Phytoplankton dynamics within Gulf Stream intrusions on the southeastern United States continental shelf during summer 1981

Continental Shelf Research, Vol. 4, No. 6, pp. 611 to 635, 1985. Printed in Great Britain.

0278~,343/85 $3.00 + 0.00 ~ 1985 Pergamon Press Ltd.

Phytoplankton dynamies within Gulf Stream intrusions on the southeastern United States continental shelf during summer 1981 JAMES A. YODER,* LARRY P. ATKINSON,* S. STEPHEN BISHOP,* JACKSON O. BLANTON,* THOMAS N. LEEt and LEONARDJ. PIETRAFESA~

(Received 11 June 1984; in revisedform 13 November 1984; accepted 15 November 1984) Abstract--During July and August 1981 subsurface intrusion of upwelled nutrient-rich Gulf Stream water was the dominant process affecting temporal and spatial changes in phytoplankton biomass and productivity of the southeastern United States continental shelf between 29 and 32°N latitude. Intruded waters in the study area covered as much as 10' km including virtually all of the middle and outer shelf and approximately 50% of the inner shelf area. Within 2 weeks followinga large intrusion event in late July, middle shelf primary production and Chl a reached 3 to 4 gC m- d - and 75 mg m - , respectively.At the peak of the bloom 80% of the water column primary production occurred below the surface mixed-layer, and new primary production (i.e., NO3-supported) exceeded 90% of the total. Chl a-normalized photosynthetic rates were very high as evidenced by high mean assimilation number (15.5 mg C mg Chl a -' h-'), high mean ct (14mg C mg Chl a - Ein-' m), and no photoinhibition. As a result of the high photosynthetic rates, mean light-utilization index (tF) was 2 to 3 times higher than reported for temperature sub-arctic and arctic waters. The results imply a seasonal(June to August) middle shelf production of 150 g C m - , about 15% higher than previous estimates of annual production on the middle shelf. Intrusions of the scale we observed in 1981 may not occur every summer. However, when such events do occur, they are by far the most important processes controlling summer phytoplankton dynamics of the middle and outer shelf and of the inner shelf in the southern half of the study area.

INTRODUCTION RECENT studies of the southeastern United States continental shelf show that offshore sources of inorganic plant nutrients are important in controlling the productivity of the planktonbased food web. Resident shelf waters are normally depleted of inorganic nitrogen and other nutrients. Thus, plankton productivity is strongly affected by upwelling and onshore movement of essential nutrients caused by eddies and other disturbances of the Gulf Stream front (LEE et al., 1981; LEE and ATKINSON, 1983). These upwelling events are the major process affecting rates and dynamics of outer shelf (40 to 200 m isobaths, Fig. 1) primary production (DUNSTAN and ATKINSON, 1976; ATKINSON et al., 1978; BISHOP et aL, 1980; YODER et aL, 1981a, 1983; POMEROY et aL, 1983; YODER, 1985). G u l f Stream-induced upwelling also occurs during summer and the effects are felt shoreward of the 40 m isobath. In response to northerly wind stress, cold (< 2 5 ° C ) upwelled

* Skidaway Institute of Oceanography, P.O. Box 13687, Savannah, GA 31416, U.S.A. t RSMAS-Universityof Miami, 4600 Rickenbacker Causeway, Miami, F133149, U.S.A. :~ North Carolina State University, Department of Marine Science and Engineering, P.O. Box 5923, Raleigh, NC 27650, U.S.A. 611

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waters intrude across the shelf beneath warmer shelf waters (ATKINS.ON, 1977). These subsurface intrusions advect plant nutrients and plankton biomass to at least the 30 m isobath off North Carolina, South Carolina, and Georgia and to the shore off north Florida. Nitrate flux associated with onshore movement of summer intrusions is the dominant source of 'new' nitrogen for the southeastern United States shelf during summer (DUNSTAN and ATKINSON, 1976; HOFMANN et al., 1981; ATKINSON et aL, 1982; ATKINSON et al., 1984). Individual summer intrusion events advect ca. 0.3 to 1.8 x 10 4 metric tons of NO3-N onto the northeastern Florida and Georgia shelves (O'MALLEY, 1981; ATKINSON et al., 1982; ATKINSON, unpublished data). The frequency and extent of cross-shelf penetration of summerl subsurface intrusions is controlled by interactions between Gulf Stream and wind forcing, density of shelf waters, and bottom topography (ATKINSON, 1977; BLANTON et al., 1981; ATKINSON et al., 1982; JANOWITZ and PIETRAFESA, 1982). Upwelling at the shelf break is forced by the Gulf Stream, but northerly wind stess is required to move upwelled waters across the shelf (ATKINSON, 1977; HOFMANN et al., 1980; HOFMANNet al., 1981; BLANTONet aL, 1981; ATKINSONet al., 1984). Thus, intrusions are not present at all times during the summer and when present, cold water is not uniformly distributed along and across the shelf. Instead, summer intrusions are mesoscale features that occur episodically with higher frequency off northeastern Florida and in the cape regions of North Carolina than off the coasts of Georgia and South Carolina (Fig. 1) (ATKINSON et al., 1978; PAFFENHOFERet al., 1980; BLANTONet al., 1981; YODER et al., 1983; ATKINSONet al., 1983; ATKINSONet al., 1984).

Phytoplankton dynamics within Gulf Stream intrusions

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The results discussed below were obtained during an interdisciplinary study (GABEX 2) of the southeastern shelf during summer 1981. The objective of GABEX 2 was to evaluate the importance of summer intrusions on shelf-wide hydrographic and circulation patterns and on the distribution and productivity of plankton. One subobjective was to obtain a long time series of observations from a single region of the shelf to describe changes in hydrographic and plankton-related variables as intruded waters moved onto the shelf. Most of the results presented were obtained along a transect extending from near the coast (ca. 15 m isobath) to the outer edge of the shelf (ca. 200 m isobath) at 30°N (transect 4, Fig. 1). The purpose of this report is, first, to describe the effect of summer intrusions on phytoplankton dynamics of the southeastern shelf. Secondly, we evaluate the contribution of summer intrusions to annual primary production of the southeastern shelf and compare the summer results (i.e., stratified shelf) with those obtained during colder months (unstratified shelf). Detailed analyses of circulation, hydrography, and zooplankton dynamics are presented elsewhere. METHODS

