FAU Institutional Repository

FAU Institutional Repository

http://purl.fcla.edu/fau/fauir

This paper was submitted by the faculty of FAU’s Harbor Branch Oceanographic Institute.

Notice: ©1998 Elsevier B.V. This manuscript is an author version with the final publication available at http://www.sciencedirect.com/science/journal/00220981 and may be cited as: Pitts, P. A. (1998). Effects of summer upwelling on the abundance and vertical distribution of fish and crustacean larvae off central Florida’s Atlantic coast. Journal of Experimental Marine Biology and Ecology, 235(1), 135‐ 146. doi:10.1016/S0022‐0981(98)00179‐8

JOURNAL OF EXPERIMENTAL MARINE BIOLOGY Journal of Experimental Marine Biology and Ecology, AND ECOLOGY ELSEVIER 235 (1999) 135-146

Effects of summer upwelling on the abundance and vertical distribution of fish and crustacean larvae off central Florida's Atlantic coast

Patrick A. Pitts* Harbor Branch Oceanographic Institution, 5600 North U.S. I, Ft. Pierce. FL 34946. USA Received 2 April 1998; received in revised form 21 September 1998; accepted 5 October 1998

Abstract

The abundance and vertical distribution of various fish and crustacean larvae in response to seasonal upwelling at a study site off the east central Florida coast were examined. Upwelling was recorded at the site from mid July to late August 1986 during which the intruded water was confined to near-bottom layers with a well-defined thermocline present near mid-depth. Near­ bottom temperatures of 20-23°C were generally 3-6°C below temperatures recorded near the surface during this time. Densities of larval fish, penaeid shrimp, stomatopods, brachyuran zoeae, brachyuran megalopae and porcellanid zoeae were quantified at near-surface and near-bottom levels prior to, during and following the upwelling event. Densities of brachyuran zoeae and penaeid larvae dominated the other four groups in both levels of the water column throughout the l4-weck study. Only brachyuran megalopae and penaeid larvae occurred in significantly higher numbers in upwelled water. All six larval groups were found in higher densities near the bottom through most of the study period. Combining these results with findings from related physical studies suggests that upwelling may play an important role in transporting these and other larval forms shoreward in this region. © 1999 Elsevier Science B.Y. All rights reserved.

Keywords: Florida's Atlantic coast; Meroplankton; Vertical distribution; Upwelling

1. Introduction

Seasonal upwelling on the southeastern United States continental shelf has been observed and reported repeatedly over the past five decades (Green, 1944; Blanton, 1971; Atkinson, 1977; Lee et aI., 1981; Smith, 1981, 1983; Pitts and Smith, 1997). During summer months the intrusion of cold, deep, nutrient-rich Gulf Stream water onto

·Tel.: + 1-561 4652400; fax: + 1-561-4680757; e-mail: [email protected]

0022-0981/99/$ - see front matter © 1999 Elsevier Science B.Y. All rights reserved. PH: 50022-0981(98)00179-8 136 P.A. Pitts / J. Exp. Mar. BioI. Ecol. 235 (1999) 135-146 the shelf displaces water of normal seasonal temperature and causes highly productive but short-lived phytoplankton blooms (Atkinson et aI., 1978; Bishop et aI., 1980; Yoder et aI., 1983). Many micro-zooplankton populations, in tum, respond positively in terms of abundance to the increased food supply. Paffenhofer (1980), (1983) has reported significant increases in abundances of most holoplankton taxa, including ostracods, small calanoid and cyclopoid copepods, cladocerans and cephalochordates, in the particle-rich layers of upwelled water on the northeastern Florida shelf. While the response of holoplankton to seasonal upwelling in this region has been well-documented, the influence of upwelling on the distribution and abundance of meroplanktonic, specifically the fish and crustacean larvae, has been scarcely studied. Blanton et aI. (1995) reported major settling events of blue crab (Callinectes sapidus) megalopae occurring in Charleston Harbor in response to downwelling, upwelling and calm winds. A study by Wenner et aI. (1995) suggests that peak densities of postlarval penaeid shrimp and blue crab megalopae coincide with downwelling conditions off the coast of South Carolina, and the transport of these larvae may be affected by upwelling events. Information on the influence of summer upwelling on these and other larval forms along Florida's coast is unavailable. The purpose of the study presented here is to investigate the variations in abundance of planktonic decapod crustacean and fish larvae before, during and after a 6-week upwelling event recorded off the central Florida Atlantic coast during the summer of 1986. Results were used to test the hypothesis that abundances of these plankton are significantly higher in the cold intruded water than in the warm water characteristically found over the inner shelf in the absence of upwelling during this time of year. The rationale of the hypothesis stems from the idea that higher concentrations of these larvae will be found in the intruded water due to their entrainment from deeper offshore sites, a response to increased food supply in the upwelled water or some other beneficial physical or biological parameter(s). Densities were determined in near-surface and near-bottom layers since the intruded cold water is often confined to near-bottom levels. Using this approach, information about the vertical distribution of these larval animals during summer months is provided.

