MARINE BIOLOGY
Marine Biology61, 119-131 (1981)
9 Springer-Verlag1981
Phytoplankton Patchiness and Frontal Regions* H. H. Seliger, K. R. McKinley**, W. H. Biggley, R. B. Rivkin and K. R. H. Aspden MeCollum Pratt Institute and Department of Biology,The Johns Hopkins University; Baltimore, MD 21218, USA
Abstract
In the Chesapeake Bay estuary there are persistent seasonal frontal and interfrontal regions that serve to deliver and retain different phytoplankton populations. The "patchiness" of phytoplankton, both in total chlorophyll a concentrations and in species compositions and abundances, is shown to be causally related to density flow forcing which results in these frontal and interfrontal regions. The delineation of these regions by on-line, twodimensional profiling of density isopleths serves to identify stations within these regions for biological and chemical sampling as opposed to sampling on an arbitrary geographical grid. It is possible, by superposition of nutrient and organism concentration isopleths upon salinity isopleths, to infer conservative and non-conservative features of the system.
Introduction
Phytoplankton distributions in the Chesapeake Bay exhibit considerable heterogeneity or "patchiness" on the order of tens of thousands of meters in horizontal extent and 1 to 5 m in vertical extent (Patten and Chabot, 1966; Marshall, 1967; Flemer, 1969; Seliger and Loftus, 1974; Zubkoff and Warinner, 1975; Tyler and Seliger, 1978; Zubkoff et al., 1979). This patchiness is further complicated by short term perturbations (storm events) (Loftus et al., 1972; Loftus and Seliger, 1976) and different sea-
* Contribution 1059 from the McCollum-PrattInstitute and Department of Biology. Research support by DOE contract DEASO2-76-EVO3278. The data in this paper have been presented at the Winter Meeting of the American Society of Limnologyand Oceanography in Corpus Christi, Texas January 2-5, 1979 and the 42nd Annual Meeting of ASLO in Stony Brook, NY, USA June 18-21, 1979 **Present address: St. John's College:AnnapolisMD 21404, USA
sonal progressions of phytoplankton communities in different regions of the estuary (Patten et al., 1963; Stross and Stottlemeyer, 1965; Flemer, 1970; Seliger et al., 1975). A number of attempts to explain patchiness have focussed on Analysis of Variance (ANOVA) (Therriault and Platt, 1978; Therriault et al., 1978) of samples taken at fixed geographic locations, on calculations of covariance of arbitrary biotic and abiotic parameters (Denman, 1976; Denman and Platt, 1975), on power spectral analysis of fluctuations (Platt and Denman, 1975; Fasham, 1978), and on biological growth coupled with turbulent diffusion (Skellam, 1951 ;Kierstead and Slobodkin, 1953 ; Denman and Platt, 1976). A causality between water circulation and stratification patterns and heterogeneous phytoplankton species distributions has been shown for the horizontal and vertical patchiness of dinoflagellates in the shallow bioluminescent bays in Jamaica, West Indies and in Puerto Rico (Seliger et al., 1969, 1970, 1971), for the temporal and spatial distributions of the dinoflagellate Prorocentrum mariae lebouriae in the Chesapeake Bay (Tyler and Seliger, 1978, 1981), and has been proposed to explain one type of patchiness in coastal waters, i.e., dinoflagellate red tides off the coasts of Florida and Maine (Seliger et al., 1979). In this paper we have attempted to establish a more precise physical basis for our original approximation of integrating chemical and biological measurement over a series of transects in a study area (Loftus et al., 1972; Seliger and Loftus, 1974). We have added, as integral parts of the measurement protocol, a) the physical hydrographic parameters of water circulation patterns, sampled on the same scales as the observed spatial and temporal variations in the phytoplanktion, and b) the enumeration of phytoplankton species compositions and abundances, in order to study the origins of phytoplankton patchiness. We have designed and constructed equipment for making rapid vertical profiles of conductivity, temperature and chlorophyll a concentrations (Biggley et al., 1981). From two-dimensional isopleths of sigma-t or salinity it is 0025-3162/81/0061/0119/$ 02.60
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possible to delineate the water circulation patterns that can result in 4 different, persistent water masses within the estuarine portion of the same tributary estuary. This delineation makes it possible to sample chemical and biological variables within these separate regions and at their interfaces "rather than at fixed geographical stations" about which these regions may oscillate semi-diurnally
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or translate seasonally. As a result horizontal and vertical "patchiness" of phytoplankton distributations are shown to be due to different phytoplankton species compositions and abundances within these different regions. The persistent regions of the tributary estuary and thek attendant phytoplankton distributions that will be described in this paper are: I. the positive (downstream-flowing), river
H. H. Seligeret al.: Phytoplankton Patchinessand Frontal Regions plume whose leading edge is a positive front; II. the negative (upstream-flowing), low-density surface water plume which originates from bay surface waters and whose leading edge is a negative front; III. the interfrontal region between I and II which is well mixed (isopycnal); and IV, the negative (upstream-flowing) high-density bottom water wedge which originates from below the major pycnocline of the two-layered central bay.