To determine circulation current meters were continuously deployed from June to September 1981. In addition, there were three shipboard sampling programs. Preliminary surveys were conducted during June using R.V. Blue Fin. R.V. Henlopen and R.V. Cape Florida were used simultaneously during July and August. The results presented here were obtained from samples collected from the R.V. Henlopen. The sampling program was divided into 7 legs, each leg consisting of ca. 5 sampling days followed by 2 da),s in port. During each leg, transect 4 was sampled first. This took ca. 10 h. During the next 3 days, six additional crossshelf transects were completed: three between 30 and 32°N (transects 1 to 3) and three between 29 and 30°N (transects 5 to 7, Fig. 1). Just before returning to port, transect 4 was sampled a second time. A CTD was used to acquire continuous vertical traces of temperature and salinity (CHANDLER et al., 1978). Water samples from discrete depths were collected with a rosette sampler equipped with 5 and 101 Niskin bottles. At stations along the transect at 30°N, discrete water samples were collected at 3 to 7 depths depending on the depth of the water column. At all other transect stations, only near-bottom and near-surface water samples were collected. Immediately after collection, the water samples were passed through 183 lam mesh Nitex netting to remove large zooplankton. To determine the relative importance of the < 10 lam size fraction, sub-samples were gently passed through 10 ~tm mesh Nitex netting. Chlorophyll a (Chl a) and pheopigment a (Pheo a) were determined using the fluorometric method of YENTSCH and MENZEL (1963) as described by STRICKLAND and PARSONS (1972). A Perkin-Elmer elemental analyzer was used to measure the C and N content of particulate matter collected on precombusted glass-fiber filters (Gelman A/E). Nutrient concentrations were determined with a Technicon Autoanalyzer following procedures described by GLmERT and LODER (1977). NO 3 was not determined separately from N O 2. Their sum is reported as NO3. Incident irradiance (400 to 700 nm) was measured with a LI-COR quantum sensor, model LI-190S, equipped with a LI-COR integrator (model LI-550). Vertical profiles of subsurface irradiance (400 to 700 nm) were determined with a LI-COR spherical quantum sensor (4 ~ collector). Water samples for isotope uptake experiments were collected from 4 to 5 depths between

614

J.A. YODERet al.

the surface and the bottom, pre-screened through 183 ~tm mesh Nitex netting and dispensed into replicate 125 ml (14C) or l 1 (lSN) glass bottles. After spiking with isotope, the bottles were covered with 1 or more layers of plastic screen (to attenuate irradiance) and then placed in 1 of 2 plexiglass water baths exposed to full sunlight on the deck of the ship. When primary production of the < 10 Bm size fraction was determined, the contents of a second set of duplicate bottles were gently passed through 10 ltm mesh Nitex netting at the end of the incubation, and the filtrate treated as described below. Seawater was pumped through one of the water baths to provide temperature control (ca. 28°C) for bottles containing surface mixed-layer samples. The second bath, cooled to ca. 20 to 22°C, was used to incubate samples collected from the colder intruded waters. Incident irradiance was measured during the incubation period. Daily rates of primary production were calculated from 3 to 6 h incubations taking into account the proportion of daily sunlight irradiance received during each incubation and after In'st subtracting 'dark bottle' from 'light bottle' uptake. Thus, the procedure should yield net light-period primary production but did not measure dark-period respiratory losses. To determine N O 3 uptake, 15NO 3 w a s added to yield a final enrichment of 0.2 BM. After incubation the contents of the bottles were filtered onto 47 mm, pre-combusted glass fiber filters (Gelman A/E). Isotope ratios were determined by Dr. Hans Paerl, University of North Carolina Marine Institute, using a Jasco emission spectrometer. The rate of N O 3 uptake was calculated using the formula given by YODERet al. (1983). The 14C-labelled sodium bicarbonate method first described by STEEMANNIELSEN (1952), was used to determine primary production. After incubation, the contents of each bottle (2 light and 1 dark from each depth) were filtered through Millipore HA filter disks (0.45 lam mean pore diameter). The filters were desiccated, fumed with HCI, and 14C activity determined with a Beckman liquid scintillation counter using New England Nuclear's 'Omnifluor' cocktail (98% PPO, 2% Bis-MSB). Quench was determined using internal standards. In addition to the uptake experiments described above, NO 3 vs NH4 preference experiments were also conducted. For these experiments, 15N-labelled NH4 and NO 3 were added (0.2 IxM enrichment) to separate sets of duplicate bottles containing water collected from a single depth. Experimental and sample processing procedures were as described above, except the bottles were incubated at constant light (100 ~tEin m -2 s -l, fluorescent bulbs) for 1 h in a small laboratory incubator maintained at the in situ temperature from which the sample was collected. Irradiance measurements are expressed in many different units which make inter-study comparisons of photosynthetic rates very difficult. For our comparisons, we used the following conversions: 1 ly h -j = 11.6 W m -2 (STRICKLAND, 1958) and for 'sun and sky' irradiance (400 to 700 nm), 1 W m -2 is ca. 0.016 Ein m -2 h -I (empirical relation; YODER, unpublished data). For interstudy comparisons of a (initial slope of the photosynthesis vs irradiance curve), we cancelled units of time [e.g., 1 m g C mgChl a -1 h -l (gEinm -2 s-l) -1 = 277.8 mg C mg Chl a -I Ein-l m2]. When expressed in this manner, the units clearly reflect the two components of a: (1) photosynthetic efficiency (mgC Ein -1 abs) and (2) mean spectral extinction coefficient of the algae (m 2 mg Chl a -~ ) (BANNISTER, 1979). Current meter results presented here are based on 6-hourly averaged currents after smoothing with a 40-h low pass (HLP) filter to remove the tidal signature (LEE et aI., 1981). The current meter coordinate system is parabathic. Thus, negative U represents cross-isobath and onshore flow. Current stability (S) given in Table 1 is the ratio of the averaged vectorial velocity to the averaged arithmetical velocity (NEOMANN, 1968). Values for current stability

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J.A. YODERet aL

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range from 0.0 to 1.0 with a value of 1.0 indicating both U and V current components did not change sign during the averaging period. A detailed analysis of current meter results during G A B E X 2 is presented elsewhere (LEE and PIETRAFESA,unpublished data). RESULTS

