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Hydrodynamic control of phytoplankton in low salinity waters of the James River estuary, Virginia, U.S.A.
-
Estuarine,
Coastal
and Shelf Science (1985) 21,653-667
_.-.
--
Hydrodynamic Control of Phytoplankton in Low Salinity Waters of the James River Estuary, Virginia, U.S.A
Margaret
J. Filardo
Old Dominion 23508, U.S.A.
University,
Received
2 November
Keywords:
mortality;
and William Department
M. Dunstan
of Oceanography,
1983 and in revised form
phytoplankton; biomass; Chesapeake Bay; Virginia
31 October
productivity;
Norfolk,
L’irginh
1984
river
flow;
salinity;
Autotrophic biomass and productivity as well as nutrient distributions and phytoplankton cell populations in the James River estuary, Virginia, were quantified both spatially and temporally over a 17-month period. Emphasis was placed on the very low salinity region of the estuary in order to gain information on the fate of freshwater phytoplankters. Differing amounts of freshwater plant biomass are advected into the estuary as living material, DOC or POC and the demonstrated variability of this input must play an important role in marine biogeochemical cycling. Late summer and fall maxima in both chlorophyll a and the photosynthetic production of particulate organic carbon in very low salinity regions were inversely correlated with river discharge. During periods of low river discharge greater than 50”,, of the chlorophyll u biomass measured at 0% disappeared within a narrow range of salinity (O--2%). Cell enumeration data suggest that species introduced from the freshwater endmember tend to comprise the bulk of the biomass removed. Confounding factors, which may contribute to the regulation of both the abundance and species of phytoplankters mid-river, include the flocculation of colloidal material with phytoplankton cells, the presence of the turbidity maximum and the growth of endemic phytoplankton populations. An inverse relationship exists between the phytoplankton abundance in very low salinity waters and the abundance of biomass measured in the lower portion of the river (estuary). Thus, autotrophic production in the fresh and very low salinity areas may indirectly regulate the onset on the spring bloom m the estuary by controlling the amount of nutrients available.
Introduction Phytoplankton in low salinity regions of estuaries may be very important to biogeochemical considerations in the marine environment. Freshwater phytoplankters are advected into low salinity estuarine waters where they must negotiate an osmotic stress or perish. ‘Mass mortalities’ with subsequent release of dissolved and particulate organic material and coincident oxygen depletion have been hypothesized (Morris er al., 1978. 1982). 653
654
M. J. Filardo & W. M. Dunstan
77000
g
76045’
76”30’ I
I "OpeweL
I
,
76”15’ I
1
,
.
--__. JF
Jamestown
Island
5-Y
c
Hog f’(
37000
km
770150
Figure
1. The James River
77000’
estuary
76’45’
study
76”30’
76”15’
area.
The phytoplankters of intermediate and higher salinity waters of estuaries have been described (e.g. Mommaerts, 1969) and many studies are available on the abundance of freshwater phytoplankton (Baker & Baker, 1979; Lack, 1971). However, little work has been done on determining the biomass and productivity of low salinity populations in spite of the need to understand the fate and impact of these populations on the biogeochemistry of marine waters. Furthermore, the significance of physical factors closely involved in controlling the low salinity biomass have not been elucidated. Several studies suggest a hydrodynamic control over phytoplankton population size in the Potomac River (Sze, 1981) and in the low salinity region of the Tamar Estuary (Morris et al., 1982). Mantoura et al. (1982) predict from a Ketchum (195 1) type tidal exchange model that endemic populations will not accumulate at the fresh/brackish water interface of the Tamar estuary because average phytoplankton division time exceeds flushing time. The James River (Figure 1) is the southernmost of the major rivers which empty into the western side of the Chesapeake Bay. The river extends the entire breadth of the state of Virginia, from its mouth at Hampton Roads to its headwaters in the Appalachian Mountains near the Virginia-West Virginia state line. Based on historical data the average flow associated with the river is 201 m3 s-r and varies from 0.28 m3 s-r to 8377 m3 s- ’ (Fang et al., 1973). The river is considered to be tidal for 169 km from its mouth in a north-west direction to Richmond. The purpose of this study was to determine the spatial and temporal variation in phytoplankton biomass and productivity along the axis of the James River estuary in order to increase our understanding of the fate of freshwater phytoplankters which are advected into the estuarine system. Materials
and methods
Sampling programme Stations in the James River estuary were occupied at approximately one month intervals from August 1981 through December 1982. Stations extended along an axial transect from Hog Point (Figure 1) to the O%Oisohaline during 1981 and from a fixed location off Newport News Shipyard (NNS) (76”25.25’W, 36”57.04’N) and the O%Oisohaline during 1982. The locations of all stations were set based on the surface salinity during each cruise rather than on geographic position. During the 1981 cruises four stations were sampled at O%o, 2%0, 4% and 8X. Eight locations were sampled per cruise during 1982 at O%O,2%0, 4%0, 8% plus two stations
Hydrodynamic
control of phytoplankton
655
evenly spaced (with respect to salinity) between 8%0and the salinity measuredat NNS. Very low salinity regions were defined as locations where the surface salinity when measured was between 0.0 and 0.75%0with a Beckman RS-5 inductive salinometer (this term is used synonomously with O%Oisohaline). Depending on weather and time, a second low salinity station was occupied at least 2 km upriver from the first low salinity station. Positions of stations were charted as latitude and longitude according to coordinates recorded on a Micrologic ML-1000 Loran C Navigator on board the R/V Linwood Holton. Water sampleswere collected from 1 m below the surface and 1 m above the bottom with 8-1 General Oceanic Go Flo bottles. Temperature and salinity profiles at each station were determined at 1 m intervals throughout the water column using a Beckman RS-5. All sampling took place during daylight hours and was not synoptic with respect to the time of day or the tidal stage. Phytoplankton
chlorophyll
and productivity
Total (unfractionated) and nanoplankton chlorophyll a were determined fluorometritally on 90” 0 acetone extractions of cells retained on a Gelman type A-E glassfiber filter (Yentsch & Menzel, 1963; Holm-Hansen et al., 1965; Strickland & Parsons, 1972) which were stored frozen until the analysis could be performed. Nanoplankton chlorophyll (I was operationally defined asthe fraction passingthrough a 28 pM Nitex net disc (Malone, 1971). Filters were ground with a serrated tissue homogenizer, centrifuged and the fluorescence of the supernatant before and after acidification was measured with a Turner Designs Model IO-OOORfluorometer calibrated with pure chlorophyll a according to the procedure described by the EPA publication number 236-2, Cincinnati, Ohio Laboratory. The fluorometer was equipped with an infrared sensitive photomultiplier and the appropriate filters. Replicate samples were obtained at every station and the value reported represents a mean. The photosynthetic production of particulate organic carbon was estimated from “C uptake measurements (Steemann Nielsen, 1952; Malone, 1971) for January through December 1982. For the analysis replicate, 125-ml light bottles and one dark bottle were drawn from each surface sample (which had been collected during daylight hours) and incubated with 5 uCi of 14C-labelled bicarbonate under artificial light for 2 h at surface water temperatures maintained with running seawater. Total CO, was estimated from a regression line derived from a knowledge of the total alkalinity and pH of the water measured during previous cruises. The use of this ‘standard’ regression line may lead to over-estimation of the specific activity of the samples. A more precise approach would require total alkalinity and pH measurementson each sample on each cruise. Due to the constraints imposed by lack of personnel, equipment and time, this ‘standard’ regression line had to be adopted. Following incubation the samples were filtered through Gelman 0.45 pM Metricel Membrane (GN-6) filters, washed with filtered seawater and fumed over HCl for 30 s. The filters were dried in a CO,-free atmosphere for at least 24 h before their activity was measuredwith a Packard Model 460C Tri-Carb liquid scintillation counter. Nutrient
analyses
Samples for nutrient analyseswere obtained from surface and bottom at all stations for the duration of the study. The water was filtered on board ship through Gelman Type
656
M. J. Filardo & W. M. Dunstan
A-E glass fiber filters, stored in polyethylene bottles and frozen until the analyses were performed. All nutrient concentrations were determined calorimetrically: phosphate by the ascorbic acid reduction of the phosphomolybdate complex (abs 885 nm) (Murphy & Riley, 1962); ammonia by the formation of an indophenol blue complex (abs 640 nm) from ammonia, phenol and hypochlorite (Liddicoat et al., 1976); nitrate by conversion to nitrite after reduction using a copper cadmium column and subsequently, nitrite by the formation of an azo dye (abs 543 nm) after reacting with sulfanilamide in an acidic solution (Wood et al., 1967). The detection limit of the phosphate, nitrite and ammonia methods were + 0.05 umol l- ‘. CelZ ident&ation At both depths at every station 250 ml of water were collected for cell identification. The samples were preserved in a modified Lugols’ solution and stored in amber glass bottles. Cell identification was accomplished using a Unitron Series N inverted microscope following a 24-h settling period in a Zeiss IO-cc combined chamber. At least 100 cells were counted and dominance was presented as a percentage of the total number of cells counted. The samples that were counted represent four different time periods of the year - January, April, July and October. The phytoplankton population present during these months would have been subjected to regimes which varied with respect to river flow, temperature and photoperiod. Samples counted included those from low surface salinities (O-2%0), moderate surface salinities (4X&) and Newport News Shipyard, whose phytoplankton population was considered to be representative of the estuarine community. River discharge Daily mean values for river discharge (obtained in ft3 s-r and converted to m3 s-r) were provided by the Virginia State Water Control Board. The discharge data were collected at station 02037500 (37”33’47”N, 77”32’5O”W) in the James River near Richmond, Virginia. For statistical analysis, daily mean discharge was used as an index of flushing time and these values were averaged over the three days prior to sampling. This assumes that three days represents sufficient time for a phytoplankton population to divide and establish itself. Results Those stations designated very low salinity (0.00 to 0.75%0), as defined in the Methods, varied in location during the course of the study over a distance of approximately 45 km, from Hog Point to just west of Windmil Point (Figure 1). The range depended on freshwater input and tidal stage and is similar to the 40-km range for the 1%0 isohaline described by Haas (1977). The water column in this region was considered to be vertically homogeneous with respect to salinity which varied less than 0.55%0. The depth at the very low salinity station varied from 5 to 13 m. The deepest station was most often less than 7 m with November (13 m) and December (8 m) being the exceptions. Surface phytoplankton chlorophyll a at the very low salinity station [Figure 2(b), Table l] ranged seasonally from a low of 0.93 ug 1-l on 15 March 1982 to a high of 38.68 ug l- ’ on 23 October 1982. The corresponding values measured 1 m above the
Hydrodynamic
35 -
control
of phytoplankton
657
(b)
1982
Month
Figure 2. (a) Mean river discharge (m3 s-r) sampling; (b) chlorophyll a (ug l- ‘) measured ( A-A ) at the very low salinity station.