2. Data and methods

The study site was located 11 km east-southeast of the Fort Pierce Inlet, Florida (27° 26.5' N, 80° 11.5' W) (Fig. 1), where water depth was approximately 15 m. Fourteen weekly hydrographic readings and plankton samples were taken from 10 June through 9 September 1986. Temperature profiles were recorded at two meter intervals using a thermistor incorporated into a Beckman model RS3-5 portable salinometer. For purposes of this study, conditions will be considered to be in an upwelled state when water temperatures are at or below 23.7°C (Table 1). Plankton samples were collected using a cone-shaped nylon plankton net with a mouth diameter of 1 m and mesh size of 335 J.1m. Three replicate, 5-min tows were taken near the bottom and three near the surface each week. Often local upwelling intrusions are confined to the bottom few meters of the water column so the near-bottom towing would assure sampling upwelled P,A, Pitts I J. Exp, Mar. Bioi. Ecol. 235 (1999) 135-146 137 lye jer ms ted lall ich

:en of ~d. Atlantic Ocean 'IS) nd tal he ng tal ce ek Study•Site of re ly he ae a al o 5 Id I kilometers s. Is

la 1. :e Is Ir Fig. I. Map of the central Florida Atlantic coast showing the study site where plankton and hydrographic data n were collected. d of water, if present. Depths of the plankton tows were determined using an Oceanics scuba e depth gauge strapped to the net ring. Near-bottom sampling depths ranged from 11.5 to v 14 m, and were always located below existing thermoclines. Near-surface sampling was j conducted with the top of the net ring just below the air-water interface. Although 138 P.A. Pitts I J. Exp. Mar. BioI. Ecol. 235 (1999) 135-146

Table I Summary of water temperatures (OC) for each weekly sample obtained during the study period (10 June-9 September 1986) Week Date Near-surface Near-bottom Upwelling 0' June 10 28.3 23.9 No I June 16 27.1 23.9 No 2 June 24 27.6 26.1 No 3 July 2 27.0 24.3 No 4 July 8 27.1 26.7 No 5 July IS 28.2 22.5 Yes 6 July 22 26.7 19.7 Yes 7 July 29 25.5 23.7 Yes 8 August 5 25.2 19.8 Yes 9 August 13 23.7 19.9 Yes 10 August 19 24.4 21.4 Yes II August 26 28.1 23.9 No 12' September 2 27.6 26.5 No 13 September 9 28.8 28.6 No Near-bottom temperatures of 23.7°C or less indicate upwelling conditions. 'No plankton were collected during these weeks. physical parameters were measured on 10 June and 2 September, plankton was not collected on these dates. Filtering efficiency (percent) was calculated by comparing the length of water 'accepted' at the mouth of the net with the length of water 'presented' to the net according to Smith et al. (1968). These measurements were made using two General Oceanics flow meters; one located off center inside the net ring and the other several centimeters to the outside of the net ring. Sample volumes ranged from 30 to 199 m 3 with filtering efficiency ranging between 17 and 100% (mean = 63%). Net clogging due to abundant gelatinous zooplankton occurred during much of the study period, particularly during upwelling. Also, filtering efficiency values should be treated with caution as the flow meter impellors sometimes did not rotate freely. Plankton samples were fixed in 3-5% buffered formalin (pH 7-8) in seawater for a minimum of 24 h. They were then washed in fresh water, concentrated and placed in 500 ml of 80% buffered ethyl alcohol (pH 7-8). Samples were agitated to create a homogenous mixture and subsamples were then taken using several techniques: (I) larger subsamples (1/4 of the total) were carefully poured from the total sample just after agitation, (2) smaller subsamples (11100-111000 of the total) were separated using a Stemple pipette. Subsamples were counted using a modified Bogorov counting tray (Wickstead, 1965) and a microscope. All macrozooplartkton included in the study belonged to six easily identifiable groups which included larval forms of fish, penaeid shrimp, stomatopods, brachyuran crab zoeae, porcellanid zoeae (a family of the Suborder Anomura), and brachyuran crab megalopae. General trends and patterns were of primary concern in this study so broad taxonomic groups were identified with little attention paid to lower taxonomic levels or various larval stages. Densities of each group (individuals m -3) for each plankton tow were extrapolated using subsample counts and volume measurements. P.A. Pitts I J. Exp. Mar. Bioi. Ecol. 235 (1999) 135-146 139