Material and Methods The study area, Fig. 1, the northern Chesapeake Bay including the Chester River, a large, slow-flowing tributary estuary on the eastern shore, serves as a microcosm within which significant patchiness in spatial and temporal distributions of nutrients and phytoplankton occurs. The area investigated and the stations occupied extended from Tolchester Beach, 39~ Lat., to below the Bay Bridge, 38 ~59'N. Lat. We made, alonghorizontal transects, series of synoptic vertical profiles of conductivity, temperature, nutrients, chlorophyll a and phytoplankton in this relatively shallow system. The monotonic spatial characteristics of variables between the separate vertical profiling stations were verfied by continuous measurements of conductivity, temperature and in vivo chlorophyll a fluorescence from water pumped from a 1 m depth during the transit from one station to the next. In addition, in separate experiments, vertical profiles of these variable were made at closely spaced intervals between stations. From these separate confirming measurements for continuity we were justified in the choice of distances between stations and the artistic license in drawing contiuous, arbitrarily straight, interpolated isopleths between vertical profiling stations. Therefore, while the true shapes of the isopleths may have been smoother and less angular than the interpolated isopleths, the latter represented a sufficiently accurate approximation of the shapes and boundaries of the variables to specify a minimum number of vertical profiling stations at which complete sampling was carried out. Measurements ofverticalprofiles of conductivity, temperature and in vivo chlorophyll a fluorescence versus depth are described separately (Biggley et al., 1981). At each station pumped water samples were collected for nutrient analysis at depths determined from on-line analog recorder plots of conductivity and in vivo chlorophyll a versus depth. Water samples for phytoplankton identification and enumeration at these specified depths were collected in Van Dorn samplers. Microscopic examinations were carried out at 100 to 400 X magnification on live samples aboard ship and with preserved samples in the laboratory. Counting was limited to approxirr/ately 20 taxa consisting of diatoms, dinoflagellates, cryptomonads and chlorophytes. Dissolved oxygen was measured with a Yellow Springs D.O. probe Model 57. Water samples from the Van Dorn casts were also used for conversion of in vivo fluorescence readings to concentration of extractable chlorophyll a (Loftus and Seliger, 1975). Dissolved inorganic nitrate was measured as nitrite (Benschneider and Robinson, 1952) after
121 Cd-Cu reduction (Wood et al., 1967) and was corrected for the presence of nitrite in the water samples;ammonia as described by Solbrzano (1969).
Results and Discussion
Fig. 2A, B and C show respectively isopleths of sigma-t [ lO00 ( p t - l ) ] , chlorophyll a concentrations [#g1-1 ] and dissolved oxygen concentrations [rag 1 -i ] during summer for a vertical section through the main channel of the Chester River from near the mouth (Station 9090) to a station approximately 13 nautical miles (24 kin) upstream (CH 12.7). The meandering of the river has been straightened in the plot. Station designations conform to Chesapeake Bay Institute nomenclature (Chesapeake Bay Institute Data Bank Report 1, 1969). From Fig. 2A isopleths characteristic of a three-layer flow system were located between Station 9070 and CH 3.9. The hatched layer (between sigma-t = 1.25 and 1.75) was intermediate in density between the negative surface plume of lower density bay water (Region II) and the negative bottom wedge of denser bay bottom water (Region IV). This intermediate layer represents the positive (downstreamflowing) outflow from the interfrontal region (Region III). The upstream boundary of the interfrontal region was the riverine positive plume (Region I). By visual comparison of the chlorophyll a isopleths of Fig. 2B with the sigma-t isopleths of Fig. 2A it can be seen that the peak of chlorophyll a concentrations lay within the interfrontal region III at CH 8.3. The subsurface chlorophyll a maximum at 2 m was the resultant of positive phototaxis and downward current flows at the convergences of positive plume I and negative plume II (Rivkin etal., 1981b). In the absence of strong frontal convergences positively phototactic dinoflagellates or positively buoyant diatoms such as Skeletonema eostatum will produce a chlorphyll maximum at the surface. From Fig. 2B for chlorophyll a and Fig. 2C for dissolved oxygen it can be seen that the bottom waters of the bay at this time of year were depleted in both phytoplankton and oxygen, while significant primary production occurred in the interfrontal region III. A dissolved oxygen concentration of 9 mg 1-I in these surface waters corresponded to approximately 110% of saturation. Table 1 presents enumerations of dominant phytoplankton species (>5 pm) at depths and locations corresponding to the progression from the central bay contiguous with plume II, to negative plume II, to interfrontal region III, to positive plume I, and to the negative wedge IV. From Table 1 and Fig. 2 it can be inferred that in summer: a) Cryptomonads and Chlamydomonas spp. are ubiquitous in surface waters, b) Dinoflagellates of the genera Prorocentrum and Gymnodinium are delivered to the interfrontal region III predominantly from the central bay via the negative plume II "and are retained or accumulated within the interfrontal region". The data are not sufficiently precise to ascertain the origins of Gymnodinium nelsoni. However we know from previous studies that they form dominant summer blooms at low salinities
