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Nutrient distributions and transport in Long Island Sound
Estuarine and Coastal Marine &ie?xe (1977) 5, 531-548
Nutrient Distributions Long Island Sound
and Transport in
Malcolm J. Bowman Marine Sciences Research Center, State University of New York at Stony Brook, Stony Brook, New York II794 Received 31 December 1975 and in revised form ab&ne
I976
Keywords: nutrient, nitrates, nitrites, ammonia, horizontal variations, sewage effluents, seasonal variations, Long Island
exchange/
Inorganic nitrogenous nutrient (NH,, NO,, NO,) distributions and transport in Long Island Sound are investigated for both winter and summer conditions with a steady state, one dimensional mass balance model. Nutrient budgets based on horizontal exchange, lateral input from sewage and agricultural sources, and first order biochemical uptake (utilization minus regeneration) are computed for each of 13 regions in the Sound. All nutrient concentrations, principally ammonia, peak sharply in the upper East River where a strong point source exists. Concentration distributions and uptake rates are consistent with previous studies of the nitrogen cycle and productivity of the Sound. However, this paper stresses horizontal exchange and sewage as important and hitherto neglected components of the nutrient budgets in various regions. Estimates of the loss of nitrogen to the sediment are presented for the western, central and eastern regions, from the Sound to the ocean at the eastern end, and into New York Harbor from the upper East River. Sewage effluents are shown to be the prime external source of nutrients for the Sound. Introduction Long Island Sound (Figure I) is a shallow estuary (mean depth ~20 m) some 165 km in length, connected to New York Harbor via the East River at its head (Bowman, rg76a) and with its second but main opening to the Atlantic Ocean located at its eastern mouth in Block Island Sound. The Sound is a highly impacted estuary ; some seventeen million people, representing about 8% of the population of the United States, reside within 40 km of Long Island Sound and New York Harbor (U. S. Dept. Commerce, 1972). The Sound has been an important navigable waterway for several centuries, serves as receiving waters for a great amount of man’s wastes (U. S. EPA, 197 I ), and as a prime recreational region for much of the above population. The purpose of this paper is to discuss experimental and theoretical aspects of nitrogenous nutrient distributions and transport in Long Island Sound obtained by the application of a mathematical model, originally designed to study the dispersion of sewage effluents released into the waters of the Sound (Bowman, rg@). All effluents contain large quantities of inorganic micronutrients and are shown in this paper to represent an important and hitherto neglected component of nutrient, budget estimates in various regions of the Sound. Contribution 162 of the Marine Sciences Research Center, of the State University of New York at Stony Brook.
532
M. J. Bowman
Kilometers
Figure I. Location map of Long Island Sound showing sampling stations and the I 3 model regions (Bowman, 1976b).
Previous
studies
A review of the oceanography of Long Island Sound suitable for the purposes of this paper, published by Riley (1956), outlines the essential physical characteristics of the estuary, and emphasizes the seasonal cycle of the nutrient supply and primary productivity. Previous studies of nutrient cycles in Long Island Sound (Conover, 1956; Riley & Conover, 1956; Harris, 1959; Riley, 1959; Hardy, 1972) have emphasized seasonal distributions and the local productivity of different regions. Exchange with continental shelf bottom waters, and the abundant freshwater supply were stressed as potentially important sources of nutrients. However, sewage effluents as sources of nutrients have been neglected in past studies. 111111111111111111111111111 = I.0 Mean NH3
0.1
0.01
JFMAMJJASONDJFMAMJJASONDJ b-1972--i j--
1973 d-1974
Figure 2. Flow weighted nutrient concentrations treatment plants (New York city EPA, 1972-74).
in upper
East River
sewage
Nutrients in Long Island Sound
Sewage eihents
533
as nutrient sources
Since no tertiary treatment of any significance exists in the region of this study, all sewage effluents contain high concentrations of nutrients. The East River daily transports a large quantity of sewage effluent into western Long Island Sound. Bowman (19766) has estimated that a mean flux of N 15 m3 s-l (400 MGD) of effluent enters the western basin from sources in the upper East River. Records of inorganic nutrient concentrations in New York City treatment plant effluents have been kept since April 1971. Samples are gathered at each plant on two consecutive days each month at 06.00, 10.00, 14.00, 18.00 and 22.00 hours, flow weighted and analysed for a mean value (Dr S. Kirshner, private communication). It is not clear what bias is involved in the scheme, as it is well known that strong diurnal and weekly cycles do exist. Combined data for the Wards Island, Hunts Point, Tallman Island, and Bowery Bay sewage treatment plants (all located on the upper East River) are shown in Figure 2 (New York City EPA, 1972-74). The upper plot shows flow weighted mean monthly ammonia concentrations for the January 1972 to January 1974 period. There is a suggestion that the ammonia and nitrate curves tend in opposite directions, and it should be noted that the abnormally low value found in November 1972, corresponds to a coincidental large peak in the nitrate distribution. The nitrate concentrations exhibit a clear seasonal cycle with a minimum occurring during the summer and fall with concentrations ~0.01 mg-at l-r, and increasing to a winter maximum with concentrations reaching as high as 0.1 mg-at 1-l. This trend qualitatively follows the nitrate cycle commonly observed in temperate coastal marine waters and estuaries. Nitrite concentrations are relatively low and display a tendency to follow the nitrate concentrations with a summer minimum and a winter maximum (see Figure 4, Bowman, 1976b, for mean effluent discharges directly and indirectly into Long Islyd Sound, including the contribution into the upper East River). It is instructive to calculate theoretical nitrate concentrations using published effluent discharge rates from treatment plants located in the Connecticut River watershed, the mean River discharge and nutrient levels similar to those measured in the New York City treatment plants. The total effluent discharge into the Connecticut River basin (drainage area -2.9 x 104 km2) is N 15 ma s-l (U.S. EPA, 1971; Wrightington, private communication). An average source concentration of 0.05 mg-at 1-l (New York City EPA, 197274) mixed into a mean River discharge of 560 m8 s-l (Garvine, 1974) leads to an estimated mean nutrient concentration of N 1.5 ug-at 1-l downstream. Riley (1959) investigated freshwater nutrient sources a short distance upstream from the mouths of the Connecticut, Thames, and Housatonic Rivers. The rivers were found to have a nitrate cycle similar to that of the Sound, with a winter maximum and a summer minimum. The range of values of nitrate values for the three rivers were 0.839, o-37, and o-29 ug-at I-1, respectively. The mean value of 17 readings spaced over a 19 months period was 15 pgat 1-l for the Connecticut River. The above calculations thus suggest that a significant fraction (N 10%) of the nutrients flowing down the Connecticut River are of municipal rather than agricultural origin.
Agricultural
and groundwater sources of nutrients
There has been much speculation on the importance of agricultural and groundwater sources of nutrients flowing into Long Island Sound. Lohr & Love (1954) published 45 chemical analyses of reservoir and other water supplies of potential commercial exploitation in the
534
M.J.
Bowman
Long Island Sound watershed. Most of their nitrate values for surface waters lay within the ranges published by Riley (1959) for the three rivers, the mean being IO pg-at 1-l. Analyses of IO wells on the New York mainland and from western Long Island revealed large variations, with values ranging from 8 to 610 pg-at 1-r with a mean of 210 l.rg-at 1-l. The recent rapid suburban development of central and eastern Long Island with waste water recharge Station
number
Hell Gote
15 2 f 20 8 25 30 35 40
Slotion 89
0
90
91
92
93
95
nwrbef 98
---_
99
100
103
l-l-
105
106
60 (b) Figure 3. Vertical longitudinal temperature sections in Long Island Sound, Cruise 7307. (a) Upper East River and western Sound. (b) Central and eastern Sound.
Nutrients in Lmg Island Sound
535
of the water table represents another potential nitrate source for the Sound. River and groundwater seepageof nutrients was incorporated into the model as a parameter by relating this flux to the freshwater drainage around the Sound’s perimeter in a proportional relationship.
