig. I) have shown annual and semi-annual periodicities. Lynn and Simpson (1987) and Chelton (1984)
10
both used harmonic analysis on CalCOFI data from 1950 through 1978 to calculate mean geostrophic flow relative to 500 dbars. They found that little or no poleward flow occurs along the continental shelf between March and June (days: 90-181). However, in July (days: 182-212), weak poleward flow «6 cm S·I) occurs along the continental shelf below about 100 m with the core of the jet (>4 cm S·I) found between 200 and 300 m. This poleward flow wealcens «3 cm S·I) through September, after which it gains in speed and begins to move progressively shallower until its velocity exceeds 10 cm S·1 during November and December (days: 305-365). During this time, the jet's core has shoaled and extends from the surface down to 250 m with the highest velocities occurring shallower than 100 m. The flow, although still strong (>8 cm S·I), begins to weaken in January (days: 1-31) and the core of the jet begins to sink. The highest velocities in January occur between 50 and 100 m with the jet continuing to extend down to 250 m. By February the maximum cun'ent speed drops off to less than 5 cm S·1 and the core of the jet continues to descend. After February, the core of the poleward flow weakens and moves offshore.
Nanomia bijuga Nanomia bijuga, a physonect siphonophore (Fig. 3) found worldwide in warm and temperate regions (Pages and Gili, 1992), is commonly observed in Monterey Bay (Robison et a!., 1998). Seventy percent of the Nanomia population in Monterey Bay is by oxygen concentration and it has not been found in waters with oxygen concentrations less than 0.1 ml 1'1 (Robison et a!., 1998). Although planktonic, Nanomia individuals are active, mobile swimmers and nndergo diel vertical migrations of several hundred meters (Barham, 1963). The Nanomia population follows an apparent seasonal pattem of
.iz
_
II
Figure 3: Illustration of the physonect siphonophore, Nanomia bijuga. Typical size range is 3 - 25 cm in length. Image courtesy of the Monterey Bay Aquarium Research Institute. Artist: Laurel Rogers.
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abundance with the peak occuning about three months after the onset of upwelling (Robison et aI., 1998). Whether this is due to advective transpOlt of offshore recruits into Monterey Bay or to resident population growth in response to seasonal food enhancement is unclear. Due to its relative abundance, Nanomia should be a good indicator of transport processes in Monterey Bay. Changes in Nanomia abundance cooccuring with changes in hydrography would suggest advection of Nanomia into or out of Monterey Bay. Spiciness Currents along the central California coast are associated with intrusions of water masses (Reid et aI., 1958). For example, the California Undercurrent can be thought of as a warm and salty jet of Pacific EquatOlial Water intruding from the south. found between 200 and 400 m. The lower limits of its population appear to be defined As this jet travels northward it mixes with surrounding waters and becomes cooler and hesher. However, its origins are still discernible, even the distal end of the jet wi1l still be warmer and saltier relative to the surrounding water masses. Spiciness (n) is an oceanographic variable used for quantifying the strength of intrusions (Munk 1981). To understand spiciness, one must keep in mind that as fluids move about in the ocean they mix along neutral surfaces (McDougall, 1987) that are the paths of minimum work. Neutral surfaces can be closely approximated by potential density (ae) surfaces (Huyer et aI., 1991; Appendix A) or coarsely approximated by depth. The 26.4 ae surface, representing the location of the core of the California t
l: F
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t
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13
Undercurrent, is found at approximately 200 m (Ramp et aI., 1997a). For reference, the 26.4 as surface will be provided on appropriate figures in this thesis. Waters of differing potential densities will be found at different depths in the oceans and therefore generally do not mix. When waters of nearly-equal potential density but differing salinity and temperatures mix, the resulting mixed water will be either saltier and warmer (higher values of spiciness) or fresher and colder (lower values of spiciness). This change in spiciness indicates the characteristics of salinity and temperature in the intruding water masses. From these charactelistics it is possible to discern the origin of the intruding waters. Along the coast of central California spiciness values greater than 0.10 on the 26.4 as surface indicate the presence of the California Undercurrent and its cargo of salty equatorial waters (Ramp et aI., 1997a). Spiciness is particularly useful because knowing the depth and timing of intruding water masses provides an opportunity to link physics and circulation with changes in ecology. Flament's (1986) derivation of1t used in this thesis is refined from work by Jackett and McDougall (1985) and Veronis (1972). Flament's derivation was chosen over others because it is mathematically well defined, it is independent of the scales chosen for the potential temperature (8) - salinity (S) diagram, and it has been used in recent publications as an indicator of the presence of the California Undercurrent (Ramp et at., 1997a). Flament started with the premise that the definition of1t(8, S) must be
invariant to changes of scale along the 8 or S axes. His first constraint (1) is that the slopes of the isopycnals and the iso-1t lines are equal and have opposite sign:
14
doP d,P
dol' d,l'
--=---
(1)
or (2)
where 8
= potential temperature, S = salinity, n: = spiciness and p = density. The
ordinary differential equation
a,pds
= doPd()
(3)
defines a unique set of isopycnallines. These isopycnallines, following constraint (1), defme a unique set of iso-n: lines, regardless of the chosen scales, which are geometrically symmetric with respect to the 8-S axes (Fig. 4). With the iso-n: lines defined, Flament assigned values to them using an approximate condition over the 8-S domain containing most oceanic water types. This condition was selected to provide a direct relation between the diapycnal gradient of spiciness, dpn:, and the diffusive stability of the water column. The least squares derived polynomial using this condition and constraint (1) can be found in appendix B while the complete derivation can be found in Flament's dissertation. The results of this polynomial assign the highest values of n: to warm, salty waters and the lowest values of n: to cold, fresh waters. Along a potential density surface, values of spiciness greater than the mean indicate the intrusion of warmer and saltier waters, lower values show intrusions of colder, fresher waters (Fig. 4).
