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Plankton productivity and biomass in a tributary of the upper Chesapeake Bay. I. Importance of size-fractionated phytoplankton productivity, biomass and species composition in carbon export
Estuarine, Coastal and Shelf Science (1983) 17, 197-206
Plankton productivity and biomass in a tributary of the upper Chesapeake Bay. I. Importance of size-fractionated phytoplankton productivity, biomass and species composition in carbon export
Kevin
G. Sellner
Academy of Natural Sciences, Benedict Estuarine Research Laborato y, Benedict, MD 20612, U. S. A. Received 19 December 1981 and in revised form 19 October 1982
Keywords: productivity;
carbon; cyanophyta; zooplankton
estuaries;
grazing;
nannoplankton;
primary
Phytoplankton productivity, community composition and biomass were determined over a nine-month period in brackish waters of the lower Gunpowder River, a tributary of Chesapeake Bay. Primary productivity followed expected seasonal magnitudes for temperate estuaries with rates exceeding 142 ‘4 mg C m-3 h-1 in July through September 1979 and minimum rates of 1.6 mg C m-3 h-1 in February 1980. Annual primary production was estimated at 45 ‘5 gC m-2. Cell numbers were highest in August, September and November with cyanophytes dominating the planktonic algae. Primary productivity, chlorophyll concentrations andcell densitieswere dominatedby nanoplanktonicforms (< 10 pm) through-
out the study. Phytoplanktoncarboncalculatedfrom cellsvolumesexceedednutritional requirementsof the pelagicherbivoresin all monthssuggesting a mean daily export (to the bay or sediments) of 1607 mg Cm-3 d-l.
Introduction Primary productivity and biomassof the phytoplankton community of ChesapeakeBay and tributaries are dominated by nanoplankton (McCarthy ef al., 1974; Seliger & Loftus, 1974; Van Valkenburg & Flemer, 1974). Within the phytoplankton assemblages of the Bay, diatoms, chlorophytes and chrysophytes comprised 60% of the total cell numbers and dinoflagellates 56.4% of total cell volume over a 17-month study period (Van Valkenburg et al., 1978). Procaryotic blue-green algaeformed only 9.5% of total cell densities. The small contribution of the procaryotes is surprising since these cells are generally associatedwith nutrient-rich, low oxygen waters, characteristics of major Chesapeake Bay tributaries (e.g. Potomac River) draining large metropolitan centers and highly developed suburbs (Jaworski et ul., 1972). The role of the bay tributaries in bay productivity has not been easily assessed.Since the brackish-water tributaries are very productive with high planktonic productivity and fairly extensive marsh communties supplying nutrients and organic matter (Biggs & Flemer, 1972; Heinle & Flemer, 1976), the contributions of inflowing organic matter and 197 0272-7714/83/080197+10$03.00/0
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Academic Press Inc. (London) Limited
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Kevin G. Sellner
nutrients to the bay could conceivably lead to more eutrophic conditions for this large drainage system if the particulate materials are not utilized within the tributaries. The role of planktonic herbivores in limiting phytoplankton standing stocks in the Chesapeake Bay region has been examined previously. Heinle (1966) reported that at least one-half of the planktonic primary production in the Patuxent River estuary was consumed by Acutiu tonsa populations in the summer. For the early spring, algal production in the same estuary was less than the carbon requirements of the spring dominant, Emytemoru a#nis (Heinle & Flemer, 1975). Considering herbivorous activity of other members of the zooplankton assemblage (e.g. ciliates, harpacticoid copepods, some cyclopoid copepods), little primary production should remain ungrazed in the system. These data were countered by research in other programs, however. Employing a deterministic feeding model, Sellner & Horwitz (1982) estimated grazing pressure of all zooplankton greater than 73 pm rarely reduced phytoplankton densities more than 20% and with greatest herbivory, only 33%, from April through November in the Patuxent River estuary. In Narragansett Bay, prevailing concentrations of phytoplankton can support A. tonsa and A. hudsonica for most times of the year, with food limitation possibly occurring at the end of the winter-spring bloom, during a July flagellate bloom and in the fall typified by low phytoplankton biomass and production (Durbin & Durbin, 1981). In the present study, size fractionated (total and < 10 pm) phytoplanktonic productivity, biomass and community composition were determined over 9 months in the lower Gunpowder River on the northwestern shore of Chesapeake Bay. Estimates of standing crop of the primary producers and species composition were employed along with zooplankton data collected previously (Grant et al., 1980) in order to approximate net phytoplankton carbon export from the brackish tributary waters. Materials
and methods
The study area is located in the lower Gunpowder River, Baltimore County, Maryland (Figure 1). Eight stations (Pl-P8) were sampled monthly from July, 1979 through March, 1980. Two composite samples (5 gal. carboys) were collected at each station by pooling 20 or more one-liter samples collected just below the surface (0.2 m) along 2 perpendicular 100-l 50 m transects. Subsamples from each carboy were removed for analysis of phytoplankton productivity, chlorophyll a and phaeopigment concentrations and species composition of the phytoplankton assemblage. Sample size fractionation through 10 pm netting was conducted at stations Pl, P3, P6 and depending on tide, P2 (flood) or P4 (ebb). Primary productivity was determined from carbon incorporation in 100 ml subsamples receiving 2 CLC;NaHiQCO,. After filling pre-cleaned and rinsed milk dilution bottles, the samples were incubated for 3-5 h in floating incubation racks l-4 cm below the surface. Samples were also filtered immediately for time zero (to) W-incorporation. Fifty-ml subsamples were flltered through 0.45 pm Millipore filters, fumed over concentrated HCl and the filters placed in scintillation vials containing 10 ml Aquasol (New England Nucler). Sample activity was monitored in an Intertechnique Model SL30 liquid scintillation counter with counting efficiencies determined with W-toluene internal standards. Absolute carbon fixation rates (mg C m-3 h-1) were calculated according to Vollenweider (1969) using alkalinity measurements (APHA, 1976) for total available inorganic carbon concentrations. Daily fixation rates were obtained by multiplying the hourly production times day length, with day lengths of 12 h in July to 6 h in March. Integrated incorporation (mg C m-2) was
Rankton productivity
and biomass in Chesapeake Bay
Figure 1. Station locations in Gunpowder River, Saltpeter Chesapeake Bay, Baltimore County, Maryland, U.S.A.
Creek
199
and Seneca Creek,
upper
calculated from planimetry of the exponential decline in productivity with depth as Cd = C,e-kd where Cd and C, are carbon fixation rates at depth (d) and the surface (s) and k = 3 .4 (secchi disc depth of 0.5 m). Pigment concentrations were determined spectrophotometrically on 100 to 250-ml aliquots filtered through 0.45 pm Millipore filters, frozen and subsequently extracted with 90% acetone. Concentrations of chlorophyll a and phaeopigment were calculated from formulas of Strickland 81Parsons(1972). Subsamples(500 ml) from each carboy were preserved and tied with buffered formalin and an I,-KI solution. After a 72-h settling period, the supernatant was siphoned off, particulates concentrated with centrifugation and cell counts and identifications determined at 400x in a Palmer-Maloney counting chamber. Carbon concentrations within the > 10 w and < 10 pm size fractions were calculated according to Strathmann (1967) assumingan average spherical diameter of 20 pm and 7 pm, respectively. Results Roductivity
Primary production in the total and < 10 um size fractions of the phytoplankton community in the study area ranged from 1.6 mgC m-3 h-i in Seneca Creek, February 1980 to 142.4 mg C m-3 h-i in Gunpowder River, September 1979 (Table 1). The carbon fixation rates over 9 months yielded an average production of 390.6 mg C m-3 d-1. The annual integrated rate for the entire study area, employing a secchi disc depth of 0.5 m, was 45.5gCm-zd-1.