Distribution
of hydrographic properties and sampling methods
Data from two cruises made by the Marine Sciences Research Center to sample water quality in Long Island Sound during January 1970 (Cruise 7001; Hardy & Weyl, rg7o), and August 1973 (Cruise 7307) were selected for study in an attempt to characterize winter and summer conditions, respectively. Dimensions of each region are given in Table I. Determinations of only surface (N I m) water properties and micronutrients were gathered during Cruise 7001. The weather during this cruise which lasted 2 days was calm, punctuated with moderate to heavy fog and broken ice fields. Sea temperatures were close to o “C throughout the Sound, with air temperatures in the range N 1-8 “C. Surface salinities varied from N 26%, near Throgs Neck, N 28x, in the central basin, to my30% in The Race. Thirteen nutrient sampling stations were taken along the longitudinal axis of the Sound (Figure I). Weather conditions during Cruise 7307 were also mild with zzerosea state. The hydrographic properties are shown for the western, and central and eastern basins in Figures 3 to 5. Temperatures attain a maximum of over 22-5 “C in the upper East River, due to the local release of power plant heated effluents. Thewell-mixed water column in the upper East River is replaced by a stratified structure in the western basin with vertical temperature contrasts ~3.5 “C! found across the thermocline between 5 and IO m. Both surface and bottom temperatures increase eastward in the central basin and a complicated thermal structure indicative of strong tidal mixing exists. Surface temperatures peak at over 22 “C at station 103 and bottom temperatures of over rg “C are found in this region. The salinity (Figure 4) and density (Figure 5) fields closely parallel each other. Salinities in the central increase rapidly from less than 22%,, in the upper East River, to -25-27x,, basin and to over 29x0 in the eastern regions. A strong horizontal salinity gradient exists in the eastern passes near the entrance to Block Island Sound. Water column stability is characteristically low at the eastern end of the Sound where tidal mixing is intense and increases in the central basin, although this region is often dominated by brief periods of intense wind driven circulation and vertical mixing (Figure 3). TABLE I. Volumes, areas and mean depths (at mean low water) of the Long Island Sound (see Figure Region Volume (ma/IO’) Area MLW (km%) Mean depth (m) below MLW
Region Volume (m”/ro’) Area MLW (km’) Mean depth (m) below MLW
I
2
3
:f
107 128
224 x86
7'3
8.4
4
5
351
611 336
242
6
7
665
889
342
446
14’5
18.2
19.4
II
12
I3
Total
34 54
6200 32"6
9
IO
942 442
804 4-J
610 276
20’1
regions of
12’0
8
21.3
13
I)
22’1
561 202 27'7
386 130 29'7
63
19'9
19'3
536
M. 3. Bowman
Station
number
Hell Gate
25-
30-
Solintty
350 40
,
t 5
km ,
I
,
,
,
,
t
1
I
I
(4
Station 89
90
91
92
93
95
number
98
99
IO0
103
105
Race 106
0
60
(b) Figure 4. Vertical longitudinal salinity sections in Long Island Sound, Cruise 7307. (a) Upper East River and western Sound. (b) Central and eastern Sound.
Nutrients in Long Ishnd Sound
70
79
80
81
82
.
.
537
03 04 85 I I .
06I
87I .
0.9 I .
89 .
25
30 Sigma-T
35
smlDll number 0
50
60,,
,
,
,
,
I:::::::::, 0 ,
km
50 I
1
04
Figure 5. Vertical longitudinal density sections in Long Island Sound, Cruise 7307. (a) Upper East River and western Sound. (b) Central and eastern Sound.
538
M.J.
Bowman
Stability decreases again in the far western Sound and upper East River, and decreases to zero in the turbulent Hell Gate region (Reynold number ~5 x 10~; Bowman, 19763). Experimental summer nutrient distributions (Figures 8, 14) (Figure II; Bowman, 1976b) generally show highest vertical contrasts within the area of increased stability (regions 3-5) The horizontal density gradient throughout the Sound (Figure 5) drives a two-layered gravitational circulation with less dense Sound water flowing eastward near the surface being replaced by saltier Block Island Sound bottom water flowing to the west at depth. The magnitude of the transport in each layer has been estimated by Wilson (1976) to be ~3 x IO* m3 s-l in the eastern reaches, dropping to N 5000 m3 s-l in the central basin, and down to ~500 m3 s-l in the Upper East River (Bowman, 1977). A submersible pumping system developed by Hulse (1975) was used during Cruise 7307 to deliver surface and bottom samples of sea water for micronutrient analysis at 25 stations spaced along the Sound. Reactive ammonia, nitrite and nitrate were determined according to the methods described in Strickland & Parsons (1972). The resolutions of these determinations are &o-25, ko.023, and fo.05 pg-at l-l, respectively. All data were projected and plotted along a line approximating the (curved) longitudinal axis of the Sound (Figure I). Nutrient
distributions
and transport
patterns
The experimental and theoretical nutrient distributions and the estimated nutrient budgets computed from a mathematical model (Bowman, 19763) and the data for each of the 13 regions of the Sound are shown in Figures 7-16. Nutrient input cPhf +r;&
Hwiiontol transport M/J 0
Figure 6. C‘ CP C, k Ml & Vr W t,r+r
Ci v
Horizontal tromport %i+f
Biochemical uptake kql+ Nutrient mass balance diagram for section i of Long Island Sound. = mean nutrient concentration in water column of section i. = nutrient concentration in sewage effluent. = nutrient concentration in runoff. = first order decay coefficient. = effluent discharge rate. = runoff discharge rate. = volume of section i. = horizontal nutrient exchange rate between sections i and i+ I.
The two source and sink terms in the transport diagrams are cumulatively plotted above and below the longitudinal axis, respectively (e.g. see Figure 9); the mirror symmetrical heavy lines represent the algebraic sum (equal in steady state) of the gains and losses in each region. A westward (negative) horizontal transport results when the heavy solid lines cross over into the two shaded areas. These budget diagrams (e.g. Figure 6) are useful in calculating the relative enrichment of each region over a given time interval t by horizontal exchange (IV,-,, I-W,, r +r)t and nutrient input across the lateral boundaries (cpMI+c,R,)t compared with consumption ktc,V,; the standing stock is given by c,V,.
Nutrients in Long Island Sound
I
2
3
4
Figure 7. Experimental centration distributions
5
6 7 0 Section number
539
9
IO
II
12
(Cruise 7001) -o- and theoretical for the winter of 1970.
13
-
ammonia
con-
Ammonia Concentrations over most of the Sound during winter (Figure 7) are relatively constant at N 5 l,tg-at l-1, with a sharp increase in the western basin with a projected peak at ~45 pg-at l-l, in the upper East River (unfortunately the data did not extend far enough westward to fix concentrations at the far western end of the Sound with much confidence). In summer, concentrations present a picture of poverty (Figure 8) ; ammonia is virtually removed from the water column over most of the Sound. However, an extremely sharp peak is found in the upper East River where experimental concentrations reach to over 145 l.tg-at 1-r. A minor secondary peak is both predicted and found in the vicinity of the Connecticut River mouth (regions IO-I I ; Figure I). These lack of ammonia and other forms of inorganic nitrogen in the water column are typically found in summer from the end of winter flowering until August or September. There is considerable patchiness in the western basin, which may be partly accounted for by incomplete mixing of (buoyant) effluents, a corresponding patchiness in phytoplankton communities, or losses across the air-sea interface. As mentioned earlier, vertical contrasts are greatest in this region of increased stability. Biological uptake of ammonia in winter is seen in Figure 9 to increase to a maximum of N 15 g-at s-l (~0.03 pg-at s-l m-a; integrated through the water column over unit
Cruise 7307 Surfocv * 0ottom = -
IO
123456709
II
I2
I:3
Section number
Figure 8. Experimental (Cruise 7307) and theoretical distributions for the summer of 1973.
ammonia
concentration
M. J. Bowman
540
Gain
75 ,
Horizontal transport
LOSS
-75
1
Figure 9. Calculated
ammoniajbudgets
for the winter
of 1970.
2. Estimated first order biochemical decay rates in Long Island Sound during the winter of 1970 (Cruise 7001) and the summer of 1973 (Cruise 7307). Uncertainties in predictions result from variations both in nutrient source concentrations and mass transport in the East River. TABLE
NH, winter (7001) NH, summer (7307) NO, winter (7001) NO, summer (7307) NO, winter
5~2&03X 4.ofr.o
(days)
k-’
k (s-l)
22i3
10-7
2~9io.8
x 10~~
1.2&0.3x
IO-’
5.0+1.0x
10-G
95123
2'5 io.5
x 10-a
1.2io.3
x IO-~
2’3
ho.5
46ozt92
(7001)
NO, summer (7307)
9.6 ir2.6
TABLE 3. Estimated gains (positive) and losses (negative) of inorganic nitrogen fractions at the western and eastern ends of Long Island Sound during the winter of 1970 (Cruise 7001) and the summer of 1973 (Cruise 7307).
Western End (Throgs Neck) (IV,,,) tons day- i kg s-r NH,
winter (7001) summer (7307) NOs winter (7001) summer (7307) NO, winter (7001) summer (7307)
0.66io.19
57&16
2'3 ko.7 0’039 fo.01 0.081
0.13
*to.023 fo.04
O.I3iO’O+
200
I
* 60
Eastern End (Block Island Sound) (-IV,,,,,) tons day-’ kg s-r --0.17&0.03
--IS%3
-0.59fo.18
-51f16
3’4+1’0
-0,067
7’0 _tz.o
+0~061~0~012
II&3 1113
i-o.005
-0~18~0~02 -0~018*0~011
--5+x&0.4
+5’3fI”J -16fz -1.6-ir1.0
Nutrients in Long Island Sound
541
cross-sectional surface area) in the region 7 of the central basin; this is a consequence of a corresponding increase in regional volume of the Sound rather than increased productivity. The utilization in the western reaches decreases, in spite of a very large increase in nutrient supply; again this is attributed to the relatively small volume of the western basin. The standing stock of ammonia is sufficient for about one month’s growth (K-l ~22 days; Table 2); however, eastward horizontal transport in the western two-thirds of the estuary is ample to replenish this consumption several times over. Horizontal transport reverses in section 9 as the nutrient supply originating mainly from region I is consumed and is replenished from sources in the Connecticut River basin (regions IO and I I). The transport pattern then reverses again near region II, resulting in a net loss of ammonia from the Sound (Table 3). The reversal in horizontal transport between regions I and 2 is due to a transport node which exists in section I (i.e. Ws,ro). Some 30~~f20~~ of effluents released into the Upper East River (region I) are transported into New York Harbor (region o) and the remaining 70%*20% flow into Long Island Sound (Bowman, 19763). Gatn
250 200 I50 100
50 ‘;, 0 F g -50 -100 -150 -200 Lass -250 f Figure
IO. Calculated
ammonia budgets for the summer of 1973.