122
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123
H. H. Seliger et al.: Phytoplankton Patchiness and Frontal Regions
Table 1. Phytoplankton species enumerations representative of the four separate regions in, and the central bay opposite the mouth of, the Chester River, July 24, 1979. Samples captured coincidently with the isopleth measurements of Fig. 2 and Fig. 5. The cell concentrations reported are based on counting single or duplicate aliquots in a Palmer-Maloney counting chamber. Coefficients of Variation were 25-50%. Starred organisms are those which appear to be retained or accumulated Species
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Plume II Sta 9079 ( 0 - 1 m) [cells ml- 1 ]
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400
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in the Rhode River (Seliger and Loflus, 1974; Seliger e t al., 1975), so that they probably are riverine in origin (positive plume I). Salinity flow rather than insolation is the driving force in this system. Therefore the sigma-t isopleths represent the conservative chloride ion concentrations. If a dissolved or particulate component of the system is not incorporated into the biota, its concentration isopleths should "parallel" the salinity (sigma-t) isopleths. If the dissolved or particulate component is utilized (incorporated into biological sinks), its concentration isopleths will deviate from parallelism with (tend to be perpendicular to) the sigma-t isopleths. Within these regions of non-conservation, the isopleths of the non-conserved component may indicate negligible concentrations within the isopleths of the biological sink or predator. In Fig. 3A and B the concentration isopleths of nitrate (NOS) and of ammonia (NH~4)have been superimposed respectively upon the sigma-t isopleths. In Fig. 4A and B these same nutrient isopleths have been superimposed upon the chlorophyll a concentration isopleths. It can be inferred from these superpositions that at this time of
year: a) The major source of NO~ in the Chester River is from the negative plume II of surface bay waters flowing into the river, b) The major source of NH~4 in the Chester River is the negative bottom wedge IV of low oxygen bay bottom waters from below the central bay pycnocline. c) The non-parallelism of the NO-~ isopleths with respect to the sigma-t isopleths (Fig. 3A), the absence (<1 #M) of NO~ within the chlorophyll a isopleths in the interfrontal region III (Fig. 4A), and the presence of NH] within the chlorophyll a isopleths (Fig. 4B) imply that during this period NO~ is utilized preferentially to NH~ by the phytoplankton. The phytoplankton enumerations of Table 1 show that the source of the dinoflagellates in the interfrontal accumulation region (Ill) is from surface waters of the central bay (Station 907) delivered in the negative plume II. This preferential utilization of NO ~ in the presence of NH ~ is consistent with the "prior acclimation" of the dinoflagellate populations to NOg-. Further experimental evidence for these implications is presented in a separate paper (Rivkin e t al., 1980). To establish that the delivery patterns implied by the isopleths of sigma-t and nutrients of Figs. 2 and 3 for the
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Chester River were consistent with those found in the contiguous bay, a further transect was made, consisting of a vertical section from the river mouth, Station 9070, west across the central channel of the bay (see Fig. 1). Fig. 5A through E shows isopleths o f sigma-t, cholorophyll a, D.O., NO 3- and NH~ respectively for this section. By comparison with the Chester River isopleths o f Figs. 2 and 3 the continuous nature o f the distributions can be seen. Chlorophyll a and NOs concentrations in the negative plume II are reflected in the surface waters of the central bay. The low D.O. values and high NH~ concentrations in the b o t t o m wedge IV(Fig. 2C and 3B) are paralleled b y the concentration isopleths below the pycnocline in the central b a y (Fig. 5C and 5E). In addition the phytoplankton enumerations in surface waters o f the central bay Station 907 (column 2 o f Table 1) are consis-
126
H.H. Seliger et al.: Phytoplankton Patchiness and Frontal Regions CliO0
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I
tent with the concentrations in the negative plume II and in the interfrontal region III (columns 3 and 4 of Table 1). In late July, the absence ofphytoplankton in the negative wedge IV (Fig. 2) is the result of the persistent deep pycnocline in the bay at this time year (see Fig. 11A of Tyler and Seliger 1978). Light intensities below this pycnocline are negligible (Tyler and Seliger, 1980) and the pycnocline inhibitsvertical advective mixing of oxygenated surface waters with bottom waters. Completely different phytoplankton distributions and water circulation patterns occur in the Chester River during spring (Fig. 6A and B). In this case the sigma-t and chlorophyll a isopleths represent a more conventional two-layer flow of river water (I) above the pycnocline at approximately 6 m and a major subsurface delivery of phytoplankton within the reverse flow bottom wedge IV as far upstream as CH 8.3. This subsurface delivery can be visualized more readily from the series of vertical planes through the sampling stations shown in Fig. 6C. In this case the width of the plane (represented by the hatching) is proportional to the chlorophyll a concentrations at the depths shown for each prof'fle.