at Richmond for the three days prior for the surface (0-O) and near-bottom
TABLE 1. Surface and bottom chlorophyll a (ug l- ‘) measured at the very low salinity station and the ‘near 2W surface salinity station. The numbers in brackets represent the actual salinity measured at the ‘near 2%~’ surface salinity stations Bottom
Surface Date
O%,
11 Aug 18 Aug 06 Ott 10 Nov 28-29
19-20 15-16 01-02 17-18
Jan Feb Mar Apr May Jun
03-04 23 July 20-2 1 Aug
12-13 22-23
19-20 15-16
Sep Ott Nov Dee
8.91 21.21 14.85 20.30 1.44 1.84 .93 7.74 5.09 8.69 18.90 17.32 20.20 38.68 11.25 5.77
2%0
13.58 8.49 13.04 14.85 1.04 2.97 1.71 4.37 5.19 3.75 6.28 11.60 8.23 5.77 3.19 1.88
(1.96) (1.72) (2.25) (3.27) (0.96) (3.44) (2.06) (2.14) (2.28) (2.22) (1.75) (1.90) (1.94) (2.00) (1.91) (2.06)
O%o
2%0
15.27 17.82 12.74 24.39 1.31 1.83 .98 8.74 5.20 9,12 23.61 18.18 13.91 34.64 11.11 6.93
7-47 (2.04) 8.91 (1.70) 5.46 (3.13) 7.12 (6.23) 1.01 (1.22) 2-97 (10.35) l-61 (6.23) 6.68 (11.10) 4.03 (2.61) 3-39 (2.52) 8.46 (2.20) lo-74 (2.36) 9.53 (2.08) 2-70 (2.38) 2-28 (2.13) l-90 (2.37)
to
658
M. J. Filardo & W. M. Dunstan
TABLE 2. Percent nanoplankton the ‘near 2%0’” surface salinity
chlorophyll stations
Surface
a measured
O%o
2%
o%a
2%
Nov
42 70 100 100 49 73 73 100 61 66 68 83 87
75 92 72 100 93 100 81 100 93 83 100 87 79
50 100 79 100 65 69 90 95 78 81 100 96 84
79 92 21 100 70 90 85 74 100 73 100 93 85
Feb Mar Apr May
Jun Jul Aw Sep Ott Nov Dee ’ See Table
28-29 Jan 19-20 Feb 15-16 Mar 01-02 Apr 17-18 May 03-04 Jun 23 Jul 20-2 1 Aug 12-13 Sep 22-23 Ott 19-20 Nov 1516 Dee
and
1 for actual salinities.
TABLE 3. Surface r4C photosynthesis stations from January through December Date
low salinity
Bottom
Date
Jan
for the very
0% 17.09 86.67 88.94 547.54 124.97 445.51 1220.93 422.40 960.32 1274.20 215.75 102.09
measured at the low salinity 1982 (ug C I- t h - ‘)
and
‘near
2%
2%0 31.92 123.31 116.15 258.60 215.67 306.74 637.06 591.04 277.31 322-77 86.16 26.40
bottom were 0.98 ugl-’ and 34.64pgl-‘, respectively [Figure 2(b)]. Nanoplankton chlorophyll a collected for November 1981 through December 1982 comprised 42-100% of the biomassat 0% and accounted for greater than 70% of the biomassat 2%0 (Table 2). Most often the nanoplankton represented a greater portion of the chlorophyll at the 2Oh stations compared to the O%O.The low 21% measurement in February 1982 (Table 2) coincides with the existence of a definite two-layered system at that time as indicated by the very high salinity of water (10.35%0,Table 1) measuredat the bottom. Both salinity and chlorophyll a data show that the water column at the 0% station was well mixed, whereas, at 2% the water column was stratified part of the time asa result of the spring-neap tidal cycle effects discussedby Haas (1977). The photosynthetic production of particulate organic carbon (Table 3) ranged from a low of 17.09 pg 1-r h- ’ to a high of 1274.20 ug I-‘h- 1 at the very low salinity station and from 31-92 ug 1-l h-’ to 637.06 pg I-’ h- ’ at the 2?& surface salinity station.
H-ydrodynamic
0
control
2
4
6 Sollnity
Figure bottom.
3. Nutrient l , NO,;
of phytoplankton
8
IO (%.A
12
14
Jonuory
_ ---.
16
18
659 -.--
20
1982
concentrations versus salinity n , NO,; 0, NH,; 0, PO,.
for January
1982;
(a) surface,
(b)
River discharge, used as an index of flushing time, ranged from 19.75 m3 s- ’ to 324.60m” SC’ [Figure 2(a)]. A Pearson Product Moment correlation coefficient calculated for the river discharge data and the river flow data (Q,) versus both surface and bottom chlorophyll a concentrations at very low salinities ( < 0.75%0) yielded significant negative correlations at a = 0.01 (Sokal & Rohlf, 1981). Identical tests calculated for river discharge data versus the chlorophyll a concentration at 2%0 yielded a significant negative correlation at a = 0.05 for the surface but a non-significant correlation with bottom values. River discharge versus surface photosynthesis measurements at very low salinity also yielded a significant negative correlation at a=0.05, whereas, a non-significant correlation was obtained using the photosynthesis measurementsobtained at 2%,. The concentration of NO; ranged from a low of < 1 umol l- ’ at the surface during May 1982 at salinities greater than 12%0to a high of 55 umol 1-l at the O%osurface salinity station during August 1982. Nitrate generally contributed the most significant portion of the inorganic nitrogen along the axis of the river from April through December, except on rare occasionswhen NH, exceeded NO; at salinities greater than 12%0and for the summer-fall months when NO; was the greater source of inorganic nitrogen in the higher salinity waters. During months which are characterized by high river discharge (e.g. January), NOT, maintains a relatively high concentration along the axis of the river (Figure 3). However, when discharge decreasesand biomassincreases(e.g. October) at the O%O isohaline, there is a removal of NO< in the O-2%0segment(Figure 4). Ammonium concentration ranged from 0 to 30 pmol l- ‘, During January and February the concentration of NH4 does exceed the concentration of NO< in the low and
M.J. Filardo & W. M. Dunstan
660
J-(a) 40 30 -f -
20
2 IO a. i. 0 .E E 40 E u”
30 20 IO
0
2
4
6
Salinity
8
IO (%d
12
14
October
16
18
20
1982
Figure 4. Nutrient concentrations versus salinity for October 1982; (a) surface, (b) bottom. l , NO,; n , NO,; 0, NH,; 0, PO,.