The summer ammonia budget (Figure IO) is qualitatively similar, except that utilization peaks in region 2 due to increased nutrient concentration in the water column offsetting the relatively smaller volume of the area, and the greatly increased utilization rate throughout the Sound (K-1 -2.9 days, sufficient for 3-4 days growth). This latter result is consistent with previous studies in the Sound (Riley, 1967) where rapid utilization of ammonia accounted for the majority of the nitrogen turnover during quasi-steady state conditions in summer. Nitrite Nitrite concentrations in winter average N I Bg-at 1-l in the central basin (Bowman, 19766; Figure 9), and display a gradual drop from a peak value of ~3 pg-at 1-l in the upper East River down to zero in the vicinity of Block Island Sound. In contrast summer concentrations exhibit very steep gradients in the western end of the Sound, with levels dropping from a sharp peak of ~4 pg-at 1-l down to virtually zero in the central basin (Bowman, 19763; Figure I I). Concentrations increase slowly again in the eastern passes to ~0.5 pg-at 1-l.
542
M. J. Bozaman
The winter budget (Figure II) reflects the essentially conservative nature of nitrite in transport gradually increasing from winter (k-l ~95 days) with a large horizontal ~0.04 kg s-l (IV,,,) in the western basin to -0.07 kg s-r (W,,,,,) near Block Island Sound (Table 3). Horizontal transport exceeds the daily biochemical demand by several times in all regions of the Sound. The winter biochemical uptake pattern (Figure I I) has some similar features to the corresponding ammonia curve (Figure 9), increasing from low values at both ends of the Sound to a maximum of ~0’5 g-at s-l in region 8 (N 1.1 X 10~~ ug-at s-l m-“).
Gam
-I
q q
Horizontal transport w,,+,
-2
Nutrient Input Biochemcol
uptoke
-3-l -4-Loss -5 Figure
Gain
I I. Calculated
nitrite
budgets
fx
the winter
of 1970.
IO 8 6 4 2 0 -2 -4 -6
Ea
q
-8
Figure
12. Calculated
nitrite
budgets
Nutrient input Biochemical uptake
for the summer
of 1973.
A drastic increase in the uptake of nitrite in summer (K-l ~2.3 days) leads to a nutrient budget (Figure 12) where the supply originating in the upper East River is gradually consumed over the western and central basins. Horizontal transport reverses east of region 8, where an influx from the Connecticut River basin supplies the relatively large biochemical demand of the eastern region.
The moat prominent feature of the winter nitrate concentration distribution is the high values found throughout the Sound (Figure r3), refkcting the fact that nitrate is highly conservative in winter (R-l -460 days). The predicted distribution thus mainly results from the dilution of nitrate as it is transported eastward from the western basin as well as post season regeneration. The experimental data is sparse; not enough stations were taken to resolve the details of the patchiness. Average concentrations in the central basin of N 8.0 ug-at 1-l are somewhat less than the x953-54 and 1954-55 mid-winter maxima of N 20 ug-at l-1 found in the central part of the Sound by Riley & Conover (1956). During the summer (Figure x4), nitrate is virtually removed from the water column over most of the Sound with concentrations dropping to -0.5 ug-at 1-r but exhibiting a sharp increase in the western end to a peak of ~8~ig-at 1-l. Cruise7OOl Surface=+
01I 2’
3
4
6
6
7
9
8
IO
12
II
I?
Section number
Figute 13. Experimental (Cruise 700x) and theoretical nitrate concentration distributions for the winter of 1970. IO.0 cruise 7307 surface=-o8oticm=-
8.0
0
I
2
3
4
5
6
7
8
9
IO
II
12
13
Section number
Figure ~4, Experimental (Cruise 7307) distributions for the summer of 1973.
and theoretical nitrate concentration
544
M. J. Bowman
The winter nitrate budget shown in Figure 15 is qualitatively similar to the winter nitrite distribution (Figure II) with horizontal exchange dominating the mass balance over most of the Sound. The consumptive demand is qualitatively similar to the other two winter distributions, peaking in the central basin (region 8) at N 1.5 pg-at s-r (~3.4 x IO-~ pg-at s-l m-2). Transport patterns in the summer (Figure 16) are qualitatively similar to winter ammonia transport and uptake (Figure 9) with the abundant eastward horizontal transport gradually consumed by the biochemical demand (K-l N 9.6 days) over the western two-thirds of the estuary. Gain
15 , 12
6 5 5 5
Horizontal transport &j
3 0 -3 Nutrient input
Horizontal transport w,i+/
-6
q
Biochemical
uptake
-9
Figure
15. Calculated
nitrate budgets for the winter
q q Figure 16. Calculated
Discussion
of 1970.
Nutrient input Biochemical uptake
nitrate budgets for the summer of
of nutrient
1973.
distributions
The computed values of the first order decay coefficient K are listed in Table 2. The fact that all values of k are greater than zero, even in mid-winter, implies that there is a continual loss of nitrogen to the sediment via zooplankton grazing, excretion and mortality. (A negative K would imply that regeneration exceeds utilization.) This result agrees with recent work on sediment chemistry in the vicinity of Eatons Neck in Long Island Sound, where the interstitial waters are very high in organic carbon (I. W. Duedall, private communication) and suggests a continual build-up of organic material in the sediment.
Nutrients in Lang Island
Sound
545
TABLE 4. Estimates of the net loss of nitrogen to the sediment based on the total inorganic nitrogen (NH,+NO,+NO,) budgets during tbe winter of 1970 (Cruise 7001) and the summer of 1973 (Cruise 7307) for the western, central and eastern basins of Long Island Sound. Region Area Loss
(ma) (7001)
(pg s-l m-3 Las (7307) (pg 5-l m- *)
I-3
4-9
3'4X 108 w83&0.29
2’2 x 100 0*57fo'16
8G3*2*3
I’.5 fo.3
IO-12 6.1 x108 0.64fo.17
2’7f’V
Table 3 lists the overall horizontal transport budgets for both ends of the Sound. During both seasonsand for all fractions (excepting NO, in the summer) there is a net eastward flux of nutrients through the western and eastern boundaries of the estuary. Table 4 gives estimates of winter and summer losses of nitrogen to the sediment based on the total inorganic nitrogen budgets (NH,+NO,+NO,), for the western, central and eastern basins of the Sound (XJs=r & K,c,,V, where j = 1,3 runs over the 3 inorganic fractions and i runs over the number of sections in each of the three regions of the Sound). Accumulation of nitrogen peaks in the eutrophic western Sound where circulation is weak and the residence time is long, drops to intermediate values in the central basin and is a little higher in the eastern region. This latter increase mainly reflects the changing estuarine cross-section from mostly broad and flat to deep narrow passes in the eastern Sound. It is likely that the upstream bottom component of the estuarine circulation deposits particulate inorganic and organic matter into the central basin away from the well-scoured eastern passages. It is probable therefore that values shown in Table 4 of loss of nitrogen to the sediment in the eastern region are over-estimated. During winter, deposition rates are lower by at least a factor of 3, and by a factor of IO in the western reaches compared to summer rates, reflecting the lowered biological productivity and associated increased eastward horizontal transport in winter. Greatly increased primary productivity during the summer growing season would result in increased amounts of nitrogen being sedimented on the bottom in combined particulate and refractory forms, unavailable for immediate regeneration. Results obtained by application of the model also indicate that all nutrient distributions can satisfactorily be accounted by assuming seepage of nutrients into the Sound from agricultural sources is negligible compared to sewage. This conclusion was established by varying the nutrient concentration in runoff c, over wide limits whilst fitting experimental and theoretical concentrations. The distribution of runoff around the Sound (apart from the indirect contribution from the Hudson River) increases from west to east (see Figure 3 ; Bowman, I&J) whereas the distribution of sewage possesses a large point source in the upper East River. The concentration distribution arising from nutrients in runoff alone thus would be expected to peak in the eastern Sound and diminish in the central and western basins. In fact, the opposite is true, and all distributions peak in the western reaches and drop to lower levels in the central and eastern basins. Reasonable fits between theory and experiment could only be obtained by assuming the runoff terms c,R,
546
M. J. Bowman
Sargasso Sea near Bermuda. Nutrient concentrations in rainfall in urban areas are, not surprisingly, an order of magnitude higher. Ammonia and nitrate concentrations in rainfall gathered at Mineola, Long Island (U.S. Geological Survey, unpublished data) over the period October 1972 to December 1973 were found to be -32 pg-at 1-r and 46 pg-at l-l, respectively. Precipitation, fairly evenly distributed throughout the year, for the same period was N 1.80 m year-l. These measurements indicate an aerosol input into the Sound of ammonia and nitrate of N 1.8 x row3 pg-at s-l m-2 and 2.6 x IO-~ pg-at s-l m-2, respectively, about 2 orders of magnitude less than the influx of nutrients from point sources in sewage, averaged over the estuary (N o-17 pg-at s-l mw2). Nitrogen in the ocean exists primarily as molecular nitrogen and as inorganic salts such as ammonia, nitrite and nitrate. Concentrations of these ions found in Long Island Sound are within the usual ranges of concentration for these salts in sea water (O~OI-50, O'OI-5, 0.1-5 pg-at l-l, respectively; Parsons & Takahashi, 1973), with the exception of the ammonium ion which exhibits very high levels in the western end of the Sound (~40-130 l,tg-at 1-l). Ammonia is utilized directly through transamination for amino acid synthesis, but nitrite and nitrate must be reduced before being utilized by the cell, principally by photosynthetically reduced ferredoxin to reduce nitrite to ammonia (Hattori & Myers, 1966) and by the action of the enzyme nitrate reductase (Hattori, 1962a, b; Epply et al., 1969~). The study of nitrogen conversions in the marine environment is intricate, e.g. see Vaccaro & Ryther (1959), and beyond the scope of this paper. Subtle differences in nutrient composition, including the presence of organic substances, may play an important role in altering the productivity and species diversity of a region (Parsons & Takahashi, 1973). Ammonia is preferentially utilized over nitrate by phytoplankton (Morris & Syrett, 1963; Dugdale & Goering, 1967; Eppley et al., 1969b). The listing of the biochemical utilization coefficients in Table I confirms this result, and the rapid utilization of ammonia is also in agreement with the findings of Riley (1967) w h o f ound that the nutrient supply in Long Island Sound in summer was sufficient for about 3 days’ growth of phytoplankton (K-l can be considered the ‘half life’ of a nutrient under isolated conditions). The rapid removal of nitrite in summer is also consistent with preferential uptake of less highly oxidized forms of nitrogen (i.e. one would expect the decay times to come out in the sequence ammonia/ nitrite/nitrate). Denitrification occurs under essentially anaerobic conditions and may be important in bottom summer water in western Long Island Sound. The nitritication reaction rates of nitrite to nitrate by the nitrobacter bacteria is not determined with the model and needs separate investigation. It is interesting to note for all the summer 1973 data, that west of section 3 surface nutrient concentrations are higher than those near the bottom. This possibility can be attributed to the initial buoyancy of those effluents, released and mixed into the surface layer of the Upper East River which has an estuarine flow component into Long Island Sound. It could also be due to reduced productivity resulting from high turbidity of these waters, chlorination of summer effluents causing synergistic toxicity due to formation of chloroamines, or the (unknown) effects of other pollutants. Ammonia levels even in the East River are well below N 600 pg-at 1-r considered toxic to marine organisms (Zgurovskaya & Kustenko, 1968). Conclusion The aim of this paper has been a limited one, namely to present results obtained from a simple but hopefully realistic model of nutrient distributions and horizontal transport in Long Island Sound.