Compared with Fig. 2A, a displaced and less pronounced distribution of regions I, II, III and IV in the Chester River is shown in Fig. 7A for late September, 1978. The negative bottom wedge IV terminates around CH 6.4. The negative bay plume II extends only as far as between CH 1.3 and CH 3.2 so that the interfrontal re. gion III overrides the negative wedge at CH 3.2, 9 kilometers downstream from its summer location (Fig. 2A). The positive plume region I is poorly defined, with weak stratification. The same 4 regions described in Fig. 2A are evident, although "they now occupy different geographical areas of the tributary estuary." Comparison of the isopleths of chlorophyll a concentrations of Fig. 7B with the sigma-t isopleths of Fig. 7A again shows significant accumulation of phytoplankton within the interfrontal region III. In this case the weak stratification of the riverine frontal region I and the advective mixing induced by the negative wedge results in extensive horizontal mixing of the phytoplankton. The species compositions and abundances (Table 2) changed from the summer distributions. Skeletonema costatum, and the small dinoflagellates Gymnodinium (<10 #m) andAmphidinium sp.
127
H. H. Seliger et al.: Phytoplankton Patchiness and Frontal Regions
Table 2. Phytoplankton species enumerations from water samples representative of the four separate regions in, and the Central bay opposite the mouth of, the Chester River, September 21, 1978. Sample captured coincidently with the isopleth measurements of Fig. 9. The cell concentrations reported are based on counts of single or duplicate aliquots in a Palmer-Maloney counting chamber. Coefficients of Variation were ca 25%. Starred organisms are those which appear to be retained or accumulated Species
Central Bay Sta 907
Plume II Sta CHOO
1m cells ml- 1
1m cells ml- 1
Interfrontal Region III Sta CH 3.2 1m cells ml- ~
Mixed Interfrontal I I I & Plume I Sta CH 6.4 0 to 1 m cells ml-
Plume I Sta CH 10.6 Sta CH 11.7 1m ceils ml- ~
Bottom Wedge IV Sta CHOO
None 200 None 900 None None 300 300
20 None 30 400 None None 700 900
20 200 None 1000 None 100 300 200
60
20
40
800 None None None 100 None None None 40 None 20
300 None None 40 40 None None None 40 None None
100 None None None 200 90 None None None None 30
7m cells ml-
Diatoms
Unidentiffed Pennate Nitzschia sp.
Unidentified Centric Skeletonema costatum Thalassiosira sp. TabeIlaria sp.
Cryptomonads
Chlamydomonas spp.
40
50
700 None 1500 None 80
100 None 2000 None None
1200
600 700
100 200 None 6000* 100 None 600 600
300
500
300
1500 None 100 None 2500 200 80 None 20 20
1500 None 70 30 1000 None None None 90 20
30
20
2000* 40 200* 80 3000* None None 20 200* None 80*
Euglenoids Eutreptia spp.
Dinofiagellates Amphidinium sp. Katodinium sp. Gonyaulax (fiat) Gonyaulax spinifera Gymnodinium sp. <10 #m
Gymnodinium sp. 15 ~m G, nelsoni G. stellatum Gyrodinium sp. Polykrikos sp. Prorocentru m mariae lebouriae
were dominant. These were not found in any of the summer samples (Table 1). The cryptomonads and Chlamydomonas spp. were again uniformly distributed. Since there were low but not negligible concentrations o f the three region III dominant phytoplankton in positive plume I, it is not unequivocal that the origin of these phytoplankton was solely the negative plume II, although it is evident that the major delivery occurred via plume II. In Fig. 8A and B, the isopleths of NO3- and NH~, respectively are superimposed upon the sigma-t isopleths. Within the interfrontal region III "both nitrogen sources" are nonconservative and are presumably utilized by the phytoplankton. Although at this time o f year the nutrient distributions are not so well defined as in Fig. 3A, it appears that nitrate which was being utilized was delivered to the interfrontal region in surface waters via positive plume I. The delivery o f ammonia from bay bottom waters within the bottom wedge IV is better defined. The isopleths of Fig. 8B imply that NH~ was vertically advected within the sigma-t = 5.0 and 5.5 region into the interfrontal region where it was apparently rapidly utilized. The NH~ = 2 ~ isopleths and the NH~ = 3.0/zM isopleths (between CH 6.4 and CH 8.2) are parallel to the sigma-t = 5 and 5.5. isopleths respectively, indicating mixing within this isopycnal region into the photic zone of region III. Over the past 6 yr in late summer we have observed persistent patches of dense phytoplankton concentrations
as high as 1000 gg 1 -1 in the area o f Belvidere Shoals, the western shore o f the bay between the Patapsco River and Sandy Pt. (see Fig. 1). A localized patch with chlorophyll a surface concentrations of I 5 0 gg 1 -~ is shown in Fig. 9 in a vertical cross section from Station 907D to Station 859B. The isopleths of chlorophyll a concentrations (dashed lines) are superimposed upon the isopleths of sigma-t (solid lines) and peak sharply at Station 905D, opposite Gibson Island (Fig. 1). Circulation patterns in the northern bay (Tyler and Setiger, 1978) have shown the area of the Bay Bridge and slightly north to be a region of strong vertical advection at this season of the year. Therefore the phytoplankton population within this region of upwelling (Station 859B to Station 901B) should have a different origin and nutrient history from the population in the southward-flowing bay surface water north of this region. In Fig. 9 the transport of the phytoplankton lens (predominantly dinoflagellates) with the lower density surface waters is consistent with the parallelism of the chlorophyll a isopleths with the sigma-t isopleths between Station 907D and Station 901C. These differences in water circulation and phytoplankton are also evident from the species enumerations o f Table 3. The surface patch at Station 905D and Station 903D is dominated by dinoflagellates, cryptomonads and Chlamydomonas spp., while the region between Station 901C and Station 859B, is dominated by Skeletonema costatum. Thus
128
H. H. Selige~et al.: Phytoplankton Patchiness and Frontal Regions
CHO0
,
CRI.~,
//
CR3~.