Physical and hydrographic setting Upwelling source water originates from below the Gulf Stream surface mixed-layer and is a mixture of water masses dominated by western North Atlantic water (LEE et al., 1981). Hereafter we will refer to this water as Gulf Stream Water (GSW) to distinguish it from resident shelf waters. When G S W < 21 °C is located below the euphotic zone off the shelf, both O2 and N O 3 concentration are linear functions of temperature (LEE et aL, 1981; ATKINSON et al., 1984). Figure 2 illustrates the O 2- and NOs-temperature relations in G S W during G A B E X 2 at offshore stations where the 21 °C isotherm was located below 50 m. The relations illustrated in Fig. 2 can be used to estimate the initial N O 3 and O2 concentrations of G S W found on the shelf from temperature measurements (ATKINSON et al., 1978; LEE et al., 1981) if one assumes: (1) water on the shelf colder than 21 °C is G S W and (2)temperature of intruded waters has not significantly changed during the period between upwelling and sampling. The first assumption is a proven fact for this area during summer months, and the temperature of the surface mixed-layer is ca. 28 °C (ATKINSON et aL, 1983). During G A B E X 2 the second assumption was probably valid for the middle and outer shelf. In these areas, the

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temperature of the surface mixed-layer (28°C) was unaffected by the presence of colder intruded waters indicating little vertical mixing between the intrusion and the overlying surface mixed-layer. However, assumption 2 is almost certainly invalid for the inner shelf during GABEX 2 (to be discussed). Throughout the text and figures we refer to temperaturederived estimates of initial 02 and N O 3 as 0 2 * and NO3*, respectively. The difference between temperature-derived estimates and actual measurements is referred to as delta NO 3 and delta O 2. Delta 02 and delta N O 3 are measures of the degree to which autotrophic processes have modified GSW between sampling time and the time that GSW first moved into the euphotic zone on the shelf. To illustrate the X , Z dimensions of intruded waters, temperature and Chl a vertical sections at 30°N on 30 July 1981 are presented in Fig. 3. Note that highest Chl a concentrations are within waters colder than 21 °C, which initially contained NO3 concentrations higher than ca. 0.5 I.tM (Fig. 2). Included in Fig. 1 is the position of the 21°C near-bottom isotherm during

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one quasisynoptic (5-day) survey of the shelf. This isotherm dermes the approximate X and Y dimensions of the intruded waters at the time of maximum penetration onto the shelf. Figure 1 shows the location of current meter moorings that describe the flow of intruded waters in areas sampled by the R.V. Henlopen. During GABEX 2, wind-forcing dominated sub tidal frequency currents on the middle shelf (20 to 40 m isobaths), whereas outer shelf (41 to 200 m isobaths) currents were forced by the Gulf Stream (LEE and PIETRAFESA, unpublished data). Mean mid-shelf flow had a strong barotropic component and was toward the north. During the period of ship operations, winds were generally northerly, and a significant fraction of the low-frequency flow variability at mid-shelf was a simple Ekman response to local wind forcing (LEE and PIETRAFESA,unpublished data). Low frequency current variability on the outer shelf was dominated by Gulf Stream disturbances such as cyclonic frontal eddies having a mean period of about 6 days during GABEX 2 (LEE and PIETRAFESA, unpublished data). Figure 4 illustrates the pronounced reversals in the nearbottom currents at the 75 m isobath that characterize the effects of these eddies. As discussed in the Introduction and illustrated in Fig. 3, nutrient-rich GSW upwells at the shelf break and moves across the shelf as subsurface intrusions during summer. Mean U and V current components from selected near-bottom current meters are given for seven consecutive episodes during the period 18 June to 19 August 1981 (Table 1). The episodes were chosen from an examination of current vector plots to discriminate periods of generally onshore from offshore near-bottom flow. The results identify three major episodes of

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sustained onshore flow (Table 1). These episodes are used to help interpret phytoplankton dynamics and are referred to as 'intrusion events' throughout the text. Intrusion event 1 ended on 27 June, approximately 2 weeks prior to shipboard sampling. Mean U was also negative (onshore) from 14 to 30 July (intrusion event 2, Table 1). Intrusion event 2 was divided into two sub-episodes (2A, 2B), since both the mean speed of the U component and current stability at the 30 and 40 m isobaths were higher during 2B than 2A (Table I). Intrusion event 3 occurred from 5 to 11 August. At stations shoreward of the 45 m isobath, the vertical distribution of solar irradiance (400 to 700 nm) generally showed an upper and lower layer, each characterized by a different attenuation coefficient (K, m-t). The upper layer with a K of 0.04 to 0.06 (.v = 0.05; S.D. = 0.0 l, n = 17), extended from the surface to a mean depth of 16 m. The lower layer extended from a mean depth of 16 m to within ca. 2 m of the bottom, the lowest depth sampled. Lower layer K values ranged from 0.09 to 0.20 with a mean of 0.14 (S.D. = 0.04, n = 17). The lower depth limit of the upper layer generally coincided with the upper depth limit of intruded GSW. Thus, the increase in K from the upper to the lower layer coincided with increasing concentrations of particles within subsurface intrusions. Based on the mean K of the upper layer, irradiance at the surface of the intruded waters averaged ca. 44% of incident. Taking into consideration the mean K of both layers, the calculated mean depth of the 1% isolume was 43 m which was deeper than the bottom at most locations sampled. Incident solar irradiance (400 to 700 nm) during the study ranged from 19 to 54 averaging 42 Ein m -2 d -l (ca. 220 ly d -l, 400 to 700 nm). Eulerian time series at the inner, middle and outer shelf at 30°N

In one important respect the response of the inner, middle, and outer shelf were similar, since in all regions highest water column biomass and productivity were observed within subsurface intrusions of GSW. However, in the following discussion we emphasize the results obtained on the middle shelf for several reasons. First of all, most phytoplankton rate measurements were obtained from middle shelf stations. Secondly, in comparison to the inner and outer shelf time series measurements obtained at a single location on the middle shelf are more likely to reflect time-dependent changes (as opposed to changes caused by advection) because (1) along-shelf gradients in Chl a and nutrients were less than those occurring on the outer and inner shelf, and (2) Gulf Stream forcing caused more current variability on the outer than on the middle shelf (previously discussed). Finally, evidence (to be discussed) suggests that vertical mixing between intruded waters and the overlying surface mixed-layer occurred to a greater extent on the inner than on the middle shelf. Extensive vertical exchange between intruded and mixed-layer waters invalidates our use of the temperature-derived estimates of initial intrusion content of O2 and NO3 (i.e., 02* and NOa*). Current meter results (Table 1) suggest that intruded waters could have been on the shelf for several weeks (i.e., from intrusion event 1) prior to the beginning of the middle shelf time series. Water column integrals (Fig. 5) for the period 12 to 20 July and vertical distributions on 12 July (Figs 6 and 7) show the following characteristics consistent with this hypothesis: (1) no NO 3 (Figs 5 and 6), (2) low Chl a (ca. 5 to 20 mg m -2, Fig. 5) which is vertically homogeneous (< 1.0 mg m -3 on 12 July, Fig. 7), (3) relatively low rates of primary production (ca. 0.5 g C m -2 d -l, Fig. 5) and NO 3 uptake (< 0.01 g NO3-N m -2 d -l, Fig. 5), and (4) super~saturation of 02 throughout the water column with delta 02 values of 0.5 ml 1-~ within the subsurface intrusion (Fig. 7). An explanation consistent with these observations is that phytoplankton 'bloomed' within the intrusion prior to 12 July, but were no longer present.