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Kevin G. Sellner
TABLE 1. Mean carbon fixation Gunpowder River area, July community, nano=cells passing to total productivity x 100, ND Seneca Creek Month
Total
Nano
JOY Aw
70.2
75.4
83.2
SePt act
71.1 10.0
Nov
29.3
DCC
(Pl)
rates* standard deviation (mg C m-3 h-l) for the lower 1979 through March 1980 (total=whole water plankton 10 w mesh net, N/T=ratio of nanoplankton productivity = not determined) Saltpeter
N/T
Total
Creek
(P2-P5)
Gunpowder
River
(P6-P8)
Nanoa
N/T
Total
Nano*
ND
136.8k2.4
103.5
N/T
107
103.3k13.6
ND
58.9
71
136.6k36.5
114.9
83
78.7+26.5
45.3
86
60.3
84
15.2k8.3
9.4
45
113.5k39.0
142.4
91
8.6
86
29.2f
23.3
81
22.3k5.3
9.7
54
23.4
80
38.7k21.8
48.7
92
27.2kl8.5
26.8
55
17.6
70
16.2k6.6
21.8
104
4.6
37
7.5k1.7
12.2
14.6
10.3
70
22.5k5.3
Jan
5.3
1.9
36
14.6f
Feb
1.6
1.7
108
6.3f5.4
7.9
24
ND
Mar
2.8
3.2
114
7.4zL4.5
10.4
95
4.5kO.7
aStations *Stations
P3 and either P6.
10.4
3.1 ND 3.6
75
58 ND 92
P2 or P4.
Small phytoplankton species were responsible for most of the carbon fixation in the phytoplankton assemblage. Nanoplankton production comprised 24-114% of total phytoplankton production at stations Pl, P2, P3, P4 and P6. For the g-month period, the mean contribution of the < 10 pm cells was 76 + 24% of total carbon fixation. Within the study area, carbon fixation rates were highest in Saltpeter Creek in 7 of 9 months while Seneca Creek was characterized by the lowest rates of the 3 areas.
Concentrations of chlorophyll a exceeded 6 *Omg m-3 throughout the study period (Table 2) with Gunpowder River stations characterized by highest concentrations, never lower than 13.9 mg m-3 for the total phytoplankton community. As in production data, high chlorophyll concentrations typified the summer months ranging from 16.9-51.2 mg m-3; however, for the 3 areas, the winter chlorophyll levels did not decline as markedly as production rates. The minimum concentration was 7.5 mg m-3 in Saltpeter Creek. As noted in the carbon fixation rates, nanoplankton chlorophyll concentrations comprised the major fraction of total chlorophyll in the area. For the g-month period, the nanoplankton chlorophyll u/total chlorophyll a ratio was 0 a81 + 0.27 for stations Pl, P2, P3, P4 and P6. Phaeopigment concentrations ranged from undetectable to 24 *8 mg m-3. The concentrations between composites and stations varied substantially and no obvious trends between phaeopigment and season, chlorophyll u or species were evident. Highest absolute concentrations were recorded in August, November and December with maximum concentrations for the study sites of 11.6 (Gunpowder River), 24.8 (Gunpowder River) and 10.9 (Seneca Creek) mg m-3.
Plankton productivity
and biomass in Chesapeake Bay
201
2. Mean chlorophyll (I concentrationsfstandard deviation (mg m-3) in the lower Gunpowder River area, July 1979 through March 1980 (nano implies < 10 pm, N/T the ratio of nanoplankton chlorophyll to total chlorophyll x 100, ND = not determined)
TABLE
Seneca Creek Month
(Pl)
Saltpeter
Total
Nano
N/T
Total
Creek
Gunpowder
(P2-P5)
Nano’
N/T ND
River
(P6-P8)
Nano’
Total
N/T
July Aw
ND
ND
ND
21.3k3.4
ND
29.7k1.6
20.8
72
19.2
8.2
43
16.9k3.8
11.9
61
18.7k4.9
16.8
122
Sept
30.8
18.5
60
37.3k7.0
19.4
53
51.2k8.5
46.2
77
03
10.9
9.7
89
10,2+3.9
13.6
150
Nov
18.6
14.7
79
20.6f2.9
17.8
93
32.4*
Dee
13.3
12-9
97
13.7f1.9
11.7
95
17.025.2
Jan
13.0
5.9
45
13~Oi5~0
7.5
86
13.9f4.4 12.4
17~Ojz3~0
5.2
55
29.3
63
17.4
89
8.7
42
Feb
7.9
5.9
75
7.5k1.6
6.0
82
ND
ND
Mar
13.5
16.6
123
18.4i7.1
13.2
107
25.9k3.2
18.5
‘Stations ‘Station
P3 and either P6.
ND 83
P2 or P4.
TABLE 3. Densities and composition of the phytoplaukton community in the lower Gunpowder River study area, July 1979 through March 1980 (mean&standard deviation presented, N/T=ratio of nanoplankton cells to total cell numbers x 100)
Month
Total cells (x 10-h I-‘)
% Blue greens
N/T
General
characteristics
July
16.34k2.82
5559
71
Other groups: Diatoms flagellates 1520%
15-20%,
Aw Sept.