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Mean ± 1 std. 03-Sep-1992
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33.2
33.4
33.6 33.8 34 Salinity (psu)
34.2
34.4
34.6
Figure 4: The mean 8o-S curve, bracketed by ± 1 stmldard deviation, overlaid with a cast from September 3, 1992. Solid contour lines are lines of const;mt ao, Le. potential density: the dashed contour lines are
lines of constant spiciness, n. A sllbsudllce maximum of 1[, on the mean curve, occurs on the 26.4 Go contour. This is due to tile intrusion of warm salty Pacilic Equatorial Waters in tile California Undercurrent
I
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16
Hypotheses The overall goal of this thesis is to examine cycles in hydrography of the midwaters of Monterey Bay and compare these with cycles in the abundance of
Nanomia bijuga. To meet these goals several specific hypotheses will be examined. I. It has been shown that time-series of hydrographic data reflect changes in the
circulation of the California Undercurrent (Ramp et al. 1997a; Lynn and Simpson, 1987; Chelton, 1984). Since the California Undercurrent transports warm, salty equatorial waters, its presence in the mesopelagic zone will be detected by spiciness values larger than the mean; likewise the absence of the California Undercurrent will be apparent through values less than the mean. 2. The most pervasive type of water found locally is Pacific Subarctic water, which is relatively fresh and cold when compared to surrounding water types. Positive anomalies in spiciness profiles will indicate the presence of intruding water masses, either NOlth Pacific Central Waters or Equatorial Waters. These intrusions can be used to define the timing of hydrographic seasons in Monterey Bay. 3. Since different currents in the California Current System are associated with different water masses (Reid et al., 1958), changes in current structure wiIl co-occur with anomalies in the spiciness profiles. High spiciness values will occur when Acoustic Doppler Current Profiler (ADCP) data from a nearby moored buoy indicate that flow is strongly northward. Near-smface current velocities will be southward during upwelling. 4. Although the siphonophore, Nanomia bijuga, is motile, it is considered planktonic and so drifts with the prevailing ocean currents. Being a relatively common
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17
zooplanter in Monterey Bay, advective transpOIt processes should affect observed abundances of Nanomia, Since intrusions of water masses are associated with CUTI'ents (Reid et aI" 1958), changes in the spiciness signal will co-occur with changes in the abundance of Nanomia" High abundances of animals co-occurring with larger spiciness values would suggest that the California Undercurrent transpOIts animals into Monterey Bay from a southern population source,
! I
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METHODS
ROV Observations Measurements of hydrography and video recordings were made using the Monterey Bay Aquarium Research Institute's (MBARI) remotely operated vehicle (ROV) Ventana. The Ventana is an ISE Hysub 40, outfitted for scientific research (Robison, 1993). Over the course of five years, from 1991 to 1995, conductivity, temperature, depth (CTD), oxygen, and video proflles were recorded at a single station in Monterey Bay from the Ve11tana (Table I). The depth at this station is approximately
1
1600 m and it is located over the axis ofthe Monterey Submarine Canyon. At 36° 42' N,
i
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i,
I
122° 02' W, it is near the center of the mouth of the bay and open to oceanic intrusions (Fig. I). Salinity, temperature, and pressure were measured with a Sea-Bird Model 9 CTD. A Sea-Bird Model 13 oxygen sensor with a Beckman electrode and a Sea-Bird ModelS pump measured oxygen concentration. Although the ROV's oxygen sensor was not calibrated using bottle casts, the measured oxygen concentrations were comparable to data from CTD casts taken during the same period using a calibrated oxygen sensor at 36° 44.9' N, 1220 02.3 'W. CTD data were recorded every 4 seconds and then stored as a IS-second average. Measurements of vertical distributions and abundances of animals were made with video, recorded on high-resolution BetaCam tapes aboard the surface SUppOit ship, RlV Point Lobos. The main video camera is a Sony DXC-3000 with a 18