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However, the organic matter synthesized during the bloom was not oxidized in the intrusion, since intruded waters were super-saturated with 02 . Onshore flow of nutrient-rich waters to the middle shelf between 22 and 30 July (intrusion event 2B, Table 1) increased subsurface NO 3 and other plant nutrients on the middle shelf. On 23 July, water column NO3-N was c a . 1.0 g m -2 compared to < 0.1 g m -2, 3 days earlier (Fig. 5). Subsurface NO 3 concentrations increased from undetectable levels on 20 July to c a . 5.0 rtM on 23 July (Fig. 6). From 20 July, Chl a steadily increased and peaked at c a . 75 mg m -2 on 3 August (Fig. 5). Primary production and NO 3 uptake measurements were not made on 3 August, but on 30 July, production reached 3 g C m -2 d -1 and NOa-N uptake exceeded 0.5 g m -2 d -] . Near the peak of the bloom, most of the Chl a, primary production, and NO3 uptake occurred within the intrusion (Fig. 7). High rates of NO3-supported primary production (i.e., 'new production' i n s e n s u DUGDALE and GOERINC, 1967) increased the 0 2 level in intruded waters (compare delta O2 of 23 July with 30 July, Fig. 7). On 6 August, vertical distributions of hydrographic and biological variables resembled those observed on 12 July (Figs 6 and 7). NO 3 was depleted, 02 was at, or above, saturation within the intrusion, NO 3 uptake and primary production were relatively low and more or less vertically homogeneous, and silicic acid was < c a . 2.0 rtM within the intrusion. The major difference between the two sampling dates was that near-bottom Chl a was much higher on 6 August than on 12 July. The vertical distribution of Chl a on 6 August (Fig. 7) suggests that

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Fig. 6. Vertical distributions o f hydrographic and nutrient data at selected stations during the middle shelf time series at 3 0 ° N (transect 4, Fig. 1). Bottom depth (Z) and sampling date are given in the lower left o f each frame.

at least some of the phytoplankton biomass synthesized during the intrusion bloom(s) had settled to, or near, the bottom. Results from the middle shelf suggest that the phytoplankton bloom (biomass increase) and its collapse (biomass decrease) occurred during a 24-day period between 20 July and 13 August (Fig. 5). Considering our 3 to 4 day sampling frequency, the observed 13-day period of biomass increase is close to the 10-day estimate obtained from a previous drogue study (YoDER et al., 1983). Concentrations of particulate carbon (PC) and nitrogen (PN) changed by a factor of 3 to 4 during the middle shelf time series as did the composition of the particulate matter (Table 2). Near the peaks of phytoplankton blooms (30 July and 6 August, Table 2), PN:PC and PC:Chl a ratios within intruded waters were near 5.5 and 35, respectively, similar to those obtained for phytoplankton cultures in good physiological state ( G O L D M A N et al., 1979; GOLDMAN, 1980). In contrast, ratios in the mixed-layer were much higher, with PN:PC ranging from 6.3 to 9.2 and PC :Chl a near 200 (Table 2). 'Old' intruded waters on 12 July had very high PC :PN and PC :Chl a ratios of ca. 11 and 490, respectively. High values of

622

J . A . YODER et al.

0 2 (ml'l" l) 0 2 4 6

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Chl a o Chl a + Pheo a

2

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Table 2.

Date 12 July SML I 30 July SML I

Vertical distributions of oxygen, chlorophyll, primary production, and NO 3 uptake at the same stations as for results illustrated in Fig. 6.

Changes in particulate matter during the middle shelf time series at 3 0 ° N . S M L is surfaced mixed layer, I is intruded waters. Delta N O 3 is NOs* minus measured N O 3 PC :~ P N (mg m -3)

Delta NO 3 (rag m -3)

PN/Delta NO 3 ( x 100)

90 130

12 12

8.0 11.0

230 490

32

38

100 170

15 30

6.3 5.7

160 32

36

83

73

40

6 August SML I

160

13 A u g u s t SML I

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7

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49

-

-

Phytoplanktondynamicswithin Gulf Stream intrusions

623

both ratios in both the surface mixed-layer and 'old' intruded waters suggest that nonphytoplankton PC was more abundant under these conditions than when the phytoplankton bloom was near its peak. Delta NO 3 (i.e., N O 3 * - N O 3 ) is an estimate of the amount of NO 3 missing from the intruded waters and presumably consumed by phytoplankton. Within c a . 1 week following an intrusion event (i.e., 30 July---event 2 and 13 August-----event 3), about 80% of delta NO 3 was present within the intrusion as PN (Table 2). However, PN accounted for only 40% of delta NO 3 within 'older' intruded waters (i.e., 12 July and 20 August). Results obtained on the middle shelf were relatively easy to interpret in that the effect of intrusion event 2B caused a large increase in NO 3 followed by a large increase in Chl a. Furthermore, increases in Chl a, PN, PC, primary production, and O2 were correlated with NO 3 removal as should occur during a phytoplankton bloom in upwelled water. Inner and outer shelf results were more difficult to explain, although in both zones highest Chl a concentrations were within the subsurface intrusion (Fig. 8). The outer is, of course, the closest of the three shelf zones to upwelling source waters. Thus the NO 3 and 02 characteristics of intruded GSW should be relatively close to initial conditions, since intruded waters on the outer shelf should generally be relatively fresh. The results presented in Fig. 9 show that observed NO3 is generally only slightly less than NO3* even at the Chi a peak on 3 August suggesting that, in general, upwelled waters do not reside for very long (i.e., < c a . 1 to 2 weeks) on the outer shelf. Using the results of 30 July in Table 2 to estimate phytoplanktonN from Chl a suggests that the NO 3 missing (i.e., delta NO3) on 3 August can only account for about 50% of the estimated phytoplankton-N. This discrepancy is probably best explained by the statistical limitations of estimating NO3* from the regression of Fig. 2, since the mean NO3* concentration on 3 August would only need to be 1.0 laM higher to yield a delta NO 3 high enough to account for all of the phytoplankton-N. The inner shelf Chl a time series was very different from those of the outer and middle shelf. Instead of well-defined peaks, inner shelf Chl a steadily increased during July and NO3(PM) and Chl ~(mg m "3) 0

. 6 ,, 12

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Fig. 8. Vertical distributions of temperature, Chl a, and NO 3 at selected stations during the inner shelf(upper panels) and outer shelf(lower panels) time series at 30°N (transect 4, Fig. 1).

