32

NOAA Technical Report NMFS 32

Nutrient Distributions for

Georges Bank and

Adjacent Waters in 1979

A. F. J. Draxler, A. Matte,

R. Waldhauer, and J. E. O'Reilly

July 1985

u.s. DEPARTMENT OF COMMERCE

National Oceanic and Atmospheric Administration

National Marine Fisheries Service
NOAA TECHNICAL REPORTS NMFS

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NOAA Technical Report NMFS 32

Nutrrent Distributions for

Georges Bank and

Adjacent Waters in 1979

A. F. J. Draxler, A. Matte,

R. Waldhauer, and J. E. O'Reilly

July 1985

u.s. DEPARTMENT OF COMMERCE

Malcolm Baldrige. Secretary

National Oceanic and Atmospheric Administration

John V. Byrne. Administrator

National Marine Fisheries Service

William G. Gordon. Assistant Administrator for Fisheries
The National Marine Fisheries Service (NMFS) does not approve, recommend or endorse any proprietary product or proprietary material mentioned in this publication. No reference shall be made to NMFS, or to this

publication furnished by NMFS, in any advertising or sales promotion

which would indicate or imply that NMFS approves, recommends or

endorses any proprietary product or proprietary material mentioned herein,

or which has as its purpose an intent to cause directly or indirectly the

advertised product to be used or purchased because of this NMFS

publication.
CONTENTS

Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Methods ,. .. .. 2

Results. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Spring depletion of nutrients from surface waters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Bottom layer Gulf of Maine nutrient reservoir. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Depletion over Georges Bank. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Regenerated nitrogen-ammonium 3

Movement of nutrients onto the shelf. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Resupply ofsurface layer from below 4

Subsurface nitrite maximum. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Literature cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Figures

1. Bathymetry of the study area and location of transects. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2. Distribution of inorganic nutrients on Transect 8, 19 March 1980 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

3. Distribution of inorganic nutrients on Transect 8, 18 May 1979 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

4. Distribution of inorganic nutrients on Transect 8, 22-23 July 1979 9

5. Distribution of inorganic nutrients on Transect 8, 22 August 1979 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

6. Distribution of inorganic nutrients on Transect 8, 18-19 September 1979 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 10

7. Distribution of inorganic nutrients on Transect 8, 17-18 October 1979 . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 10

8. Distribution of inorganic nutrients on Transect 8, 10 December 1979 .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. II

9. Distribution of inorganic nutrients on Transect 8, 19 December 1979 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 11

10. Distribution of inorganic nutrients on Transect 5, 24-25 March 1980 12

11. Distribution ofinorganic nutrients on Transect 5, 19-20,27-28 May 1979 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 13

12. Distribution ofinorganic nutrients on Transect 5, 9-10 July 1979 14

13. Distribution of inorganic nutrients on Transect 5, 23-24 July 1979 15

14. Distribution of inorganic nutrients on Transect 5, 24, 30-31 August 1979 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 16

15.. Distribution of inorganic nutrients on Transect 5, 12-13, 18 September 1979 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 17

16. Distribution of inorganic nutrients on Transect 5, 2-3 October 1979 18

17. Distribution of inorganic nutrients on Transect 5, 20-21,27 October 1979 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 19

18. Distribution of inorganic nutrients on Transect 5, 24-26 November 1979 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 20

19. Distribution ofinorganic nutrients on Transect 6, 27-28 March 1980 21

20. Distribution of inorganic nutrients on Transect 6, 22-23, 26 May 1979 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 22

21. Distribution of inorganic nutrients on Transect 6, 12-13 July 1979 23

22. Distribution of inorganic nutrients on Transect 6, 24-25 July 1979 24

23. Distribution of inorganic nutrients on Transect 6, 27-28 August 1979 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 25

24. Distribution of inorganic nutrients on Transect 6, 15-17 September 1979 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 26

25. Distribution of inorganic nutrients on Transect 6, 23-24, 26 October 1979 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 27

26. Distribution of inorganic nutrients on Transect 6, 2, 12 December 1979 ... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 28

27. Distribution of inorganic nutrients on Transect 7,2-3 April 1980. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 29

28. Distribution of inorganic nutrients on Transect 7,24-25 May 1979 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 30

29. Distribution of inorganic nutrients on Transect 7,25-26 October 1979 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 31

30. Distribution of inorganic nutrients on Transect 7,6-7, 16 December 1979 32

31. Nitrate-orthophosphorus relationships, July-October 1979 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 33

32. Frequency with which the nitrite maximum at a station was found in a given depth interval and the concentration of that maximum for the Gulf of Maine, July-October 1979 at stations deeper than 120 m . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 34

Tables

1. Extremes of topography in the study area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

2. New England fish landings and value :..................................................... 2

iii

Nutrient Distributions for Georges Bank and

Adjacent Waters in 1979

A. F. 1. DRAXLER, A. MATTE, R. WALDHAUER, and

J. E. O'REILLyt

ABSTRACT

In this report we describe the temporal and spatial distributions of inorganic nutrients over Georges Bank

and in adjacent waters and discuss major features with respect to tbe nutrient environments of pbytoplankton.

Nitrate and ortbophosphorus were rapidly depleted from the surface layer of much of the study area in spring,

but major ditTerences were found between the shallow areas on Georges Bank and the surrounding stratified

waters. In the "well-mixed" area ....f Georges Bank, the depletion encompassed the entire water column and ammonium became the dominant form of inorganic nitrogen throughout. Dissolved silicon was depleted slowly over

central Georges Bank, reacbing a minimum concentration in September while orthophosphorus gradually increased during the summer. The nutrient environment of phytoplankton over central Georges Bank may be

described as vertically uniform but temporally changing in the relative availability of the various nutrients. In

areas that undergo stratification (e.g., the central Gulf of Maine), a quasi-steady state was established as the surface water layer formed, consisting of declining nutrient gradients from below the euphotic layer to the top ofthe

water column. These intergrading nutrient environments are relatively stable through time. Destratification

reintroduced nutrients to depleted areas beginning in October; however, dissolved silicon WaS again depleted

over shallow Georges Bank in late autumn though nitrate remained abundant. Slope water has been found to

enter the bottom layer of the Gulf of Maine via the Northeast Channel. High nutrient concentrations observed in

the bottom water of the Northeast Channel are consistent with this mechanism being the nutrient source for the

Gulf of Maine.