mid-salinities and during March there is a greater amount of NIT!, present in the higher salinity water. However, its concentration is lessthan that for NC3 during the remainder of the year. Phosphate was generally present in concentrations of less than 7 pmol l- ‘. The amount of phosphate was lowest during the winter-spring, at the higher salinities, when phytoplankton biomassexhibited a maximum at these stations. The results of the cell counts (Table 4) lists the four most abundant speciesin the surface waters for January, April, July and October 1982 at the 0% (considered to be representative of the freshwater phytoplankton community), 2%0,4%0, 8% and NNS stations. Samplescollected 1 m above the bottom were analyzed for the April 1982 cruise and for the O%oand 2%0surface salinity stations collected during October 1982. In general, those specieswhich exhibited a high percent abundance at either the OX or NNS station gradually decreased in overall percent abundance in the mid-estuary. At the mid-salinity stations specieswhich are different from either endmember tend to be present. During January, the freshwater sampleswere dominated by the diatom Cyclotella sp. which gradually disappeared from the sampleswith an increase in salinity. Euglena type microflagellates were an important contributor to the mid-estuarine community while diatoms dominated the higher salinity stations. In April the chain forming diatom, MeZosira sp. accounts for 68% of the biomass present in the freshwater. However, at a surface salinity of 2%~ only about 18% asmany Melosira sp. cells were enumerated suggesting a rapid removal of this species. The dinoflagellate, Prorocentrum minimum, was numerous in mid-estuary and the diatom Rhizosoleniafma, along with Cryptomonas sp. provided the bulk of the biomassat the higher salinity stations. It should be noted (Table 1) that during April the water
Hydrodynamic
control of phytoplankton
4. The four abundance
TABLE
percent
major
species
enumerated
Approximate 0
Month
surface
2
at each station
salinity
and their
associated
(%a)
8
4
43.2 20.5 18.2 15.9
CYI
Nav Mf Mel
Mf Nav Pr
26.5 16.8 15.6 13.2
Mf Nav CY 1 Mel
36.7 16.8 14.8 5.9
Mf UC cy I cy II
30.0 12.6 11.6 8.7
UC cy II
rn~
Mel cy I Micr Mf
67.9 10.1 6.8 4-l
Mel Mf Sk Nav
21.1 19.5 15.6 11.7
Pr Mel UC Sk
16.6 15.5 11.9 9.5
Rh Pr Mf Cry
24.2 23-l 16.8 9.5
Rh Crv UCSC
Apr I hot )
Mel cy I Nav Mf
47.0 13.7 12-9 5.2
Mel Rh Sk CY 1
9.5 24.8 10.4 8.5
Rh Sk UC Lep
50.0 18.3 7.1 6.1
Rh UC Ni Mf
59.3 7-8 7.4 6.5
Ri-. Sk Cr> Pr
July
Mel cy I Mf
34.8 22-o 15-7 7.6
Sk Mf Pr Micr
62.4 16.2 5.9 5.3
Sk Mf Micr Gym
53.6 11.7 6.6 4-o
Sk Mf Rod Sph
31.3 20 0 128 7.2
Sk Mf Ca Ni
39.3 21.3 14.8 14.8
Ni Cry UC Sk
37.0 32.7 10.3 6.9
[an (! m ,
Apr!l
1 in;
CY 1
MiC,
Ott
I 1 m,
Mel cy I Micr Ped
26.5 25.6 15.5 7.7
Micr Mf CY 1 Mer
13.6 il.9 11.0 7.6
Ott
(hot)
Mel CY 1 Micr Chr
41.4 19.9 9.4 56
Mel Micr Mf Mer
41.7 14.3 7.6 5.5
Crv Lep
0) Ul3 Mf Sal
Key to abbreviations:
Cy I, Cyclotella sp. I; Mel, Melosira sp.; Nav, Navicula sp.; Rh, fragilissima; UC, unidentified centrales; Sk, Skeletonema costarurn; Ni, sp.; Cy II, Cyclotella sp. II; Lep, Leptocylindrus minimus; Cry, Cryptomonas sp.; Mf, microflagellates; Pr, Prorocentrum minimum; Gym, Gymodinium sp.; Sph, unidentified blue green spheres; Rod, unidentified blue green rods; Micr, Microqstis sp.; Mer, Merismopedia sp.; Chr, Chroococcus dispersus; SC, Scenedesmus dimorphus; Red, Pedtastrum sp.
Rhizosolenia Nitzchia
column was extremely stratified. The surface to bottom salinity varied as much as 9%0, while at NNS the surface salinity only measured 8.86%0 compared to the more common 1518%0. During July and October, when biomass was relatively high in the freshwater, Melosira sp. and Cyclotella sp.1 dominated the biomass. However, during this time the The remainder was diatoms only comprised about 500,, of the total percent abundance. composed of porportionately greater amounts of the blue-green and green algae. The NMS station was not sampled during July, but the mid-river populations were almost entirely made up of the neritic diatom, Skeletonema costatum and microflagellates. In October, Cryptomonas sp. accounted for greater than 70”,, of the population at NNS, while mid-river diatoms and microflagellates dominated the phytoplankton community. The October bottom samples again illustrate the change in population structure from the freshwater to an increased salinity station. While the relative percentage of
662
M.J. Filardo & W. M. Dunstan
--‘zQ 38 b i 0
0 5
IO Salinity
15
20
25
(no)
Figure 5. Chlorophyll (I (pg I-‘) versus salinity for periods of low flow (September, October) and high flow (January, February).
Melosira sp. was the same at O%Oand 2%0, it should be noted that the biomass decreased by approximately a factor of 10 across this segment and the total number of Melosira sp. decreased by a factor of 5. Discussion Our data show that autotrophic biomass in the very low salinity waters of the James River estuary reached a maximum during the late summer and autumn months and these peak populations were coincident with a decrease in river discharge (Figure 2). Morris et al. (1982) suggest a pattern of seasonal phytoplankton abundance in the Tamar estuary based on oxygen data, which, after allowing for local climatic differences, is similar to that found in the James River estuary. Theoretically, for biomass to accumubte, growth must occur at a rate which exceeds flushing (where flushing is defined as the rate at which freshwater is being removed from the estuary driven by the influx of river discharge). When using a Ketchum (1951) type model to assess flushing rates in the very low salinity areas, the river discharge and the geometry of the basin dominate the calculations. Therefore, river discharge is related to the flushing rate and the statistically significant negative correlation between discharge and biomass strongly supports a hydrodynamic control over phytoplankton abundance. Similar variations in phytoplankton abundance and river discharge were noted by Sze ( 198 1) in the Potomac River. An inverse relationship was demonstrated between river flow and seasonal zooplankton abundance in the Hudson River estuary by Malone et al. (1980). The amount of zooplankton biomass, as a function of the hydrodynamics of the system, would therefore be expected to be low when phytoplankton biomass is low. Consequently, zooplankton grazing was not considered to be an important contributor to the reduction of phytoplankton biomass measured during months of increased river flow. It has been implied (Morris et al., 1978) that freshwater phytoplankton suffer mass mortality when subjected to the osmotic changes encountered at the freshwater-seawater interface. Our data (Figure 5) show that biomass is often decreased by a factor of 2 or more at a salinity of 296, (Table 1). Surface biomass along the length of the river, for periods of low discharge (September-October), maintains a relatively constant
Hydrodynamic
control of phytoplankton
Salinity
Lo&l
Figure6. Percentdiatomabundance andpercentmicroflagellate abundance (shaded area)for O%O, 2%0,4%0,8%0 andtheNNS stationsfor January,April, July andOctober surfacesamples. A-A, Y, flagellates; O-O, O0diatoms. concentration after the initial decrease. In addition, biomass exhibits little variation in concentration when a second very low salinity (0.00-0.75’?&) was occupied further upriver suggesting that dilution was not an important factor. If the decreasein biomassthat was measured was simply due to dilution then the relative percentage of the fresh water speciesthat had been advected acrossthe O-2960salinity segment would remain the same. Referring to Table 4, we note that in all the samples that were enumerated there is a change in the relative percentage of the speciespresent at O%O to those present at 2960.In order to ensure that the change in the relative percentage was not due to the addition of a new speciesnot present at O%,,the percent abundance using only those species that were present at 0% was calculated. Again the relative percent abundance of the community composition was altered in all casesalong the 0%0 to 2%0segment. The April and October bottom samples (Table 4) were enumerated in order to determine if the heavier cells were settling out of the water column. There was no evidence to suggestthat this occurred. Again, it appearsthat dilution doesnot seemto be an important contributory mechanism for the decreasein biomassmeasured along the segment from O%O to 2%0. The amount and type of endmember phytoplankton species which are advected mid-river are dependent on discharge as was demonstrated by Cloern et al. (1983) for San Francisco Bay. Their smaller winter mid-river populations contained many freshwater diatoms and microflagellates as did our samples.During summer when discharge decreased to within what they considered to be a ‘critical discharge’, neritic diatoms bloomed and replaced the freshwater types. In our study this was evidenced by the large numbers of Skeletonemucostatumpresent at low salinities in our July samples(Table 4). Additionally, it appears that the predominant cell types present in the mid-river communities were usually different than either endmember. There was a general
664
M. 7. Filardo & W. M. Dunstan
increase in the percent abundance of flagellated cells (Figure 6) in the surface waters, which during certain months exceeds or equals diatom percent abundance. These data suggest that there is a phytoplankton community which resides mid-river whose structure may be a function of salinity tolerance and hydrodynamics. The fact that freshwater phytoplankton may exhibit a mass mortality in such a narrow range of salinity seems incongruous when considering the laboratory studies which have demonstrated the ability of several species of phytoplankton to tolerate large variations in osmotic pressure (McLachlan, 1961; Hellebust, 1976; Brown & Borowitzka, 1979; Paul, 1979; Setter & Greenway, 1979; Greenway & Setter, 1979; Takuda, 1968; Tomas, 1978). However, laboratory studies which employ batch culture and single salinity changes may not be acceptable analogs of the natural system when considering osmotic stress. If we consider the absolute percentage of biomass that survives, rather than the abundance, it becomes apparent that a greater percentage of phytoplankton negotiate the osmotic stress during periods of high discharge, i.e. January to February (Table 1). Since discharge plays an important role in the determination of survival we suggest that the rate of change of salinity, the time associated with the osmotic stress and exposure to a repeated osmotic stress (such as that being encountered by a cell being advected downriver) are more significant factors in determining a cell’s survival than the absolute magnitude of the salinity change. The occurrence of the turbidity maximum (Sharp et al., 1982) and the tendency of clay minerals to form floes incorporating phytoplankters (Avnimelech et al., 1982) present alternative hypotheses to which the decrease in biomass in the low salinities may be attributed. However, in this study particular care was taken to sample further upriver than the turbidity maximum when one was formed (accounting for the variability in the surface salinity of those stations designated 2%0 surface salinity). Additionally, the role of the turbidity maximum, in terms of limiting growth by a reduction in the depth of the photic zone, is dubious since the same physical processes which are responsible for the formation of the turbidity maximum, would also serve to readvect phytoplankters from the deeper waters into the photic zone (Cloem et al., 1982). Avnimelech et al. (1982) present evidence that aggregation with clay particles for the species they tested only occurs at a NaC concentration greater than 2 x lo-’ mol 1-l (S> 15%0). Therefore, it appears that both of these mechanisms may play an important role in mediating the amount of biomass present at intermediate salinities, however, their role in accounting for the initial decrease in biomass along the O-2%0 salinity segment is questionable. During periods of high discharge (January-February) biomass increases downriver in the more estuarine water (Figure 5). At the Newport News station increased discharge is correlated with an increase in growth. While river discharge is not directly responsible for estuarine phytoplankton abundance (in a hydrodynamic sense) it may, by controlling determine whether nutrient-rich or nutrientphytoplankton abundance upriver, depleted water is available down river. This is evidenced by the inverse relationship that exists between the biomass measured at O%Osalinity and that measured at the Newport News Shipyard station (Figure 7) and the concurrent removal of NO; together with biomass during October 1982 (Figure 4) along the low salinity (O-2%0) segment. The onset of the spring bloom in the more estuarine portion of the river, while mediated by light and temperature, appears to be a direct function of river discharge which by regulating the abundance of phytoplankton upriver may control the amount of nutrients reaching this area.