624

J.A. YODERetal. 5

•

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Outer shelf(40 to 45 m isobaths) time series at 30°N (transect 4, Fig. I).

August from c a . 2 to 50 mg m -2 (Fig. 10). Most of the increase was within intruded waters occupying the lower portion of the water column (Fig. 8). One difficulty in interpreting inner shelf results is that vertical exchange between the subsurface intrusion and the overlying surface mixed-layer occurred to an extentnot observed on the middle and outer shelf (Fig. 3). During the latter part of July, and in August, inner shelf surface mixed-layer temperatures were 26 to 27.5 °C, at least 1 to 2°C colder than mixedlayer temperatures on the middle and outer shelf (e.g., compare results from 30 July, Figs 8 and 10). Vertical heat exchange of this magnitude between intruded waters and the overlying surface mixed-layer invalidates the use of delta NO 3 and delta O: as indicators of previous autotrophic activity. Thus, the apparent correlation between increasing Chl a with decreasing 1.o

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625

Phytoplankton dynamics within Gulf Stream intrusions

NO3* from

10 to 20 August does not necessarily mean that high Chl a on the inner shelf was not related to intrusion production. In fact, the evidence discussed below suggests that onshore flow of intrusion nutrients and Chl a caused the Chl a increase observed on the inner shelf during August. Northerly flow of intruded waters appeared to have had a major impact on temporal al 3e I ff~.3 e

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Fig. 11. Maps of near-bottom Chl a + Pheo a derived from four different surveys spaced 1 week apart. Results obtained from transect 4 showed a good linear relation (r = 0.91, n = 66) between near-bottom Chl and the water column integral. The effects of intrusion events 2A and 2B are clearly reflected in the increase in Chl between 20 to 23 July and 27 to 30 July. Chl-rich intruded waters at the coast near 290N on 27 to 30 July appear to have moved north along the coast during the following 2 weeks causing high inner shelf Chl concentrations near 30°N on 10 to 13 August. Also present in the map of 10 to 13 August is an offshore band of high near-bottom Chl resulting from intrusion event 3.

626

J . A . YODER et al.

Table 3.

Shelf zone

Ranges and means during inner, middle and outer shelf time series at 30°N

NOa*-N (g m -2) Range Mean

Inner Middle Outer

0.0-0.9 0.0-2.1 0.6-4.2

0.2 0.8 1.8

NO3-N ( g m -2) Range Mean 0.0-0.4 0.008-1.4 0.1-4.6

0.05 0.30 1.6

Chl a (mg m -2) Range Mean 1.6-71.0 4.0-84.0 15.0-130.0

29.0 30.0 54.0

Chl a + Pheo a (mg m -2) Range Mean 5.7-80.0 10.0-110.0 32.0-219.0

42.0 45.0 89.0

changes of inner shelf Chl a. Weekly, shelfwide maps of near-bottom Chl a for the period 20 July to 13 August are shown in Fig. 11. These suggest that Chl a-rich intruded waters, originally located inside the 20 m isobath near 29°N on 27 to 30 July, migrated up the coast during the next 2 weeks. This conclusion is supported by current meter records (mooring 7) which show that the near-bottom currents at the 20 m isobath at 30°N generally showed northerly flow (5 to 10 cm s -t) from 27 July to 10 August. Thus, the increase of inner shelf Chl a during August at 30°N (Fig. 10) probably resulted from two processes: (1) subsurface, onshore flow of nutrients and Chl a-rich waters and (2) alongshore advection of Chl a-rich waters originally located near the coast at 29°N in the latter part of July. Outer, middle, and inner shelf NO 3 and Chl a values are summarized in Table 3. The outer shelf averaged several times more NO 3 than the middle shelf which, in turn, averaged several times more NO 3 than the inner shelf. However, the difference between mean NO3* and mean observed N O 3 suggests that a much higher proportion of intruded NO 3 was consumed by phytoplankton on the middle and inner than on the outer shelf. Mean Chl a on the outer shelf (54 mg m -2) was almost twice that on the middle and inner shelf (ca. 30 mg m-2). In all three zones, mean Chl a was about 2 times higher than mean Pheo a (Table 3).

Primary production Mean values for assimilation number (mean Pm b = 15.5 mg C mg Chl a -1 h -l, S.D. = 2.6) and initial slope of the photosynthesis vs irradiance curve (mean ct = 14 mg C mg Chl a -~ Ein -l m 2, S.E. = 2) were similar to mean Pm b and a values (15.8 and 8.0, respectively) obtained during an earlier study of upwelled waters on the outer southeastern shelf during April 1979 (YODER et al., 1983). In both studies, euphotic zone primary production Table 4. Photosynthetic index [rag C m -2 h-l)/(mg Chl a m-2)] vs irradiance (Ein m -2 h -1) regression results from this and other studies. The slope o f the regression line (°d) is the light utilization index (Falkowski, 1981)

Study area Southeastern shelf (GABEX 2 and April 1979) New York Bight* Sub-arctic Pacifict Canadian Arctic:[: Models

Incubation time (h)

W (S.E.) (mg C mg Chl a -I Ein -l m 2)

r

3-6 3-6 3-6 24 24

1.5 (0.18) 0.43 0.56 0.30 ca. 0.43

0.92 ca. 0.93 -

* FALKOWSKI( 1981). ~" LARRANCE (1971), irradiance converted from energy to quanta using empirical relation: 1 ly (400 to 700 nm) = 0.19 Ein m -2 (400 to 700 nm). :~ HARRISON et aL (1982), irradiance conversion as above. § From model of RYTHER and YENTSCH (1957), irradiance conversions as above.