INTRODUCTION

Table I.-Extremes of topography in the study area

(Uchupi 1965).

slope water over the sill of the Northeast Channel and vertical exchange with MIW which is driven by the dissipation of tidal

energy.

On Georges Bank, a clockwise gyre is established in spring and

persists through autumn when the entire bank may be swept by

MSW (Bumpus 1976). Within the 60 m isobath, the water column

is vertically well-mixed at all times (Colton et al. 1968) and sufficiently well-isolated that the surface temperature may be several

degrees warmer in spring or colder in summer and autumn than

surrounding waters (Loder et al. 1982). There is a net southwestward movement of water from the Great South Channel and the

south side of Georges Bank toward the New York Bight (Bumpus

1976). Slope water south of Georges Bank and southern New

England is subject to periodic perturbations by passing Gulf

Stream eddies (Christ and Chamberlin 1980).

The study area includes the Gulf of Maine which is bounded by

the coasts of the United States from Cape Cod through Maine,

New Brunswick and southwestern Nova Scotia, Canada, from

Long Island to Cape Sable, with the mouth of the Bay of Fundy

between (Fig. I). The water perimeter southwest from Nova

Scotia consists of a series of banks and channels. Clockwise from

Cape Sable to Cape Cod they are: the channel south of Cape

Sable, Browns Bank, Northeast Channel, Georges Bank, Great

South Channel, and Nantucket Shoals. Minimum depths over the

banks and maximum sill depths in the channels are given in Table

I. There are three major basins in the Gulf (Georges, Wilkinson,

and Jordan) which are separated by a system of swells which rise,

in the shallowest case (Cashes Ledge at lat. 42°53'N, long.

68°57'W), to within 8 m of the surface.

Circulation within the Gulf of Maine has been described as a

large counterclockwise gyre (Bumpus 1976) divided vertically into three layers (Hopkins and Garfield 1979). A surface layer about

50 m deep, known as the Gulf of Maine surface water (MSW), is

formed'in spring from the winter mixed layer «120 m deep) due

to a buoyancy increase resulting from freshwater runoff and increasing temperatures. This leaves Maine Intermediate Water

(MIW) between 50 and 120 m. The amount of Scotian shelf water

entering the Gulf of Maine to become a progenitor of these layers

is unclear and a matter of some controversy (see Hopkins and

Garfield 1979, for discussion). Maine Bottom Water (MBW) is by

definition "unaffected directly by air-sea interactions" and exchange of MBW water with other systems is limited to inflow of

1Northeast Fisheries Center. Sandy Hook Laboratory, National Marine Fisheries

Service. NOAA, Highlands, NJ 07732.

Minimum depths over banks

Browns Bank

Georges Bank

Nantucket Shoals

Maximum depths of channels

Channel south of Cape Sable

Northeast Channel

Great South Channel

Maximum depths of basins

Georges Basin

Wilkinson Basin

Jordan Basin

29 m

3m

about 6 m

about 130 m

234 m

71 m

377 m

295 m

311 m
Landings

METHODS

Table 2.-New England fish landings and value

(from U.S. Department of Commerce 1982).

'Estimate based on New England landings being

about 75% of total North Atlantic commercial landings

and foreign landings of 77 x 10· kg for the North

Atlantic Fishery Conservation Zone (FCZ).

'Estimate based on the ratio of New England wholesale to U.S. wholesale value for fish landed.

Spring Depletion of Nutrients From Surface Waters

RESULTS

Nitrate nitrogen, orthophosphorus, and dissolved silicon are

depleted seasonally from the upper water column of the Gulf of

Maine and the waters south and west of Georges Bank. By our

May (1979) sampling, depletion was well established, to a depth

of 20 m or more at some stations, but was not discernible in

waters deeper than about 60 m in March (1980). The distribution

of depleted water in the Gulf of Maine during May was not uniform. Intense (NO) concentrations <0.2 liM) and deep depletions

were found on transects 5 and 7 in the southern Gulf of Maine

(Figs. 11,28) while along transect 6, the only low nutrient content

was in the surface (I m) samples at the northern stations (Fig. 20).

treated with phenol solution, which is the first reagent of the

colorometric analysis (Liddicoat et al. 1975), and then frozen

(Degobbis 1973). After filtering, the samples to be used for analysis of the other nutrients were frozen at -20°C within approximately 0.5 h of Niskin bottle closure, depending on station depth.

Parallel measurements made from the same bottle cast included

temperature, salinity, chlorophyll a and phaeopigments, dissolved

oxygen, and, at two stations per day, t4C primary production. Pigment estimates were made to a depth of 75 m and productivity to

the depth of 1% light level as determined using a submersible

quantum sensor (400-700 nm).

Nutrient analyses were performed at the shore laboratory. Ammonium analyses were completed manually (Liddicoat et al.

1975); other nutrient concentrations were estimated on a Technicon Autoanalyzer. Nitrite and nitrate were determined with the

naphthylethylenediamine-sulfanilamide system using copper

amalgamated cadmium reduction of nitrate (Wood et al. 1967).

To measure reactive phosphorus the molybdate-ascorbic acid

method was used (Murphy and Riley 1962). The reactive silicon

measurement was similar with appropriate addition of oxalic acid

(Brewer and Riley 1966). On two intercalibration exercises with

Brookhaven National Laboratory (BNL), we compared concentration estimates for the five nutrient species on unfiltered, fresh

(BNL) and filtered, frozen (SHL) paired samples (n 150). Coefficients of correlation (r) were 0.95, 0.99, 0.95, 0.83, and 0.91 for

nitrate, nitrite, ammonium, phosphorus, and silicon, respectively,

indicating that methods in this study are comparable with others

currently in use (Walsh et al. in press).

Isopleths of nutrient concentration at stations along the four

transects shown in Figure I were generated by a computer program - SYMAP (Harvard University 1971). Bottom profiles

were taken from Uchupi (1965) with additional points on the

shallow areas of Georges Bank from recent navigation charts. In

the rendering of the concentration contours (Figs. 2-30) from the

computer printouts, care was taken to include each value (represented by a dot) in the appropriate concentration interval, and emphasis was given to horizontal homogeneity where the computer

program tended to obscure these features because of the greater

distance (in mapping units) between stations than between depths

at a station.

In some analyses, data from the stratified season were combined by analyzing the July to October data together, we do not mean

to imply that the concentrations of nutrients in such areas are independent of time; however, the variation with time at a given

depth (relative to the I% light level depth) from July to early

October was small compared with gradients in the vertical direction.