Nutrients
in Long Island
Sound
547
The study has provided, from an admittedly limited amount of experimental data, some first order estimates of nutrient transport and biological uptake rates in summer and winter and has focused on the role of horizontal advective and dispersive transport and the importance of sewage as a significant nutrient supply, especially in the western Sound, where a strong point source exists. With the exception of aspects of the horizontal transport patterns and the relative importance of nutrient sources in runoff, results presented are qualitatively consistent with previous studies. The application and interpretation of the model are both facilitated by the existence of large horizontal gradients in the nutrient distributions in the Sound. The method is especially useful in that it is hard to see how direct measurements of nutrient transports applicable to seasonal time scales could be obtained in Long Island Sound. It is difficult enough to gather direct verification data for simple models, for example of daily primary production in much smaller marine embayments, such as Bedford Basin (Platt & Conover, 1971). Many of the details of nutrient cycling particularly in the eutrophic western end of the estuary need to be investigated with a two dimensional model. The question of the relative productivity of different regions within the Sound needs to be reopened and the validity of the assumption of steady state and spatially uniform biochemical utilization coefficients examined more closely. These variations will affect the predicted areal and temporal distributions of nutrients, but they probably would not alter the general features and processes described in this paper.
Acknowledgements I wish to thank Trevor Platt and Charles D. Hardy for their helpful comments on the manuscript and Iver W. Duedall for his assistance in the gathering of the hydrographic and nutrient data on Cruise 7307. This work is a result of research partially sponsored by the New York State Sea Grant Institute, NOAA Office of Sea Grant, Department of Commerce, under Grant Number 2-35281. The U.S. Government is authorized to produce and distribute reprints for governmental purposes notwithstanding any copyright notation that may appear hereon.
Bowman,
M. J. 1976~. The tides of the East River,
New York. Journul
of Geophysical Research 81,
1609-1616.
Bowman, M. J. 19763. Pollution prediction model of Long Island Sound. In Proceedings of civil Engineering in the Oceans/III, Newark, Delaware, g-13 June, 1975. American &ciety of Civil Engineers, New York, pp. 1084-1103. Bowman, M. J. 1977. Scales of variability of non tidal characteristics of the East River, New York. (In press). Conover, S. A. M. 1956 Oceanography of Long Island Sound, 1952-1954. IV. Phytoplankton. Bulletin of the Bingham Oceanographic collection 15, 156-233. Dugdale, R. C. & Goering, J. J. 1967 Uptake of new and regenerated forms of nitrogen in primary productivity. Limnology and Oceanography 12, 19G2c6. Eppley, R. W., Coatsworth, J. L. & Solonano, L. 1969~ Studies of nitrate reductase in marine phytoplankton. hh?WlOgy and Oceanography 14, 194-205. Eppley, R. W., Rogers, J. N. & McCarthy, J. J. 19693 Half saturation constants for uptake of nitrate and ammonium by marine phytoplankton. Limnology and Oceanography 14, 192-220. Garvine, R. W. 1974 Physical features of the Connecticut River outiow during high discharge. JourrruI of Geophysical Research 80 (9), I 176-83. Hardy, C. D. & Weyl, P. K. 1970 Hydrographic data report: Long Island Sound-x970. Part I. Murine Sciences Research Center, Technical Report No. 6. Stony Brook, New York.
Ad. J. Bozwnan
548
Hardy, C. D. 1972 Movement and quality of Long Island Sound waters, 1971. Marine Sciences Research Center, Technical Report No. 17. Stony Brook, New York. Harris, E. 1959 The nitrogen cycle in Long Island Sound. Bulletin of the Bingham Oceanographic Collection 17, 3 1-64. Hattori, A. 1962a Light-induced reduction of nitrate, nitrite and hydroxylamine in a blue-green alga, Anabaena cylindrica. Plant and Cell Physiology 3, 355-369. Hattori, A. 1962b Adaptive formation of nitrate reducing system in Anabaena cylindrica. Plant and Cell Physiology 3, 371-377. Hattori, A. & Myers, J. 1966 Reduction of nitrate and nitrite by subcellular preparations of Anabaena cylindrica. I. Reduction of nitrite to ammonia. Plant Physiology 41, 1031-1036. Hulse, G. 1975 The Plunket: A shipboard water quality monitoring system. Marine Sciences Research Center Technical Report No. 22. Stony Brook, New York. Lohr, E. W. & Love, S. K. 1954 The industrial utility of public water supplies in the United States, 1952. Part I. States east of the Mississippi River. Water Supply (Irrigation) Paper U.S. Geological &mvey No. 1299, 1-639. Menzel, D. W. & Spaeth, J. P. 1962 Occurrence of ammonia in Sargasso Sea waters and in rain water at Bermuda. Limnology and Oceanography 7, 159-162. Morris, I. & Syrett, P. J. 1963 The development of nitrate reductase in Chlorella and its repression by ammonium. Archiw fuer Mikrobiologie 47, 32-41. New York City Environmental Protection Administration, 1972-74. Sewage and analytical records. Unpublished. Parsons, T. & Takahashi, M. 1973 Biological Oceanographic Processes. Pergamon Press, Oxford. Platt, T. & Conover, R. J. 1971 Variability and its effect on the 24 hour chlorophyll budget of a small marine basin. Marine Biology IO, 52-65. Riley, G. A. 1956 Review of the oceanography of Long Island Sound. Deep Sea Research Supplement 3, 224-238.
Riley, G. A. & Conover, S. A. M. 1956 Oceanography of Long Island Sound, 1952-1954. III. Chemical Oceanography. Bulletin of the Bingham Oceanographic Collection 15, 47-61. Riley, G. A. 1959 Oceanography of Long Island Sound, 1954-1955. Bulletin of the Binghom Oceanographic ColkCtiOn 17, 9-29. Riley, G. A. 1967 Mathematical model of nutrient conditions in coastal waters. Bulletin of the Bingham Oceanographic Collection 19, 72-80. Strickland, J. D. H. & Parsons, T. R. 1972 A Practical Handbook of Seawater Analysis. Fisheries Research Board of Canada, Bulletin 167 (2nd ed.). U.S. Dept. Commerce 1972 1970 Census of Population. Characteristics of Population. Vol. 1, Part A. Sections land Z. Bureau of the Census, Social and Economic Statistics. U.S. Government Printing Office, Washington, D.C. U.S. Environmental Protection Agency 1971 Report on the Water quality of Long Island Sound. Dot. No. CWT, 10-29. Vaccaro, R. P. & Ryther, J. H. 1959 Marine phytoplankton and the distribution of nitrite in the sea. Journal du Conseil, Conseil International pour I’Exploration de la Mer 25,260-271. Wilson, R. E. 1976 Gravitational circulation in Long Island Sound. Estuarine and Coastal Marine Science 4,443~453. Wrightington, R. 1973 Private communication. (U.S. Environmental Protection Agency, Region I, Waltham, Mass.) Zgurovskaya, L. N. & Kustenko, N. G. 1968 The effect of ammonia nitrogen on cell division, photosynthesis, and pigment accumulation in Sceletonema costatum (grev) cl., Chatoceros spp. and Prorocentrum micans ehr. Oceanology 8, 90-98.