Phytoplanktondynamics within Gulf Stream intrusions

627

(g C m --2 h -~) was related to incident irradiance (Ein m -2 h -~) and Chl a (mg m -2) as observed by others (FALKOWSKI, 1981). Depth-integrated primary production, normalized by depthintegrated Chl a (photosynthetic index) and regressed vs incident irradiance yielded a high linear correlation coefficient (r = 0.92, Table 4). The slope of the regression line (light utilization index, ~) is ca. 3 times higher than reported by others for temperate, sub-arctic, or arctic waters (Table 4). During the middle shelf time series, more Chl a measurements were obtained than were measurements of primary production (Fig. 5). Thus, the regression results summarized in Table 4, daily records of incident irradiance, and Chl a measurements were used to generate a primary production time series for the middle shelf at 30°N during the period 12 July to 20 August. Primary production ranged from 0.3 (12 July) to 4.2 (3 August) gC m -2 d -l, excluding dark-period respiratory losses. The mean rate of middle shelf primary production at 30°N was 1.9 gC m -2 d -I over the 40-day period of our study. NO 3 u p t a k e a n d n e w p r o d u c t i o n

Rates of NO 3 uptake were probably underestimated, since all measurements were from relatively short incubations (h) during the light-period. NO 3 uptake continues at a reduced, but significant, rate during darkness (DU~DALE and GOERING, 1967; CONWAV and WmTLEDG~., 1979), whereas organic carbon synthesis does not. Though rates of NO 3 uptake are commonly presented as a function of ambient NO 3 concentration, uptake is also affected by irradiance when light levels are below ca. 10 to 20% of incident (MAclSAAC and DtJGDALE, 1972). As demonstrated above, NO 3 concentrations higher than ca. 0.2 taM never occurred within the surface mixed-layer. Thus, high concentrations of NO 3, when present, were always within intruded waters that were generally located below the 44% irradiance isolume. Because insufficient NO 3 uptake data were available to consider interactions of concentration and irradiance on uptake, NO 3 uptake measurements were pooled into 1 of 3 categories: (1) surface mixed-layer (i.e., low NOa,~high irradiance), (2) intrusion when NO 3 was < 1.0 taM (low NO3, low irradiance), and (3) intrusion when NO3 exceeded 1.0 taM (high NO3, low irradiance). Simultaneous measures of primary production and NO3 uptake showed that the C :NO3-N uptake ratios in intruded waters with NO 3 < 1.0 taM were very similar to ratios in the surface mixed-layer. The C:NO3-N uptake ratio (by weight) in the surface mixed-layer averaged 56 (S.D. = 28, n = 19) compared to an average of 60 (S.D. = 51, n = 11) in intruded waters when NO 3 was < 1.0 taM. In contrast, the C :NO3-N uptake ratio in intruded waters having NO 3 > 1.0 taM was significantly (t-test, P < 0.05) less than that of the other two categories, averaging 7.5 (S.D. = 6.7, n = 11). Very high C:NO3-N uptake ratios in intruded waters depleted of NO 3 suggests that other forms of N satisfied phytoplankton requirements. This is supported by the results obtained from the NH4 :NO3 uptake preference experiments conducted in a laboratory incubator. When NO 3 was < 0.5 taM within intruded waters, the NH 4:NO 3 uptake ratio averaged 3.7 (n = 4, S.D. = 1.6). The same uptake ratio was only 0.45 (n = 3, S.D. = 0.6) when NO 3 exceeded 0.5 taM. To determine the importance of NO 3 as a source of N, uptake rates of NH4 and other forms of recycled N should be known but were not routinely measured. To estimate 'new production' (in sensu DUGDALE and GOERING, 1967) we assumed that C and all forms of N (total N) were assimilated according to the Redfield ratio (5.7 by weight), a representative C:N ratio of marine phytoplankton (GOLDMAN et al., 1979). Comparing our depth-integrated C :NOa-N uptake ratios with the Redfield ratio, we calculated that the percentage of 'new

628

J.A. YODERet aL 50 = •U ,,~

// // //' •

40

,

30

/"

/

~

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///

o

20

~ o

10

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¢1

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NO 3 Consumed (gM) Fig. 12. Delta 02 (02 production) vs delta NO3 (NO3 consumption) from middle shelf stations. Dashed line shows theoretical relation based on Redfield ratio, whereas solid line is from linear rcgrcssion (r = 0.86, n = 16). See text for explanation.

production' was 97% of the total near the peak of the phytoplankton bloom on 30 July but was < 10% within 'old' intruded waters present on the middle shelf at 30°N on 20 August. Oxygen production is a byproduct of photosynthesis, whereas N H 4 and other forms of recycled N are byproducts of zooplankton metabolism which consumes 02. Thus, when phytoplankton use recycled N as a source, O2 content of the water may not change significantly since zooplankton consume 02 whereas excretion stimulates phytoplankton photosynthesis which produces 02. However, when phytoplankton use primarily 'new' N (i.e., intrusion NO3), 02 production should be related to NO 3 consumption. If we assume a photosynthetic quotient of 1.25 to convert 0 2 production to net photosynthetic CO2 assimilation, and use the Redfield ratio to estimate C :N uptake ratio, the predicted molar ratio of 02 produced:NO3 consumed is 8.6 (MINAS et al., 1982). For intruded waters on the middle shelf, 02 production (delta 02) was linearly correlated (r = 0.86, n = 16, P < 0.01 from F-statistic) with NO 3 consumption (delta NO a) (Fig. 12). The slope of the regression line is 6.1 (S.E. = 1.0) compared to the predicted 8.6. Phytoplankton cell size

At the inner, middle, and outer shelf stations at 30°N, phytoplankton passing through a 10 lam mesh net averaged, respectively, 91% (S.D. = 14%, n = 6), 92% (S.D. = 10%, n = 10), and 88% (S.D. = 15%, n = 7) of total Chl a in the mixed-layer. The importance of the < 10 Bm fraction in intruded waters depended upon the state of the phytoplankton bloom with its relative contribution decreasing with increasing Chl a (Fig. 13). In general, the < 10 lam fraction accounted for at least 50% of euphotic zone Chl a during the time series on the inner, middle, and outer shelf. At some stations on the middle shelf we also determined the contribution of the < 10 tam size fraction to primary production. In the surface mixed-layer, the < 10 Bm size fraction averaged 93% (S.D. = 12%, n = 6) of total Chl a and contributed 83% (S.D. = 12%, n = 6) of total primary production. In intruded waters, a linear regression of < 10 lam Chl a (as percent of total) vs < 10 ~tm primary production (as percent of total) had a correlation coefficient (r) of 0.89 (n = 6), a slope of 1.01 (S.E. = 0.26), and a Y-axis intercept near zero (2.3%). Thus, in intruded waters and in the surface mixed-layer, the < 10 Ixm size fraction contributed equally to biomass (Chl a) and production.

