portions of spring and summer when mean surface Ekman transport is ... of Biology, DeLand, Florida 32720. ..... break where internal waves are generated ... level variations at Monterey, California. NOAA. Tech. Rep. NMFS SSRF-76 1. 50 p.
Limnol. Oceanogr.. 36(Z), 1991, 279-288 0 1991, by the American Society of Limnology
and Oceanography,
Inc.
Cross-shelf transport causes recruitment populations in central California
to intertidal
Terence M. Farrell,’ David Bracher, and Jonathan Roughgarden Hopkins Marine Station, Stanford University, Pacific Grove, California 93950 Abstract The control of recruitment to intertidal barnacle populations along the central California coast was examined from April to mid-October 1988. Four recruitment pulses occurred during periods of relaxation in alongshore winds and cessation of coastal upwelling. In each case recruitment ended when strong equatorward winds reappeared -and upwelling resumed. Data on SST, salinity, adjusted sea level, and satellite (AVHRR) images revealed alternating periods of onshore and offshore transport of the surface water layer. The onset of the largest recruitment pulse was associated with the advection of warm, clear, low-salinity water into the neat-shoreregion. This oceanic water mass also contained a different zooplankton assemblage than the water mass it replaced.
Most fish and invertebrates that inhabit nearshore waters have a two-phase life history. Adults are sessile or relatively sedentary, while larvae are planktonic and capable of widespread dispersal. For many of these species,the rate at which larvae return to the adult habitat has a large influence on adult abundance. Settling larvae in both fish and invertebrates tend to arrive in pulses lasting from one to several days. These short recruitment pulses may result from mesostale or smaller oceanic events. In this study we investigate recruitment of common intertidal barnacles, Balanus glandula and Chthamalus spp., on the central California coast. On the basis of laboratory growth studies, development from the first naupliar stageto the cyprid lasts 1O-20 d for both B. glandula (Brown and Roughgarden 1985) and Chthamalus (Miller et al. 1989). The breeding season of these species in central California lasts for at least 6 months (Hines 1978; Page 1984), including portions of spring and summer when mean surface Ekman transport is strongly offshore (Parrish et al. 1981; Strub et al. 1987). Pre’ Present address: Stetson University, Department of Biology, DeLand, Florida 32720. AcknowIedgments We thank A. Deck, M. Graham, J. Leichter, and M. Vassar for assistance in collecting and sorting samples. The manuscript was improved by discussions with C. Baxter, M. Denny, C. Harrold, L. West, R. Zimmerman, and two anonymous reviewers. This research was supported by the U.S. Department of Energy (DE FG03-85ER60362).
vious work on B. gland&a in this area indicates that Ekman transport affects recruitment becausehigh total recruitment during spring occurs only when mean spring upwelling is weak. Intertidal barnacle larvae are carried far from shore in years of high upwelling but are retained nearshore, where the probability of encountering the adult habitat is high, in years with weak upwelling (Roughgarden et al. 1988). Similarly, in this study we demonstrate that the great variation in barnacle recruitment occurring on the scale of days results primarily from alternating periods of onshore and offshore transport of surface water associated with relaxation and resumption of upwelling. Methods Barnacle recruitment was determined along 10 km of shore on the southern edge of Monterey Bay. Three settling plates were placed at each of six sites (Fig. 1). Tidal heights of plates ranged from 1.0 to 2.5 m above mean lower low water. Sites exposed to strong wave action had settling plates placed higher in the intertidal zone than sites in more protected areas. In this study the plates were used to determine the timing of recruitment. These plates were collected and replaced every 2 d for 6.5 months (1 April13 October 1988). Plates each had a surface area of 50 cm2 and were made of safetywalk tape (3M Co., product No. 7740) on fiber glass. Cyprid larvae and newly metamorphosed barnacles were counted and identified to species (for B. glandula) and genus (for Chthamalus) with Sltanding
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Farrell et al. 122”55’W +
36”40’N
MONTEREY
BAY
PP
MONTEREY
PENINSULA
Fig. 1. Southern Monterey Bay. Recruitment monitoring stations are marked with site abbreviations. Hydrographic data were collected off three of these sites (HMS, PP, PJ).