Nutrient Distributions Long Island Sound
and Transport in
Malcolm J. Bowman Marine Sciences Research Center, State University of New York at Stony Brook, Stony Brook, New York II794 Received 31 December 1975 and in revised form ab&ne
I976
Keywords: nutrient, nitrates, nitrites, ammonia, horizontal variations, sewage effluents, seasonal variations, Long Island
exchange/
Inorganic nitrogenous nutrient (NH,, NO,, NO,) distributions and transport in Long Island Sound are investigated for both winter and summer conditions with a steady state, one dimensional mass balance model. Nutrient budgets based on horizontal exchange, lateral input from sewage and agricultural sources, and first order biochemical uptake (utilization minus regeneration) are computed for each of 13 regions in the Sound. All nutrient concentrations, principally ammonia, peak sharply in the upper East River where a strong point source exists. Concentration distributions and uptake rates are consistent with previous studies of the nitrogen cycle and productivity of the Sound. However, this paper stresses horizontal exchange and sewage as important and hitherto neglected components of the nutrient budgets in various regions. Estimates of the loss of nitrogen to the sediment are presented for the western, central and eastern regions, from the Sound to the ocean at the eastern end, and into New York Harbor from the upper East River. Sewage effluents are shown to be the prime external source of nutrients for the Sound. Introduction Long Island Sound (Figure I) is a shallow estuary (mean depth ~20 m) some 165 km in length, connected to New York Harbor via the East River at its head (Bowman, rg76a) and with its second but main opening to the Atlantic Ocean located at its eastern mouth in Block Island Sound. The Sound is a highly impacted estuary ; some seventeen million people, representing about 8% of the population of the United States, reside within 40 km of Long Island Sound and New York Harbor (U. S. Dept. Commerce, 1972). The Sound has been an important navigable waterway for several centuries, serves as receiving waters for a great amount of man’s wastes (U. S. EPA, 197 I ), and as a prime recreational region for much of the above population. The purpose of this paper is to discuss experimental and theoretical aspects of nitrogenous nutrient distributions and transport in Long Island Sound obtained by the application of a mathematical model, originally designed to study the dispersion of sewage effluents released into the waters of the Sound (Bowman, rg@). All effluents contain large quantities of inorganic micronutrients and are shown in this paper to represent an important and hitherto neglected component of nutrient, budget estimates in various regions of the Sound. Contribution 162 of the Marine Sciences Research Center, of the State University of New York at Stony Brook.
532
M. J. Bowman
Kilometers
Figure I. Location map of Long Island Sound showing sampling stations and the I 3 model regions (Bowman, 1976b).
Previous
studies
A review of the oceanography of Long Island Sound suitable for the purposes of this paper, published by Riley (1956), outlines the essential physical characteristics of the estuary, and emphasizes the seasonal cycle of the nutrient supply and primary productivity. Previous studies of nutrient cycles in Long Island Sound (Conover, 1956; Riley & Conover, 1956; Harris, 1959; Riley, 1959; Hardy, 1972) have emphasized seasonal distributions and the local productivity of different regions. Exchange with continental shelf bottom waters, and the abundant freshwater supply were stressed as potentially important sources of nutrients. However, sewage effluents as sources of nutrients have been neglected in past studies. 111111111111111111111111111 = I.0 Mean NH3
0.1
0.01
JFMAMJJASONDJFMAMJJASONDJ b-1972--i j--
1973 d-1974
Figure 2. Flow weighted nutrient concentrations treatment plants (New York city EPA, 1972-74).
in upper
East River
sewage
Nutrients in Long Island Sound
Sewage eihents
533
as nutrient sources
Since no tertiary treatment of any significance exists in the region of this study, all sewage effluents contain high concentrations of nutrients. The East River daily transports a large quantity of sewage effluent into western Long Island Sound. Bowman (19766) has estimated that a mean flux of N 15 m3 s-l (400 MGD) of effluent enters the western basin from sources in the upper East River. Records of inorganic nutrient concentrations in New York City treatment plant effluents have been kept since April 1971. Samples are gathered at each plant on two consecutive days each month at 06.00, 10.00, 14.00, 18.00 and 22.00 hours, flow weighted and analysed for a mean value (Dr S. Kirshner, private communication). It is not clear what bias is involved in the scheme, as it is well known that strong diurnal and weekly cycles do exist. Combined data for the Wards Island, Hunts Point, Tallman Island, and Bowery Bay sewage treatment plants (all located on the upper East River) are shown in Figure 2 (New York City EPA, 1972-74). The upper plot shows flow weighted mean monthly ammonia concentrations for the January 1972 to January 1974 period. There is a suggestion that the ammonia and nitrate curves tend in opposite directions, and it should be noted that the abnormally low value found in November 1972, corresponds to a coincidental large peak in the nitrate distribution. The nitrate concentrations exhibit a clear seasonal cycle with a minimum occurring during the summer and fall with concentrations ~0.01 mg-at l-r, and increasing to a winter maximum with concentrations reaching as high as 0.1 mg-at 1-l. This trend qualitatively follows the nitrate cycle commonly observed in temperate coastal marine waters and estuaries. Nitrite concentrations are relatively low and display a tendency to follow the nitrate concentrations with a summer minimum and a winter maximum (see Figure 4, Bowman, 1976b, for mean effluent discharges directly and indirectly into Long Islyd Sound, including the contribution into the upper East River). It is instructive to calculate theoretical nitrate concentrations using published effluent discharge rates from treatment plants located in the Connecticut River watershed, the mean River discharge and nutrient levels similar to those measured in the New York City treatment plants. The total effluent discharge into the Connecticut River basin (drainage area -2.9 x 104 km2) is N 15 ma s-l (U.S. EPA, 1971; Wrightington, private communication). An average source concentration of 0.05 mg-at 1-l (New York City EPA, 197274) mixed into a mean River discharge of 560 m8 s-l (Garvine, 1974) leads to an estimated mean nutrient concentration of N 1.5 ug-at 1-l downstream. Riley (1959) investigated freshwater nutrient sources a short distance upstream from the mouths of the Connecticut, Thames, and Housatonic Rivers. The rivers were found to have a nitrate cycle similar to that of the Sound, with a winter maximum and a summer minimum. The range of values of nitrate values for the three rivers were 0.839, o-37, and o-29 ug-at I-1, respectively. The mean value of 17 readings spaced over a 19 months period was 15 pgat 1-l for the Connecticut River. The above calculations thus suggest that a significant fraction (N 10%) of the nutrients flowing down the Connecticut River are of municipal rather than agricultural origin.
Agricultural
and groundwater sources of nutrients
There has been much speculation on the importance of agricultural and groundwater sources of nutrients flowing into Long Island Sound. Lohr & Love (1954) published 45 chemical analyses of reservoir and other water supplies of potential commercial exploitation in the
534
M.J.
Bowman
Long Island Sound watershed. Most of their nitrate values for surface waters lay within the ranges published by Riley (1959) for the three rivers, the mean being IO pg-at 1-l. Analyses of IO wells on the New York mainland and from western Long Island revealed large variations, with values ranging from 8 to 610 pg-at 1-r with a mean of 210 l.rg-at 1-l. The recent rapid suburban development of central and eastern Long Island with waste water recharge Station
number
Hell Gote
15 2 f 20 8 25 30 35 40
Slotion 89
0
90
91
92
93
95
nwrbef 98
---_
99
100
103
l-l-
105
106
60 (b) Figure 3. Vertical longitudinal temperature sections in Long Island Sound, Cruise 7307. (a) Upper East River and western Sound. (b) Central and eastern Sound.
Nutrients in Lmg Island Sound
535
of the water table represents another potential nitrate source for the Sound. River and groundwater seepageof nutrients was incorporated into the model as a parameter by relating this flux to the freshwater drainage around the Sound’s perimeter in a proportional relationship.