Phytoplankton dynamics within Gulf Stream intrusions

ISOBATH o 20-25m • 30-35m

.o "S 100 Fe~ ~ A

)

629

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zx I

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Relative contribution (%) of the < 10 m size fraction to total Chl a vs the amount of Chl a. All X, Y pairs are based on water column integrals,

DISCUSSION

Regional significance o f summer intrusions

During summer 1981 phytoplankton dynamics on the middle and outer southeastern United States continental shelf were dominated by changes that occurred within subsurface intrusions of Gulf Stream water. As a first approximation, changes within intruded waters were analagous to those which occur when a sealed flask containing nutrient-rich seawater is innoculated with phytoplankton and exposed to light. Source waters (GSW) < 21°C are NO 3-rich and 02-poor when below the euphotic zone seaward of the shelf break. When GSW upwells and intrudes onto the southeastern shell nutrients, Chl a, and primary production in the overlying surface mixed-layer are not measurably affected. However, rapid and dramatic changes occur within intruded waters located only a few meters below the mixed layer. Shoreward of the 40 to 50 m isobaths, phytoplankton bloom for ca. 2 weeks within newly intruded waters. During the bloom, primary production and phytoplankton biomass increase at least 10-fold depleting the water of NO 3 and other inorganic nutrients. High rates of new primary production lead to increases in 0 2 within intruded waters. Near the peak of the phytoplankton bloom ca. 80% of middle shelf primary production occurs below the surface mixed-layer and is supported by NO3 (i.e., new primary production), the rate of primary production is ca. 3 to 4 gC m -2 d -1, Chl a is ca. 75 mg m -2, and most phytoplankton are > 10 ~tm. At bloom termination, intruded waters are supersaturated with O 2. The mean increase of intrusion 02 (delta 02) over the duration of the bloom is ca. 1 ml (i.e., 44.6 ~tmoles) 02 1- l Assuming a photosynthetic quotient of 1.25, an intrusion 15 m deep, and no net gas exchange across the very strong thermocline, net water column productivity (phytoplankton CO2 assimilation minus all respiratory losses) is ca. 6.5 gC m -2 during the bloom. The significance of intrusion primary production to seasonal and annual estimates for the southeastern United States shelf depends upon the rates of primary production within intruded waters, the frequency of intrusion events, and the spatial extent of intruded waters. Of particular interest is the importance of summer intrusions to middle shelf (20 to 40 m isobaths) productivity. Previous studies showed that upwelling and intrusion of GSW is the principal process controlling outer shelf (40 to 200 m isobaths) rates of total and 'new' primary production, temporal changes in phytoplankton biomass and its size composition, and related autotrophic processes (DUNSTAN and ATKINSON, 1976; ATKINSON et al., 1978;

630

J.A. YODERet al.

BISHOP et al., 1980; YODER et aL, 1981b, 1983; POMEROY et aL, 1983; YODER, 1985). However, intrusions of GSW to the middle and inner shelf generally occur only in the warmer months of the year (ca. May to October) when resident shelf waters are thermally stratified (ATKINSON,1977; ATKINSONet al., 1983, 1984). The mean rate of primary production (1.9 gC m -2 d -~) during the 40-day middle shelf time series reported here was 25 times higher than mean rates reported for the 20 to 200 m isobaths in the summer (July) of 1973 (HAINES and DUNSTAN, 1975). The 1973 study was relatively short (ca. 7-day cruises) and was completed before physical processes that cause subsurface penetration of upwelleci GSW were understood (YoDER, 1985). Thus, the biological impact of intruded waters was not appreciated or considered in the sampling strategy of Haines and Dunstan (YODER, 1985). The means and ranges of both primary production and euphotic zone Chl a during our study are essentially identical to those reported for upwelled GSW on the outer shelf during spring (BISHOPet al., 1980; YODERet al., 1983) and during a 10-day drogue study of an intrusion that penetrated to the middle shelf during summer 1978 (YODERet al., 1983). Current meter results (Table 1) and hydrographic surveys (PAFFENHOFER,unpublished data) showed that intruded waters were also extensively distributed across and along the shelf south of ca. 31 °N during June 1981, implying that middle shelf primary production was high before we began our time series studies on 12 July. If our 40-day average rate of primary production is extrapolated through June and early July 1981, the resulting 80-day production would be ca. 150 gC m -2, about 15% higher than the HAINES and DUNSTAN (1975) annual estimate for shelf area between the 20 and 200 m isobaths and 50% of their annual estimate for the inner shelf off Georgia. This comparison illustrates the potential importance of event time-scale phenomena, such as summer intrusions, to annual productivity of the southeastern United States continental shelf. The importance of intrusions to shelf production during any given summer obviously depends upon the frequency of events and the area of the shelf that intrusions influence. The frequency of summer intrusions onto the middle shelf is controlled by the number of upwelling-favourable Gulf Stream frontal meanders or eddies that coincide with periods of northerly wind stress (O'MALLEY, 1981; HOFMANN et al., 1981; ATKINSON et al., 1984). Taking both processes into account suggests that intrusions generally occur twice per month during June to August (O'MALLEY,1981 ; ATKINSONet al., 1984). During summer 1981, three intrusion events occurred during ca. 80 days, approximately 50% of the frequency predicted by earlier studies. However, a much greater area of the shelf was affected by intruded waters during summer 1981 than previously observed. South of ca. 31 to 32°N, intruded waters penetrated shoreward to, or beyond, the 20 m isobath during summer 1981, and at times covered an area of about l0 4 km 2. In comparison, intrusions during the summers of 1978 and 1979 covered considerably smaller areas (ca. 0.3 x 104 km 2 or less) (BLANTONet al., 1981; O'MALLEY, 1981; YODER et aL, 1983; ATKINSONet aL, 1984). Thus, temporal and spatial scales associated with summer intrusions differ significantly from year to year. However, we show that intrusions of the scale observed during 1981 are by far the most important process controlling summer phytoplankton dynamics of the middle and outer shelf south of ca. 31 to 32 ° N, and the inner shelf south of ca. 30 to 31 °N. Comparison with other continental shelf ecosystems

Table 5 lists mean or 'typical' rates of phytoplankton production and biomass (Chl a) for seasonal blooms at temperate and higher latitudes and for some major wind-driven upwelling

631

Phytoplankton dynamics within Gulf Stream intrusions

Table 5. Comparison of mean primary production and phytoplankton biomass ( Chl a) from the 40-day middle shelf time series with mean or typical values from recent studies of wind-driven upwelling systems and the seasonal blooms of temperate and arctic shelves.