Two-way multivariate analysis of variance (MANOVA) models were employed to determine if significant differences in larval abundance occurred between conditions (upwelling vs. non-upwelling) and position in the water column (near-surface vs. near-bottom) (Sokal and Rohlf, 1981). SYSTAT software (Leland, 1992) was used and all statistical assumptions were met.

3. Results

The weekly hydrographic data indicate a homogenous water column at the beginning of the study period with temperatures near 28°C (Fig. 2 and Table 1). Two moderate cooling periods were recorded in mid June and early July when near-bottom tempera­ tures decrease to approximately 24°C. A dramatic decrease in near-bottom temperatures occurred at the beginning of the third week of July, signaling the onset of a 6-week upwelling event. Near-bottom temperatures remained below 24°C in the lower 5 m of the water column through most of August and temperatures were below the 23.7°C upwelling threshold for most of that time. A minimum of just under 20°C was recorded near the bottom on 22 July and 13 August. By comparison, near-surface temperatures were generally 3-7°C warmer during this 6-week time period. A minimum surface temperature of 23.7°C was recorded on 13 August. The plot shows a distinct thermocline about 5 m above the bottom during the first 2 weeks of the upwelling event. The thermocline broke down somewhat at the end of July, then became re-established higher in the water column for the last 2-3 weeks of the event. Water temperatures remained above 24°C from late August to mid September. By the end of the study temperatures had returned to values normal for the late summer season for this region and the thermocline had virtually disappeared.

28 ~z, ~8, o , 27 2f3 I ?4, ,,,?I;l 2{3 ,t' \, " I \ I I \/ : \ 1\/ \ , : /I \ I 1\1 "\ IIII I /"', \ 1\ ('I :' : \ / \! ,/25" "'- \\\ E ',', /' " \\ ,: I J -j~,," _ _ :::..... \\" 5 I\ I // I III \' ..... ,', \\ " ---- I \ I \ \ J I ~ / I I" \" \\ \ " " I j I I \, ,i /;~";""', ",' I/ " /11 \\\\\\ '\ 'f \ \, ../ /' / / II \\ ,,\ " I ,\' I,': \\ \. //1 J / 1/11 \\\\\\ I ~ /~I I / \I " \ \ ...... / ..",',,I //\\\\\ , , o 10 ",',- ...... , " ... ) ,/: : \ /-:~~-:3-~1"f------/"1:/ //-, \, \ \ /" \ :: : I I~/~:':""" /(\\\, //:?'/ \ \ \ \ ", ' ,/ / ,.-\ \ !: 1 ~ ,'/,~I /,' \\ \ I I II ,\I\ ,II' ,1 / ,',' \\ :: : I If{t /: 1\ \"::l \ \ \ \ ,I I I r ,\, r II f 1,1 I 1'-': \ \ ,',',", I ',I " : I 15 27' '24 '26 ' 25 ' 26'" 2'2 '2d' '2'3 2023 27 I I