356

'1,125

Million

dollars

317

8

'58

Million

kg

Commercial (1981)

Recreational (1979)

Foreign in FCZ (1981)

Value added to GNP

Seawater samples were collected using 5 L Niskin bottles on 11

National Marine Fisheries Service cruises, as part of the

MARMAP and Ocean Pulse programs, between May 1979 and

March 1980. In water depths < 100 m, we use a Niskin bottle rigged to close upon contacting the seabed. This bottle was "equilibrated" at up to 10 m from the bottom for 5 min along with the

other bottles on the cast. After the upper bottles were tripped by

messenger, the entire string was slowly lowered until the depressor

touched, closing the bottom bottle. Subsamples of 30 ml each

were pressure filtered «0.07 atm) through Whatman GF/F glass

fiber filters (nominal pore size 0.7 lim) into acid-cleaned glass

tubes for ammonium determinations and polyethylene tubes for

nitrite, nitrate, reactive phosphorus, and dissolved silicon determinations. We found that 10 ml of sample was sufficient to

remove silicon contamination from the glass fiber filter and therefore we allowed approximately 30 ml to pass. Sample containers

were then rinsed twice with sample water before a 30 ml subsample was collected. Ammonium samples were immediately

Upwelling has been reported in two areas. One is immediately

north of the Great South Channel and involves MIW (Magnel et

al. 1981); the other is an area of centrifugal upwelling south of

Cape Sable that presumably brings up slope water (Garrett and

Loucks 1976). Onwelling has been observed at several points

along the shelf-slope interface (Metcalf 1958; Worthington 1958;

Wright 1976; Christ and Chamberlin 1979,1980) and is probably

a frequent, episodic event.

The importance of the area to fisheries may be appreciated

from the value attached to commercial landings (Table 2). At a

lower level of the food chain, phytoplankton primary productivity

in the central region of Georges Bank is greater than in any region

on the continental shelf between Cape Hatteras and Nova Scotia

(O'Reilly and Busch 1984), except for the sewage-enriched New

York Bight apex. As we have indicated above, the hydrography is

related closely to the bathymetry as are phytoplankton biomass

distributions (O'Reilly and Evans-Zetlin 1982). These are, in tum,

interrelated with the availability of inorganic nutrients, both

through supply and removal. This paper describes the coarse-scale

:listribution of nutrients and patterns of change in these distributions in Georges Bank and adjacent waters in 1979, and how these

affect the nutrient environments of the phytoplankton.

2
Of particular interest is the nitrate minimum on transect 7 which

appears to have been subsurface. Its origin may have been hydrological, as well as biological; i.e., it may have resulted from water,

depleted of nitrate further west, having been transported eastward

by the surface currents in the Northeast Channel reported by

Ramp et aU

On the south side of Georges Bank in May, depletion in the upper water column was more uniform and generally shallower (10

m or less) than in the Gulf of Maine. Along transect 8, in May

(Fig. 3), the depleted layer was present at all stations and extended

to between 10 and 20 m.

By July the depleted layer had deepened to > 20 m over most of

transect 8 (Fig. 4) and was more than 50 m deep over the continental slope. In the Gulf of Maine, in August (there were no

samples taken in July), the layer was deeper than 20 m everywhere except at the very northernmost stations where, for example, on transect 6 (Fig. 23) nitrate stratification was completely

absent.

Bottom-Layer Gulf of Maine Nutrient Reservoir

High concentrations of nutrients were present in the deep

waters of the Gulf of Maine throughout the year. Nitrate concentrations in excess of 151lM (and once on transect 6 in September,

>25 IlM) were found in the deep water (> I00 m) of the central

and western Gulf of Maine at all times except March 1980. The

same was true for dissolved silicon at a concentration of at least

10 IlM and frequently >151lM and for orthophosphorus at 0.6 and

1.0 IlM.

The movement of the nutrient contour lines follows seasonal

patterns. Along transects 5 and 6, the 15 IlM nitrate isopleth progressed southward and upward from May through August to its

maximum southward position in September. It then retreated

northward from October through December. Along transect 5,

orthophosphorus increased from >0.6 IlM in May through August

to > 1.0 IlM from September through November. If only the

southern part of the Gulf of Maine on this transect is considered,

the migration of the phosphorus isopleths follows the same pattern

as those of nitrate.

The variations in dissolved silicon in this water mass are not as

readily amenable to general description. While the temporal

changes in the positions of the silicon isopleths have some characteristics in common with nitrate and phosphorus, apparently

moving northward and southward, no simple progression is evident in these data for this species. For example, the largest volume

of water containing more than 15 IlM silicon was observed on

transects 5 and 6 in May in the northern half of the Gulf (Figs. 11,

20). Nitrate and phosphorus distributions were similar to silicon

on transect 6 but not on transect 5.

Depletion Over Georges Bank

The beginning of the depletion of (presumably) uniform winter

concentrations of nitrate throughout the water column over

Georges Bank was evident in March (Figs. 10, 19). Orthophosphorus and dissolved silicon concentrations were also lower over

the Bank (transects 5 and 6) in March than in the waters to the

'Ramp, S., R. Schlitz, and W. R. Wright. 1985. The deep flow through the Northeast Channel of the Gulf of Maine. Unpubl. manuscr., 54 p. Northeast Fisheries

Center Woods Hole Laboratory, National Marine Fisheries Service, NOAA, Woods

Hole, MA 02543.

3

north and south. A similar condition was present at the inshore

end of transect 8, at this time, where nitrate and silicon were

depleted. Along this transect, the shallow water «60 m) appeared

like central Georges Bank in May, the 60 to 80 m area like Great

South Channel (transect 5), and the deeper than 80 m area like the

eastern end of Georges Bank (transect 7). Phosphorus distributions, however, were uniform except for two samples at the shelf

break (200 m).

By May, the patterns in the various areas of the Bank had

started to diverge. Over central Georges Bank «60 m) nitrate and

phosphorus were reduced to <0.2 and 0.1 IlM, respectively (Fig.

20). Nitrite was present at <0.05 IlM throughout the water column

while silicon concentrations were higher (except at 1 m) than at

adjacent stations on the transect. Over the deeper areas of the

Bank (transects 5 and 7; Great South Channel and the eastern end)

nitrate and, to a lesser extent, phosphorus concentrations in the

bottom waters were low, relative to adjacent stations; in the surface layer (to "'30 m) they were higher than at adjacent stations

(Figs. 11, 28). From the data available, silicon concentrations in

Great South Channel appear not to be substantially different than

in March; however, on transect 7 the entire water column over the

Bank was lower in silicon concentration «5 IlM) than at adjacent

stations.