Hydrodynamic
2 0
25-
<
zo
control
Newport
665 -
of phytoplankton
low s~llnlty stot1on / f-.. .I’
News
P”
: ’
: 1 ‘(
,;
ii
\ :
/’
99
.- __. I 7 ’
JFMAMJJASON
’
’
’ Month
Figure 7. Monthly and at the Newport
’
’
’
’
’
(1982)
surface chlorophyll News station.
a (pg l- I) values for the very
low salinity
station
The photosynthetic production of particulate organic carbon in the very low salinity areas exhibit the same pattern as biomass, which implies that this population is viable and is producing at a rate comparable to those characteristic of productive nearshore regions. These high rates of productivity may, in part, be a function of the predominance of nanoplankton (Table 2) which have high surface to volume ratios and hence, higher growth rates. Caution must be used in viewing the production data, however; total CO, may have been over-estimated since total CO, was not measured on every cruise (a regression between total CO, and salinity was substituted) and may have changed over the duration of the study as discussedin the Methods. In summary, autotrophic biomassand productivity in very low salinity waters exhibits a dynamic and variable pattern which to a great extent is hydrodynamically controlled. It is important that this be taken into account when considering the biogeochemistry of marine and estuarine systems. The amount of carbon produced in the very low salinity waters during the month of highest discharge (corresponding to lowest biomass) was 0.012 g C m-’ day-’ and when biomass was at a maximum (low discharge) the amount increased two orders of magnitude to 3.61 g C m-* day- ‘. The decomposition of this biomass causesa down river displacement firstly of nutrients, which at some point will be reutilized in the euphotic zone and, secondly, of detritus which ultimately enters a detrital food chain. There is good evidence to suggestthat osmotic stress,causedby an increasein salinity, selectively effects phytoplankton survival and at times plays an important role in the initial decline of biomass along the low salinity (O-2%) segment as shown in our September to October data (Figure 5). Cell enumerations have shown that the types of phytoplankters vary considerably seasonally and in their distribution along the river axis. Also, productivity measurements indicate that autotrophic production is taking place all along the salinity gradients on the axes of the river. Lastly, the abundance of phytoplankton downriver is inversely related to the amount of biomass upriver, thus the status of autotrophic activity at very low salinities may be the controlling factor in the determination of the onset of the estuarine spring bloom. Acknowledgements The authors extend their appreciation to the crew of the R/V Linwood Holton - Bob Bray, Donnie Padgett and Nelson Griffin - and to all those individuals who helped in the
M. J. Filardo
666
& W. M. Dunsran
collection of samples, among them Howard Schaller, Beth Hester, Emily Deaver, Richard Lacouture, Pete Spence, David Velinsky and Kazufumi Takayanagi. This research was in part supported by the Samuel and Fay Slover Funds at Old Dominion University. References Avnimelech, Y., Troeger, B. W. & Reed, L. W. 1982 Mutual flocculation of algae and clay. Evidence and implications. Science, 216,63-65. Baker, A. W. & Baker, A. J. 1979 Effects of temperature and current discharge of the concentration and photosynthetic activity of phytoplankton in the upper Mississippi river. Freshwater Biology, 9, 191-198. Brown, A. D. & Borowitzka, L. J. 1979 Halotolerance of DunalieIla. In Biochemistry and Physiology of Prorozoa, Vol. 1 (Levandowsky, M. & Humer, S. S. eds). Academic Press, London and Orlando. pp. 139-190. Cloem, J. E., Alpine, A. E., Cole, B. E., Wong, R. L. J., Arthur, J. F. & Ball, M. D. 1983 River discharge controls phytoplankton dynamics in the northern San Francisco Bay estuary. Estuarine, Coastal and Shelf Science.
16,415-429.
Fang, C. S., Kuo, A. Y., Hyer, P. V. & Hargis, Jr., W. J. 1973 Hydrography and hydrodynamics of Virginia estuarines. IV. Mathematical Model Studies of Water Quality in the James Estuary. Special Report No. 41, Virginia Institute of Marine Science, September. Greenway, H. & Setter, T. L. 1979 Accumulation of proline and sucrose during the first hours after transfer of Cklorella emersonii to high NaCl. Auswalian Journal of Plant Physiology, 6,69-79. Haas, L. W. 1977 The effect of the spring-neap tidal cycle on the vertical salinity structure of the James, York and Rappahannock rivers, Virginia, U.S.A. Estuarine and Coastal Marine Science, 5,485-496. Hellebust, J. A., 1976 Effect of salinity on photosynthesis and mannitol synthesis in the green flagellate Platymonas sue&a. Canadian Journal of Botany, 54,1735-1741. Hahn-Hansen, O., Lorenzen, C. J., Holmes, R. W. & Strickland, J. D. H. 1965 Fluorometric determination of chlorophyll. Journal du Consceil,Con.& Inremarionalpour PExpIoration de la Mer, 30,~15. Ketchum, B. H. 1951 The exchange of fresh wafer and salt water in tidal estuaries. %urnal of Marine Research, 10,18-38. Lack, T. J. 1971 Quantitative studies on the phytoplankton of the rivers Thames and Kennet at Reading. Freshwater Biology, 1,213-224. Liddicoat, M. I., Tibbits, S. 81 Butler, E. I. 1976 The determination of ammonia in natural waters. wafer Research, 10,567-568. M&a&an, J. 1961 The effect of salinity on growth and chlorophyll content in representative classes of unicellular marine algae. Canadian Journal of Microbiology, 7,399-406. Malone, T. C. 1971 The relative importance of nanoplankton as primary producers in the California current system. Fisheries Bulletin, 69,799-820. Malone, T. C., Neale, P. J. & Boardman, D. C. 1980 Inl%nces of estuarine circulation on the distribution and biomass of phytoplankton size fractions. In Esruarine Perspecriues (Kennedy, V. S., ed.). Academic Press, Orlando and London. Mantoura, R. F. C., Morris, A. W., Burkill, P. H. & Owens, N. J. P. 1982 Chemical and biological reactivity at the freshwater, brackishwater interphase (FBI) of estuaries. Estuarine and Brackish Water Sciences Association Bulletin, No. 32. Mommaerts, J. P. 1969 The distribution of major nutrients and phytoplankton in the Tamar Estuary. Journal of the Marine Biological Association of the Um’ted Kingdom, 49,749-765. Morris, A. W., Mantoura, R. F. C., Bale, A. J., Howland, R. J. M. 1978 Very low salinity regions of estuaries; important sites for chemical and biological reactions. Nature, 274, (5672), 678-680. Morris, A. W., Bale, A. J. & Howland, R. J. M. 1982 Chemical variability in the Tamar Estuary South-west England. Estuarine, Coastal and Shelf Science, 14,649-661. Murphy, J. & Riley, J. P. 1962 A modified single solution method for the determination of phosphate in natural waters. Analytical Chimica Acta, 27,31-36. Paul, J. S. 1979 Osmoregulation in the marine diatom Cylindrotheca fus~ownis. 3ow-d of Phycology, 15, 280-284.
Setter, T. L. & Greenway, H. 1979 Growth and osmoregulation of Chlorefla emersonii in NaCl and neutral osmotica. Ausrralian3ournal of Plant Physiology, 6,47-60. Sharp, J. H., Culberson, C. H. & Church, T. M. 1982 The chemistry of the Delaware estuary. General considerations. Limnologv and Oceanography, 27(6), 1015-1028. Sokal, R. R. i? Rohlf, F. J. 1981 Biameny, W. H. Freeman and Company. Steeman Neilsen, E. 1952 The use of radioactive carbon (%) for measuring organic production in the sea. 3ournal du Conseil Inremationalpour I’Exploration de la Mer, l&117-140.
Hydrodynamic
control
of phytoplankton
I)07
Strickland, J. D. H. & Parsons, T. R. 1972 A practical handbook of seawater analysis. Bulletin of the Fisheries Research Board of Canada, 167. Sze, I’. 1981 A culture model for phytoplankton succession in the Potomac River near Washington, D.C. (U.S.A.). Phycologia, 20(3), 285-291. Takuda, H. 1968 Effects of salinity on the growth of a marine diatom Nitzchia closterium. Bulletin of the Plankton Society ofJapan. 15,13-19. Tomas, C. R. 1978 Olisthodiscus luteus (Chrysophyceae) I. Effects of salinity and temperature on growth, motility and survival. Journal ofl’hycology, 14,309-313. Wood, E. D., Armstrong, F. A. J. St Richards, F. A. 1967 Determination of nitrate in sea water by cadmium copper reduction to nitrate. Journal of the Marine Biological Association of the United Kingdom, 41, 23-31. Yentsch, C. S. & Menzel, D. W. 1963 A method for the determination of phytoplankton chlorophyll and phaeophytin by fluorescence. Deep Sea Research, 10,221-231.