6R6,4
(5S,
Cb18.2
CRIO.6 CH11.7
,,o> . -
' ~----~,~,L
\
..__
~11
/
X
.,-,W'~\
1~ 14 SIGMA-t ANO NO"~ [#lld' " / ~
-1.o,
.-..~'.~o'..'~,,..'.'~,,
~
'2.E~,..2,~Ep,,,-~
CRt4.9
T'~I"
- \
/
~
,'
"'
~
'~
M
17 18 19 20
CHO0 CHI.3
_oi i , // 2 1/
~,~
CH3.2
CH6.4
CH8.2
l
~
1
\ (5.0)
I
~"
,./
,5 14 ,5
\
i,,
15.5)
--<,~.-~._-~
i 14,0)
..1 /
\
,~
(.5,
'~t\ ~X , '
4
CHt4.9
I
\
x~,\ \
Si61tlA-f ANO NH,~ [ / . d d / ~ / ~ o , ~3
i
X
~ " ,.
6 ~.-------"~4/~-L-~----~ " ' / i i
~ ~ ,'r,,,, 1
CHIO.6CHIL7
V
B
A
i"
/~
~
Fig. 8. A: Isopleths of NO~ [#M] (dashed lines) superimposed upon isopleths of sigma-t (solid lines);B: Isopleths ofNH~ [/xM] (dashed lines) superimposed upon isopleths of sigma-t (solid lines). Chester River, 21 September 1978
20
907D
905D
_ol
903D
~,5o I
~~~--,0-"
2
"
~
. . . . .
9OlC
l
859B
./'/(5r
50----
4 /14.51
~ " (6.0)/
6 7
9 CHESAPEAKEBAY 22 SEPT7 8 ~ IO SIGMA-IAND ")~f~~ 5" /
" C.LOROP.~_A [,<,tF']
J/
?.5,/
,,
18 19 20 2' Z2 25 24 :;:'5 26 7'7 28
-"'Jj J
~ 18O ) / J/
km
/7"7~'~ (lO.O)
even when isopleths o f total chlorophyll a concentrations appear continuous a significant patchiness in phytoplank. ton species compositions can exist. The differences in species compositions in these 2 contiguous areas also give rise to significant differences in surface concentrations o f p h y t o p l a n k t o n as reflected b y chlorophyll a concentrations. As shown in Fig. 10A through C, the dinoflagellates at Stations 907D, 905D and 903D show strongphototactic accumulations at the surface during the day. S k e l e t o n e m a c o s t a t u m at Station 859B are not as strongly p h o t o b u o y a n t and are more subject to mixing within the water column (Fig. 10D). The shallow Belvidere Shoals region is bounded by steep bathymetric gradients on its eastern and southern extents. It is shallow enough to be wind mixed so that
9 Fig. 9. Isopleths of chlorophyll a [pgl- 1 ] (dashed lines) superimposed upon isopleths of sigma-t (solid lines) for a vertical cross section in a north-south transect (parallel to the axis of the Chesapeake Bay), along the western shore, including the Beli~idere Shoals and into a region of strong vertical advection (Station 859B) below the Bay Bridge
129
H. H. Seliger et al.: Phytoplankton Patchiness and Frontal Regions
Table 3. Phytoplankton species enumerations representative of stations along a north-south transect 907D ~ 905D ~ 901C ~ 859B and an east-west transect 901C ~ 901B in the Chesapeake Bay, September 22, 1978. Samples captured coincidentally with the isopleth measurements of Figs. 9 and 11. Cell concentrations are based on counting single aliquots in a Palmer-Maloney counting chamber. Coefficients of Variation were ca. 25%. Starred organisms are those which appear to be retained or accumulated Sta 907D 0-1 m cells ml -~
Species
Sta 905D 0-1 m cells ml- a
Sta 901C 0-1 m cells ml- ~
Sta 859B 0-1 m ceils ml- ~
Sta 901B 0-1 m cells ml-
10000
8000
Diatoms Skeletonema costatum
400
800
1400
Cryptomonads Chlamydomonas spp.