Similar conditions existed through the summer except that

silicon concentrations on transect 6 declined to <0.5 IlM in the

shallowest area by mid-September (Fig. 24) and phosphorus on

transect 6 and phosphorus, silicon, and nitrate on transect 5

stratified by September (Fig. 15). The vertical homogeneity of

nitrate over the shallow area of central Georges Bank persisted into late October (Fig. 25).

Regenerated Nitrogen-Ammonium

Ammonium concentrations were almost always higher in portions of the study area with a depth between approximately 40

and 60 m than at deeper or,shallower stations. These waters with

higher ammonium concentration were usually in the lower half of

the water column, frequently most concentrated at depths above

the bottom sample and present from March through November.

Because this ammonium is isolated from any advective source, it

is likely part of the recycled nitrogen as defined by Dugdale and

Goering (1967).

In the upper water column, high ammonium concentrations

(2-10 IlM) were found in five samples. Three were from the upper

10m near the shelf break and of these, two were found in December on transects 6 and 7 (Figs. 26, 30). On transect 6 the high

ammonium was associated with a relatively high phosphorus concentration. There appears to have been a 10-fold increase in

ammonium between 10 and 19 December on transect 8 (Figs. 8,

9) at the most northern stations sampled on 10 December.

However, the >2 IlM concentration found on 19 December occurred in only one sample (20 m) and the station on the former date

was located 6 km further south, so it might have been missed on

10 December. Further seaward, however, the 0.21lM isopleth was

clearly displaced by the 0.5 IlM isopleth between the two samplings. The time period for the activity was therefore about 9 d.

The last of the five upper water column observations was found at

a depth of 1 m in August on transect 5 (Fig. 14) and was the highest of the study (> 10 IlM). It was found in an area characterized

by intense hydrographic and, therefore, chemical activity (the

upwelling along the north edge of Great South Channel). The 20

m sample contained nearly 2 IlM ammonium and the upper 30 m

of water column was also rich in nitrate, phosphorus, and silicon
Regeneration of inorganic nitrogen in deeper waters of the Gulf

of Maine is suggested by elevated ammonium concentrations in

two samples, one> 1.5 J.lM and one between 0.2 and 0.5 J.lM (Fig.

24) found on transect 6 in September. The vertical gradient may

indicate transport upward or restriction of ammonification to

near-bottom waters.

Movement of Nutrients Onto the Shelf

The presence of slope water nutrients on the continental shelf is

most clearly seen along transect 8 (Figs. 2-9). The 0.5, 1,2, and 5

J.lM nitrate isopleths successively occupied the bottom of the inshore station between March (when the concentration there was

0.2 J.lM) and mid-September. A sample with a nitrate concentration of > 10 J.lM was found in October but the isopleth was no

longer continuous with the offshore water. Also in September, the

highest concentration of nitrate (>25 J.lM) on this transect was

observed in the bottom sample near the shelf break. Nitrate concentrations were in the range of 15 J.lM before and after this maximum. Further east, on transects 5 and 6, a similar, though less

distinct, pattern was found. The maximum northward excursion of

the 10 J.lM nitrate isopleth appears to have occurred in September

and a concentration in excess of 25 J.lM was also found at the bottom of the deepest station on transect 5 at this time (Fig. 15).

These movements coincide with those in the MBW discussed

above.

In bottom waters of the Northeast Channel (the presumed

source for MBW), high nitrate (> 15 J.lM), phosphorus (> I J.lM),

and silicon (> 15 J.lM) concentrations were present during March

and May. These had decreased to > 10, 0.2-0.6, and 5-10 J.lM,

respectively, by autumn.

Resupply of Surface Layer From Below

Destratification in autumn, beginning in shallower areas and

progressing to deeper areas, introduced large quantities of nitrate

into the previously depleted surface layer. At the inshore stations

on transect 8 between mid-September and mid-October (Figs. 6,

7), surface nitrate concentration increased from 0.2 to 2-5 and 1-2

J.lM, respectively. The process continued through both samplings

in December when most of the upper water column nitrate values

were between 2 and 5 J.lM on this transect. Chlorophyll a concentrations (O'Reilly and Evans-Zetlin 1982) suggested that phytoplankton activity continues throughout the winter so that vertical

nutrient uniformity may never have been attained in some areas.

It is clear, however, from the positions of the 5 and 10 J.lM

isopleths in March 1980 that the offshore end of the transect was

essentially destratified with respect to nutrients.

On the north side of Georges Bank, early indications of resupply of nitrate to the upper water column are evident along

transect 5 in late October (Fig. 17). There was a slight increase in

nitrate on the south flank of the Bank along transect 6 at that time

while large amounts of dissolved silicon had been introduced (Fig.

25). By December (Fig. 26) considerable nitrate was present over

central Georges Bank (2-5 J.lM) with higher values to the north. In

the Gulf of Maine, based on the incomplete sampling along

transect 6 and nitrate concentrations on transect 5 in March 1980,

nearly uniform distributions are probably achieved at some time

during the winter.

With no summer data for transect 7, we cannot determine

whether resupply in autumn is a factor over the eastern end of

Georges Bank; however, values in December were higher than in

4

October and higher over the Bank than in water to the north and

south.

Upwelling of bottom water to the depleted surface layer was

observed in May and August on transect 5 (Figs. 11, 14). At one

station ("-'90 m deep), just north of Great South Channel, the

nitrate isopleths curve upward so that the 5 and 1 J.lM isopleths

reach the sea surface. Similar patterns of distribution were observed for phosphorus and silicon in August and for phosphorus in

May. The phenomenon was not observed at stations south of lat.

41 ON nor north of lat. 41 °30'N. It was not observed in March and

the area was not sampled in June, July, or September. There was,

however, some upward bending of isopleths in October and

November, particularly for phosphorus, but increased vertical

mixing in the surface layer associated with destratification at this

time may have diluted concentrated nutrients near the surface

even if upwelling had continued.

Subsurface Nitrite Maximum

A subsurface maximum nitrite concentration was found on all

transects in the Gulf of Maine. It was most frequently noted in the

30 m sample but was deeper in May on transect 6 (to 75 m). It

was also observed on transect 5 in October (Figs. 16, 17) but not

in March, May, August, September, or November. On transect 6 it

was observed in May, August, and October, but not in December.

