Distribution of nutrients in Louisiana's coastal waters influenced by the Mississippi River

Es&urine and Coastal Marine Science (x977) 5, 173-195

Distribution of Nutrients in Louisiana’s Coastal Waters Influenced by the Mississippi River

Clara L. Ho Department of Marine Sciences, Louisiana State University, Baton Rouge, LA 70803, U.S.A.

and Barney B. Barrett Louisiana Wildlife and Fisheries, Division of Oysters, Waterbottoms and Seafoods, Baton Rouge, LA 70808, U.S.A. Received 22 November 1975 and in revised form 26 April 1976

The volume of freshwater introduced into Louisiana’s coastal zone during 1973, by rainfall and river discharge, was the highest in the past 35 years. Water samples were taken from inshore estuarine areas as well as in the open Gulf of Mexico during 1973. Three of these sampling trips coincided with high river discharge and the last trip was made during low river discharge. Analysis of the water samples showed that the nutrient content (NO;, PO;‘, dissolved SiOZ, and organic matter) of the water within the zone of the Mississippi River influence is directly related to the volume of water discharged to the Gulf by the river. Drainage water from the marshes into the upper regions of Barataria and Caminada Bays was characterized by high levels of NH:-N and organic-N, but low values of (NO;+NO;)-N as compared to waters influenced by the Mississippi River. During the low discharge period of the Mississippi River, coastal waters adjacent the river mouth were dominated by low nutrient seawater. However, inorganic nutrients and organic-N in the bays and their adjacent nearshore waters were higher than at the river mouth during low river discharge. The primary source of nutrients and organic matter to the bays is the surrounding marshes. The estimated combined weight of (NO;+NO;)-N, PO,‘-P, dissolved SiO,, organic-N and organic-C discharged into the Gulf by the Mississippi River during the period from January through July 1973 was 28.45 billion kg. The high fishery productivity of the water adjacent the river mouth is a result of nutrient contribution by the Mississippi River. However, marshes play a key role in providing organic matter and nutrients to the estuaries which serves as nursery grounds for many marine species.

Introduction Louisiana coastal waters influenced by nutrients discharged by the Mississippi River system (including the Atchafalaya River) are among the most productive fishery areas in the United States. Gunter (1967) termed the area ranging from Mobile Bay westward around the mouth of the Mississippi River to Sabine Lake the ‘Fertile Fisheries Crescent’. The discharge of I73

Ii4

C. L. Ho & B. B. Barrett

Figure I. Sampling locations in Caminada and Barataria Bays.

the AtchafalayaRiver inRuencesmostof southwesterncoastalLouisiana.The mainstreamof the Mississippi River spreadsits watemovermuch of southeasterncoastalLouisiana. The areainvestigatedduring this study waslimited to Caminadaand BaratariaBays and the near offshorewaters betweenthese bays and the mouth of the Mississippi River at SouthwestPass(Figures I and a). This area,ascomparedto other regionsequalin size,is probablythe richestin both volumeandvariety of natural resourceswithin the United States. Sulphur is mined in the areaand hundredsof oii and gaswells arelocatedboth inshoreand offshore.Private and charterboatsfish thesewatersyear-round,and deep-seasport fishing is exceptionallyproductivesouth of the river mouth. The areaproducesgreat quantitiesof commercialandsport fish, oysters,crabsandshrimp. Since1970,the annualcatchof shrimp alonehas exceeded9 milIion kilogramsin the areafrom CaminadaBay to SouthwestPass. With the exceptionof work by the LouisianaWildlife and FisheriesCommissionin 1968 and 1969, there are few comprehensivestudies concerningorganic and inorganic-N concentrationsand their distribution in Louisiana’s coastalwaters. The re.sultsof the Commission’swork showedthat NO; levelsin areasinfluencedby the river were directly related

Nutrients in Louisiana’s coastal waters

I75

to the volume of river discharge. During high Mississippi River discharge, most frequently occurring during the spring, much of the fresh river water flows westward along the coastline and enters the bays through passes open to the Gulf. The distribution of nutrients emptied into the Gulf by the Mississippi River depends primarily on the amount of water discharged and the prevailing nearshore currents (Barrett et al., 1971).