I I , , , It' I I I , I 10 16 24 2 8 15 22 29 5 13 19 26 2 9 JUNE JULY AUG SEPT Fig. 2. Temperature profiles recorded weekly at the study site from 10 June to 9 September 1986. Temperatures were recorded at 2-m intervals from the surface to a bottom depth of 15 m. The bracketed region indicates period of upwelling. 140 P.A. Pitts I J. Exp. Mar. Bioi. Ecol. 235 (1999) 135-146

Brachyuran zoeae and penaeid shrimp larvae dominated the other groups at both levels of the water column during the entire 14-week study (Fig. 3). At times, the densities of these two meroplankton groups were 1-2 orders of magnitude greater than the other four. Brachyuran zoeae abundance was relatively high near the bottom during the upwelling period, yet reached a maximum during sample week 11, after the upwelling event had ended. Penaeid abundance was noticeably higher near the bottom during most of the upwelling period and peaked during week 6 at 346 larvae m-3. Their numbers declined suddenly during week 10 and remained relatively low for the remainder of the study. Like the penaeids, brachyuran megalopae were found in highest

300 Brachyuran 400 Penaeids zoeae 300 200 200 100 ...... 100 <:' E O...j.....C~c4"-l.pu..jl§l.L:ji!l...j:II..CIlJl-ClllL.qlU.p.....,..--41l en 0 1 2 3 4 5 6 7 8 9 1011 12 13 1 2 3 4 5 6 7 8 910111213 ClS ':l "0 10 Brachyuran 8 Fish :~ megalopae "0 8 c 6 --- 6 4 4 2

1 2 3 4 5 6 7 8 9 10111213 1 2 3 4 5 6 7 8 9 10111213

12 Porcellanid 2 Stomatopods zoeae

8

4

o+--FL,...... 4'i....p.-jlL::jlLIll-Q!L.,lli....jml~,...jiJ o J Ir I~ iJ o 1 2 3 4 5 6 7 8 9 10111213 o 1 2 3 4 5 6 7 8 9 10111213 Week

Fig. 3. Densities of larval groups (mean::!:S.E., n = 3) near the surface (white bars) and near the bottom (shaded bars) during weekly sampling periods at the study site from 16 June to 9 September 1986. Week I corresponds in time with the 16 June sampling period. No samples were collected during week 12 (2 September). Underlined numbers indicate the upwelling period. Note that the y-axis scale varies between plots. P.A. Pitts I J. Exp. Mar. Bioi. Ecol. 235 (1999) 135-146 141 densities near the bottom during the weeks when upwelled water was present. They reached a peak abundance of about 10 animals m -3 during the seventh week of sampling. Densities of porcellanid zoeae were sporadic throughout the study period. They reached a maximum of about 10 zoeae m - 3 near the bottom during week 11. Like the porcellanids, the density of larval fish showed no apparent pattern with respect to water temperature. They were found in highest numbers near the bottom during week 6 of the study (7 larvae m -3). Unlike the other five groups, qsh larvae were found in similar densities at both levels in the water column throughout the study period. Stomatopods were the least abundant of the larval groups examined. They reached a peak of only 2 animals m -3 during the sixth week and their numbers were noticeably higher during the first 7 weeks of the study. It is noteworthy that the greatest densities of most larval groups coincided with the period of coldest water - week 6 of the plankton sampling. During this time in the near-bottom sampling layer fish larvae tripled in numbers from the previous sampling week, brachyuran and porcellanid zoeae exhibited a 5-fold increase and penaeid larvae increased by an order of magnitude (Fig. 3). Abundances of all four groups exhibited distinct declines the following week, coinciding with a generalized warming of the water column. Densities of brachyuran zoeae and penaeid larvae decreased by one half and two-thirds, respectively, while the numbers of fish larvae and porcellanid zoeae decreased by an order of magnitude during this time. Larval densities rebounded somewhat with the return of cold water during week 8, but not to the levels observed during week 6. All larval groups were significantly more abundant in near-bottom waters (Fig. 4 and Table 2). Densities of brachyuran megalopae, penaeid larvae, porcellanid zoeae and stomatopod larvae are approximately an order of magnitude higher in the near-bottom layer, regardless of the upwelling condition. Near-bottom densities of fish and brach­ yuran larvae are 1.5-3 times greater than those observed near the surface during both upwelling and normal conditions. Only brachyuran megalopae and penaeid larvae are significantly more abundant in upwelled water (Table 2 and Fig. 4). Penaeids exhibit significant increases in abundance in both near-surface and near-bottom water during upwelling, while brachyuran megalopae show a significant increase only in the near-bottom layers.