DISCUSSION

Of the nutrient distributional characteristics observed, the nearly complete removal of nitrate nitrogen from the surface layer in

spring is the most important in defining the nutrient environment

of phytoplankton. In some areas the decrease in concentration is

nearly 100-fold between March and May. Since the waters over

central Georges Bank do not stratify, depletion in the "wellmixed" area takes place throughout the water. column and therefore has an even greater effect on -the nutrient environment of

phytoplankton there. If one assumes a primary production rate of

1 g carbon/m2 per d (O'Reilly and Busch 1984), a carbon to nitrogen uptake ratio of 106: 16 (Redfield et al. 1963), and a semienclosed circulation in the well-mixed area over Georges Bank,

the March nitrate stock will supply the phytoplankton nutrient demand for only 1 to 3 mo. Since primary production is sustained at

high levels through the summer over central Georges Bank, inorganic nitrogen must be supplied from: 1) recycled nitrogen

released from the seabed as settled organic matter is mineralized;

2) ammonium nitrogen recycled in situ from earlier planktonic

production; 3) nitrate-rich slope water crossing the shelf from the

south along the bottom followed by vertical mixing; or 4) nitrate

advected from MIW and vertically mixed by the dissipation of

tidal energy. (It is unlikely that horizontal transport of surface

waters onto Georges Bank contributes significantly since surface

waters of surrounding stratified areas are also depleted.) In the

case of areas that do stratify (e.g., central Gulf of Maine), the

resupply of nitrogen is limited to transport through the pycnocline

and remineralization that takes place before dead cells, detritus,

and zooplankton pellets settle out of the euphotic zone.

Aerobic mineralization of organic matter by the seabed constitutes < 10% of the total mineralization on an areal basis (Thomas

et al. 1978) and observed rates. of ammonium flux from the seabed

are low (Draxler and Phoel 1980); i.e., most of the mineralization

occurs in the water column. This is also suggested by the fact that

the majority of our ammonium maxima for central Georges Bank
were found off the bottom. However, unlike phytoplankton in the

deeper, vertically stratified areas surrounding Georges Bank

which benefit from nutrients released by the seabed only upon

turnover, phytoplankton in the well-mixed area over Georges

Bank may benefit immediately from these nutrients because of

continuous vertical mixing,

Ammonium concentrations for MBW indicate that while

regeneration of inorganic nitrogen does take place (Fig. 24), this is

an insignificant source for primary production because it is

eclipsed by the amount of nitrate available in the overlying MIW.

This is not the case in the "well-mixed" area over Georges Bank.

Ammonium concentrations generally increased in the bottom

water at stations 40 to 60 m deep between spring and late summer

(Figs. 19-23). Ammonium I).itrogen was the dominant form of inorganic nitrogen in the well-mixed area between July and

September (Draxler and Waldhauer 1982), at times constituting

more than 90% of the inorganic nitrogen pool throughout the

water column, both because its concentration increased and

because the concentration of nitrate decreased. If the rate of ammonium formation is relatively uniform in the waters of central

Georges Bank, its abundance in the lower water column of certain

areas implies that ti:lis nitrogen is less rapidly reused by the system

there than it is in shallower areas. We infer from the low levels of

all forms of inorganic nitrogen at shallow central Georges Bank

stations (e.g., lat. 41 °30'N, Fig. 23) that nitrogen from any of the

possible sources is rapidly assimilated due to continuous vertical

mixing and high rates of primary production. The paucity of inorganic nitrogen in this area may be compared with transect 8

where cross-shelf movement introduced more than 10 /-Imole/L of

nitrate to the 50 m level in July (Fig. 4) and where in excess of 2

/-1M ammonium was observed to a depth of 30 m in May (Fig. 3).

Nitrogen in the bottom waters of transect 8 is of limited availability to phytoplankton due to stratification as Ketchum et al.

(1958) showed in an area to the west, off Block Island. In contrast,

the movements of nutrient isopleths on the southern flank of

Georges Bank and in the MBW provide evidence for nutrient

transport from deep reservoirs to the central Bank where it can be

introduced into the euphotic zones by vertical mixing.

Whereas nitrate nitrogen was depleted from the well-mixed

area of Georges Bank during a short period in spring, the dissolved

silicon concentration was highest in spring and declined gradually

through the summer. Silicon concentrations recovered rapidly as

nitrate replenishment began in October, but were again depleted

by December though nitrate was by then abundant. Phosphorus

generally increased from late spring through summer in the area.

The competing and complementing processes of removal and

resupply (plankton assimilation, remineralization, advective and

diffusive transport) affect the concentration of each of the

nutrients at different rates with the result that phytoplankton in

the well-mixed area are presented with a nutrient environment

that is continually changing in the relative availability of nitrogen,

phosphorus, and silicon. This, in turn, implies that at anyone

place there will be continually changing, "best competitor,"

phytoplankton assemblage through time.

In stratified areas, nutrient depletion was persistent, as seen in

the nutrient profiles, but was not uniform nor exhaustive

throughout the euphotic zone and identifiable nutrient gradients

developed. Using Figure 31, we can examine the conditions more

closely. In Figure 31 A we have arbitrarily divided the data from

shelf and Gulf of Maine deep stations (> 120 m), sampled during

the highly stratified season (July-early October), into three depth

intervals: I) from 10m above the I% light level to the surface; 2)

the I% light depth ± 5 m; and 3) from 10m to 45 mbelow the 1%

5

light level. Not only was there a relationship between nutrient

concentrations and depth, but the relationship between nutrients

within depth strata changed with depth; i.e., the slope of the

nitrate:phosphorus lines at the I% light level and in the water

below were both about 16:I but the I% light depth region was

depleted by about 4.0 /-Imole of nitrate per liter and/or 0.2 /-Imole

of phosphorus per liter when compared with the deeper water. In

the top layer, the nitrate concentration was independent of

phosphorus suggesting that they are related to the I% light level

data but collapsed along the functional regression line.

The depiction in Figure 31 A of discrete strata was actually a

continuum of intergrading layers. As may be seen in Figure 31B,

while phosphorus changed continually with depth, nitrate was

essentially exhausted at about 13 m above the I% light depth.