3000
>3000*
3000*
700
700
Dinoflagellates Prorocentrum mariae lebouriae Amphidinium sp. Gonyaulax sp. (flat) Gymnodinium sp. (<10 #m) G. nelsoni
4000 3000 300 600 200
300 >3000* 200 3000* 100
100 3000* None 5000* None
30 100 50 1100 20
50 1000 30 1500 20
SlGMA-t CHL A [/~g[-lx I0 -/]
SIGMA-I CHL A [p. gj[-I= i0-I] 0 2 4 6
0
2
6
4
90i C SANDY PT. ~
8
90IB ~
901 I KENT IS.
2V--oo611/i//11//////
7
SIGMA-I CHL A [/~gL-Ix i0-1 ] 2 4 6 8
0
I0
2
2
2.
I (8)
SIGMA-t
CHL A [/~g~-i io-I] 4
6
8
I0
1
/ c r"
~l:r ~ ~ "~
~64 r
' I ....
ii/'i/i
0
I
2
3
4
km
SIGMA-I AND CHLOROPHYLL A [/~g[-i]
lO-
Fig. 11. Two-dimensional isopleths of chlorophyll a [#g 1-1 ] (dashed lines) superimposed upon isopleths of sigma-t(solid lines) East-west cross section of the Chesapeake Bay at its narrowest portion along 39001 ' N. Lat., 20 September, 1978
t214-
16-
18-
20-
22-
6
5
HUNTER-I CHESAPEAKE BAY 20 SEPT 78
8i
14 15
//)l///l///////////~
bottom-regenerated nutrients are continually made available to its resident phytoplankton population. Therefore water in this region, while subject to tidal excursions, is only slowly exchanged with the more rapidly flowingmain channel of the bay and retained phytoplankton can grow to high concentrations. The different distributions of phytoplankton along the transect of Fig. 9 require that the Skeletonernaeostaturn alsong the eastern shore (as represented by Station 859B) are being advected into this region via b o t t o m waters while southward flow of surface waters which do not
'4Fig. 10. Vertical profiles of chlorophyll a [#g 1-1 ] at mid-day (dashed lines) and sigma-t (solid lines) for the stations along the north-south transect of Fig. 9. A: Station 907D;B: Station 905D; C: Station 903D; D: Station 859B
contain S. costatum is mainly along the western shore. In order to verify this circulation pattern an east-west transect across the narrowest section of the bay was made (Stations 901C, 901B, 901 between Sandy Pt. and Kent Island) and is shown in Fig. 11. The sigma-t isopleths confirm that the lower density southward flow was along the western shore. The chlorophyll a concentration isopleths and the phytoplankton enumerations in the 5th and 6th columns of Table 3 confirm that S. costaturn is vertically advected into surface waters in the eastern portion of the bay, while the phytoplanktonenumerations
130 for the western shore stations (columns 2, 3, 4 o f Table 3) show the dominance o f dinoflagellates, cryptomonads and Chlamydomonas spp.
Conclusions In a tributary estuary o f the Chesapeake Bay the stratification-circulation patterns show persistent seasonal features, ranging from a two-layer flow extending into the tributary from the two-layer system in the bay proper, to a three-layer flow, resulting in the delivery to and retention and growth of p h y t o p l a n k t o n within an interfrontal region. In the bay proper, shoal regions serve as retention areas for the accumulation and growth o f phytoplankton, and regions o f rapid shallowing o f b a t h y m e t r y result in vertical advective mixing o f b o t t o m waters i n t o the photic zone, In all of these cases significantly different distributions o f nutrients and p h y t o p l a n k t o n species compositions are observed. This patchiness is characteristic of these physically and chemically distinct water masses. By comparing isopleths of dissolved oxygen, nutrient and p h y t o p l a n k t o n concentrations with isopleths o f conservative (salinity) properties, it is possible to infer the conservative and nonconservative features o f the system, the selective utilization o f chemical species o f nitrogen b y the p h y t o p l a n k t o n , the utilization of oxygen b y the benthos and the delivery o f p h y t o p l a n k t o n . We have not yet taken into account possible modifications of the nutrient concentration distributions due to rapid regeneration processes within the water column, nor have we considered modifications to the p h y t o p l a n k t o n concentrations due to grazing. A detailed analysis of the effects o f these processes will be necessary in order to provide a complete description o f the system. The dimensions o f the fronts and frontal regions and the bathymetric dimensions in the Chesapeake Bay are small enough to permit the detailed and essentially synoptic sampling that we have described to be carried out with small ships and a small group. The physical dimensions o f the coastal waters o f the British Isles (Simpson and Hunter, 1974: Pingree et al., 1975, 1976; Holligan and Harbour, 1977; Holligan, 1979) and the Bering Sea (Coachman and Charnell, 1979; Iverson et al.,1979 a, b), are much larger, the physical driving forces are quite different and the extents o f the fronts and frontal regions are observed on progressively larger scales, up to hundreds of kilometers. In all systems, however, the delineation o f the fronts and frontal regions b y physical hydrographic analysis was essential to understanding the observed heterogeneity in the distributions o f p h y t o p l a n k t o n .