(1980) Brown and Roughgarden(1985), and Miller et al. (1989) as guides. A boat was available for daily oceanographic sampling from 16 to 25 July. CTD (Sea-Bird SBE19) casts were made offshore of three of the recruitment sites [Fig. l), and zooplankton were sampled by pumping water from - 1 m deep and passing it through a flowmeter (mean volume, 1.8 m3) and plankton net (70-pm mesh). This depth was used to collect near-surface samples while keeping the pump continuously submerged in choppy conditions. Windspeed and sea-surface temperature (SST) data were obtained from National Data Buoy Center buoys. One buoy (No. 46042; 36.8”N, 122.4”W) was located offshore of Santa Cruz -35 km northwest of the study area; the other buoy (No. 46028; 35.8”N, 121.9”W) was offshore of Cape San Martin -70 km south of the study area. Windspeed data were recorded as vector (No. 46028) or scalar (No. 46042) averaged windspeed and direction for 8-min periods
taken every 6 h. Daily SST was also obtained for a nearshore area 20 km south of the study area at the California Game Commission’s Marine Resources Laboratory at Granite Canyon. Conditions at Granite Canyon reflect the direction of Ekman transport since it is frequently the site of strong coastal upwelling (Breaker and Mooers 1986). Water temperature at 8 m adjacent to the HMS site was monitored continuously during the study via the Monterey Bay Aquarium Weather and Oceanographic Data Acquisition System. These data were used, as by Breaker and Broenkow (1989), to determine the Frequencyof internal waves in the study area. Internal waves often result in rapid, large changesin water temperature as the position of the thermocline moves past the fixed mooring depth of the sensor. Temperature changes of >2”C in ~20 m were assumed to indicate imemal waves. This method of determining the frequency of internal waves provides an underestimate because the absenceof large, rapid changes in water temperature does not necessarily indicate the absence of internal waves. Specifically, when the thermocline is well below 8 m, internal waves will not move cold water past the sensor. Satellite images of SST produced by the advanced very high resolution radiometers (AVHRR, band 4) on the NOAA 9 and NOAA 10 weather satellites were obtained through the Scripps Satellite Oceanographic Facility for several cloud-free periods in July-August. The ima.ges were ground truthed with SST data from buoys 46042 and 46028 and false colored with exponential transformations. Sea-level data were obtained from a tidal gaugethat is maintained by NOAA in Monterey Harbor (Fig. 2F). The influences of tides and barometric pressure on sea level were removed as by Bretschneider and McLain (1983) to adjust measured sealevel. Determining the statistical significance of correlation coefficients between time-series data was influenced by the fact that sequential pairs of observations are not completely independent. The number of observations (N) therefore overestimates the actual degrees of freedom (df). True df for a corre-
Barnacle recncitment
281
Fig. 2. A. Recruitment of intertidal barnacles. Plotted values are the mean number of settling Balanus glanduia and Chthamalus spp. on 18 settling plates. The bars across the top of the figure indicate periods when internal waves occurred. B. Alongshore windspeed at buoy 46042 (positive values indicate equatotward winds in m s-l). C. SST at buoy 46042. D. SST at Granite Canyon. E. Salinity (%) at Granite Canyon. F. Adjusted sea level at Monterey Harbor expressed as deviation (cm) from mean sea level.
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Farrell et al.
lation was determined by the artificial correlation method (Davis 1976, 1977; see also Chelton 1982). The statistical significance of the principal component analyses was determined with Monte Carlo simulations.
FIRST
PULSE
b;;]
Results The recruitment events -Barnacle
recruitment displayed high temporal variation from April to mid-October. The total abundance of settling larvae ranged from 0 to 985 cyprids per 2-d sampling period. We observed four pulses of high recruitment separated by periods of low settlement (Fig. 2A). Balanus glandula and Chthamalus spp. settled concurrently in each of the four pulses. The recruitment pulses were usually initiated by rapid (>4-fold) increases in settlement (three of four pulses) and ended with similarly rapid declines in recruitment (all four pulses). In all four pulses, recruitment first appeared at sites near the mouth of Monterey Bay (such as HMS) and then moved to the SS site in the interior of the bay (Fig. 3).
RECRUITMENT
lo
14
SECOND
18 22 APRIL
26
30
RECRUITMENT
4 MAY
8
PULSE
0.3 1
I
2
6
10
14
18
22
26
6
10
30
JUNE
THIRD
PULSE
Detailed view of the onset of the largest recruitment pulse-Temperature increased throughout the water column during the onset of the largest recruitment pulse at each of the three hydrographic stations (Fig. 4AC). Salinity decreased greatly during this event (Fig. 4D). Warm, clear water of low salinity appeared first at the southernmost hydrographic station, replacing a cold, highsalinity water mass that contained a large standing crop of phytoplankton. This ocean water mass contained large numbers of siphonophores (probably Apolemia) and scyphozoans (Aurelia and Pelagia). This water mass, like barnacle recruitment, subsequently moved into the interior of the bay. The zooplankton species assemblage also showed large changes slightly after the onset of the largest recruitment event. The abundance of shelled bivalve and gastropod larvae increased rapidly between 24 and 25 July (Fig. 5). The abundances of seven taxa were quantified from the pumped samples. These data were analyzed with principal components analysis. The first principal component accounted for a statistically significant proportion of the variation among samples (PC 1 = 0.94 larval bivalves + 0.84
17
21
25
29
2
14
AUGUS’I
J II LY
FOURTH
PULSE
0.6 7
16
22
26
30
AUGUST Fig. 3. Recruitment of barnacles at two of the shore sites for the four recruitment pulses. Plotted values are expressed as the mean number of barnacles settling at a site in a 48-h period divided by the total recruitment at that site during the recruitment pulse.