Also shown in the figure are the average upper and lower limits of

the thermocline (with 95% confidence intervals) and the 95%

confidence interval about the 1% light depth from the 10 stations

for which hydrographic data are available. The confidence intervals of the upper and lower thermocline limits decrease from ±9.1

and ± 17.0 m, respectively, when calculated on the basis of depth

from the sea surface, to ±6.9 and ±15.0 m when calculated

relative to the I% light level depth. The position of the average

I% light level depth and the average center of the thermocline at

these 10 stations differed by only 2.5 m.

These relationships serve to illustrate in a qualitative way the

thermocline- nutrient- phytoplankton-light interactions operating

in this area that determine the nutrient environment of the phytoplankton. These include: I) the pycnocline acting as an impediment to the vertical movement of nutrients, the unavailability of

which limits phytoplankton densities; 2) the extent of light penetration being partially dependent on phytoplankton densities arid

vice versa, while nutrient concentrations (in the euphotic zone)

are partially determined by rates of phytoplankton uptake as well

as rates of remineralization of organic material; and 3) the density

gradient in the pycnocline providing a means for some phytoplankters to control their vertical position in order to make optimal use of light and nutrients. The cohesiveness of the July

through early October data, as seen i'n Figure 31, implies a temporal stability through the period. We can view the nutrient environment of deeper parts of the study area as relatively constant

through time (when located relative to the I% light level) but vertically graduated through the water column.

While nitrate, phosphorus, and silicon existed in monotonic

vertical gradients, nitrite was found as a subsurface maximum

layer on some transects in the Gulf of Maine. Figure 32 shows the

frequency with which the maximum nitrite concentration in the

upper 120 m (MSW and MIW) occurred in depth intervals defined on the basis of our normal sampling depths (I, 10, 20, 30, 50,

75, and 100 m) during July through September in the deep,

stratified portion of the Gulf of Maine (bottom depths> 120 m).

The average number of observations in each depth interval is 30

(ranging from 37 in 0-5 m to 26 in the 62.5-87.5 m interval). The

average depth of the I% light leyel at these stations was 28.8 m. It

may be seen that most of the nitrite maxima are at the limit of or

below the euphotic zone and that the maxima in the second most

frequent interval (40-62.5 m) are in waters with higher nitrite

concentrations than other intervals. The nitrite layer may have

developed by microbial oxidation of ammonium as described in

Pacific coastal areas (Ward et al. 1982) or by phytoplankton

reduction of the abundant nitrate nitrogen being transported upward (Olson 1981), or by both. Of the two transects on which we

observed a midwater nitrite maximum, and had ammonium

measurements (Figs. 18, 25), one (transect 5) had an ammonium
maximum and the other did not. Data collected in 1980 and 1981

include more measurements of ammonium which may be useful

in addressing this question further.

In general then, while the well-mixed area of Georges Bank has

a temporally changing nutrient environment, areas that stratify

seasonally (e.g., the central Gulf of Maine) reach a quasi-steady

state that persists for several months, in which nutrient environments are vertically arrayed in parallel with light, temperature,

and salinity layering. Areas that are homogeneous with respect to

phytoplankton biomass segregate along similar lines. The wellmixed area of Georges Bank has the highest annual average

chlorophyll concentration of 20 subareas examined in our study

area, while stratified subareas generally have low standing stocks

of phytoplankton (O'Reilly and Evans-Zetlin 1982). The subareas

along the north side of Georges Bank also have high chlorophyll

concentrations (second only to the central, well-mixed area).

Phytoplankton in these transitional subareas may benefit from the

nutrient reserves below the euphotic zone because of vertical mixing associated with the dissipation of tidal energy in stratified

water masses that impinge on the northern flank of Georges Bank

again indicating the close association of bathymetry, nutrients,

and phytoplankton. This may be important in determining the

standing stocks of phytoplankton biomass and rates of primary

production.

Finally, with respect to the flow of nutrients into and out of the

study area, Hopkins and Garfield (1979) calculated that the influx

of water to the Gulf of Maine across the Scotian Shelf is episodic

and of low magnitude. Our 1979 nutrient data support this view in

that we observed no highly concentrated nutrient inputs to the

MIW which might be attributable to this source. Our data for the

Northeast Channel are consistent with both Hopkins and

Garfield's (1979) analysis that slope water, of high nutrient content, enters the Gulf of Maine through the channel and is the

source for MBW and with Ramp et al.'s (footnote 2) report of

outflow from the Gulf of Maine in the surface layer of the Northeast Channel.

LITERATURE CITED

BREWER. P. G., and J. P. RILEY.

1966. The automatic determination of silicate-silicon in waters with special

reference to sea water. Anal. Chim. Acta 35:514-519.

BUMPUS, D. F.

1976. Review of the physical oceanography of Georges Bank. ICNAF Res.

Bull. 12:119-134.

CHRIST, R. W., and J. L. CHAMBERLIN.

1979. Bottom temperatures on the continental shelf and slope south of New

England in 1977. Ann. BioI. 34:21-27.

1980. Bottom temperatures on the continental shelf south of New England in

1978. Ann. Bioi. 35:25-30.

COLTON, J. B., Jr., R. R. MARAK, S. R. NICKERSON, and R. R. STODDARD.

1968. Physical, chemical, and biological observations on the continental

shelf, Nova Scotia to Long Island, 1964-66. U.S. Fish Wildl. Serv., Data

Rep. 23, 195 p. on 3 microfiche.

DEGOBBIS, D.

1973. On the storage of seawater samples for ammonia determination.

Limnol. Oceanogr. 18:146-150.

DRAXLER, A. F. J., and W. C. PHOEL.

1980. Seabed ammonium flux. Section 4. Environmental Chemistry

Investigation, Northeast Monitoring Program Annual Report No. 1I1-80-

ABG-0015, p. 19-22.

DRAXLER, A. F. J., and R WALDHAUER.

1982. A preliminary analysis of the annual inorganic nitrogen cycle. Section

IV. Environmental Chemistry Investigation, Northeast Monitoring Program Annual Report No. 1I1-81-BFG-OIII, 6 p.

6

DUGDALE, R. C., and J. J. GOERING.

1967. Uptake of new and regenerated forms of nitrogen in primary productivity. Limnol. Oceanogr. 12:196-206.

GARRETT, C. J. R, and R. H. LOUCKS.

1976. Upwelling along the Yarmouth shore of Nova Scotia. J. Fish. Res.

Board Can. 33:116-117.

HARVARD UNIVERSITY.