15,13+4.11
64+8
70
Microtlagellates
31.97f28.31
61k18
79
Diatoms i-10%, chlorophytes microiIagellates lO-15%
act
8.18k3.75
28fll
46
Diatoms lo-32%
20-30%,
microflagellates
Nov
18.99+6.14
17&5
63
Diatoms 45-59%, 8-15%, cryptophytes
microflagellates 7-15%
Dee
9.83k3.40
35*15
36
Diatoms 12-44%, chlorophytes lO-2?%, cryptophytes 6-12%, microflagellates %12%
Jan
6.59k2.20
6+4
64
Diatoms 23-49%, chlorophytes 20-51%, cryptophytes 4-l l%, microflagellates 1 l-28%
Feb
3.11+2-02
If1
65
Diatoms 42-47%, chlorophytes 12-18%, microflagellates 9-16%, dinoflagellates lo-15%
Mar
4.12f1.17
6*4
62
Diatoms 14-39%; chlorophytes 6-15%, cryptophytes 3-15%, microtlagellates 17-57%, dinoffagellates &39%
20%, chlorophytes
microlow lo%,
Kevin G. Sellner
202
Community
composition
and cell densities
Procaryotic blue-green algae dominated the phytoplankton assemblage in July through September forming 55-64% of total cell numbers (Table 3). The principal cyanophytes were Schizothrix calcicola, Spirulina sub&a and Anacystis montana. Gunpowder River stations, particularly P6, typically had greater densities of blue-greens than found in Seneca or Saltpeter Greeks with the highest numbers recorded in September. Chlorophytes, diatoms and unidentified microflagellates comprised the remaining portions of the community; diatoms formed the largest component of the community in November, 1979 and January and February, 1980. Total cell numbers (eucaryotes and blue-green algae) were highest in the area for September with a mean density of 32.0 ( f 28.3) x 106 cells l-1, principally due to a bluegreen algal density of 78.4 x 106 1-l at P6. Total cell densities greater than 15 x 106 l-1 were recorded in July, August, September and November for the 8 stations of the study area. Minimum cell numbers were recorded in February with a mean of 3.1 (*2*0)x 1061-l. Cells less than 10 pm formed 36-79% of cell numbers for the 9 months, with the summer months typified by less than 30% of the total cells exceeding 10 pm. The contributions of the blue-green algae and the two cell size classes to the total phytoplankton carbon pool are summarized in Table 4. Although blue-greens and nanoplankton dominated cell densities, cells greater than 10 pm contributed substantially more biomass than the more numerous smaller cells. TABLE 4. Carbon content and productivity of the phytoplankton community, zooplankton carbon demand and the export of carbon from surface waters of the lower Gunpowder River study area, July 1979 through March 1980 Mean
phytoplankton
biomass
(mg C m-3)
Eucaryotes Month
JOY A% SePt act Nov Dee
Jan Feb Mar
Blue-green@ 9.4 10.1 48.2 4.9 9.8 10.9 1.1 0.1 0.7
> 10 pn1@7~ 883.1 784.5 366.9 1305.9 3101.4 1419.2 1362.8 368.3 723.2
< 10 pmQJC 164.2 109.5 266.0 89.3 281.2 94.9 103.0 71.3 72.7
Phytoplsnkton productivity (mg Cm-r d-l)d
Zooplankton ( > 76 pm) requirements (mg C m-r d-r)e
Carbon export (mg C m-3 d-1 >r
1328.6 1189.9 590.6 217.8 265.5 130.6 64.6 25.5 34.4
35.8 13.7 17.2 5.5 7.3 31.5 61.8 104.2 362.9
2349.5 2080 3 1254.5 1612.4 3650 6 1624.1 1469.7 361 ,O 468.1
‘%alculated from Strathman (1967) using diameters of 1.2-l. 65 pm for S. caZcic&z. bRadius of 10 um assumed for cells retained on 10 pm mesh net. CRadius of 3.5 ttnr assumed for cells passing 10 pm mesh net. dAssuming decreasing daylength from 12 h in July to 6 h in March, to account for intensity of light available for photosynthesis. eZooplankton densities from Grant et al. (1980); estimated carbon contents (weight x 0.5) for A. ronsa from Heinle (1966). E. uffinis from Allsn et al. (1977) and Richman t-r al. (1977), with carbon consumption d-1 calculated from carbon content x0.5 (Paffenhofer i? Harris, 1976); assumed carbon consumption of 0.85 pg C/animal/d for Moina micrura, 1 ug C/ animal/d for other cladocerans, 6 ng C/rotifer/d, 0.375~ C/Scorrolana cunadensis nauplii/d. Ingestion rates are not corrected for temperature. fCarbon export (mg C m-3 d-1) =(blue-green carbon+eucaryote carbon) +daily productivity-zooplankton demand.
Plankton productivity
and biomass in Chesapeake Bay
203
Discussion Nanoplankton were responsible for 76%of total primary productivity, 81%of total chlorophyll a and greater than 70% of total cell numbers over the g-month study period in the low Gunpowder River study area. These results are similar to observations made in the mid-region of ChesapeakeBay (McCarthy et al., 1974; Van Valkenburg & Flemer, 1974; Van Valkenburg et al., 1978) and the West and Rhode River estuaries (Seliger & Loftus, 1974). However, the large contributions of procaryotic blue-green algae noted in the nanoplankton and total phytoplankton community of the lower Gunpowder River have not been observed in previous down-bay studies. Over a 17-month period, Van Valkenburg et al. (1978) reported that cyanophytes only comprised 9.5% of total cell numbers and 0.3% of total cell volumes off Drum Point. Two other programs failed to note any significant blue-green algae component (Van Valkenburg & Flemer, 1974; Seliger et al., 1981). The absence of blue-greens in bay samplesindicates the contribution from the Gunpowder River system, reaching 7.8 x 107procaryotes 1-l at station P6 in September, never forms a major portion of the pelagic flora. Where do these cells go? Ingestion of blue-green algae by pelagic herbivores does not appear likely as an explanation for the paucity of procaryotic phytoplankton in ChesapeakeBay. Blue-green algae reportedly inhibit feeding and reduce cladoceran growth relative to growth accompanying ingestion of green algae (Arnold, 1971; Porter i? Orcutt, 1980; Lampert, 1981). These cells passthrough the digestive tracts of salps, pteropods and possibly small crustaceans intact (Silver & Bruland, 1981). Filter-feeding herbivores in the study area might also experience similar feeding responsesfor the procaryotes S. calcicola, S. subsalsaand A. montana. In addition, the dominant numbers of the zooplankton in the study area, Acartia tonsa and Eurytemora u@is (Grant et al., 1980), do not effectively filter particles smaller than 2 l.nn (Allan et al., 1977; Richman et al., 1977; Conover, 1979; Richman et al., 1980). Diameters of S. culcicolu ranged from 1.2-I .65 ltm in July, September and December. By calculation, rotifers, another numerically important herbivore group in the zooplankton community (4.2-104.2 x 104m-3, Grant et al., 1980), would require approximately 500 pg Cm-3 d-1 for maintenanceand reproduction at peak densities (seeTable 4), assuming an ingestion rate of 6 ng C animal-1 d-1 (adapted from Dewey, 1976). The meanbluegreen algal biomassin the summer months was 22.6 mg Cm-3 (Table 4), well above the needs of the summer protozoan community. Total phytoplankton carbon (as blue-greens and all eucaryotes) available to the pelagic herbivores for the study period is alsopresented in Table 4. The low abundance of the procaryotes in bay water could reflect dilution of the freshwater with saline bay water and/or sedimentation of the cells to more denseinflowing bottom saline waters and eventually the sediments.Recently, Avnimelech et al. (1982) noted rapid flocculation of clay particles with Anabaena sp. and sedimentation with increasing concentrations of CaCl, or NaCl. If local sedimentation does occur, the large influx of organic matter to the sedimentsshould result in high sediment oxygen demand, deposits of reduced carbon in the sedimentsand low dissolved oxygen concentrations in the area. However, low dissolved oxygen concentrations were never observed in the present nor previous programs (Grant et al., 1980). The combination of low herbivory and abundant oxygen in bottom waters support the concept of dilution of freshwater standing crops as an explanation for low densitiesof blue-green algae in the bay. Transport of plankton biomassdownbay hasbeen observed previously in ChesapeakeBay (Flemer et al., 1976; Biggs & Flemer, 1972), the Hudson River estuary (Duedall et ul, 1977), WassawSound estuary