Literature Cited Bendschneider, K. and R. J. Robinson: A new speetrophotometric method for the determination of nitrite in sea water. J. mar. Res. 11, 87-92 (1952) Biggley, W. H., H. H. Seliger and R. B. Rivkin: Instrumentation for rapid profiling of water densities: Structure of fronts. Limnol. Oceanogr. to be submitted (1981)
H.H. Seliger et al.: Phytoplankton Patchiness and Frontal Regions Chesapeake Bay Institute, Johns Hopkins University. Data Bank Report 1. Data Bank Inventory Vol. 1, Chesapeake Bay Tributaries, Edition 1, 1949 through 1969, Marweit, M. and C. Feister, eds. 1969 Coachman, L. K. and R. L. Charnell: On lateral water mass interaction - a case study, Bristol Bay, Alaska. J. phys. Oceanogr. 9,278-297 (1979) Denman, K. L.: Covariability of chlorophyll and temperature in the sea. Deep Sea Res. 23, 539-550 (1976) Denman, K. L. and T. Platt: Coherences in the horizontal distributions of phytoplankton and temperature in the upper ocean. Mere. Soc. r. Sci. Liege 6 e serie 7, 19-30 (1975) Denman, K. L. and T. Platt: The variance spectrum of phytoplankton in a turbulent ocean. J. mar. Res. 34,593-601 (1976) Fasham, M. J. R.: The statistical and mathematical analysis of phytoplankton patchiness. Oceanogr. mar. Biol. 16, 93 112 (1978) Flemer, D. A.: Continuous measurement of in vim chlorophyll of a dinoflagellate bloom in Chesapeake Bay. Chesapeake Sci. 10, 99-103 (1969) Flemer, D. A.: Primary production in the Chesapeake Bay. Chesapeake Sci. 11,117-129 (1970) Holligan, P. M.: Dinoflagellate blooms associated with tidal fronts around the British Isle~In: Toxic dinoflagellate blooms, pp 249 -256. Ed. by. D. L. Taylor and H. H. Seliger. New York: Elsevier 1979 Holligan, P. M. and D. S. Harbour: The vertical distribution and succession of phytoplankton in the western English Channel in 1975 and 1976. J. mar. biol. Ass. U.K. 57, 1075-1093 (1977) Iverson, R. L., L. K. Coachman, R. T. Cooney, T. S. English, J. J. Goering, G. L. Hunt, Jr., M. C. Macauley, C. P. McRoy, W. S. Reeburg and T. E. Whitledge: Ecological significance of fronts in the southeastern Bering Sea.In: Ecological processes in coastal and marine systems, pp 437-466. Ed. by R. J. Livingston. New York: Plenum Pub. 1979a Iverson, R. L., T. E. Whitledge and J. J. Goering: Chlorophyll and nitrate fine structure in the southeastern Bering Sea shelf break front. Nature, Lond. 281,664-666 (1979b) Kierstead, H. and L. B. Slobodkin: The size of water masses containing plankton blooms. J. mar. Res. 12,141-147 (1953) Loftus, M. E. and H. H. Seliger: Some limitations of the in vim fluorescence technique. Chesapeake Sci. 16, 79- 92 (1975) Loftus, M. E. and H. H. Seliger: A comparative study of primary production and standing crops of phytoplankton in a portion of the upper Chesapeake Bay subsequent to Tropical Storm Agnes. In: The effects of Tropical Storm Agnes on the Chesapeake Bay estuarine system. Chesapeake Res. Consortium Pub. 54, pp 509-521 Baltimore, Md.: Johns Hopkins Press 1976 Loftus, M. E., D. V. Subba Rao and H. H. Seliger: Growth and dissipation of phytoplankton in Chesapeake Bay. I. Response to a large pulse of rainfall. Chesapeake Sci. 13, 282-299 (1972) Marshall, H. G.: Plankton in James River estuary, Virginia. I. Phytoplankton in Willoughby Bay and Hampton Roads. Chesapeake Sci. 8, 90-101 (1967) Patten, B. C. and B. F. Chabot: Factorial productivity experiment in a shallow estuary: Characteristics of response surfaces. Chesapeake Sci. 7, 117-136 (1966) Patten, B. C., R. A. Mulford and J. E. Warinner: An annual phytoplankton cycle in the lower Chesapeake Bay. Chesapeake Sci. 4, 1-20 (1963) Pingree, R. D., P. M. Holligan, G. T. Mardell and R. N. Head: The influence of physical stability on spring, summer and autumn phytoplankton blooms in the Celtic Sea. J. mar. biol. Ass. U.K. 56,845-873 (1976) Pingree, R. D., P. R. Pugh, P. M. Holligan and G. R.Foster: Summer phytoplankton blooms and red tides along tidal fronts in the approaches to the English Channel. Nature Lond., 258,672-677 (1975) Platt, T., and K. L. Denman: Spectral analysis in ecology. Ann. Rev. Ecol. Syst. 6, 189-210 (1975)