4. Discussion

The physical nature of the upwelling observed in this study is characteristic of events described in earlier studies from this region's inner continental shelf. Smith (1981), (1983) has reported that upwelling events off Florida's central Atlantic coast typically last on the order of several weeks and are spatially variable; that is, they can cover a wide geographic area or be relatively 'patchy'. The mechanisms driving upwelling in this area have been examined by a number of investigators. Pitts and Smith (1997) have shown that upwelling off the east central Florida coast can be wind driven. The studies of Smith (1981), (1983) support the hypothesis that a meandering of the Florida Current advects cold, deep water onto Florida's continental shelf. Still another hypothesis 142 P.A. Pitts I J. Exp. Mar. Bioi. Ecol. 235 (1999) 135-146

B C 80 200

60 150

40 100

20 50 '?-- A E 0 0 en ro B :::J C 1:J 6 3 .:; Brachyuran megalopae 1:J C 4 2 >. :<=: en c 2 Q) 1:J A ro 0 0 ....> ro B B c 3 1.0 ro Q) ~ 2 0.5

0.0'--'----"-- Normal Upwelling Normal Upwelling

Fig. 4. Near-surface (white bars) and near-bottom (shaded bars) larval density (mean±S.E., n = 5) during upwelling and normal conditions. Within plots, treatments sharing a symbol (A, B or C) are not significantly different (MANOVA) (Table 2). Note that the y-axis scale varies between plots.

Table 2 Results of the two-way multivariate analysis of variance of the densities of each larval group relative to depth and upwelling state

Depth Upwelling Depth X Upwelling Brachyuran zoeae 4.43* 1.90NS 0.62 NS Penaeids 16.59** 15.87** 10.96** Brachyuran megalopae 49.16** 23.79** 22.05** Fish 5.11 * 0.04NS 0.02 NS Porcellanid zoeae 13.80** 0.06NS 0.07 NS Stomatopods 36.13** 0.67 NS 0.24NS

F values only are provided (* P < 0.05; ** P < 0.01; NS P > 0.05; df = 1.68). P.A. Pitts I J. Exp. Mar. Bioi. Ecol. 235 (1999) 135-146 143