t f
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k
_
19
Table 1: Dates of the ROV Ventana dives at 36 0 42' N, 1220 02' W ± 2 km used for this study. The date, cOlTesponding day of the year, and maximum depth (m) for each ROV dive are included. 1991 Dives Dale
30-Jan-1991 14-Mar-1991 19-Mar-1991 29-Mar-1991 16-Apr-1991 07-May-1991 14-May-1991 30-May-1991 27-Jun-1991 02-Jul-1991 11-Jul-1991 01-Aug-1991 08-Aug-1991 09-Sep- I 991 24-0ct-1991 07-Nov-1991 25-Nov-1991 05-Dec-1991 13-Dec-1991
Day 30 73 78 88 106 127 134 150 178 183 192 213 220 252 297 311 329 339 347
520 172 483 474 438 479 477 614 253 >1000 988 >1000 978 903 >1000 >1000 >1000 >1000 >1000
1992 Dives Date
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09-Jan-1992 16-Jan-1992 20-Jan-1992 06-Mar-1992 10-Apr-1992 13-Apr-1992 08-May-1992 28-May-1992 15-Jun-1992 16-Jul-1992 10-Aug-1992 20-Aug-1992 21-Aug-1992 03-Sep-1992 1O-Sep-1992 09-Nov-1992 19-Nov-1992 23-Nov-1992 03-Dec-1992
1994 Dives
1993 Dives
Max. Depth
Day
Max. Deptb
9 16 20 66 101 104 129 149 167 198 223 233 234 247 254 314 324 328 338
>1000 >1000 >1000 926 865 >1000 >1000 >1000 865 >1000 . >1000 >1000 569 >1000 >1000 >1000 958 >1000 851
Dale
29-Jan-1993 08-Feb-1993 18-Feb·1993 22-Feb-1993 04-Mar-1993 08-Mar-1993 12-Mar-1993 22-Apr-1993 23-Apr-1993 28-May-1993 07-Jun-1993 01-Jul-1993 05-Aug-1993 12-Aug-1993 15-Sep-1993 30-Sep-I 993 07-0ct-1993 15-0ct-1993 28-0ct-1993 16-Nov-1993 18-Nov-1993 22-Nov-1993 20-Dcc-1993 21-Dec-1993 28-Dec-1993
Day
Max. Depth
29 39 49 53 63 67 71 112 113 148 158 182 217 224 258 273 280 288 301 320 322 326 354 355 362
>1000 >1000 702 442 >1000 >1000 970 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000
Date
14-Jan-1994 18-Jan-1994 25-Jan-1994 27-Jan-1994 14-Feb-1994 28-Feb-1994 07-Mar-1994 10-Mar-1994 07-Apr-1994 14-Apr-1994 26-Apr-1994 19-May-1994 20-May-1994 31-May-1994 07-Jun-1994 15-Jul-1994 26-Jul-1994 15-Aug-1994 16-Aug-1994 31-Aug-1994 08-Sep- I 994 23-Sep-1994 18-0ct-1994 25-0ct-1994 26-0ct-1994 10-Nov-1994 01-Dec-1994 19-Dec-1994
Day
Max. Depth
14 18 25 27 45 59 66 69 97 104 116 139 140 lSI 158 196 207 227 228 243 251 266 291 298 299 314 335 353
>1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 808 861 530 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 >1000 546 829
1995 Dives Date
19-Jan-1995 31-Jan-1995 I 6-Feb-1995 23-Feb-1995 06-Mar-1995 20-Mar-1995 02-Juu-1995 13-Jun-1995 20-Jul-1995 25-Jul-1995 31-Jul-1995
Day
Max. Depth
19 31 47 54 65 79 153 164 201 206 212
641 >1000 >1000 706 >1000 425 >1000 923 >1000 656 794
20
Fujinon 5.5-49 mm zoom lens; signals from the camera were transmitted through optical fibers in the ROY's tether to a video recorder aboard the surface SUppOlt ship. Only the CTD downcast data were used for data analysis. Downcasts are preferred because the sensors are located in the middle of the ROY's frame, on the underside of the syntactic foam floatation; water parcels sampled when descending are less perturbed by the motion of the ROY and its tether than parcels measured by the ascending ROY. CTD and oxygen data were interpolated to I m increments between a and 1000 m to allow direct comparisons between different cruises. CTD time series data were gridded on a 7 day by 10 m grid, from January 1, 1991 to July 31, 1995 and from the surface to 1000 m, using Kriging, an optimal interpolation method commonly used in geostatistics (Keckler, 1995). Mean profiles for salinity, temperature, oxygen, potential temperature (6), density, potential density (O'e), and spiciness (n) were calculated from the gridded data between January 1,1991 and December 31,1994. Anomalies from these mean profiles provide evidence of seasonal variation. However, in deeper waters, variations are small compared to the variations of measurements made near the surface. Therefore, anomalies were calculated both as standard units, for near-surface variation, and in tenns of standard deviations (4), to resolve seasonal variation at deeper depths (Fig. 5). x-x
Anomaly=-x.rtd
(4)
Where x is the measured value, and x and x"rd are the mean and standard deviation, respectively, at the same depths as x.
21
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34
33.5
Spiciness
Temperature (0C)
Salinity (psu) 34.5
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