204
Kevin G. Sellner
in Georgia (Turner et al., 1979) and possibly in Narragansett Bay, R. I. (Durbin & Durbin, 1981). There was also sufficient edible eucaryotic cell carbon and daily productivity (except in February and March) for maintenance of the greater than 73 pm zooplankton community in the study area (Table 4). Phytoplankton production in several other areas has generally been sufficient to meet the needs of Acur&dominated zooplankton assemblages, e.g. Patuxent River (Heinle, 1966, 1974; Sellner & Horwitz, 1982) and Narragansett Bay (Durbin & Durbin, 1981). However, Heinle & Flemer (1975) found that the demands of E. affinis populations obligated ingestion of detrital matter due to insufficient phytoplankton production in the Patuxent River. In the lower Gunpowder River area, highest carbon requirements by the zooplankton community were also recorded when E. uffinis dominated the herbivore densities at 1.6 and 2.4 x 105 nauplii m-3 in February and March, respectively (Grant et ul., 1980). Phytoplankton production in excess of herbivore requirements averaged 1607 mg C m-3 d-1 in the lower Gunpowder River area resulting in export of carbon to the bay or sediments from the euphotic zone. Highest export was noted in November (3.65 g Cm-3 d-i) when diatoms dominated the phytoplankton assemblage and few grazers were observed. The abundance of diatoms probably resulted from low grazing pressure in October as populations of A. tonsu were declining and E. uffiis were just beginning to increase (Grant et al., 1980). On a yearly basis, 587 g Cm-3 y-i would be flushed from the study area. For comparison, annual export of total particulate carbon (detrital plus plankton) from Gott’s marsh in the Patuxent River was 2.4 x 106 g C y-i (Heinle & Flemer, 1976). Annual phytoplankton productivity for the lower Gunpowder River study area was 45.5 g C m-2 y-i, lower than recorded in other estuaries on the eastern seaboard (Table 5). The lower rates are probably functions of a shallow euphotic zone, relatively low summer phosphorus concentrations (0.27 k 0.11 pg-at Pl-1; Sellner, unpublished data) and the assumed exponential decline in productivity with depth. In summary, the upper Chesapeake Bay may receive significant carbon inllux as ungrazed phytoplankton cells, including high densities of procaryotic blue-green algae, from the lower Gunpowder River estuary. Calculated herbivory would remove from 0 *2-44% of the total phytoplankton carbon with maximum grazing pressure relative to algal production in February and March when E. uffiis juveniles dominated the zooplankton community. The fates of the ungrazed carbon and in particular the high procaryote densities in Chesapeake Bay remain to be determined. TABLE 5. Net phytoplankton
Annual productivity (g C m-2 y-i)
Location Lower Gunpowder River estuary, Patuxent River estuary, MD Beaufort estuary, NC North Inlet estuary, SC Wassaw Sound estuary, GA Narragansett Bay, RI
productivity
MD
45.5 193-330 53 67 346 90 269 99-440
for eastern
seaboard
estuaries,
continental
U.S.A.
Reference Present study Stross & Stottlemeyer (1966) Williams (1966) Thayer (1971) Sellner et al. (1976) Turner et al. (1979) Oviatt et al. (1981) See Oviatt er al. (1981) for references
Plankton productivity
and biomass in Chesapeake Bay
205
Acknowledgements
The author expresses his gratitude to the Department of Natural Resources, State of Maryland for funding the field effort under contract P 35-80-02. The author is also indebted to the efforts of F. Acker, S. Halterman, K. Johnson, M. Kachur, L. Lyons, E. Perry and M. Olson in collection and preliminary analysis of the data. R. Mahoney deserves special praise for phytoplankton identifications and enumerations while special thanks are extended to J. Arciprete for typing the final manuscript.
References Allan,
J. D., Richman, S., Heinle, D. R. & Huff, R. 1977 Grazing in juvenile stages of some estuarine calanoid copepods. Marine Biology 43, 317-331. APHA 1976 Standard Methods for the Examination of Water and Waste-water. 14th edition. Published for the American Public Health Association, American Water Works Association and Water Pollution Control Federation. American Public Health Association, Washington, D.C. 1193 pp. Arnold, D. E. 1971 Ingestion, assimilation, survival, and reproduction by Du#mia pulex fed seven species of blue-green algae. Limnology and Oceanography 16,906-920. Avnimelech, Y., Troeger, W. W. & Reed, W. 1982 Mutual flocculation of algae and clay: evidence and implications. Science 216,63-65. Biggs, R. B. & Flemer, D. A. 1972 The flux of particulate carbon in an estuary. Marine Bialogy 12, 11-17. Conover, R. J. 1979 Secondary production as an ecological phenomenon. In Zoogeography and LXversity in Plankton (Van der Speol, S. & Pierrot-Bults, A. (2, eds). Bunge Scientific Publishers, Utrect. pp. 50-86. Dewey, J. M. 1976 Rates of feeding, respiration, and growth of the rotifer Brachionus plicasilis and the dinoflagellate Nocriluca miliaris in the laboratory. PhD dissertation, University of Washington, 117 pp. Duedall, I. W., O’Connors, H. B., Parker, J. B., Wilson, R. E. & Robbins, A. S. 1977 The abundances, distribution and flux of nutrients and chlorophyll a in the New York Bight Apex. Estuarine and Coastal Marine Science 5, 81-105. Durbin, A G. & Durbiu, E. G. 1981 Standing stock and estimated production rates of phytoplankton and zooplankton in Narragansett Bay, Rhode Island. Estuaries 4,24-H. Flemer, D. A, Ulanowicz, R E. & Taylor, D. L., Jr. 1976 Some effects of tropical storm Agnes on water quality in the Patuxent River estuary. In The Effects of Tropical storm Agnes on the Chesapeake Bay Estwrine System. Chesapeake Research Consortium Publication 54. Johns Hopkins University Press, Baltimore, MD. pp. 251-287. Grant, G. C, Womack, C. J. & Olney, J. E. 1980 Zooplankton of the waters adjacent to the C. P. Crane generating station. Final report for State of Maryland, Department of Natural Resources and Power Plant Siting Program. Virginia Institute of Marine Science, Gloucester Point, Virginia. 134 pp. Heinle, D. R. 1966 Production of a calanoid copepod, Acarria fonsu, in the Patuxent River estuary. Chesapeake Science 7, 59-74. Heinle, D. R. 1974 An alternate grazing hypothesis for the Patuxent estuary. Chesapeake Science 15, 146-150. Heinle, D. R. & Flemer, D. A. 1975 Carbon requirements of a population of the estuarine copepod Eurytenwra a@nis. Marine Biology 31, 235-247. Heinle, D. R. & Flemer, D. A. 1976 Flows of materials between poorly flooded tidal marshes and an estuary. Marine Biology 35,359-373. Jaworski, N. A, Lear, D. W., Jr. & Villa, O., Jr. 1972 Nutrient management in the Potomac estuary. In Nunrents and Eutrophication: the Limiting Nmrient Consroversy. (Likens, G. E., ed.). Limnology and Oceanography Special Symposium 1. pp. 246-273. Lampert, W. 1981 Inhibitory and toxic effects of blue-green algae on Baphnia. Internationale Revue dergesamten Hydrobiologie 66,285-298. McCarthy, J. J., Taylor, W. R. & Loftus, M. E. 1974 Significance of nanoplankton on the Chesapeake Bay estuary and problems associated with the measurement of nanoplankton productivity. Marine Biology 24,7-16. Oviatt, C., Buckley, B. & Nixon, S. 1981 Armual phytoplankton metabolism in Narragansett Bay calculated from survey field measurements and microcosm observations. Estuaries 4, 167-175. Paffenhofer, G.-A. & Harris, R. P. 1976 Feeding, growth and reproduction of the marine planktonic copepod Pseua’ocalanus elongatus Boeck. Journal of the Marine Biological Association, United Kingdom 56,327-344. Porter, K. G. & Orcutt, J. D., Jr. 1980 Nutritional adequacy, manageability, and toxicity as factors that determine the food quality of green and blue-green algae for Lkphnia. In Evolution ana’ Ecology of Zooplankton Communiries (Kerfoot, W. C., ed). University Press of New England, Hanover, N. H. pp. 268-281.