Estuarine,
Coastal
and Shelf Science (1985) 21,653-667
_.-.
--
Hydrodynamic Control of Phytoplankton in Low Salinity Waters of the James River Estuary, Virginia, U.S.A
Margaret
J. Filardo
Old Dominion 23508, U.S.A.
University,
Received
2 November
Keywords:
mortality;
and William Department
M. Dunstan
of Oceanography,
1983 and in revised form
phytoplankton; biomass; Chesapeake Bay; Virginia
31 October
productivity;
Norfolk,
L’irginh
1984
river
flow;
salinity;
Autotrophic biomass and productivity as well as nutrient distributions and phytoplankton cell populations in the James River estuary, Virginia, were quantified both spatially and temporally over a 17-month period. Emphasis was placed on the very low salinity region of the estuary in order to gain information on the fate of freshwater phytoplankters. Differing amounts of freshwater plant biomass are advected into the estuary as living material, DOC or POC and the demonstrated variability of this input must play an important role in marine biogeochemical cycling. Late summer and fall maxima in both chlorophyll a and the photosynthetic production of particulate organic carbon in very low salinity regions were inversely correlated with river discharge. During periods of low river discharge greater than 50”,, of the chlorophyll u biomass measured at 0% disappeared within a narrow range of salinity (O--2%). Cell enumeration data suggest that species introduced from the freshwater endmember tend to comprise the bulk of the biomass removed. Confounding factors, which may contribute to the regulation of both the abundance and species of phytoplankters mid-river, include the flocculation of colloidal material with phytoplankton cells, the presence of the turbidity maximum and the growth of endemic phytoplankton populations. An inverse relationship exists between the phytoplankton abundance in very low salinity waters and the abundance of biomass measured in the lower portion of the river (estuary). Thus, autotrophic production in the fresh and very low salinity areas may indirectly regulate the onset on the spring bloom m the estuary by controlling the amount of nutrients available.
Introduction Phytoplankton in low salinity regions of estuaries may be very important to biogeochemical considerations in the marine environment. Freshwater phytoplankters are advected into low salinity estuarine waters where they must negotiate an osmotic stress or perish. ‘Mass mortalities’ with subsequent release of dissolved and particulate organic material and coincident oxygen depletion have been hypothesized (Morris er al., 1978. 1982). 653
654
M. J. Filardo & W. M. Dunstan
77000
g
76045’
76”30’ I
I "OpeweL
I
,
76”15’ I
1
,
.
--__. JF
Jamestown
Island
5-Y
c
Hog f’(
37000
km
770150
Figure
1. The James River
77000’
estuary
76’45’
study
76”30’
76”15’
area.
The phytoplankters of intermediate and higher salinity waters of estuaries have been described (e.g. Mommaerts, 1969) and many studies are available on the abundance of freshwater phytoplankton (Baker & Baker, 1979; Lack, 1971). However, little work has been done on determining the biomass and productivity of low salinity populations in spite of the need to understand the fate and impact of these populations on the biogeochemistry of marine waters. Furthermore, the significance of physical factors closely involved in controlling the low salinity biomass have not been elucidated. Several studies suggest a hydrodynamic control over phytoplankton population size in the Potomac River (Sze, 1981) and in the low salinity region of the Tamar Estuary (Morris et al., 1982). Mantoura et al. (1982) predict from a Ketchum (195 1) type tidal exchange model that endemic populations will not accumulate at the fresh/brackish water interface of the Tamar estuary because average phytoplankton division time exceeds flushing time. The James River (Figure 1) is the southernmost of the major rivers which empty into the western side of the Chesapeake Bay. The river extends the entire breadth of the state of Virginia, from its mouth at Hampton Roads to its headwaters in the Appalachian Mountains near the Virginia-West Virginia state line. Based on historical data the average flow associated with the river is 201 m3 s-r and varies from 0.28 m3 s-r to 8377 m3 s- ’ (Fang et al., 1973). The river is considered to be tidal for 169 km from its mouth in a north-west direction to Richmond. The purpose of this study was to determine the spatial and temporal variation in phytoplankton biomass and productivity along the axis of the James River estuary in order to increase our understanding of the fate of freshwater phytoplankters which are advected into the estuarine system. Materials
and methods
Sampling programme Stations in the James River estuary were occupied at approximately one month intervals from August 1981 through December 1982. Stations extended along an axial transect from Hog Point (Figure 1) to the O%Oisohaline during 1981 and from a fixed location off Newport News Shipyard (NNS) (76”25.25’W, 36”57.04’N) and the O%Oisohaline during 1982. The locations of all stations were set based on the surface salinity during each cruise rather than on geographic position. During the 1981 cruises four stations were sampled at O%o, 2%0, 4% and 8X. Eight locations were sampled per cruise during 1982 at O%O,2%0, 4%0, 8% plus two stations
Hydrodynamic
control of phytoplankton
655
evenly spaced (with respect to salinity) between 8%0and the salinity measuredat NNS. Very low salinity regions were defined as locations where the surface salinity when measured was between 0.0 and 0.75%0with a Beckman RS-5 inductive salinometer (this term is used synonomously with O%Oisohaline). Depending on weather and time, a second low salinity station was occupied at least 2 km upriver from the first low salinity station. Positions of stations were charted as latitude and longitude according to coordinates recorded on a Micrologic ML-1000 Loran C Navigator on board the R/V Linwood Holton. Water sampleswere collected from 1 m below the surface and 1 m above the bottom with 8-1 General Oceanic Go Flo bottles. Temperature and salinity profiles at each station were determined at 1 m intervals throughout the water column using a Beckman RS-5. All sampling took place during daylight hours and was not synoptic with respect to the time of day or the tidal stage. Phytoplankton
chlorophyll
and productivity
Total (unfractionated) and nanoplankton chlorophyll a were determined fluorometritally on 90” 0 acetone extractions of cells retained on a Gelman type A-E glassfiber filter (Yentsch & Menzel, 1963; Holm-Hansen et al., 1965; Strickland & Parsons, 1972) which were stored frozen until the analysis could be performed. Nanoplankton chlorophyll (I was operationally defined asthe fraction passingthrough a 28 pM Nitex net disc (Malone, 1971). Filters were ground with a serrated tissue homogenizer, centrifuged and the fluorescence of the supernatant before and after acidification was measured with a Turner Designs Model IO-OOORfluorometer calibrated with pure chlorophyll a according to the procedure described by the EPA publication number 236-2, Cincinnati, Ohio Laboratory. The fluorometer was equipped with an infrared sensitive photomultiplier and the appropriate filters. Replicate samples were obtained at every station and the value reported represents a mean. The photosynthetic production of particulate organic carbon was estimated from “C uptake measurements (Steemann Nielsen, 1952; Malone, 1971) for January through December 1982. For the analysis replicate, 125-ml light bottles and one dark bottle were drawn from each surface sample (which had been collected during daylight hours) and incubated with 5 uCi of 14C-labelled bicarbonate under artificial light for 2 h at surface water temperatures maintained with running seawater. Total CO, was estimated from a regression line derived from a knowledge of the total alkalinity and pH of the water measured during previous cruises. The use of this ‘standard’ regression line may lead to over-estimation of the specific activity of the samples. A more precise approach would require total alkalinity and pH measurementson each sample on each cruise. Due to the constraints imposed by lack of personnel, equipment and time, this ‘standard’ regression line had to be adopted. Following incubation the samples were filtered through Gelman 0.45 pM Metricel Membrane (GN-6) filters, washed with filtered seawater and fumed over HCl for 30 s. The filters were dried in a CO,-free atmosphere for at least 24 h before their activity was measuredwith a Packard Model 460C Tri-Carb liquid scintillation counter. Nutrient
analyses
Samples for nutrient analyseswere obtained from surface and bottom at all stations for the duration of the study. The water was filtered on board ship through Gelman Type
656
M. J. Filardo & W. M. Dunstan
A-E glass fiber filters, stored in polyethylene bottles and frozen until the analyses were performed. All nutrient concentrations were determined calorimetrically: phosphate by the ascorbic acid reduction of the phosphomolybdate complex (abs 885 nm) (Murphy & Riley, 1962); ammonia by the formation of an indophenol blue complex (abs 640 nm) from ammonia, phenol and hypochlorite (Liddicoat et al., 1976); nitrate by conversion to nitrite after reduction using a copper cadmium column and subsequently, nitrite by the formation of an azo dye (abs 543 nm) after reacting with sulfanilamide in an acidic solution (Wood et al., 1967). The detection limit of the phosphate, nitrite and ammonia methods were + 0.05 umol l- ‘. CelZ ident&ation At both depths at every station 250 ml of water were collected for cell identification. The samples were preserved in a modified Lugols’ solution and stored in amber glass bottles. Cell identification was accomplished using a Unitron Series N inverted microscope following a 24-h settling period in a Zeiss IO-cc combined chamber. At least 100 cells were counted and dominance was presented as a percentage of the total number of cells counted. The samples that were counted represent four different time periods of the year - January, April, July and October. The phytoplankton population present during these months would have been subjected to regimes which varied with respect to river flow, temperature and photoperiod. Samples counted included those from low surface salinities (O-2%0), moderate surface salinities (4X&) and Newport News Shipyard, whose phytoplankton population was considered to be representative of the estuarine community. River discharge Daily mean values for river discharge (obtained in ft3 s-r and converted to m3 s-r) were provided by the Virginia State Water Control Board. The discharge data were collected at station 02037500 (37”33’47”N, 77”32’5O”W) in the James River near Richmond, Virginia. For statistical analysis, daily mean discharge was used as an index of flushing time and these values were averaged over the three days prior to sampling. This assumes that three days represents sufficient time for a phytoplankton population to divide and establish itself. Results Those stations designated very low salinity (0.00 to 0.75%0), as defined in the Methods, varied in location during the course of the study over a distance of approximately 45 km, from Hog Point to just west of Windmil Point (Figure 1). The range depended on freshwater input and tidal stage and is similar to the 40-km range for the 1%0 isohaline described by Haas (1977). The water column in this region was considered to be vertically homogeneous with respect to salinity which varied less than 0.55%0. The depth at the very low salinity station varied from 5 to 13 m. The deepest station was most often less than 7 m with November (13 m) and December (8 m) being the exceptions. Surface phytoplankton chlorophyll a at the very low salinity station [Figure 2(b), Table l] ranged seasonally from a low of 0.93 ug 1-l on 15 March 1982 to a high of 38.68 ug l- ’ on 23 October 1982. The corresponding values measured 1 m above the
Hydrodynamic
35 -
control
of phytoplankton
657
(b)
1982
Month
Figure 2. (a) Mean river discharge (m3 s-r) sampling; (b) chlorophyll a (ug l- ‘) measured ( A-A ) at the very low salinity station.