Distribution
of hydrographic properties and sampling methods
Data from two cruises made by the Marine Sciences Research Center to sample water quality in Long Island Sound during January 1970 (Cruise 7001; Hardy & Weyl, rg7o), and August 1973 (Cruise 7307) were selected for study in an attempt to characterize winter and summer conditions, respectively. Dimensions of each region are given in Table I. Determinations of only surface (N I m) water properties and micronutrients were gathered during Cruise 7001. The weather during this cruise which lasted 2 days was calm, punctuated with moderate to heavy fog and broken ice fields. Sea temperatures were close to o “C throughout the Sound, with air temperatures in the range N 1-8 “C. Surface salinities varied from N 26%, near Throgs Neck, N 28x, in the central basin, to my30% in The Race. Thirteen nutrient sampling stations were taken along the longitudinal axis of the Sound (Figure I). Weather conditions during Cruise 7307 were also mild with zzerosea state. The hydrographic properties are shown for the western, and central and eastern basins in Figures 3 to 5. Temperatures attain a maximum of over 22-5 “C in the upper East River, due to the local release of power plant heated effluents. Thewell-mixed water column in the upper East River is replaced by a stratified structure in the western basin with vertical temperature contrasts ~3.5 “C! found across the thermocline between 5 and IO m. Both surface and bottom temperatures increase eastward in the central basin and a complicated thermal structure indicative of strong tidal mixing exists. Surface temperatures peak at over 22 “C at station 103 and bottom temperatures of over rg “C are found in this region. The salinity (Figure 4) and density (Figure 5) fields closely parallel each other. Salinities in the central increase rapidly from less than 22%,, in the upper East River, to -25-27x,, basin and to over 29x0 in the eastern regions. A strong horizontal salinity gradient exists in the eastern passes near the entrance to Block Island Sound. Water column stability is characteristically low at the eastern end of the Sound where tidal mixing is intense and increases in the central basin, although this region is often dominated by brief periods of intense wind driven circulation and vertical mixing (Figure 3). TABLE I. Volumes, areas and mean depths (at mean low water) of the Long Island Sound (see Figure Region Volume (ma/IO’) Area MLW (km%) Mean depth (m) below MLW
Region Volume (m”/ro’) Area MLW (km’) Mean depth (m) below MLW
I
2
3
:f
107 128
224 x86
7'3
8.4
4
5
351
611 336
242
6
7
665
889
342
446
14’5
18.2
19.4
II
12
I3
Total
34 54
6200 32"6
9
IO
942 442
804 4-J
610 276
20’1
regions of
12’0
8
21.3
13
I)
22’1
561 202 27'7
386 130 29'7
63
19'9
19'3
536
M. 3. Bowman
Station
number
Hell Gate
25-
30-
Solintty
350 40
,
t 5
km ,
I
,
,
,
,
t
1
I
I
(4
Station 89
90
91
92
93
95
number
98
99
IO0
103
105
Race 106
0
60
(b) Figure 4. Vertical longitudinal salinity sections in Long Island Sound, Cruise 7307. (a) Upper East River and western Sound. (b) Central and eastern Sound.
Nutrients in Long Ishnd Sound
70
79
80
81
82
.
.
537
03 04 85 I I .
06I
87I .
0.9 I .
89 .
25
30 Sigma-T
35
smlDll number 0
50
60,,
,
,
,
,
I:::::::::, 0 ,
km
50 I
1
04
Figure 5. Vertical longitudinal density sections in Long Island Sound, Cruise 7307. (a) Upper East River and western Sound. (b) Central and eastern Sound.
538
M.J.
Bowman
Stability decreases again in the far western Sound and upper East River, and decreases to zero in the turbulent Hell Gate region (Reynold number ~5 x 10~; Bowman, 19763). Experimental summer nutrient distributions (Figures 8, 14) (Figure II; Bowman, 1976b) generally show highest vertical contrasts within the area of increased stability (regions 3-5) The horizontal density gradient throughout the Sound (Figure 5) drives a two-layered gravitational circulation with less dense Sound water flowing eastward near the surface being replaced by saltier Block Island Sound bottom water flowing to the west at depth. The magnitude of the transport in each layer has been estimated by Wilson (1976) to be ~3 x IO* m3 s-l in the eastern reaches, dropping to N 5000 m3 s-l in the central basin, and down to ~500 m3 s-l in the Upper East River (Bowman, 1977). A submersible pumping system developed by Hulse (1975) was used during Cruise 7307 to deliver surface and bottom samples of sea water for micronutrient analysis at 25 stations spaced along the Sound. Reactive ammonia, nitrite and nitrate were determined according to the methods described in Strickland & Parsons (1972). The resolutions of these determinations are &o-25, ko.023, and fo.05 pg-at l-l, respectively. All data were projected and plotted along a line approximating the (curved) longitudinal axis of the Sound (Figure I). Nutrient
distributions
and transport
patterns
The experimental and theoretical nutrient distributions and the estimated nutrient budgets computed from a mathematical model (Bowman, 19763) and the data for each of the 13 regions of the Sound are shown in Figures 7-16. Nutrient input cPhf +r;&
Hwiiontol transport M/J 0
Figure 6. C‘ CP C, k Ml & Vr W t,r+r
Ci v
Horizontal tromport %i+f
Biochemical uptake kql+ Nutrient mass balance diagram for section i of Long Island Sound. = mean nutrient concentration in water column of section i. = nutrient concentration in sewage effluent. = nutrient concentration in runoff. = first order decay coefficient. = effluent discharge rate. = runoff discharge rate. = volume of section i. = horizontal nutrient exchange rate between sections i and i+ I.
The two source and sink terms in the transport diagrams are cumulatively plotted above and below the longitudinal axis, respectively (e.g. see Figure 9); the mirror symmetrical heavy lines represent the algebraic sum (equal in steady state) of the gains and losses in each region. A westward (negative) horizontal transport results when the heavy solid lines cross over into the two shaded areas. These budget diagrams (e.g. Figure 6) are useful in calculating the relative enrichment of each region over a given time interval t by horizontal exchange (IV,-,, I-W,, r +r)t and nutrient input across the lateral boundaries (cpMI+c,R,)t compared with consumption ktc,V,; the standing stock is given by c,V,.
Nutrients in Long Island Sound
I
2
3
4
Figure 7. Experimental centration distributions
5
6 7 0 Section number
539
9
IO
II
12
(Cruise 7001) -o- and theoretical for the winter of 1970.
13
-
ammonia
con-
Ammonia Concentrations over most of the Sound during winter (Figure 7) are relatively constant at N 5 l,tg-at l-1, with a sharp increase in the western basin with a projected peak at ~45 pg-at l-l, in the upper East River (unfortunately the data did not extend far enough westward to fix concentrations at the far western end of the Sound with much confidence). In summer, concentrations present a picture of poverty (Figure 8) ; ammonia is virtually removed from the water column over most of the Sound. However, an extremely sharp peak is found in the upper East River where experimental concentrations reach to over 145 l.tg-at 1-r. A minor secondary peak is both predicted and found in the vicinity of the Connecticut River mouth (regions IO-I I ; Figure I). These lack of ammonia and other forms of inorganic nitrogen in the water column are typically found in summer from the end of winter flowering until August or September. There is considerable patchiness in the western basin, which may be partly accounted for by incomplete mixing of (buoyant) effluents, a corresponding patchiness in phytoplankton communities, or losses across the air-sea interface. As mentioned earlier, vertical contrasts are greatest in this region of increased stability. Biological uptake of ammonia in winter is seen in Figure 9 to increase to a maximum of N 15 g-at s-l (~0.03 pg-at s-l m-a; integrated through the water column over unit
Cruise 7307 Surfocv * 0ottom = -
IO
123456709
II
I2
I:3
Section number
Figure 8. Experimental (Cruise 7307) and theoretical distributions for the summer of 1973.
ammonia
concentration
M. J. Bowman
540
Gain
75 ,
Horizontal transport
LOSS
-75
1
Figure 9. Calculated
ammoniajbudgets
for the winter
of 1970.
2. Estimated first order biochemical decay rates in Long Island Sound during the winter of 1970 (Cruise 7001) and the summer of 1973 (Cruise 7307). Uncertainties in predictions result from variations both in nutrient source concentrations and mass transport in the East River. TABLE
NH, winter (7001) NH, summer (7307) NO, winter (7001) NO, summer (7307) NO, winter
5~2&03X 4.ofr.o
(days)
k-’
k (s-l)
22i3
10-7
2~9io.8
x 10~~
1.2&0.3x
IO-’
5.0+1.0x
10-G
95123
2'5 io.5
x 10-a
1.2io.3
x IO-~
2’3
ho.5
46ozt92
(7001)
NO, summer (7307)
9.6 ir2.6
TABLE 3. Estimated gains (positive) and losses (negative) of inorganic nitrogen fractions at the western and eastern ends of Long Island Sound during the winter of 1970 (Cruise 7001) and the summer of 1973 (Cruise 7307).
Western End (Throgs Neck) (IV,,,) tons day- i kg s-r NH,
winter (7001) summer (7307) NOs winter (7001) summer (7307) NO, winter (7001) summer (7307)
0.66io.19
57&16
2'3 ko.7 0’039 fo.01 0.081
0.13
*to.023 fo.04
O.I3iO’O+
200
I
* 60
Eastern End (Block Island Sound) (-IV,,,,,) tons day-’ kg s-r --0.17&0.03
--IS%3
-0.59fo.18
-51f16
3’4+1’0
-0,067
7’0 _tz.o
+0~061~0~012
II&3 1113
i-o.005
-0~18~0~02 -0~018*0~011
--5+x&0.4
+5’3fI”J -16fz -1.6-ir1.0
Nutrients in Long Island Sound
541
cross-sectional surface area) in the region 7 of the central basin; this is a consequence of a corresponding increase in regional volume of the Sound rather than increased productivity. The utilization in the western reaches decreases, in spite of a very large increase in nutrient supply; again this is attributed to the relatively small volume of the western basin. The standing stock of ammonia is sufficient for about one month’s growth (K-l ~22 days; Table 2); however, eastward horizontal transport in the western two-thirds of the estuary is ample to replenish this consumption several times over. Horizontal transport reverses in section 9 as the nutrient supply originating mainly from region I is consumed and is replenished from sources in the Connecticut River basin (regions IO and I I). The transport pattern then reverses again near region II, resulting in a net loss of ammonia from the Sound (Table 3). The reversal in horizontal transport between regions I and 2 is due to a transport node which exists in section I (i.e. Ws,r
250 200 I50 100
50 ‘;, 0 F g -50 -100 -150 -200 Lass -250 f Figure
IO. Calculated
ammonia budgets for the summer of 1973.