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G. Sellner
Richman, S., Bohon, S. A. & Robbins, S. E. 1980 Grazing interactions among freshwater calanoid copepods. In Evolution and Ecology of Zooplankton Commumiies (Kerfoot, W. C., ed.). University Press of New England, Hanover, N. H. pp. 219-233. Richman, S., Heinle, D. R. & Huff, R. 1977 Grazing by adult estuarine calanoid copepodes of the Chesapeake Bay. Marine Biology 42,69-84. Seliger, H. H. & Loftus, M. E. 1974 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 Chesapeake Bay. Chesapeake Science 15, 185-204. Seliger, H. H., McKinley, K. R, Biggley, W. H., Rivkin, R. B. & Aspden, K. R. H. 1981 Phytoplankton patchiness and frontal regions. Marine EioZogy 61, 119-131. Sellner, K. G. & Horwitx, R. J. 1982 Plankton interactions in the Patuxent River estuary: Field studies of community composition and density, with a deterministic model of the effects of zooplankton grazing on phytoplankton carbon, production of fecal matter, sediment oxygen demand and nutrient regeneration. Report 82-14D, Academy of Natural Sciences for Department of Health and Mental Hygiene, Maryland. 111 pp. Sellner, K. G., Zingmark, R G. & Miller, T. E. 1976 Interpretations of the r*C method of measuring the total annual production of phytoplankton in a South Carolina estuary. Boranica Marina 19, 119-125. Silver, M. W. & Bruland, K. W. 1981 Differential feeding and fecal pellet composition of salps and pteropods, and the possible origin of deep-water flora and olive-green ‘ cells ‘. Marine Biology 62, 263-273. Strathmann, R R. 1967 Estimating the organic carbon content of phytoplankton from cell volume or plasma volume. Limnology and Oceanography 12,41 l-418. Strickland, J. D. H. & Parsons, T. R. 1972 A practical handbook of sea water analysis. Fisheries Research Board of Canada, Bulletin 167. 310 pp. Stress, R. G. & Stottlemeyer, J. R. 1965 Primary production in the Patuxent River. Chesapeake Science 6,125-140. Thayer, G. W. 1971 Phytoplankton production and the distribution of nutrients in a shallow unstratified estuarine system near Beaufort, N.C. Chesapeake Science 12, 240-253. Turner, R. E., Woo, S. W. & Jitts, H. R. 1979 Phytoplankton production in a turbid, temperate salt marsh estuary. Estuarine and Coastal Marine Science 9,603-613. Van Valkenburg, D. E. & Flemer, D. A. 1974 The distribution and productivity of nanoplankton in a temperate estuarine area. Estuarine and Coastal Marine Science 2,311-322. Van Valkenburg, S. D., Jones, J. K. & Heinle, D. R. 1978 A comparison by size class and volume of detritus versus phytoplankton in Chesapeake Bay. Estuarine and Coastal Marine Science 6, 569-582. Vollenweider, R. A. 1969 A Manual on Methods for Measuring Prima?y Production in Aquatic Environments. I.B.P. Handbook No. 12. Blackwell Scientific Publishers, Oxford 213 pp. Williams, R B. 1966 Annual phytoplanktonic production in a system of shallow temperate estuaries. In Some Contemporary Studies in Marine Science (Barnes, H., ed.). George Allen & Unwin Ltd., London. pp. 699-716.
Plankton productivity and biomass in a tributary of the upper Chesapeake Bay. I. Importance of size-fractionated phytoplankton productivity, biomass and species composition in carbon export
Kevin
G. Sellner
Academy of Natural Sciences, Benedict Estuarine Research Laborato y, Benedict, MD 20612, U. S. A. Received 19 December 1981 and in revised form 19 October 1982
Keywords: productivity;
carbon; cyanophyta; zooplankton
estuaries;
grazing;
nannoplankton;
primary
Phytoplankton productivity, community composition and biomass were determined over a nine-month period in brackish waters of the lower Gunpowder River, a tributary of Chesapeake Bay. Primary productivity followed expected seasonal magnitudes for temperate estuaries with rates exceeding 142 ‘4 mg C m-3 h-1 in July through September 1979 and minimum rates of 1.6 mg C m-3 h-1 in February 1980. Annual primary production was estimated at 45 ‘5 gC m-2. Cell numbers were highest in August, September and November with cyanophytes dominating the planktonic algae. Primary productivity, chlorophyll concentrations andcell densitieswere dominatedby nanoplanktonicforms (< 10 pm) through-
out the study. Phytoplanktoncarboncalculatedfrom cellsvolumesexceedednutritional requirementsof the pelagicherbivoresin all monthssuggesting a mean daily export (to the bay or sediments) of 1607 mg Cm-3 d-l.