283
Barnacle recruitment A. PJ
ZOOPLANKTON
l’s
20 JULY
22
24 23 25 JULY Fig. 5. Zooplankton community structure. Plotted values are mean (N = 3) abundances of the shelled larvae of bivalves and gastropods and mean PC 1 scores. 1!I
B. PP
1s
24
20 22 J u LY
JlJI,Y
Cross-shelf transport and recruitment -
AT 10 M
34.2
-PJ
34.0 -
1
33.4 33.2
I I 20 22 24 JULY Fig. 4. A-C. Temperature profiles for three hydrographic stations. Water between 12” and 13°C is shaded. D. Salinity at 10 m for the three stations. 16
21
abundance of all zooplankton (all coefficients are positive) with emphasis on larval bivalves, larval gastropods, and pelagic polychaetes. The mean daily PC 1 scores showed large increases toward the end of the sampling period (Fig. 5). The pumping did not yield quantitative samples of intertidal barnacle larvae (water was pumped from l-m depth-below the usual depth range of intertidal barnacle cyprids). The increase in late-stage larval molluscs suggests that a recruitment pulse for many benthic invertebrates, not only barnacles, occurred at this time.
C. IIMS
D. SALINITY
ABUNDANCE
1 18
pelagic polychaetes + 0.80 larval gastropods + 0.68 fish eggs + 0.63 cladocerans + 0.60 larval polychaetes + 0.30 calanoid copepods; PC 1 accounted for 50% of the total variation, the critical value at cy= 0.05 was 3 1.9%). This component measures the
The start of all four recruitment pulses coincided with a large decrease in the equator-ward component of the offshore wind field (Fig. 2B). Furthermore, each recruitment pulse ended when strong equatorward winds reappeared. The longest and strongest wind-field relaxation corresponded with the longest and strongest recruitment pulse. A significant negative correlation (r = -0.43) occurred between the alongshore wind velocity at buoy 46042 and total barnacle recruitment during the study period, with recruitment lagging 2 d behind the wind. Recruitment is expected to lag wind velocity because of inertia and the time required for advection to bring larvae to shore. Winds along the California coast were highly coherent. The maximal correlation coefficient for alongshore windspeed at the offshore buoys was 0.90 and occurred at a lag of zero days. Alongshore winds appeared to control the
Farrell et al. Table 1. Correlations among physical parameters and recruitment. Lag given in days; positive lag indicates the first variable leads the second variable. GC is the Granite Canyon Marine Laboratory. -Comparison
Max r
Lag (4
Wind at buoy 46042 and 46028 Wind and SST at buoy 46042 Wind and SST at buoy 46028 Wind at buoy 46042 and SST at GC SST and salinity at GC Wind at buoy 46042 and recruitment SST at GC and recruitment
0.90 -0.40 -0.52 -0.59 -0.58 -0.43 0.59
0 1 2 1 -1 2 2
direction of cross-shelf advection. Strong north winds resulted in upwelling, while relaxation of the wind field resulted in onshore advection of the surface layer. SST at an offshore buoy (Fig. 2C) and nearshore at Granite Canyon (Fig. 2D) showed strong negative correlations with alongshore wind (Table l), indicating pulses of upwelling caused by equator-ward winds. Salinity at Granite Canyon (Fig. 2E) showed a significant positive correlation (r = 0.41) with alongshore wind. The low-salinity ( < 3 3.3Y~) water that replaced high-salinity water (> 33.87’) nearshore during the third relaxation event also indicates onshore advection at this time. The first principal component (PC 1 = 0.86 SST + 0.62 recruitment + 0.45 sea level - 0.745 salinity - 0.766 north wind velocity) accounted for a’ statistically significant portion (50.1%; the critical value at 01= 0.05 was 29.4%) of the total variation. As expected if upwelling intensity is controlling the system, most of the variation is spread along an axis that contrasts variables having high values during relaxation events (nearshore SST, recruitment, and sea level) with variables having low values during relaxation events (nearshore salinity and north wind velocity). Principal components analysis of salinity, SST, and alongshore wind revealed that > 70% of the variation in these three parameters was explained by the first principal component (PC 1 = 0.87 SST 0.83 salinity - 0.8 1 north wind; critical value for significance was 44.5%).
Effective
24 20 17 48 12 38 17
N
P value
co.01 co.05 co.05 co.01 co.05 co.01