The summer ammonia budget (Figure IO) is qualitatively similar, except that utilization peaks in region 2 due to increased nutrient concentration in the water column offsetting the relatively smaller volume of the area, and the greatly increased utilization rate throughout the Sound (K-1 -2.9 days, sufficient for 3-4 days growth). This latter result is consistent with previous studies in the Sound (Riley, 1967) where rapid utilization of ammonia accounted for the majority of the nitrogen turnover during quasi-steady state conditions in summer. Nitrite Nitrite concentrations in winter average N I Bg-at 1-l in the central basin (Bowman, 19766; Figure 9), and display a gradual drop from a peak value of ~3 pg-at 1-l in the upper East River down to zero in the vicinity of Block Island Sound. In contrast summer concentrations exhibit very steep gradients in the western end of the Sound, with levels dropping from a sharp peak of ~4 pg-at 1-l down to virtually zero in the central basin (Bowman, 19763; Figure I I). Concentrations increase slowly again in the eastern passes to ~0.5 pg-at 1-l.
542
M. J. Bozaman
The winter budget (Figure II) reflects the essentially conservative nature of nitrite in transport gradually increasing from winter (k-l ~95 days) with a large horizontal ~0.04 kg s-l (IV,,,) in the western basin to -0.07 kg s-r (W,,,,,) near Block Island Sound (Table 3). Horizontal transport exceeds the daily biochemical demand by several times in all regions of the Sound. The winter biochemical uptake pattern (Figure I I) has some similar features to the corresponding ammonia curve (Figure 9), increasing from low values at both ends of the Sound to a maximum of ~0’5 g-at s-l in region 8 (N 1.1 X 10~~ ug-at s-l m-“).
Gam
-I
q q
Horizontal transport w,,+,
-2
Nutrient Input Biochemcol
uptoke
-3-l -4-Loss -5 Figure
Gain
I I. Calculated
nitrite
budgets
fx
the winter
of 1970.
IO 8 6 4 2 0 -2 -4 -6
Ea
q
-8
Figure
12. Calculated
nitrite
budgets
Nutrient input Biochemical uptake
for the summer
of 1973.
A drastic increase in the uptake of nitrite in summer (K-l ~2.3 days) leads to a nutrient budget (Figure 12) where the supply originating in the upper East River is gradually consumed over the western and central basins. Horizontal transport reverses east of region 8, where an influx from the Connecticut River basin supplies the relatively large biochemical demand of the eastern region.
The moat prominent feature of the winter nitrate concentration distribution is the high values found throughout the Sound (Figure r3), refkcting the fact that nitrate is highly conservative in winter (R-l -460 days). The predicted distribution thus mainly results from the dilution of nitrate as it is transported eastward from the western basin as well as post season regeneration. The experimental data is sparse; not enough stations were taken to resolve the details of the patchiness. Average concentrations in the central basin of N 8.0 ug-at 1-l are somewhat less than the x953-54 and 1954-55 mid-winter maxima of N 20 ug-at l-1 found in the central part of the Sound by Riley & Conover (1956). During the summer (Figure x4), nitrate is virtually removed from the water column over most of the Sound with concentrations dropping to -0.5 ug-at 1-r but exhibiting a sharp increase in the western end to a peak of ~8~ig-at 1-l. Cruise7OOl Surface=+
01I 2’
3
4
6
6
7
9
8
IO
12
II
I?
Section number
Figute 13. Experimental (Cruise 700x) and theoretical nitrate concentration distributions for the winter of 1970. IO.0 cruise 7307 surface=-o8oticm=-
8.0
0
I
2
3
4
5
6
7
8
9
IO
II
12
13
Section number
Figure ~4, Experimental (Cruise 7307) distributions for the summer of 1973.
and theoretical nitrate concentration
544
M. J. Bowman
The winter nitrate budget shown in Figure 15 is qualitatively similar to the winter nitrite distribution (Figure II) with horizontal exchange dominating the mass balance over most of the Sound. The consumptive demand is qualitatively similar to the other two winter distributions, peaking in the central basin (region 8) at N 1.5 pg-at s-r (~3.4 x IO-~ pg-at s-l m-2). Transport patterns in the summer (Figure 16) are qualitatively similar to winter ammonia transport and uptake (Figure 9) with the abundant eastward horizontal transport gradually consumed by the biochemical demand (K-l N 9.6 days) over the western two-thirds of the estuary. Gain
15 , 12
6 5 5 5
Horizontal transport &j
3 0 -3 Nutrient input
Horizontal transport w,i+/
-6
q
Biochemical
uptake
-9
Figure
15. Calculated
nitrate budgets for the winter
q q Figure 16. Calculated
Discussion
of 1970.
Nutrient input Biochemical uptake
nitrate budgets for the summer of
of nutrient
1973.
distributions
The computed values of the first order decay coefficient K are listed in Table 2. The fact that all values of k are greater than zero, even in mid-winter, implies that there is a continual loss of nitrogen to the sediment via zooplankton grazing, excretion and mortality. (A negative K would imply that regeneration exceeds utilization.) This result agrees with recent work on sediment chemistry in the vicinity of Eatons Neck in Long Island Sound, where the interstitial waters are very high in organic carbon (I. W. Duedall, private communication) and suggests a continual build-up of organic material in the sediment.
Nutrients in Lang Island
Sound
545
TABLE 4. Estimates of the net loss of nitrogen to the sediment based on the total inorganic nitrogen (NH,+NO,+NO,) budgets during tbe winter of 1970 (Cruise 7001) and the summer of 1973 (Cruise 7307) for the western, central and eastern basins of Long Island Sound. Region Area Loss
(ma) (7001)
(pg s-l m-3 Las (7307) (pg 5-l m- *)
I-3
4-9
3'4X 108 w83&0.29
2’2 x 100 0*57fo'16
8G3*2*3
I’.5 fo.3
IO-12 6.1 x108 0.64fo.17
2’7f’V
Table 3 lists the overall horizontal transport budgets for both ends of the Sound. During both seasonsand for all fractions (excepting NO, in the summer) there is a net eastward flux of nutrients through the western and eastern boundaries of the estuary. Table 4 gives estimates of winter and summer losses of nitrogen to the sediment based on the total inorganic nitrogen budgets (NH,+NO,+NO,), for the western, central and eastern basins of the Sound (XJs=r & K,c,,V, where j = 1,3 runs over the 3 inorganic fractions and i runs over the number of sections in each of the three regions of the Sound). Accumulation of nitrogen peaks in the eutrophic western Sound where circulation is weak and the residence time is long, drops to intermediate values in the central basin and is a little higher in the eastern region. This latter increase mainly reflects the changing estuarine cross-section from mostly broad and flat to deep narrow passes in the eastern Sound. It is likely that the upstream bottom component of the estuarine circulation deposits particulate inorganic and organic matter into the central basin away from the well-scoured eastern passages. It is probable therefore that values shown in Table 4 of loss of nitrogen to the sediment in the eastern region are over-estimated. During winter, deposition rates are lower by at least a factor of 3, and by a factor of IO in the western reaches compared to summer rates, reflecting the lowered biological productivity and associated increased eastward horizontal transport in winter. Greatly increased primary productivity during the summer growing season would result in increased amounts of nitrogen being sedimented on the bottom in combined particulate and refractory forms, unavailable for immediate regeneration. Results obtained by application of the model also indicate that all nutrient distributions can satisfactorily be accounted by assuming seepage of nutrients into the Sound from agricultural sources is negligible compared to sewage. This conclusion was established by varying the nutrient concentration in runoff c, over wide limits whilst fitting experimental and theoretical concentrations. The distribution of runoff around the Sound (apart from the indirect contribution from the Hudson River) increases from west to east (see Figure 3 ; Bowman, I&J) whereas the distribution of sewage possesses a large point source in the upper East River. The concentration distribution arising from nutrients in runoff alone thus would be expected to peak in the eastern Sound and diminish in the central and western basins. In fact, the opposite is true, and all distributions peak in the western reaches and drop to lower levels in the central and eastern basins. Reasonable fits between theory and experiment could only be obtained by assuming the runoff terms c,R,
546
M. J. Bowman
Sargasso Sea near Bermuda. Nutrient concentrations in rainfall in urban areas are, not surprisingly, an order of magnitude higher. Ammonia and nitrate concentrations in rainfall gathered at Mineola, Long Island (U.S. Geological Survey, unpublished data) over the period October 1972 to December 1973 were found to be -32 pg-at 1-r and 46 pg-at l-l, respectively. Precipitation, fairly evenly distributed throughout the year, for the same period was N 1.80 m year-l. These measurements indicate an aerosol input into the Sound of ammonia and nitrate of N 1.8 x row3 pg-at s-l m-2 and 2.6 x IO-~ pg-at s-l m-2, respectively, about 2 orders of magnitude less than the influx of nutrients from point sources in sewage, averaged over the estuary (N o-17 pg-at s-l mw2). Nitrogen in the ocean exists primarily as molecular nitrogen and as inorganic salts such as ammonia, nitrite and nitrate. Concentrations of these ions found in Long Island Sound are within the usual ranges of concentration for these salts in sea water (O~OI-50, O'OI-5, 0.1-5 pg-at l-l, respectively; Parsons & Takahashi, 1973), with the exception of the ammonium ion which exhibits very high levels in the western end of the Sound (~40-130 l,tg-at 1-l). Ammonia is utilized directly through transamination for amino acid synthesis, but nitrite and nitrate must be reduced before being utilized by the cell, principally by photosynthetically reduced ferredoxin to reduce nitrite to ammonia (Hattori & Myers, 1966) and by the action of the enzyme nitrate reductase (Hattori, 1962a, b; Epply et al., 1969~). The study of nitrogen conversions in the marine environment is intricate, e.g. see Vaccaro & Ryther (1959), and beyond the scope of this paper. Subtle differences in nutrient composition, including the presence of organic substances, may play an important role in altering the productivity and species diversity of a region (Parsons & Takahashi, 1973). Ammonia is preferentially utilized over nitrate by phytoplankton (Morris & Syrett, 1963; Dugdale & Goering, 1967; Eppley et al., 1969b). The listing of the biochemical utilization coefficients in Table I confirms this result, and the rapid utilization of ammonia is also in agreement with the findings of Riley (1967) w h o f ound that the nutrient supply in Long Island Sound in summer was sufficient for about 3 days’ growth of phytoplankton (K-l can be considered the ‘half life’ of a nutrient under isolated conditions). The rapid removal of nitrite in summer is also consistent with preferential uptake of less highly oxidized forms of nitrogen (i.e. one would expect the decay times to come out in the sequence ammonia/ nitrite/nitrate). Denitrification occurs under essentially anaerobic conditions and may be important in bottom summer water in western Long Island Sound. The nitritication reaction rates of nitrite to nitrate by the nitrobacter bacteria is not determined with the model and needs separate investigation. It is interesting to note for all the summer 1973 data, that west of section 3 surface nutrient concentrations are higher than those near the bottom. This possibility can be attributed to the initial buoyancy of those effluents, released and mixed into the surface layer of the Upper East River which has an estuarine flow component into Long Island Sound. It could also be due to reduced productivity resulting from high turbidity of these waters, chlorination of summer effluents causing synergistic toxicity due to formation of chloroamines, or the (unknown) effects of other pollutants. Ammonia levels even in the East River are well below N 600 pg-at 1-r considered toxic to marine organisms (Zgurovskaya & Kustenko, 1968). Conclusion The aim of this paper has been a limited one, namely to present results obtained from a simple but hopefully realistic model of nutrient distributions and horizontal transport in Long Island Sound.