explaining upwelling involves Florida Current and Gulf Stream frontal spin-off eddies which occur sporadically every several days to 2 weeks from Miami to Cape Hatteras (Lee and Mayer, 1977; Vukovich et aI., 1979; Lee et aI., 1981). Regardless of the cause, upwelling in this geographic area results in nutrient-rich intruded water stimulating phytoplankton productivity, followed by a succession of holoplankton blooms (Atkinson et aI., 1978; Bishop et aI., 1980; Yoder et aI., 1983; PaffenhOfer, 1980, 1983). Of particular interest in this study, however, is whether the abundance and/or distribution of planktonic fish and crustacean larvae are affected by upwelling. Results indicate that of the six groups studied here, only penaeid shrimp larvae and brachyuran megalopae exhibit a significant change in abundance in response to upwelling. The relatively high numbers of penaeid larvae and brachyuran megalopae observed in the intruded water mass may be due to the entrainment of these animals from deeper offshore locations. One of the mechanisms by which fish and invertebrate larvae are moved perpendicular to the coast from the open ocean to nearshore waters where settlement occurs is upwelling. Coastal upwelling areas are generally located on the eastern boundaries of oceans where the equator-ward trade winds induce a quasi-steady offshore Ekman transport. Upwelling-induced larval recruitment in these areas is well documented (Hamann et aI., 1981; Roughgarden et aI., 1988; Farrell et aI., 1991; Roughgarden et aI., 1991). Although the upwelling that occurs on the central Atlantic coast of Florida is seasonal and ephemeral, it may be an important mechanism for transporting larvae to the inner continental shelf and adjacent estuaries where settlement occurs. An alternative explanation for the increased abundances of the two groups in the upwelled water lies in the swimming ability of these animals and the availability of food in the intruded water mass. PaffenhOfer (1980) reported an increase in the abundance of cyclopoid copepods and other microzooplankton as a response to increased food supply (phytoplankton) in near-bottom intrusions in Onslow Bay, North Carolina. Microzoop­ lankton blooms, in tum, represent an increased abundance of prey items available to crustacean larvae. Most crustacean larvae are relatively mobile and often exhibit vertical g migration patterns whose adaptational significance has been linked to predator avoid­ y ance, regulation of metabolism and other beneficial physical and/or biological parame­ ter(s) (Zaret and Suffern, 1976; Cronin and Forward, 1986). Although the near-surface samples showed relatively few penaeid larvae and brachyuran megalopae prior to or during upwelling, these animals could have inhabited unsampled midwater levels during h this time and migrated into the food-rich water upon its arrival. Alternately, the animals could have resided in large numbers very close to the bottom - below the sampling level - prior to upwelling. Although abundances of fish larvae and brachyuran zoeae did not change significantly during upwelling, it is possible that the species composition of these relatively broad groups varied over the course of the study. Shenker (1988) reported a rapid change in the composition of ichthyofauna following the onset of upwelling off the coast of Oregon. Results from his study showed that some fish species that were found abundant prior to upwelling disappeared following the onset of upwelling, presumably due to settlement of these fishes in demersal habitats. While classification of fish and zoea 144 P.A. Pitts I J. Exp. Mar. Bioi. Eco!. 235 (1999) 135-146 groups to the species level was beyond the scope of this paper, an important follow-up study would examine the change in species composition relative to upwelling. Our use of the 23.7°C as the threshold value for upwelling was not arbitrary; it allowed the temperature data to be interpreted as just one upwelling event, as opposed to two or more events. This is consistent with the primary objective of the study, to examine the general trends and patterns of larval distribution in response to upwelling, as indicated by the relatively low-frequency (weekly) sampling rates and use of broad taxonomic groups. By using a threshold value of, for example, 23.0°C the study would have described two discreet upwelling events and week 8 of the study period would then fall into the non-upwelling treatment group. A close examination of Fig. 3, however, reveals that five of the six larval groups showed no dramatic changes in abundance during week 8. Also, the odd group out, brachyuran megalopae, displayed a marked decrease in abundance near the bottom during week 8, when compared to the preceding and following weeks. This group showed significant increases in abundance in the near-bottom layers during upwelling, so removal of the week 8 sample from the upwelling treatment group would only increase that significance. Regardless of how upwelling in this region might affect the abundance or distribution of fish and crustacean larvae, the process can certainly provide an important onshore transport mechanism for whatever larvae are residing in the near-bottom layers. Calculations from an earlier upwelling study (Pitts and Smith, 1997) indicate that upwelled water moves shoreward across Florida's central Atlantic continental shelf at a rate of 7-9 cm S-I (6-8 km day-I). Using the 75 m isobath as the beginning of the continental slope, the width of the shelf at the study site is 30 km, and it follows that larvae could be transported from the outer shelf to nearshore waters in 4-5 days by the upwelling process. The effectiveness of upwelling in larval transport, however, depends on the position of larvae in the water column, a position that is governed mainly by turbulent mixing and larval behavior, i.e. vertical migration (Blanton et aI., 1995). Unfortunately, little is known about the vertical migration patterns of these larval forms or how they become distributed through the water column due to mixing. Results from this study indicate a significantly higher concentration of larvae in near-bottom layers, at least during the daylight hours when sampling occurred.

5. Conclusions

Of the six meroplankton groups studied here, brachyuran zoeae and penaeid larvae were much more abundant than the other four groups in both levels of the water column throughout the 14-week study. At times, densities of brachyuran zoeae and penaeids were 1-2 orders of magnitude greater than the other meroplankton examined. All six larval groups were found in higher densities near the bottom through most of the study period. While only brachyuran megalopae and penaeid larvae occurred in significantly higher densities throughout the 6-week upwelling event, all six groups exhibited a dramatic increase in abundance with the arrival of the coldest intruded water. These findings, P.A. Pitts I J. Exp. Mar. Bioi. Ecol. 235 (1999) 135-146 145 p together with results from earlier related studies, suggest that upwelling in this region may play an important role in advecting larvae to the inner shelf and adjacent estuaries where settlement can occur.