at Richmond for the three days prior for the surface (0-O) and near-bottom
TABLE 1. Surface and bottom chlorophyll a (ug l- ‘) measured at the very low salinity station and the ‘near 2W surface salinity station. The numbers in brackets represent the actual salinity measured at the ‘near 2%~’ surface salinity stations Bottom
Surface Date
O%,
11 Aug 18 Aug 06 Ott 10 Nov 28-29
19-20 15-16 01-02 17-18
Jan Feb Mar Apr May Jun
03-04 23 July 20-2 1 Aug
12-13 22-23
19-20 15-16
Sep Ott Nov Dee
8.91 21.21 14.85 20.30 1.44 1.84 .93 7.74 5.09 8.69 18.90 17.32 20.20 38.68 11.25 5.77
2%0
13.58 8.49 13.04 14.85 1.04 2.97 1.71 4.37 5.19 3.75 6.28 11.60 8.23 5.77 3.19 1.88
(1.96) (1.72) (2.25) (3.27) (0.96) (3.44) (2.06) (2.14) (2.28) (2.22) (1.75) (1.90) (1.94) (2.00) (1.91) (2.06)
O%o
2%0
15.27 17.82 12.74 24.39 1.31 1.83 .98 8.74 5.20 9,12 23.61 18.18 13.91 34.64 11.11 6.93
7-47 (2.04) 8.91 (1.70) 5.46 (3.13) 7.12 (6.23) 1.01 (1.22) 2-97 (10.35) l-61 (6.23) 6.68 (11.10) 4.03 (2.61) 3-39 (2.52) 8.46 (2.20) lo-74 (2.36) 9.53 (2.08) 2-70 (2.38) 2-28 (2.13) l-90 (2.37)
to
658
M. J. Filardo & W. M. Dunstan
TABLE 2. Percent nanoplankton the ‘near 2%0’” surface salinity
chlorophyll stations
Surface
a measured
O%o
2%
o%a
2%
Nov
42 70 100 100 49 73 73 100 61 66 68 83 87
75 92 72 100 93 100 81 100 93 83 100 87 79
50 100 79 100 65 69 90 95 78 81 100 96 84
79 92 21 100 70 90 85 74 100 73 100 93 85
Feb Mar Apr May
Jun Jul Aw Sep Ott Nov Dee ’ See Table
28-29 Jan 19-20 Feb 15-16 Mar 01-02 Apr 17-18 May 03-04 Jun 23 Jul 20-2 1 Aug 12-13 Sep 22-23 Ott 19-20 Nov 1516 Dee
and
1 for actual salinities.
TABLE 3. Surface r4C photosynthesis stations from January through December Date
low salinity
Bottom
Date
Jan
for the very
0% 17.09 86.67 88.94 547.54 124.97 445.51 1220.93 422.40 960.32 1274.20 215.75 102.09
measured at the low salinity 1982 (ug C I- t h - ‘)
and
‘near
2%
2%0 31.92 123.31 116.15 258.60 215.67 306.74 637.06 591.04 277.31 322-77 86.16 26.40
bottom were 0.98 ugl-’ and 34.64pgl-‘, respectively [Figure 2(b)]. Nanoplankton chlorophyll a collected for November 1981 through December 1982 comprised 42-100% of the biomassat 0% and accounted for greater than 70% of the biomassat 2%0 (Table 2). Most often the nanoplankton represented a greater portion of the chlorophyll at the 2Oh stations compared to the O%O.The low 21% measurement in February 1982 (Table 2) coincides with the existence of a definite two-layered system at that time as indicated by the very high salinity of water (10.35%0,Table 1) measuredat the bottom. Both salinity and chlorophyll a data show that the water column at the 0% station was well mixed, whereas, at 2% the water column was stratified part of the time asa result of the spring-neap tidal cycle effects discussedby Haas (1977). The photosynthetic production of particulate organic carbon (Table 3) ranged from a low of 17.09 pg 1-r h- ’ to a high of 1274.20 ug I-‘h- 1 at the very low salinity station and from 31-92 ug 1-l h-’ to 637.06 pg I-’ h- ’ at the 2?& surface salinity station.
H-ydrodynamic
0
control
2
4
6 Sollnity
Figure bottom.
3. Nutrient l , NO,;
of phytoplankton
8
IO (%.A
12
14
Jonuory
_ ---.
16
18
659 -.--
20
1982
concentrations versus salinity n , NO,; 0, NH,; 0, PO,.
for January
1982;
(a) surface,
(b)
River discharge, used as an index of flushing time, ranged from 19.75 m3 s- ’ to 324.60m” SC’ [Figure 2(a)]. A Pearson Product Moment correlation coefficient calculated for the river discharge data and the river flow data (Q,) versus both surface and bottom chlorophyll a concentrations at very low salinities ( < 0.75%0) yielded significant negative correlations at a = 0.01 (Sokal & Rohlf, 1981). Identical tests calculated for river discharge data versus the chlorophyll a concentration at 2%0 yielded a significant negative correlation at a = 0.05 for the surface but a non-significant correlation with bottom values. River discharge versus surface photosynthesis measurements at very low salinity also yielded a significant negative correlation at a=0.05, whereas, a non-significant correlation was obtained using the photosynthesis measurementsobtained at 2%,. The concentration of NO; ranged from a low of < 1 umol l- ’ at the surface during May 1982 at salinities greater than 12%0to a high of 55 umol 1-l at the O%osurface salinity station during August 1982. Nitrate generally contributed the most significant portion of the inorganic nitrogen along the axis of the river from April through December, except on rare occasionswhen NH, exceeded NO; at salinities greater than 12%0and for the summer-fall months when NO; was the greater source of inorganic nitrogen in the higher salinity waters. During months which are characterized by high river discharge (e.g. January), NOT, maintains a relatively high concentration along the axis of the river (Figure 3). However, when discharge decreasesand biomassincreases(e.g. October) at the O%O isohaline, there is a removal of NO< in the O-2%0segment(Figure 4). Ammonium concentration ranged from 0 to 30 pmol l- ‘, During January and February the concentration of NH4 does exceed the concentration of NO< in the low and
M.J. Filardo & W. M. Dunstan
660
J-(a) 40 30 -f -
20
2 IO a. i. 0 .E E 40 E u”
30 20 IO
0
2
4
6
Salinity
8
IO (%d
12
14
October
16
18
20
1982
Figure 4. Nutrient concentrations versus salinity for October 1982; (a) surface, (b) bottom. l , NO,; n , NO,; 0, NH,; 0, PO,.