Figure 2. Sampling locations in the nearshore zone.

The phosphate contribution by the Mississippi River to the offshore waters adjacent to the river mouth was reported by Riley (1937) and recently by Alberts (1970) who concluded that this river is a major source of P to the Gulf of Mexico. Analysis of water collected from Southwest Pass in February 197~ showed high concen(1ess than 0.06 mg/l) (Ho, 1972). POi3-P trations of NO;-N (14 mgil), but low NH:-N and dissolved SiO, were also high (0.07 mg/l and 5.0 mg/l, respectively) and salinity was low

176

C. L. Ho & B. B. Barrett

(less than I%~). The NO,-N-enriched river water is characteristic of the drainage waters from agricultural lands as a result of washout of fertilizers and feedlot wastes (Willrich & Smith, 1970; Kohl et al., 1971). The objectives of this investigation were threefold: (I) to determine the concentration and distribution of selected nutrients and organic matter discharged into the Gulf of Mexico by the Mississippi River at various discharge stages; (2) to evaluate the importance of nutrient supply sources other than the Mississippi River; and (3) to provide basic information for future assessment of man’s impact on fisheries production in the Gulf of Mexico and associated estuarine systems.

Procedures Field sampling Water samples were taken from IO stations in Caminada Bay and from 12 stations in the adjacent nearshore waters during rising tide. The nearshore stations consisted of three transects of four stations each (Figures I and 2). Water samples were’collected from these areas in January, March, May and September 1973. Barataria Bay was included in the sampling during the March, May and September trips, and the nearshore zone between Quatre Bayou Pass and the mouth of the Mississippi River at Southwest Pass was sampled in May and September. The bay water was shallow (less than 1.5 m in depth), therefore, only surface water was collected for analysis. Water depths in the nearshore area varied from 4 to 12 m. As stratification, especially during high river discharge and low wind velocity, was pronounced in these offshore waters, both surface and bottom waters were collected for analysis. All surface water samples were taken by dipping a soo-ml Pyrex glass bottle by hand to a depth of 15 cm below the surface. A Van Dorn sampler was used for sampling bottom water. All water samples were preserved with 5 ml of chloroform, placed on ice during transport and stored at 4 “C in the laboratory. Our tests have shown that filtration causes serious contamination to NH: and retention of POi3-P by millipore filters. The error resulting from filtration may be very significant when low concentrations of NH:-N and POi3-P are involved. All samples were analyzed for NH;, NO, and POi3 concentrations within 3 to 4 days after collection.

Analytical methods Ammonium-N in 10-25 ml water sample was steam-distilled for 3 min in presence of MgO. (Ammonium+nitrite+nitrate)-N in 10-25 ml water was quantitatively converted to NHZ-N by Devarda’s Alloy in presence of MgO. Our tests have shown that presence of sea salts does not interfere with the recoveries of any of the inorganic forms of nitrogen individually or in a mixture (Ho & Schneider, 1975) although most of the analyses were conducted in salt-free media (Vogel, 1960; Bremner, 1965; Golterman, 1971; Kohl et al., 1971). Blanks and standards were analyzed in exactly the same manner during the analysis of each batch of samples. Ammonium-N in distillate was measured spectrophotometrically using the procedure of Strickland & Parsons (1965) and Truesdale (1971) further modified by our laboratory (Ho & Schneider, 1974). Kjeldahl-N was determined by a micro-Kjeldahl method (Bremner, 1965). Complete recovery of organic-N required only 30 min at 375 “C after clearing. NH:-N distilled was measured spectrophotometrically after 15 min Nessler’s reaction.