H. H. Seliger et al.: Phytoplankton Patchiness and Frontal Regions Rivkin, R. B., K. R. McKinley, K. R. H. Aspen and H. H. Seliger: Differential nutrient uptake in natural phytoplankton populations. Mar. Biol. submitted 1980 Rivkin, R. B., H. H. Seliger, W. H. Biggley and K. R. H. Aspden: Recirculation mechanism for dinoflagellates. Mar. Biol. to be submitted 1981 Seliger, H. H., J. H. Carpenter, M. Loftus, W. H. Biggley and W. D. McEkoy: Bioluminescence and phytoplankton successions in Bahia Fosforescente, Puerto Rico, Limnol. Oceanogr. 16, 608-622 (1971) Seliger, H. H., J. H. Carpenter, M. Loftus and W. D. McElroy: Mechanisms for the accumulation of high concentrations of dinoflagellates in a bioluminescent bay. Limnol. Oceanogr. 15, 234-245 (1970) Seliger, H. H., W. G. Fastie and W. D. McElroy: Towable photometer for rapid area mapping of concentrations of bioluminescent marine dinoflageUates. Limnol. Oceanogr. 14,809-813 (1969) Seliger, H. H. and M. E. Loftus. Growth and dissipation of phytoplankton in Chesapeake Bay II. A statistical analysis of phytoplankton standing crops in the Rhode and West Rivers and an adjacent section of the Chesapeake Bay. Chesapeake Sci. 15,185-205 (1974) Seliger, H. H., M. E. Loftus and D. V. Subba Rao: Dinoflagellate accumulations in Chesapeake Bay. In : Proc. 1st Internat. Conf. on Toxic DinoflageUate Blooms, pp 181 205. Ed. by V. R. LoCicero. Wakefield, Mass.: Mass. Sci. Tech. Found. 1975 Seliger, H. H., M. A. Tyler and K. R. McKinley: Phytoplankton distributions and red tides resulting from frontal circulation patterns. In: Toxic Dinoflagellate Blooms, pp 239-248. Ed. by D. L. Taylor and H. H. Seliger. New York: Elsevier 1979 Simpson, J. H. and J. R. Hunter: Fronts in the Irish Sea. Nature, Lond. 250,404-406 (1974) Skellam, J. G.: Random dispersal in theoretical populations. Biometrika 38, 196-219 (1951) ! Solorzano, L.: Determination of ammonia in natural waters by the phenolhypochlorite method. Limnol. Oceanogr. 14,799801 (1969)
131 Stross, R. G. and J. R. Stottlemeyer: Primary production in the Patuxent River. Chesapeake Sci. 6 , 1 2 5 - 1 4 0 (1965) Therriault, J. C., D. J. Lawrence and T. Platt: Spatial variability of phytoplankton turnover in relation to physical processes in a coastal environment. Limnol. Oceangr. 23, 900-911 (1978) Therriault, J. C. and T. Platt: Spatial heterogeneity of phytoplankton biomass and related factors on the near-surface waters of an exposed coastal embayment. Limnol. Oceangr. 23, 888-899 (1978) TyIer, M. A. and H. H. Seliger: Annual subsurface transport of a red tide dinoflageUate to its bloom area: Water circulation patterns and organism distributions in the Chesapeake Bay. Limnol. Oceanogr. 23, 227-246 (1978) Tyler, M. A. and H. H. Seliger: Selection for a red tide organsm: Physiological responses to the physical environment. Limnol. Oceanogr. 26, (1981) Wood, E. P., F. A. J. Armstrong and R. A. Richards: Determination of nitrate in seawater by cadmium-copper reduction to nitrite. J. mar. biol. Ass. U.K. 47, 23-31 (1967) Zubkoff, P. L., J. C. Munday Jr., R. G. Rhodes and J. E. Warinner III: Meso scale features o f summer (1975 - 1977 ) dinoflagellate blooms in the York River, Virginia (Chesapeake Bay Estuary). ln: Toxic dinoflagellate blooms, pp 279-286. Ed. by D. L. Taylor and H. H. Seliger. New York: Elsevier 1979 Zubkoff, P. L. and J. E. Warinner III: Synoptic sightings of red waters of the lower Chesapeake Bay and its tributary rivers. ln: Proc. 1st lnternat. Conf. on Toxic dinoflageUate Blooms, pp 105-112. Ed. by V. R. LoCicero. Wakefiedl, Mass.: Mass. Sci. Technol. Found. 1975
Date of final manuscript acceptance: October 7, 1980. Communicated by I. Morris, West Boothbay Harbor