Location This study (middle shelf) Wind-driven upwelling Peru* NW Africa'[" Baja California:~ Temperate-arctic shelves Bering Sea (spring bloom)§II Canadian Arctic (summer bloom)¶ Hudson River Plume (March)** New York Bight (March-April spring bloom)~'~" Apex <40 m 41-80 m 81-1000 m

Approximatewater temp. (°C)

Primary production (gC m-2 d-1)

Chl a (mg m-z)

17-28

1.9

30

?(ca. 15-20) ?(ca. 15-20) ca. 15

5.3 2.0 4.1-7.5

120-275 68 108-200

< 10

1-4

25-400

<5

0.2

57

<5

0.4-1.0

75-600

2.6-5.0 1.0-1.75 1.0-1.75 1.01-1.1

170-190 47-114 153-242 166-340

< 10

* HENRIKSONet aL (1982), ~"HUNTSMANand BARBER(1977), :~WALSHet aL (1977), § IVERSONet aL (1979), IIWALSH(1983), ¶ HARRISONet aL (1982), **MALONEet aL (1983b), ~'J"MALONEet al. (1983a).

systems. The mean rate of primary production during our 40-day middle shelf study is significantly lower than generally observed in wind-driven upwelling systems (Table 5). However, with the exception of the Apex of the New York Bight, our mean is similar to rates reported for spring blooms in temperate waters of the New York Bight and sub-arctic waters of the Bering Sea, and is ca. 10 times higher than that of the summer bloom in the Canadian Arctic (Table 5). In contrast, our 40-day mean Chl a and the maximum we observed is significantly lower than almost all other values listed in Table 5. Relatively high production from relatively low Chl a is reflected in high values for the light utilization index (¥, Table 4). Photosynthetic index (PI = depth-integrated primary production divided by depthintegrated Chl a) is similar to W except that the latter is also normalized by incident irradiance to account for experiment-to-experiment differences in incident irradiance (FAU~OWSKI, 1981). Thus, a relatively high value for ¥ means that a given amount of phytoplankton biomass (Chl a) yields a relatively high rate of primary production per incident quanta. In general, ¥ is a function of the vertical distribution of Chl a and the photosynthetic parameters (e.g., a, Pm b) that define the photosynthesis vs irradiance (P-I) relation. Our relatively high ¥ did not result from the effects of high nutrient concentrations on pb rate as observed in the wind-driven upwelling systems (SMALLet al., 1972; HUNTSMANand BARBER, 1977; HARRISONet aL, 1981). Mean nutrient concentrations at the other study sites (Table 5) were at least a s high, and generally higher, than levels we observed. Furthermore, Pm b always occurred in the nutrientdepleted surface mixed-layer during our study. Two effects or characteristics of intruded

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waters contribute to the physiological responses that lead to high values of ¥. First, the temperature of intruded waters is generally above 15°C, which is relatively high compared to spring bloom conditions at higher latitudes. The temperature of the overlying surface mixedlayer during summer is ca. 28°C, close to the maximum for ocean waters. Pm B generally increases with temperature in laboratory studies of individual phytoplankton species (VERITY, 1981 ; LI and MORRIS, 1982) and in natural phytoplankton assemblages (DURBIN et al., 1975; PLATT and JASSBY, 1976; MALONE,1977; MALONE and NEALE, 1981; HARRISONet al., 1982). Our mean Pm B (15.5 mg C mg Chl a -l h -~) is 65% of the theoretical maximum of 24 (FALKOWSKI, 1981). Our mean is higher than most mean values from other coastal and shelf waters (1.3 to 22; PLATT and JASSBY, 1976; MALONE, 1977; P L A r r e t al., 1980, 1982; FALKOWSKI, 1981; MALONE and NEALE, 1981; HARRISONet al., 1981, 1982) and approaches those reported for nanoplankton in the lower Hudson estuary and Hudson River plume (22 and 16.6, respectively) at water temperatures of 17 to 26 ° C (MALONEand NEALE, 1981). Our mean Pm B is the same as that reported for the surface mixed-layer in the equatorial Atlantic (HERBLANDand LEBOUTEILLER,1983), where nutrient and Chl a concentrations, temperature, and water clarity are very similar to the surface mixed-layer located above intruded waters on the southeastern shelf. Stratification of the euphotic zone into two distinct vertical layers is the second effect of intrusions contributing to high ~. Intrusions divide the euphotic zone into two irradiance regimes: (1) surface mixed-layer where phytoplankton are vertically mixed between the ca. 44 and 100% isolumes, and (2) the subsurface layer where irradiance is generally < 44%, but > 1% of incident. As a result, phytoplankton can adapt to either relatively high or low mean irradiance but generally do not experience both irradiance regimes over time-scales of days to weeks. Our high mean p , B and the lack of significant photoinhibition may reflect adaption to the relatively high mean irradiance of the surface mixed layer. The ability to efficiently harvest relatively low levels of irradiance is reflected by a. Our summer 1981 mean a (14 mg C mg Chl a -l Ein -1 m E) is ca. 60% of the theoretical maximum (23.9; PLA.TTand JASSBY, 1976). It is generally higher than, or approximately equal to the annual mean for Nova Scotian coastal waters (12.9; PLATT and JASSaY, 1976), summer mean for Arctic waters (3.0; PLATT et al., 1982), spring mean for nano- and netphytoplankton of the Hudson River plume (6.4 and 2.7, respectively; MALONE et al., 1983b), annual means for nano- and netphytoplankton of the New York Bight (16.7 and 13.9, respectively; MALONE and NEALE, 1981), and various other locations (12.3; PLATT et al., 1980). Thus, mean phytoplankton photosynthetic rates, as reflected in mean values of a and PmB, within the upper and lower vertical layers during summer 1981 were about equivalent to or exceeded maximum m e a n rates reported for coastal and shelf waters from a number of other locations, including major wind-driven upwelling systems.

CONCLUSIONS Subsurface intrusions of nutrient-rich GSW controlled temporal and spatial variability of phytoplankton biomass and rates of new and total primary production on the southeastern United States continental shelf during summer 1981. During summer, phytoplankton blooms within intrusions may contribute as much as 150 gC m -2 to middle shelf productivity off Georgia and northeastern Florida. Mean levels of phytoplankton biomass within intruded waters are low compared to spring and summer blooms in temperate, sub-arctic, and arctic waters, and to those in major wind-driven upwelling systems. However, relatively high rates

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o f C h l a - n o r m a l i z e d p h o t o s y n t h e s i s lead to m e a n daily rates o f w a t e r c o l u m n p r i m a r y p r o d u c t i o n that are e q u i v a l e n t to, or greater than, m o s t m e a n s r e p o r t e d for s e a s o n a l b l o o m s at t e m p e r a t e and higher latitudes.

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