Introduction Primary productivity and biomassof the phytoplankton community of ChesapeakeBay and tributaries are dominated by nanoplankton (McCarthy ef al., 1974; Seliger & Loftus, 1974; Van Valkenburg & Flemer, 1974). Within the phytoplankton assemblages of the Bay, diatoms, chlorophytes and chrysophytes comprised 60% of the total cell numbers and dinoflagellates 56.4% of total cell volume over a 17-month study period (Van Valkenburg et al., 1978). Procaryotic blue-green algaeformed only 9.5% of total cell densities. The small contribution of the procaryotes is surprising since these cells are generally associatedwith nutrient-rich, low oxygen waters, characteristics of major Chesapeake Bay tributaries (e.g. Potomac River) draining large metropolitan centers and highly developed suburbs (Jaworski et ul., 1972). The role of the bay tributaries in bay productivity has not been easily assessed.Since the brackish-water tributaries are very productive with high planktonic productivity and fairly extensive marsh communties supplying nutrients and organic matter (Biggs & Flemer, 1972; Heinle & Flemer, 1976), the contributions of inflowing organic matter and 197 0272-7714/83/080197+10$03.00/0
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1983
Academic Press Inc. (London) Limited
198
Kevin G. Sellner
nutrients to the bay could conceivably lead to more eutrophic conditions for this large drainage system if the particulate materials are not utilized within the tributaries. The role of planktonic herbivores in limiting phytoplankton standing stocks in the Chesapeake Bay region has been examined previously. Heinle (1966) reported that at least one-half of the planktonic primary production in the Patuxent River estuary was consumed by Acutiu tonsa populations in the summer. For the early spring, algal production in the same estuary was less than the carbon requirements of the spring dominant, Emytemoru a#nis (Heinle & Flemer, 1975). Considering herbivorous activity of other members of the zooplankton assemblage (e.g. ciliates, harpacticoid copepods, some cyclopoid copepods), little primary production should remain ungrazed in the system. These data were countered by research in other programs, however. Employing a deterministic feeding model, Sellner & Horwitz (1982) estimated grazing pressure of all zooplankton greater than 73 pm rarely reduced phytoplankton densities more than 20% and with greatest herbivory, only 33%, from April through November in the Patuxent River estuary. In Narragansett Bay, prevailing concentrations of phytoplankton can support A. tonsa and A. hudsonica for most times of the year, with food limitation possibly occurring at the end of the winter-spring bloom, during a July flagellate bloom and in the fall typified by low phytoplankton biomass and production (Durbin & Durbin, 1981). In the present study, size fractionated (total and < 10 pm) phytoplanktonic productivity, biomass and community composition were determined over 9 months in the lower Gunpowder River on the northwestern shore of Chesapeake Bay. Estimates of standing crop of the primary producers and species composition were employed along with zooplankton data collected previously (Grant et al., 1980) in order to approximate net phytoplankton carbon export from the brackish tributary waters. Materials
and methods
The study area is located in the lower Gunpowder River, Baltimore County, Maryland (Figure 1). Eight stations (Pl-P8) were sampled monthly from July, 1979 through March, 1980. Two composite samples (5 gal. carboys) were collected at each station by pooling 20 or more one-liter samples collected just below the surface (0.2 m) along 2 perpendicular 100-l 50 m transects. Subsamples from each carboy were removed for analysis of phytoplankton productivity, chlorophyll a and phaeopigment concentrations and species composition of the phytoplankton assemblage. Sample size fractionation through 10 pm netting was conducted at stations Pl, P3, P6 and depending on tide, P2 (flood) or P4 (ebb). Primary productivity was determined from carbon incorporation in 100 ml subsamples receiving 2 CLC;NaHiQCO,. After filling pre-cleaned and rinsed milk dilution bottles, the samples were incubated for 3-5 h in floating incubation racks l-4 cm below the surface. Samples were also filtered immediately for time zero (to) W-incorporation. Fifty-ml subsamples were flltered through 0.45 pm Millipore filters, fumed over concentrated HCl and the filters placed in scintillation vials containing 10 ml Aquasol (New England Nucler). Sample activity was monitored in an Intertechnique Model SL30 liquid scintillation counter with counting efficiencies determined with W-toluene internal standards. Absolute carbon fixation rates (mg C m-3 h-1) were calculated according to Vollenweider (1969) using alkalinity measurements (APHA, 1976) for total available inorganic carbon concentrations. Daily fixation rates were obtained by multiplying the hourly production times day length, with day lengths of 12 h in July to 6 h in March. Integrated incorporation (mg C m-2) was
Rankton productivity
and biomass in Chesapeake Bay
Figure 1. Station locations in Gunpowder River, Saltpeter Chesapeake Bay, Baltimore County, Maryland, U.S.A.
Creek
199
and Seneca Creek,
upper
calculated from planimetry of the exponential decline in productivity with depth as Cd = C,e-kd where Cd and C, are carbon fixation rates at depth (d) and the surface (s) and k = 3 .4 (secchi disc depth of 0.5 m). Pigment concentrations were determined spectrophotometrically on 100 to 250-ml aliquots filtered through 0.45 pm Millipore filters, frozen and subsequently extracted with 90% acetone. Concentrations of chlorophyll a and phaeopigment were calculated from formulas of Strickland 81Parsons(1972). Subsamples(500 ml) from each carboy were preserved and tied with buffered formalin and an I,-KI solution. After a 72-h settling period, the supernatant was siphoned off, particulates concentrated with centrifugation and cell counts and identifications determined at 400x in a Palmer-Maloney counting chamber. Carbon concentrations within the > 10 w and < 10 pm size fractions were calculated according to Strathmann (1967) assumingan average spherical diameter of 20 pm and 7 pm, respectively. Results Roductivity
Primary production in the total and < 10 um size fractions of the phytoplankton community in the study area ranged from 1.6 mgC m-3 h-i in Seneca Creek, February 1980 to 142.4 mg C m-3 h-i in Gunpowder River, September 1979 (Table 1). The carbon fixation rates over 9 months yielded an average production of 390.6 mg C m-3 d-1. The annual integrated rate for the entire study area, employing a secchi disc depth of 0.5 m, was 45.5gCm-zd-1.
200
Kevin G. Sellner
TABLE 1. Mean carbon fixation Gunpowder River area, July community, nano=cells passing to total productivity x 100, ND Seneca Creek Month
Total
Nano
JOY Aw
70.2
75.4
83.2
SePt act
71.1 10.0
Nov
29.3
DCC
(Pl)
rates* standard deviation (mg C m-3 h-l) for the lower 1979 through March 1980 (total=whole water plankton 10 w mesh net, N/T=ratio of nanoplankton productivity = not determined) Saltpeter
N/T
Total
Creek
(P2-P5)
Gunpowder
River
(P6-P8)
Nanoa
N/T
Total
Nano*
ND
136.8k2.4
103.5
N/T
107
103.3k13.6
ND
58.9
71
136.6k36.5
114.9
83
78.7+26.5
45.3
86
60.3
84
15.2k8.3
9.4
45
113.5k39.0
142.4
91
8.6
86
29.2f
23.3
81
22.3k5.3
9.7
54
23.4
80
38.7k21.8
48.7
92
27.2kl8.5
26.8
55
17.6
70
16.2k6.6
21.8
104
4.6
37
7.5k1.7
12.2
14.6
10.3
70
22.5k5.3
Jan
5.3
1.9
36
14.6f
Feb
1.6
1.7
108
6.3f5.4
7.9
24
ND
Mar
2.8
3.2
114
7.4zL4.5
10.4
95
4.5kO.7
aStations *Stations
P3 and either P6.
10.4
3.1 ND 3.6
75
58 ND 92
P2 or P4.
Small phytoplankton species were responsible for most of the carbon fixation in the phytoplankton assemblage. Nanoplankton production comprised 24-114% of total phytoplankton production at stations Pl, P2, P3, P4 and P6. For the g-month period, the mean contribution of the < 10 pm cells was 76 + 24% of total carbon fixation. Within the study area, carbon fixation rates were highest in Saltpeter Creek in 7 of 9 months while Seneca Creek was characterized by the lowest rates of the 3 areas.
Concentrations of chlorophyll a exceeded 6 *Omg m-3 throughout the study period (Table 2) with Gunpowder River stations characterized by highest concentrations, never lower than 13.9 mg m-3 for the total phytoplankton community. As in production data, high chlorophyll concentrations typified the summer months ranging from 16.9-51.2 mg m-3; however, for the 3 areas, the winter chlorophyll levels did not decline as markedly as production rates. The minimum concentration was 7.5 mg m-3 in Saltpeter Creek. As noted in the carbon fixation rates, nanoplankton chlorophyll concentrations comprised the major fraction of total chlorophyll in the area. For the g-month period, the nanoplankton chlorophyll u/total chlorophyll a ratio was 0 a81 + 0.27 for stations Pl, P2, P3, P4 and P6. Phaeopigment concentrations ranged from undetectable to 24 *8 mg m-3. The concentrations between composites and stations varied substantially and no obvious trends between phaeopigment and season, chlorophyll u or species were evident. Highest absolute concentrations were recorded in August, November and December with maximum concentrations for the study sites of 11.6 (Gunpowder River), 24.8 (Gunpowder River) and 10.9 (Seneca Creek) mg m-3.