)

J Acknowledgements j i Paul Mikkelsen and David Inglin are acknowledged for their help both in the field and with sorting and counting plankton samples. Special thanks to Drs Adele Pile and Joseph Dineen for statistical assistance and helpful suggestions for the manuscript. This is Harbor Branch Oceanographic Institution Contribution No. 1259. 1

References

Atkinson, L.P., 1977. Modes of Gulf Stream intrusion into the South Atlantic Bight shelf waters. Geophys. Res. Lett. 4, 583-586. Atkinson, L.P., Paffenhiifer, G.-A., Dunstan, W.M., 1978. The chemical and biological effect of a Gulf Stream intrusion off St. Augustine, Florida. Bull. Mar. Sci. 28, 667-679. Bishop, S.S., Yoder, J.A., Paffenhiifer, G.-A., 1980. Phytoplankton and nutrient variability along a cross-shelf transect off Savannah, Georgia, USA. Estuar. Coast. Mar. Sci. 11, 359-368. Blanton, J.O., 1971. Exchanges of Gulf Stream water with North Carolina shelf water in Onslow Bay during stratified conditions. Deep-Sea Res. 18, 167-178. Blanton, J.O., Wenner, E., Werner, F., Knott, D., 1995. Effects of wind-generated coastal currents on the transport of blue crab megalopae on a shallow continental shelf. Bull. Mar. Sci. 57 (3), 739-752. Cronin, TW., Forward, R.B., 1986. Vertical migration in crab larvae and their role in larval dispersal. Bull. Mar. Sci. 39 (2), 192-201. Farrell, T.M., Bracher, D., Roughgarden, J., 1991. Cross-shelf transport causes recruitment to intertidal populations in Central California. Limnol. Oceanogr. 36, 279-288. Green, C., 1944. Summer upwelling-northeast coast of Florida. Science 100, 546-547. Hamann, I., John, H.-Ch., Middelstaedt, E., 1981. Hydrography and its effect on fish larvae in the Mauritanian upwelling area. Deep-Sea Res. 28A (6), 561-575. Lee, T.N., Mayer, D.A., 1977. Low-frequency current variability and spin-off eddies along the shelf off Southeast Florida. J. Mar. Res. 35, 193-220. Lee, T.N., Atkinson, L.P., Legeckis, R., 1981. Observations of a Gulf Stream frontal eddy on the Georgia continental shelf, April 1977. Deep-Sea Res. 28, 347-378. Leland, w., 1992. SYSTAT for Windows: Statistics, verso 5 ed. SYSTAT, Inc., Evanston, IL, pp. 750. Paffenhiifer, G.-A., 1980. Zooplankton distribution as related to summer hydrographic conditions in Onslow Bay, North Carolina. Bull. Mar. Sci. 30 (4), 819-832. Paffenhiifer, G.-A., 1983. Vertical zooplankton distribution on the northeastern Florida Shelf and its relation to temperature and food abundance. J. Plankton Res. 5 (I), 15-33. Pitts, P.A., Smith, N.P., 1997. An investigation of summer upwelling across central Florida's Atlantic coast: the case for wind stress forcing. J. Coast. Res. 13 (I), 105-110. Roughgarden, J., Gaines, S., Possingham, H., 1988. Recruitment dynamics in complex life cycles. Science 241, 1460-1466. Roughgarden, J., Pennington, J.T., Stoner, D., Alexander, S., Miller, K., 1991. Collisions of upwelling fronts with the intertidal zone: the cause of recruitment pulses in barnacle populations of central California. Acta Oecol. 12, 35-51. Shenker, J.M., 1988. Oceanographic associations of neustonic larval and juvenile fishes and dungeness crab megalopae off Oregon. Fish. Bull. 86 (2), 299-317.