Nutrients
in Long Island
Sound
547
The study has provided, from an admittedly limited amount of experimental data, some first order estimates of nutrient transport and biological uptake rates in summer and winter and has focused on the role of horizontal advective and dispersive transport and the importance of sewage as a significant nutrient supply, especially in the western Sound, where a strong point source exists. With the exception of aspects of the horizontal transport patterns and the relative importance of nutrient sources in runoff, results presented are qualitatively consistent with previous studies. The application and interpretation of the model are both facilitated by the existence of large horizontal gradients in the nutrient distributions in the Sound. The method is especially useful in that it is hard to see how direct measurements of nutrient transports applicable to seasonal time scales could be obtained in Long Island Sound. It is difficult enough to gather direct verification data for simple models, for example of daily primary production in much smaller marine embayments, such as Bedford Basin (Platt & Conover, 1971). Many of the details of nutrient cycling particularly in the eutrophic western end of the estuary need to be investigated with a two dimensional model. The question of the relative productivity of different regions within the Sound needs to be reopened and the validity of the assumption of steady state and spatially uniform biochemical utilization coefficients examined more closely. These variations will affect the predicted areal and temporal distributions of nutrients, but they probably would not alter the general features and processes described in this paper.
Acknowledgements I wish to thank Trevor Platt and Charles D. Hardy for their helpful comments on the manuscript and Iver W. Duedall for his assistance in the gathering of the hydrographic and nutrient data on Cruise 7307. This work is a result of research partially sponsored by the New York State Sea Grant Institute, NOAA Office of Sea Grant, Department of Commerce, under Grant Number 2-35281. The U.S. Government is authorized to produce and distribute reprints for governmental purposes notwithstanding any copyright notation that may appear hereon.
Bowman,
M. J. 1976~. The tides of the East River,
New York. Journul
of Geophysical Research 81,
1609-1616.
Bowman, M. J. 19763. Pollution prediction model of Long Island Sound. In Proceedings of civil Engineering in the Oceans/III, Newark, Delaware, g-13 June, 1975. American &ciety of Civil Engineers, New York, pp. 1084-1103. Bowman, M. J. 1977. Scales of variability of non tidal characteristics of the East River, New York. (In press). Conover, S. A. M. 1956 Oceanography of Long Island Sound, 1952-1954. IV. Phytoplankton. Bulletin of the Bingham Oceanographic collection 15, 156-233. Dugdale, R. C. & Goering, J. J. 1967 Uptake of new and regenerated forms of nitrogen in primary productivity. Limnology and Oceanography 12, 19G2c6. Eppley, R. W., Coatsworth, J. L. & Solonano, L. 1969~ Studies of nitrate reductase in marine phytoplankton. hh?WlOgy and Oceanography 14, 194-205. Eppley, R. W., Rogers, J. N. & McCarthy, J. J. 19693 Half saturation constants for uptake of nitrate and ammonium by marine phytoplankton. Limnology and Oceanography 14, 192-220. Garvine, R. W. 1974 Physical features of the Connecticut River outiow during high discharge. JourrruI of Geophysical Research 80 (9), I 176-83. Hardy, C. D. & Weyl, P. K. 1970 Hydrographic data report: Long Island Sound-x970. Part I. Murine Sciences Research Center, Technical Report No. 6. Stony Brook, New York.
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Hardy, C. D. 1972 Movement and quality of Long Island Sound waters, 1971. Marine Sciences Research Center, Technical Report No. 17. Stony Brook, New York. Harris, E. 1959 The nitrogen cycle in Long Island Sound. Bulletin of the Bingham Oceanographic Collection 17, 3 1-64. Hattori, A. 1962a Light-induced reduction of nitrate, nitrite and hydroxylamine in a blue-green alga, Anabaena cylindrica. Plant and Cell Physiology 3, 355-369. Hattori, A. 1962b Adaptive formation of nitrate reducing system in Anabaena cylindrica. Plant and Cell Physiology 3, 371-377. Hattori, A. & Myers, J. 1966 Reduction of nitrate and nitrite by subcellular preparations of Anabaena cylindrica. I. Reduction of nitrite to ammonia. Plant Physiology 41, 1031-1036. Hulse, G. 1975 The Plunket: A shipboard water quality monitoring system. Marine Sciences Research Center Technical Report No. 22. Stony Brook, New York. Lohr, E. W. & Love, S. K. 1954 The industrial utility of public water supplies in the United States, 1952. Part I. States east of the Mississippi River. Water Supply (Irrigation) Paper U.S. Geological &mvey No. 1299, 1-639. Menzel, D. W. & Spaeth, J. P. 1962 Occurrence of ammonia in Sargasso Sea waters and in rain water at Bermuda. Limnology and Oceanography 7, 159-162. Morris, I. & Syrett, P. J. 1963 The development of nitrate reductase in Chlorella and its repression by ammonium. Archiw fuer Mikrobiologie 47, 32-41. New York City Environmental Protection Administration, 1972-74. Sewage and analytical records. Unpublished. Parsons, T. & Takahashi, M. 1973 Biological Oceanographic Processes. Pergamon Press, Oxford. Platt, T. & Conover, R. J. 1971 Variability and its effect on the 24 hour chlorophyll budget of a small marine basin. Marine Biology IO, 52-65. Riley, G. A. 1956 Review of the oceanography of Long Island Sound. Deep Sea Research Supplement 3, 224-238.
Riley, G. A. & Conover, S. A. M. 1956 Oceanography of Long Island Sound, 1952-1954. III. Chemical Oceanography. Bulletin of the Bingham Oceanographic Collection 15, 47-61. Riley, G. A. 1959 Oceanography of Long Island Sound, 1954-1955. Bulletin of the Binghom Oceanographic ColkCtiOn 17, 9-29. Riley, G. A. 1967 Mathematical model of nutrient conditions in coastal waters. Bulletin of the Bingham Oceanographic Collection 19, 72-80. Strickland, J. D. H. & Parsons, T. R. 1972 A Practical Handbook of Seawater Analysis. Fisheries Research Board of Canada, Bulletin 167 (2nd ed.). U.S. Dept. Commerce 1972 1970 Census of Population. Characteristics of Population. Vol. 1, Part A. Sections land Z. Bureau of the Census, Social and Economic Statistics. U.S. Government Printing Office, Washington, D.C. U.S. Environmental Protection Agency 1971 Report on the Water quality of Long Island Sound. Dot. No. CWT, 10-29. Vaccaro, R. P. & Ryther, J. H. 1959 Marine phytoplankton and the distribution of nitrite in the sea. Journal du Conseil, Conseil International pour I’Exploration de la Mer 25,260-271. Wilson, R. E. 1976 Gravitational circulation in Long Island Sound. Estuarine and Coastal Marine Science 4,443~453. Wrightington, R. 1973 Private communication. (U.S. Environmental Protection Agency, Region I, Waltham, Mass.) Zgurovskaya, L. N. & Kustenko, N. G. 1968 The effect of ammonia nitrogen on cell division, photosynthesis, and pigment accumulation in Sceletonema costatum (grev) cl., Chatoceros spp. and Prorocentrum micans ehr. Oceanology 8, 90-98.