1971. Program description and availability memorandum for SYMAP.

Laboratory for Computer Graphics and Spatial Analysis, Cambridge, MA,

157 p.

HOPKINS, T. S., and N. GARFIELD III.

1979. Gulf of Maine intermediate water. J. Mar. Res. 37:103-139.

KETCHUM, B. H., R. F. VACCARO, and N. CORWIN.

1958. The annual cycle of phosphorus and nitrogen in New England coastal

waters. J. Mar. Res. 17:282-301.

UDDICOAT,M. I., S. TIBBITTS, and E. I. BUTLER.

1975. The determination of ammonia in seawater. Limnol. Oceanogr. 20:

131-132.

LODER, J. w., D. G. WRIGHT, C. GARRETT, and B.-A. JUSZKO.

1982. Horizontal exchange on central Georges Bank. Can. J. Fish. Aquat.

Sci. 39:1130-1137.

MAGNELL, B. A., J. J. CURA, C. N. FLAGG, and D. FRYE.

1981. Interpretation of physical oceanography of Georges Bank-preliminary results. Prepared for U.S. Dep. Inter., BLM, Wash., DC, Contract AA851-CTI-39.

METCALF, w. G.

1958. Oceanographic data from Crawford cruise, 16 October-II December

1957. Woods Hole Oceanogr.lnst. Ref. 58-31,132 p.

MURPHY, J., and J. P. RILEY.

1962. A modified single solution method for the determination of phosphate

in natural waters. Anal. Chim. Acta 27:31-36.

OLSON, R. J.

198 I. ISN tracer studies of the primary nitrite maximum. J. Mar. Res. 39:

203-226.

O'REILLY, J. E., and D. BUSCH.

1984. The annual cycle of phytoplankton primary production (netplankton,

nannoplankton and release of dissolved carbon) for the northwestern Atlantic

shelf. Rapp. P.-v. Roun. Cons. int. Explor. Mer 183:255-268.

O'REILLY, J. E., and C. EVANS-ZETLIN.

1982. A comparison of the abundance (chlorophyll 0) and size composition

of the phytoplankton communities in 20 subareas of Georges Bank and

surrounding water. ICES C.M. I982/L:49, 9 p.

REDFIELD, A. C., B. H. KETCHUM, and F. A. RICHARDS.

1963. The influence of organisms on the composition of sea-water. In M. N.

Hill (editor), The sea, Vol. 2, p. 26-77. Intersci. Publ., N.Y.

RICKER, w. E.

1973. Linear regressions in fishery research. J. Fish. Res. Board Can.

30:409-434.

THOMAS, J. P., J. E. O'REILLY, C. N. ROBERTSON, and w. C. PHOEL.

1978. Primary productivity and respiration over Georges Bank during March

and July 1977. ICES C.M. 1978/L:37, lOp.

UCHUPI, E.

1965. Map showing relation of land and submarine topography Nova Scotia

to Florida. Dep. Inter., U.S. Geol. Surv., 3 sheets.

U.S. DEPARTMENT OF COMMERCE.

1982. Fisheries of the United States, 198 I. U.S. Dep. Commer., NOAA,

Natl. Mar. Fish. Serv., Curro Fish. Stat. 8200, 131 p.

WALSH, J. J., T. E. WHITLEDGE, J. E. O'REILLY, W. C. PHOEL, and A. F. J.

DRAXLER.

In press. Nitrogen cycling within seasonally stratified and tidally mixed

regions of the Mid-Atlantic Bight. The New York Shelf and Georges Bank.

In R H. Backus (editor), Georges Bank.

WARD, B. B., R J. OLSON, and M. J. PERRY.

1982. Microbial nitrification rates in the primary nitrite maximum off

southern California. Deep-Sea Res. 29A:247-255.

WOOD, E. D., F. A. J. ARMSTRONG, and F. A. RICHARDS.

1967. Determination of nitrate in sea water by cadmium-copper reduction

to nitrite. J. Mar. BioI. Assoc. U.K. 47:23-31.

WORTHINGTON, L. V.

1958. Oceanographic data from the R.R. Discovery II international geophysical year cruises one and two, 1957. Woods Hole Oceanogr. Inst. Ref.

58-30, 151 p.

WRIGHT, W. R.

1976. The limits of shelf water south of Cape Cod, 1941 to 1972. J. Mar.

Res. 34:1-14.
200

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KILOMETERS

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Figure I.-Bathymetry (from Uchupi 1965) of the study area and location of transects.

7
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Figure 2.-Distribution of inorganic nutrients on Transect 8, 19 March 1980. Figure 3.-Distribution of inorganic nutrients on Transect 8, 18 May 1979.

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Figure 4.-Distribution of inorganic nutrients on Transect 8, 22-23 July 1979. Figure 5.-Distribution of inorganic nutrients on Transect 8, 22 August 1979.

9
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Figure 6.-Distribution of inorganic nutrients on Transect 8, 18-19 September

1979.

Figure 7.-Distribution of inorganic nutrients on Transect 8, 17-18 October

1979.

10
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Figure 8.-Distribution of inorganic nutrients on Transect 8, 10 December 1979. Figure 9.-Distribution of inorganic nutrients on Transect 8,19 December 1979.

11
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12
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Figure ll.-Di&tribution of inorganic nutrients on Transect 5, 19·20, 27-28 May 1979.

13
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Figure 12.,.--Distribution of inorganic nutrients on Transect 5, 9-10 July 1979.

14
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15
16

Figure 14.-Dist·brI utlOn . of inorgamc. nutrients on Transect 5• 24.30-31 August 1979.

43° 0.,---

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Figure IS.-Distribution of inorganic nutrients on Transect S, 12·13, 18 September 1979.

17
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Figure 16.-Distribution of inorganic nutrients on Transect 5, 2-3 October 1979.

18
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'f

Ia.

UJ

a 400

E

'f

ln.

UJ

a

E

'f

Ia.

UJ

a

E

'f

Ia.

UJ

a

Figure 17.-Distribution of inorganic nutrients on Transect 5, 20·21, 27 October 1979.

19
E

I

...

Q.

W

o

E

I

...

Q.

W

o

E

I

...

Q.

W

o

E

I

...

Q.

W

o

E

I

...

Q.

W

o

('0.2

to.2 •

~4/• 0.2

Figure IS.-Distribution of inorganic nutrients on Transect 5, 24-26 November 1979.

20
E

I

....

Q.

W

o

E

I

....

Q.

W

o

E

I

....

Q.