mid-salinities and during March there is a greater amount of NIT!, present in the higher salinity water. However, its concentration is lessthan that for NC3 during the remainder of the year. Phosphate was generally present in concentrations of less than 7 pmol l- ‘. The amount of phosphate was lowest during the winter-spring, at the higher salinities, when phytoplankton biomassexhibited a maximum at these stations. The results of the cell counts (Table 4) lists the four most abundant speciesin the surface waters for January, April, July and October 1982 at the 0% (considered to be representative of the freshwater phytoplankton community), 2%0,4%0, 8% and NNS stations. Samplescollected 1 m above the bottom were analyzed for the April 1982 cruise and for the O%oand 2%0surface salinity stations collected during October 1982. In general, those specieswhich exhibited a high percent abundance at either the OX or NNS station gradually decreased in overall percent abundance in the mid-estuary. At the mid-salinity stations specieswhich are different from either endmember tend to be present. During January, the freshwater sampleswere dominated by the diatom Cyclotella sp. which gradually disappeared from the sampleswith an increase in salinity. Euglena type microflagellates were an important contributor to the mid-estuarine community while diatoms dominated the higher salinity stations. In April the chain forming diatom, MeZosira sp. accounts for 68% of the biomass present in the freshwater. However, at a surface salinity of 2%~ only about 18% asmany Melosira sp. cells were enumerated suggesting a rapid removal of this species. The dinoflagellate, Prorocentrum minimum, was numerous in mid-estuary and the diatom Rhizosoleniafma, along with Cryptomonas sp. provided the bulk of the biomassat the higher salinity stations. It should be noted (Table 1) that during April the water
Hydrodynamic
control of phytoplankton
4. The four abundance
TABLE
percent
major
species
enumerated
Approximate 0
Month
surface
2
at each station
salinity
and their
associated
(%a)
8
4
43.2 20.5 18.2 15.9
CYI
Nav Mf Mel
Mf Nav Pr
26.5 16.8 15.6 13.2
Mf Nav CY 1 Mel
36.7 16.8 14.8 5.9
Mf UC cy I cy II
30.0 12.6 11.6 8.7
UC cy II
rn~
Mel cy I Micr Mf
67.9 10.1 6.8 4-l
Mel Mf Sk Nav
21.1 19.5 15.6 11.7
Pr Mel UC Sk
16.6 15.5 11.9 9.5
Rh Pr Mf Cry
24.2 23-l 16.8 9.5
Rh Crv UCSC
Apr I hot )
Mel cy I Nav Mf
47.0 13.7 12-9 5.2
Mel Rh Sk CY 1
9.5 24.8 10.4 8.5
Rh Sk UC Lep
50.0 18.3 7.1 6.1
Rh UC Ni Mf
59.3 7-8 7.4 6.5
Ri-. Sk Cr> Pr
July
Mel cy I Mf
34.8 22-o 15-7 7.6
Sk Mf Pr Micr
62.4 16.2 5.9 5.3
Sk Mf Micr Gym
53.6 11.7 6.6 4-o
Sk Mf Rod Sph
31.3 20 0 128 7.2
Sk Mf Ca Ni
39.3 21.3 14.8 14.8
Ni Cry UC Sk
37.0 32.7 10.3 6.9
[an (! m ,
Apr!l
1 in;
CY 1
MiC,
Ott
I 1 m,
Mel cy I Micr Ped
26.5 25.6 15.5 7.7
Micr Mf CY 1 Mer
13.6 il.9 11.0 7.6
Ott
(hot)
Mel CY 1 Micr Chr
41.4 19.9 9.4 56
Mel Micr Mf Mer
41.7 14.3 7.6 5.5
Crv Lep
0) Ul3 Mf Sal
Key to abbreviations:
Cy I, Cyclotella sp. I; Mel, Melosira sp.; Nav, Navicula sp.; Rh, fragilissima; UC, unidentified centrales; Sk, Skeletonema costarurn; Ni, sp.; Cy II, Cyclotella sp. II; Lep, Leptocylindrus minimus; Cry, Cryptomonas sp.; Mf, microflagellates; Pr, Prorocentrum minimum; Gym, Gymodinium sp.; Sph, unidentified blue green spheres; Rod, unidentified blue green rods; Micr, Microqstis sp.; Mer, Merismopedia sp.; Chr, Chroococcus dispersus; SC, Scenedesmus dimorphus; Red, Pedtastrum sp.
Rhizosolenia Nitzchia
column was extremely stratified. The surface to bottom salinity varied as much as 9%0, while at NNS the surface salinity only measured 8.86%0 compared to the more common 1518%0. During July and October, when biomass was relatively high in the freshwater, Melosira sp. and Cyclotella sp.1 dominated the biomass. However, during this time the The remainder was diatoms only comprised about 500,, of the total percent abundance. composed of porportionately greater amounts of the blue-green and green algae. The NMS station was not sampled during July, but the mid-river populations were almost entirely made up of the neritic diatom, Skeletonema costatum and microflagellates. In October, Cryptomonas sp. accounted for greater than 70”,, of the population at NNS, while mid-river diatoms and microflagellates dominated the phytoplankton community. The October bottom samples again illustrate the change in population structure from the freshwater to an increased salinity station. While the relative percentage of
662
M.J. Filardo & W. M. Dunstan
--‘zQ 38 b i 0
0 5
IO Salinity
15
20
25
(no)
Figure 5. Chlorophyll (I (pg I-‘) versus salinity for periods of low flow (September, October) and high flow (January, February).
Melosira sp. was the same at O%Oand 2%0, it should be noted that the biomass decreased by approximately a factor of 10 across this segment and the total number of Melosira sp. decreased by a factor of 5. Discussion Our data show that autotrophic biomass in the very low salinity waters of the James River estuary reached a maximum during the late summer and autumn months and these peak populations were coincident with a decrease in river discharge (Figure 2). Morris et al. (1982) suggest a pattern of seasonal phytoplankton abundance in the Tamar estuary based on oxygen data, which, after allowing for local climatic differences, is similar to that found in the James River estuary. Theoretically, for biomass to accumubte, growth must occur at a rate which exceeds flushing (where flushing is defined as the rate at which freshwater is being removed from the estuary driven by the influx of river discharge). When using a Ketchum (1951) type model to assess flushing rates in the very low salinity areas, the river discharge and the geometry of the basin dominate the calculations. Therefore, river discharge is related to the flushing rate and the statistically significant negative correlation between discharge and biomass strongly supports a hydrodynamic control over phytoplankton abundance. Similar variations in phytoplankton abundance and river discharge were noted by Sze ( 198 1) in the Potomac River. An inverse relationship was demonstrated between river flow and seasonal zooplankton abundance in the Hudson River estuary by Malone et al. (1980). The amount of zooplankton biomass, as a function of the hydrodynamics of the system, would therefore be expected to be low when phytoplankton biomass is low. Consequently, zooplankton grazing was not considered to be an important contributor to the reduction of phytoplankton biomass measured during months of increased river flow. It has been implied (Morris et al., 1978) that freshwater phytoplankton suffer mass mortality when subjected to the osmotic changes encountered at the freshwater-seawater interface. Our data (Figure 5) show that biomass is often decreased by a factor of 2 or more at a salinity of 296, (Table 1). Surface biomass along the length of the river, for periods of low discharge (September-October), maintains a relatively constant
Hydrodynamic
control of phytoplankton
Salinity
Lo&l
Figure6. Percentdiatomabundance andpercentmicroflagellate abundance (shaded area)for O%O, 2%0,4%0,8%0 andtheNNS stationsfor January,April, July andOctober surfacesamples. A-A, Y, flagellates; O-O, O0diatoms. concentration after the initial decrease. In addition, biomass exhibits little variation in concentration when a second very low salinity (0.00-0.75’?&) was occupied further upriver suggesting that dilution was not an important factor. If the decreasein biomassthat was measured was simply due to dilution then the relative percentage of the fresh water speciesthat had been advected acrossthe O-2960salinity segment would remain the same. Referring to Table 4, we note that in all the samples that were enumerated there is a change in the relative percentage of the speciespresent at O%O to those present at 2960.In order to ensure that the change in the relative percentage was not due to the addition of a new speciesnot present at O%,,the percent abundance using only those species that were present at 0% was calculated. Again the relative percent abundance of the community composition was altered in all casesalong the 0%0 to 2%0segment. The April and October bottom samples (Table 4) were enumerated in order to determine if the heavier cells were settling out of the water column. There was no evidence to suggestthat this occurred. Again, it appearsthat dilution doesnot seemto be an important contributory mechanism for the decreasein biomassmeasured along the segment from O%O to 2%0. The amount and type of endmember phytoplankton species which are advected mid-river are dependent on discharge as was demonstrated by Cloern et al. (1983) for San Francisco Bay. Their smaller winter mid-river populations contained many freshwater diatoms and microflagellates as did our samples.During summer when discharge decreased to within what they considered to be a ‘critical discharge’, neritic diatoms bloomed and replaced the freshwater types. In our study this was evidenced by the large numbers of Skeletonemucostatumpresent at low salinities in our July samples(Table 4). Additionally, it appears that the predominant cell types present in the mid-river communities were usually different than either endmember. There was a general
664
M. 7. Filardo & W. M. Dunstan
increase in the percent abundance of flagellated cells (Figure 6) in the surface waters, which during certain months exceeds or equals diatom percent abundance. These data suggest that there is a phytoplankton community which resides mid-river whose structure may be a function of salinity tolerance and hydrodynamics. The fact that freshwater phytoplankton may exhibit a mass mortality in such a narrow range of salinity seems incongruous when considering the laboratory studies which have demonstrated the ability of several species of phytoplankton to tolerate large variations in osmotic pressure (McLachlan, 1961; Hellebust, 1976; Brown & Borowitzka, 1979; Paul, 1979; Setter & Greenway, 1979; Greenway & Setter, 1979; Takuda, 1968; Tomas, 1978). However, laboratory studies which employ batch culture and single salinity changes may not be acceptable analogs of the natural system when considering osmotic stress. If we consider the absolute percentage of biomass that survives, rather than the abundance, it becomes apparent that a greater percentage of phytoplankton negotiate the osmotic stress during periods of high discharge, i.e. January to February (Table 1). Since discharge plays an important role in the determination of survival we suggest that the rate of change of salinity, the time associated with the osmotic stress and exposure to a repeated osmotic stress (such as that being encountered by a cell being advected downriver) are more significant factors in determining a cell’s survival than the absolute magnitude of the salinity change. The occurrence of the turbidity maximum (Sharp et al., 1982) and the tendency of clay minerals to form floes incorporating phytoplankters (Avnimelech et al., 1982) present alternative hypotheses to which the decrease in biomass in the low salinities may be attributed. However, in this study particular care was taken to sample further upriver than the turbidity maximum when one was formed (accounting for the variability in the surface salinity of those stations designated 2%0 surface salinity). Additionally, the role of the turbidity maximum, in terms of limiting growth by a reduction in the depth of the photic zone, is dubious since the same physical processes which are responsible for the formation of the turbidity maximum, would also serve to readvect phytoplankters from the deeper waters into the photic zone (Cloem et al., 1982). Avnimelech et al. (1982) present evidence that aggregation with clay particles for the species they tested only occurs at a NaC concentration greater than 2 x lo-’ mol 1-l (S> 15%0). Therefore, it appears that both of these mechanisms may play an important role in mediating the amount of biomass present at intermediate salinities, however, their role in accounting for the initial decrease in biomass along the O-2%0 salinity segment is questionable. During periods of high discharge (January-February) biomass increases downriver in the more estuarine water (Figure 5). At the Newport News station increased discharge is correlated with an increase in growth. While river discharge is not directly responsible for estuarine phytoplankton abundance (in a hydrodynamic sense) it may, by controlling determine whether nutrient-rich or nutrientphytoplankton abundance upriver, depleted water is available down river. This is evidenced by the inverse relationship that exists between the biomass measured at O%Osalinity and that measured at the Newport News Shipyard station (Figure 7) and the concurrent removal of NO; together with biomass during October 1982 (Figure 4) along the low salinity (O-2%0) segment. The onset of the spring bloom in the more estuarine portion of the river, while mediated by light and temperature, appears to be a direct function of river discharge which by regulating the abundance of phytoplankton upriver may control the amount of nutrients reaching this area.