Plankton productivity
and biomass in Chesapeake Bay
201
2. Mean chlorophyll (I concentrationsfstandard deviation (mg m-3) in the lower Gunpowder River area, July 1979 through March 1980 (nano implies < 10 pm, N/T the ratio of nanoplankton chlorophyll to total chlorophyll x 100, ND = not determined)
TABLE
Seneca Creek Month
(Pl)
Saltpeter
Total
Nano
N/T
Total
Creek
Gunpowder
(P2-P5)
Nano’
N/T ND
River
(P6-P8)
Nano’
Total
N/T
July Aw
ND
ND
ND
21.3k3.4
ND
29.7k1.6
20.8
72
19.2
8.2
43
16.9k3.8
11.9
61
18.7k4.9
16.8
122
Sept
30.8
18.5
60
37.3k7.0
19.4
53
51.2k8.5
46.2
77
03
10.9
9.7
89
10,2+3.9
13.6
150
Nov
18.6
14.7
79
20.6f2.9
17.8
93
32.4*
Dee
13.3
12-9
97
13.7f1.9
11.7
95
17.025.2
Jan
13.0
5.9
45
13~Oi5~0
7.5
86
13.9f4.4 12.4
17~Ojz3~0
5.2
55
29.3
63
17.4
89
8.7
42
Feb
7.9
5.9
75
7.5k1.6
6.0
82
ND
ND
Mar
13.5
16.6
123
18.4i7.1
13.2
107
25.9k3.2
18.5
‘Stations ‘Station
P3 and either P6.
ND 83
P2 or P4.
TABLE 3. Densities and composition of the phytoplaukton community in the lower Gunpowder River study area, July 1979 through March 1980 (mean&standard deviation presented, N/T=ratio of nanoplankton cells to total cell numbers x 100)
Month
Total cells (x 10-h I-‘)
% Blue greens
N/T
General
characteristics
July
16.34k2.82
5559
71
Other groups: Diatoms flagellates 1520%
15-20%,
Aw Sept.
15,13+4.11
64+8
70
Microtlagellates
31.97f28.31
61k18
79
Diatoms i-10%, chlorophytes microiIagellates lO-15%
act
8.18k3.75
28fll
46
Diatoms lo-32%
20-30%,
microflagellates
Nov
18.99+6.14
17&5
63
Diatoms 45-59%, 8-15%, cryptophytes
microflagellates 7-15%
Dee
9.83k3.40
35*15
36
Diatoms 12-44%, chlorophytes lO-2?%, cryptophytes 6-12%, microflagellates %12%
Jan
6.59k2.20
6+4
64
Diatoms 23-49%, chlorophytes 20-51%, cryptophytes 4-l l%, microflagellates 1 l-28%
Feb
3.11+2-02
If1
65
Diatoms 42-47%, chlorophytes 12-18%, microflagellates 9-16%, dinoflagellates lo-15%
Mar
4.12f1.17
6*4
62
Diatoms 14-39%; chlorophytes 6-15%, cryptophytes 3-15%, microtlagellates 17-57%, dinoffagellates &39%
20%, chlorophytes
microlow lo%,
Kevin G. Sellner
202
Community
composition
and cell densities
Procaryotic blue-green algae dominated the phytoplankton assemblage in July through September forming 55-64% of total cell numbers (Table 3). The principal cyanophytes were Schizothrix calcicola, Spirulina sub&a and Anacystis montana. Gunpowder River stations, particularly P6, typically had greater densities of blue-greens than found in Seneca or Saltpeter Greeks with the highest numbers recorded in September. Chlorophytes, diatoms and unidentified microflagellates comprised the remaining portions of the community; diatoms formed the largest component of the community in November, 1979 and January and February, 1980. Total cell numbers (eucaryotes and blue-green algae) were highest in the area for September with a mean density of 32.0 ( f 28.3) x 106 cells l-1, principally due to a bluegreen algal density of 78.4 x 106 1-l at P6. Total cell densities greater than 15 x 106 l-1 were recorded in July, August, September and November for the 8 stations of the study area. Minimum cell numbers were recorded in February with a mean of 3.1 (*2*0)x 1061-l. Cells less than 10 pm formed 36-79% of cell numbers for the 9 months, with the summer months typified by less than 30% of the total cells exceeding 10 pm. The contributions of the blue-green algae and the two cell size classes to the total phytoplankton carbon pool are summarized in Table 4. Although blue-greens and nanoplankton dominated cell densities, cells greater than 10 pm contributed substantially more biomass than the more numerous smaller cells. TABLE 4. Carbon content and productivity of the phytoplankton community, zooplankton carbon demand and the export of carbon from surface waters of the lower Gunpowder River study area, July 1979 through March 1980 Mean
phytoplankton
biomass
(mg C m-3)
Eucaryotes Month
JOY A% SePt act Nov Dee
Jan Feb Mar
Blue-green@ 9.4 10.1 48.2 4.9 9.8 10.9 1.1 0.1 0.7
> 10 pn1@7~ 883.1 784.5 366.9 1305.9 3101.4 1419.2 1362.8 368.3 723.2
< 10 pmQJC 164.2 109.5 266.0 89.3 281.2 94.9 103.0 71.3 72.7
Phytoplsnkton productivity (mg Cm-r d-l)d
Zooplankton ( > 76 pm) requirements (mg C m-r d-r)e
Carbon export (mg C m-3 d-1 >r
1328.6 1189.9 590.6 217.8 265.5 130.6 64.6 25.5 34.4
35.8 13.7 17.2 5.5 7.3 31.5 61.8 104.2 362.9
2349.5 2080 3 1254.5 1612.4 3650 6 1624.1 1469.7 361 ,O 468.1
‘%alculated from Strathman (1967) using diameters of 1.2-l. 65 pm for S. caZcic&z. bRadius of 10 um assumed for cells retained on 10 pm mesh net. CRadius of 3.5 ttnr assumed for cells passing 10 pm mesh net. dAssuming decreasing daylength from 12 h in July to 6 h in March, to account for intensity of light available for photosynthesis. eZooplankton densities from Grant et al. (1980); estimated carbon contents (weight x 0.5) for A. ronsa from Heinle (1966). E. uffinis from Allsn et al. (1977) and Richman t-r al. (1977), with carbon consumption d-1 calculated from carbon content x0.5 (Paffenhofer i? Harris, 1976); assumed carbon consumption of 0.85 pg C/animal/d for Moina micrura, 1 ug C/ animal/d for other cladocerans, 6 ng C/rotifer/d, 0.375~ C/Scorrolana cunadensis nauplii/d. Ingestion rates are not corrected for temperature. fCarbon export (mg C m-3 d-1) =(blue-green carbon+eucaryote carbon) +daily productivity-zooplankton demand.