W

o

E

I

....

Q.

W

o

E

I

....

Q.

W

o

0'lr 4.L4_O 4"'3_° ~42:..0__~-__,r_-4:.i1._0 --.__-.:.4,::.0°---,

2

r

0.6 •

"--

Figure 19.-Distribution of inorganic nutrients on Transect 6, 27-28 March 1980.

21
E

I

....

a.

w

o

40·

E

I

....

a.

w

o

1000

0.05 0.1.

0.6~-··: 02.

0.4~0.4

06)

~06~:

E

I

....

a.

w

o

E

I

....

a.

w

o

E

I

....

a.

w

o

0.05~0.1 0.1 • r---:

0.2

'--.:

Figure 20.-Distribution of inorganic nutrients on Transect 6, 22-23, 26 May 1979.

22
E

I

...

a.

w

o

Figure 2l.-Distributlon of Inorganic nutrients on Transect 6, 12-13 July 1979.

23
44° 43° 42°

20 ';:0.1

60

E

I

....

"-w EO. 1 Q

/.0.1

800

20

60

E

I

....

"-w

Q

800

E

I

w

Q

40

!

r

Figure 22.-Distribution of inorganic nutrients on Transect 6, 24-25 July 1979.

24
E

I

....

a.

w

o

E

I

....

a.

w

o

E

I

....

a.

w

o

E

I

....

a.

w

o

41·

Figure 23.-Distribution of inorganic nutrients on Transect 6, 27-28 August 1979.

25
E

J:

ln.

w

Cl

E

J:

ln.

w

Cl

40·

Figure 24.-0istribution of inorganic nutrients on Transect 6, 15-17 September 1979.

26
E

:f

....

0.

W

o

20

800

44' 43'

----5 42' 41' 40'

E

:f

....

0.

W

o

20

60

E

:f

....

0.

W

0

800

20

40

60

E 80

:f

....

0.

W

0

600

800

Figure 25.-Distribution of inorganic nutrients on Transect 6, 23·24, 26 October 1979.

27
E

J:

...

Q.

W

C

E

J:

...

Q.

W

C

~0.2

c

.0.1

20 • 0·~21.5

<0:0.2

40

60

E

J:

... 0.2 Q.

W

c

800

E

J:

...

Q.

W

C

E

J:

...

Q.

W

C

Figure 26.-Distribution of inorganic nutrients on Transect 6, 2, 12 December 1979.

28
E

I

....

a.

w

o

E

I

....

a.

w

o

E

I

....

a.

w

o

E

I

....

a.

w

o

o:s:!o.s -0.2

Figure 27.-Distribution of inorganic nutrients on Transect 7, 2-3 April 1980.

29
41·

0.2 •• ·01

: c;: <::0.4 ---=02 ___--:---0.2 •• .0.4

44·

0

20

E

I

fa.

w

0

800

1000

0

20

E

I

fa.

w

o

E

I

fa.

w

o

E

I

fa.

w

o

o.------'-----.-..-::;;;..----,-t-----.-----.---r---',~___r-_._'C"'~-----____l

20

E 80

I

fa.

w

o

800

Figure 28.-Distribution of inorganic nutrients on Transect 7, 24-25 May 1979.

30
0.,._-..:4--.:4_· -.:.4!=-3. ......,,,-__4.:J2:...-.-- --r_4.J.l:.... ~40·

i

5~

<&0.2 .0.5

C O.5

C:0.2

20

600

1000JL.LL.L..LLL..LL.L..<~:.LL..LLLL.LL...~:.LLLLLL.LL...LL....'...LJ.'.L.LL- --l

600

100°JL..(LtLLLLLLLLL~~::L<::L<(L(LtLLLLLLLL:L~~2...

E

I

...

Q.

W

o

E

:£

...

Q.

W

o

E

I

...

Q.

W

o

E

I

...

Q.

W

o

E

I

...

Q.

W

o

Figure 29.-Distribution of inorganic nutrients on Transect 7. 25-26 October 1979.

31
44' 43' 41" 40" 0 5. 'l 20

E 1

I <:2 fa.

w

0

800

1000

E

I

fa.

w

o

E

r:

fa.

w

o

E

r:

fa.

w

o

Figure 30.-Distribution of inorganic nutrients on Transect 7. 6-7. 16 December 1979.

32
20..-----------------------------,

15 • •

• • •

• • •• .. • /

10 6' •

-- /

2 / /

w /D O. f-

« •

0

--- a: /

f- 0 //0

Z 5 • cJI /

• &/0 /

0 + + /'

0 //0

+ /+.

0 t .1fM&:JGI ..···et-·{f'HElIt+-....·.. /'

/'

/

A

0.4 0,6 0.8 1,0 1.2

ORTHOPHOSPHORUS CUM)

0,0 0,2

+

++

:]

-.J

W

>

W

-.J

fI

o

::J

o

fW

>

i=«-.J

W

a:

I

b::

w

o

-40

-20

o

20

40

I

I

I

I

I

I

I

I

I

I

I

I

"-

"-

"-

\

\

\

+ \

\

\

\

\

\

I

\ +

+

+

+

LT

B

o 10 20 30

NITRATE/PHOSPHORUS

Figure 31.-Nitrate-orthophosphorus relationships in stratified areas, July to October 1979. A) Samples rrom

10 m above the 1% light level depth to the surface (+); the 1% light depth ± 5 m (circles); and rrom 10 to 45 m

below the 1% light level depth (squares); lines fitted by runctional regression (Ricker 1973; least :l:l\X·l\Y). B)

Two line segment third order polynomial regression fit or NIP versus depth relative to 1% light level depth

(positive downward) with 95% confidence bands; means and 95% confidence intervals ror 1% light level depth

(1%) and upper (UT) and lower (LT) limits or the thermocline (n = 10 stations).

33
60 70

FREQUENCY OF OBSERVATIONS

OF NITRITE MAXIMUM (%)

O~~ o __10L-- 20L-- 30L-- 40L-- 50L-- L--__

5

15

40

NITRITE CONCENTRATION

IN WATER COLUMN MAXIMUM (uM)

t/::-:/J < 0.05

Irrr~~rl 0.05-0.10

BI 0.10-0.20

_ 0.20-0.40

_>0.40

Figure 32.-Frequency with which the nitrite maximum at a station was found in a given depth interval and the concentration of that maximum for the Gulf of Maine,

July-October 1979 at stations deeper than 120 m.

34