Hydrodynamic
2 0
25-
<
zo
control
Newport
665 -
of phytoplankton
low s~llnlty stot1on / f-.. .I’
News
P”
: ’
: 1 ‘(
,;
ii
\ :
/’
99
.- __. I 7 ’
JFMAMJJASON
’
’
’ Month
Figure 7. Monthly and at the Newport
’
’
’
’
’
(1982)
surface chlorophyll News station.
a (pg l- I) values for the very
low salinity
station
The photosynthetic production of particulate organic carbon in the very low salinity areas exhibit the same pattern as biomass, which implies that this population is viable and is producing at a rate comparable to those characteristic of productive nearshore regions. These high rates of productivity may, in part, be a function of the predominance of nanoplankton (Table 2) which have high surface to volume ratios and hence, higher growth rates. Caution must be used in viewing the production data, however; total CO, may have been over-estimated since total CO, was not measured on every cruise (a regression between total CO, and salinity was substituted) and may have changed over the duration of the study as discussedin the Methods. In summary, autotrophic biomassand productivity in very low salinity waters exhibits a dynamic and variable pattern which to a great extent is hydrodynamically controlled. It is important that this be taken into account when considering the biogeochemistry of marine and estuarine systems. The amount of carbon produced in the very low salinity waters during the month of highest discharge (corresponding to lowest biomass) was 0.012 g C m-’ day-’ and when biomass was at a maximum (low discharge) the amount increased two orders of magnitude to 3.61 g C m-* day- ‘. The decomposition of this biomass causesa down river displacement firstly of nutrients, which at some point will be reutilized in the euphotic zone and, secondly, of detritus which ultimately enters a detrital food chain. There is good evidence to suggestthat osmotic stress,causedby an increasein salinity, selectively effects phytoplankton survival and at times plays an important role in the initial decline of biomass along the low salinity (O-2%) segment as shown in our September to October data (Figure 5). Cell enumerations have shown that the types of phytoplankters vary considerably seasonally and in their distribution along the river axis. Also, productivity measurements indicate that autotrophic production is taking place all along the salinity gradients on the axes of the river. Lastly, the abundance of phytoplankton downriver is inversely related to the amount of biomass upriver, thus the status of autotrophic activity at very low salinities may be the controlling factor in the determination of the onset of the estuarine spring bloom. Acknowledgements The authors extend their appreciation to the crew of the R/V Linwood Holton - Bob Bray, Donnie Padgett and Nelson Griffin - and to all those individuals who helped in the
M. J. Filardo
666
& W. M. Dunsran
collection of samples, among them Howard Schaller, Beth Hester, Emily Deaver, Richard Lacouture, Pete Spence, David Velinsky and Kazufumi Takayanagi. This research was in part supported by the Samuel and Fay Slover Funds at Old Dominion University. References Avnimelech, Y., Troeger, B. W. & Reed, L. W. 1982 Mutual flocculation of algae and clay. Evidence and implications. Science, 216,63-65. Baker, A. W. & Baker, A. J. 1979 Effects of temperature and current discharge of the concentration and photosynthetic activity of phytoplankton in the upper Mississippi river. Freshwater Biology, 9, 191-198. Brown, A. D. & Borowitzka, L. J. 1979 Halotolerance of DunalieIla. In Biochemistry and Physiology of Prorozoa, Vol. 1 (Levandowsky, M. & Humer, S. S. eds). Academic Press, London and Orlando. pp. 139-190. Cloem, J. E., Alpine, A. E., Cole, B. E., Wong, R. L. J., Arthur, J. F. & Ball, M. D. 1983 River discharge controls phytoplankton dynamics in the northern San Francisco Bay estuary. Estuarine, Coastal and Shelf Science.
16,415-429.
Fang, C. S., Kuo, A. Y., Hyer, P. V. & Hargis, Jr., W. J. 1973 Hydrography and hydrodynamics of Virginia estuarines. IV. Mathematical Model Studies of Water Quality in the James Estuary. Special Report No. 41, Virginia Institute of Marine Science, September. Greenway, H. & Setter, T. L. 1979 Accumulation of proline and sucrose during the first hours after transfer of Cklorella emersonii to high NaCl. Auswalian Journal of Plant Physiology, 6,69-79. Haas, L. W. 1977 The effect of the spring-neap tidal cycle on the vertical salinity structure of the James, York and Rappahannock rivers, Virginia, U.S.A. Estuarine and Coastal Marine Science, 5,485-496. Hellebust, J. A., 1976 Effect of salinity on photosynthesis and mannitol synthesis in the green flagellate Platymonas sue&a. Canadian Journal of Botany, 54,1735-1741. Hahn-Hansen, O., Lorenzen, C. J., Holmes, R. W. & Strickland, J. D. H. 1965 Fluorometric determination of chlorophyll. Journal du Consceil,Con.& Inremarionalpour PExpIoration de la Mer, 30,~15. Ketchum, B. H. 1951 The exchange of fresh wafer and salt water in tidal estuaries. %urnal of Marine Research, 10,18-38. Lack, T. J. 1971 Quantitative studies on the phytoplankton of the rivers Thames and Kennet at Reading. Freshwater Biology, 1,213-224. Liddicoat, M. I., Tibbits, S. 81 Butler, E. I. 1976 The determination of ammonia in natural waters. wafer Research, 10,567-568. M&a&an, J. 1961 The effect of salinity on growth and chlorophyll content in representative classes of unicellular marine algae. Canadian Journal of Microbiology, 7,399-406. Malone, T. C. 1971 The relative importance of nanoplankton as primary producers in the California current system. Fisheries Bulletin, 69,799-820. Malone, T. C., Neale, P. J. & Boardman, D. C. 1980 Inl%nces of estuarine circulation on the distribution and biomass of phytoplankton size fractions. In Esruarine Perspecriues (Kennedy, V. S., ed.). Academic Press, Orlando and London. Mantoura, R. F. C., Morris, A. W., Burkill, P. H. & Owens, N. J. P. 1982 Chemical and biological reactivity at the freshwater, brackishwater interphase (FBI) of estuaries. Estuarine and Brackish Water Sciences Association Bulletin, No. 32. Mommaerts, J. P. 1969 The distribution of major nutrients and phytoplankton in the Tamar Estuary. Journal of the Marine Biological Association of the Um’ted Kingdom, 49,749-765. Morris, A. W., Mantoura, R. F. C., Bale, A. J., Howland, R. J. M. 1978 Very low salinity regions of estuaries; important sites for chemical and biological reactions. Nature, 274, (5672), 678-680. Morris, A. W., Bale, A. J. & Howland, R. J. M. 1982 Chemical variability in the Tamar Estuary South-west England. Estuarine, Coastal and Shelf Science, 14,649-661. Murphy, J. & Riley, J. P. 1962 A modified single solution method for the determination of phosphate in natural waters. Analytical Chimica Acta, 27,31-36. Paul, J. S. 1979 Osmoregulation in the marine diatom Cylindrotheca fus~ownis. 3ow-d of Phycology, 15, 280-284.
Setter, T. L. & Greenway, H. 1979 Growth and osmoregulation of Chlorefla emersonii in NaCl and neutral osmotica. Ausrralian3ournal of Plant Physiology, 6,47-60. Sharp, J. H., Culberson, C. H. & Church, T. M. 1982 The chemistry of the Delaware estuary. General considerations. Limnologv and Oceanography, 27(6), 1015-1028. Sokal, R. R. i? Rohlf, F. J. 1981 Biameny, W. H. Freeman and Company. Steeman Neilsen, E. 1952 The use of radioactive carbon (%) for measuring organic production in the sea. 3ournal du Conseil Inremationalpour I’Exploration de la Mer, l&117-140.
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of phytoplankton
I)07
Strickland, J. D. H. & Parsons, T. R. 1972 A practical handbook of seawater analysis. Bulletin of the Fisheries Research Board of Canada, 167. Sze, I’. 1981 A culture model for phytoplankton succession in the Potomac River near Washington, D.C. (U.S.A.). Phycologia, 20(3), 285-291. Takuda, H. 1968 Effects of salinity on the growth of a marine diatom Nitzchia closterium. Bulletin of the Plankton Society ofJapan. 15,13-19. Tomas, C. R. 1978 Olisthodiscus luteus (Chrysophyceae) I. Effects of salinity and temperature on growth, motility and survival. Journal ofl’hycology, 14,309-313. Wood, E. D., Armstrong, F. A. J. St Richards, F. A. 1967 Determination of nitrate in sea water by cadmium copper reduction to nitrate. Journal of the Marine Biological Association of the United Kingdom, 41, 23-31. Yentsch, C. S. & Menzel, D. W. 1963 A method for the determination of phytoplankton chlorophyll and phaeophytin by fluorescence. Deep Sea Research, 10,221-231.