Plankton productivity
and biomass in Chesapeake Bay
203
Discussion Nanoplankton were responsible for 76%of total primary productivity, 81%of total chlorophyll a and greater than 70% of total cell numbers over the g-month study period in the low Gunpowder River study area. These results are similar to observations made in the mid-region of ChesapeakeBay (McCarthy et al., 1974; Van Valkenburg & Flemer, 1974; Van Valkenburg et al., 1978) and the West and Rhode River estuaries (Seliger & Loftus, 1974). However, the large contributions of procaryotic blue-green algae noted in the nanoplankton and total phytoplankton community of the lower Gunpowder River have not been observed in previous down-bay studies. Over a 17-month period, Van Valkenburg et al. (1978) reported that cyanophytes only comprised 9.5% of total cell numbers and 0.3% of total cell volumes off Drum Point. Two other programs failed to note any significant blue-green algae component (Van Valkenburg & Flemer, 1974; Seliger et al., 1981). The absence of blue-greens in bay samplesindicates the contribution from the Gunpowder River system, reaching 7.8 x 107procaryotes 1-l at station P6 in September, never forms a major portion of the pelagic flora. Where do these cells go? Ingestion of blue-green algae by pelagic herbivores does not appear likely as an explanation for the paucity of procaryotic phytoplankton in ChesapeakeBay. Blue-green algae reportedly inhibit feeding and reduce cladoceran growth relative to growth accompanying ingestion of green algae (Arnold, 1971; Porter i? Orcutt, 1980; Lampert, 1981). These cells passthrough the digestive tracts of salps, pteropods and possibly small crustaceans intact (Silver & Bruland, 1981). Filter-feeding herbivores in the study area might also experience similar feeding responsesfor the procaryotes S. calcicola, S. subsalsaand A. montana. In addition, the dominant numbers of the zooplankton in the study area, Acartia tonsa and Eurytemora u@is (Grant et al., 1980), do not effectively filter particles smaller than 2 l.nn (Allan et al., 1977; Richman et al., 1977; Conover, 1979; Richman et al., 1980). Diameters of S. culcicolu ranged from 1.2-I .65 ltm in July, September and December. By calculation, rotifers, another numerically important herbivore group in the zooplankton community (4.2-104.2 x 104m-3, Grant et al., 1980), would require approximately 500 pg Cm-3 d-1 for maintenanceand reproduction at peak densities (seeTable 4), assuming an ingestion rate of 6 ng C animal-1 d-1 (adapted from Dewey, 1976). The meanbluegreen algal biomassin the summer months was 22.6 mg Cm-3 (Table 4), well above the needs of the summer protozoan community. Total phytoplankton carbon (as blue-greens and all eucaryotes) available to the pelagic herbivores for the study period is alsopresented in Table 4. The low abundance of the procaryotes in bay water could reflect dilution of the freshwater with saline bay water and/or sedimentation of the cells to more denseinflowing bottom saline waters and eventually the sediments.Recently, Avnimelech et al. (1982) noted rapid flocculation of clay particles with Anabaena sp. and sedimentation with increasing concentrations of CaCl, or NaCl. If local sedimentation does occur, the large influx of organic matter to the sedimentsshould result in high sediment oxygen demand, deposits of reduced carbon in the sedimentsand low dissolved oxygen concentrations in the area. However, low dissolved oxygen concentrations were never observed in the present nor previous programs (Grant et al., 1980). The combination of low herbivory and abundant oxygen in bottom waters support the concept of dilution of freshwater standing crops as an explanation for low densitiesof blue-green algae in the bay. Transport of plankton biomassdownbay hasbeen observed previously in ChesapeakeBay (Flemer et al., 1976; Biggs & Flemer, 1972), the Hudson River estuary (Duedall et ul, 1977), WassawSound estuary
204
Kevin G. Sellner
in Georgia (Turner et al., 1979) and possibly in Narragansett Bay, R. I. (Durbin & Durbin, 1981). There was also sufficient edible eucaryotic cell carbon and daily productivity (except in February and March) for maintenance of the greater than 73 pm zooplankton community in the study area (Table 4). Phytoplankton production in several other areas has generally been sufficient to meet the needs of Acur&dominated zooplankton assemblages, e.g. Patuxent River (Heinle, 1966, 1974; Sellner & Horwitz, 1982) and Narragansett Bay (Durbin & Durbin, 1981). However, Heinle & Flemer (1975) found that the demands of E. affinis populations obligated ingestion of detrital matter due to insufficient phytoplankton production in the Patuxent River. In the lower Gunpowder River area, highest carbon requirements by the zooplankton community were also recorded when E. uffinis dominated the herbivore densities at 1.6 and 2.4 x 105 nauplii m-3 in February and March, respectively (Grant et ul., 1980). Phytoplankton production in excess of herbivore requirements averaged 1607 mg C m-3 d-1 in the lower Gunpowder River area resulting in export of carbon to the bay or sediments from the euphotic zone. Highest export was noted in November (3.65 g Cm-3 d-i) when diatoms dominated the phytoplankton assemblage and few grazers were observed. The abundance of diatoms probably resulted from low grazing pressure in October as populations of A. tonsu were declining and E. uffiis were just beginning to increase (Grant et al., 1980). On a yearly basis, 587 g Cm-3 y-i would be flushed from the study area. For comparison, annual export of total particulate carbon (detrital plus plankton) from Gott’s marsh in the Patuxent River was 2.4 x 106 g C y-i (Heinle & Flemer, 1976). Annual phytoplankton productivity for the lower Gunpowder River study area was 45.5 g C m-2 y-i, lower than recorded in other estuaries on the eastern seaboard (Table 5). The lower rates are probably functions of a shallow euphotic zone, relatively low summer phosphorus concentrations (0.27 k 0.11 pg-at Pl-1; Sellner, unpublished data) and the assumed exponential decline in productivity with depth. In summary, the upper Chesapeake Bay may receive significant carbon inllux as ungrazed phytoplankton cells, including high densities of procaryotic blue-green algae, from the lower Gunpowder River estuary. Calculated herbivory would remove from 0 *2-44% of the total phytoplankton carbon with maximum grazing pressure relative to algal production in February and March when E. uffiis juveniles dominated the zooplankton community. The fates of the ungrazed carbon and in particular the high procaryote densities in Chesapeake Bay remain to be determined. TABLE 5. Net phytoplankton
Annual productivity (g C m-2 y-i)
Location Lower Gunpowder River estuary, Patuxent River estuary, MD Beaufort estuary, NC North Inlet estuary, SC Wassaw Sound estuary, GA Narragansett Bay, RI
productivity
MD
45.5 193-330 53 67 346 90 269 99-440
for eastern
seaboard
estuaries,
continental
U.S.A.
Reference Present study Stross & Stottlemeyer (1966) Williams (1966) Thayer (1971) Sellner et al. (1976) Turner et al. (1979) Oviatt et al. (1981) See Oviatt er al. (1981) for references
Plankton productivity
and biomass in Chesapeake Bay
205
Acknowledgements
The author expresses his gratitude to the Department of Natural Resources, State of Maryland for funding the field effort under contract P 35-80-02. The author is also indebted to the efforts of F. Acker, S. Halterman, K. Johnson, M. Kachur, L. Lyons, E. Perry and M. Olson in collection and preliminary analysis of the data. R. Mahoney deserves special praise for phytoplankton identifications and enumerations while special thanks are extended to J. Arciprete for typing the final manuscript.
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