Nutrients in Louisiana’s coastal waters

177

Phosphate-P was determined by the procedure of Strickland & Parsons (1965). This procedure was modified in that ethyl acetate was used for extracting the phosphomolybdate complex (Ho & Schneider, 1975). Total-P in water was evaporated to dryness in an evaporating dish followed by ignition at 550 “C for z h to convert organic-P to the inorganic form which was then dissolved in 25% H,SO, on a steam bath. Then determination of phosphate-P was made by the same procedure as described above (Ho & Schneider, 1975). Dissolved SiO, and salinity were measured by Strickland & Parsons’ methods

(1965). Distribution of nutrients during various discharge stages of the Mississippi River The year 1973 was an extremely high water year in Louisiana. Coastal rainfall was 190 cm with heaviest precipitation occurring during the months of March, April and September (U.S. Department of Commerce, 1956-1973). Average discharge of the Atchafalaya and

o-

’

2 ;= = P .r e -5 ‘i x 0

15. (b) ,()-

I

I

I

I

,I

Y

3

I

I

.--4 1 ‘1 \ :

5-LboL.-~,:t-.+--d L-: A--.’

A 11 ’ \ ’ , : ‘, ‘-*Le<;l&a

I I I I I I I I I I I %n. Feb. Mor.Apr.Moy.JunJul.Aug.Sep.Oct.Nov.Dec.

Figure 3. Mean monthly Mississippi River discharge and total monthly coastal rainfall during 1973. River discharge data taken from Stages and Discharges of the Mississippi River and Tributaries and Other Watersheds in the New O&am District, IgJo-Ig73. Rainfall data taken from Climatological Data, Louisiana, rg55-rg73. -, Long-term monthly mean; - - - -, 1973 monthly mean.

Mississippi Rivers during 1973 was IO 800 m3/s and 20 800 m3/s, respectively (U.S. Army Engineers, 1973). Comparison of the 1973 average monthly Mississippi River discharge and coastal rainfall with the long-term averages are shown in Figure 3(a) and (b), respectively. The tidal range during each sampling period, the average coastal rainfall, and the Mississippi River discharge prior to and during the sampling months are given in Table I.

C. L. Ho & B. B. Barrett

178

TABLE I. Tidal range during sampling periods, average coastal rainfall and discharge of the Mississippi River prior to and during the sampling months

Sampling period

Tidal range (cm)

17 Jan. 1973

25-69 rising 53-69 rising 38-51 rising 5 I-43 during 32 low

29 March

1973

22 May I973

17 ranuary

15.3 Dec. 1972

1st part falling latter part slack 32-43 rising during latter part

30 Sept. I973

Ig73-Caminada

Average monthly rainfall (cm)

12.7 8.6 25.6 26.4 16.0

Average monthly Mississippi River discharge (m”/s) 400 Dec. 1972 26 000 Jan. 1973 24 000 Feb. 1973 22 400 March 1973 37 000 April 1973 38 000 May 1973 22

Jan 1973 Feb. 1973 March 1973 April 1973 May 1973

12.5 Aug. 1973 36.3 Sept. 1973

6840 Aug. 1973 6280 Sept. 1973

Bay and the adjacent oflszore area

The variation between salinities in Caminada Bay and the near offshore zone suggests that waters in these two areas were not well mixed [Figures z and 4(a)]. The low nearshore salinities were the result of dilution by freshwater discharged from the Mississippi River into

f

‘4 (a)

-32

1.5 -

-30

I.4 -

- 28

1.3 -

- 26

1.2-

-24

0.16~ o.,+(b) 0.14-

-1.6 j I I

-4.0

- 3.5

- 22 -20

I+ Caminada Bay

Caminada Pass

Nearshore zane Distance

+_I_ Caminada Caminada Pass Bay from bay to river mouth (km)

Nearshore zane

Figure 4. Distribution of chemical components at selected stations in Caminada Bay and its nearshore zone, 17 January, 1973: (a) salinity and inorganic-N; (b) dissolved SiO,, organic-N and PO ;a-P.

the Gulf. These near-shore waters were characteristically high in (NO, +NO,)-N, but low in NHf-N. The low salinity in the northern region of Caminada Bay is the result of freshwater drained from nearby marshes [Figures I and 4(a)]. Dissolved SiO, andPOi3-P concentrations

Nutrients in Louisiana’s coastal

I79

waters

were lowest in the bay where salinity was highest; whereas the near-shore area (Station 4-1) which received most Mississippi water showed the highest levels of SiO, and POi3-P [Figure 4(b)]. Water near the marsh contained much lower SiO, and POi3-P than in the nearshore area but higher than in the center of the bay. Dissolved organic-N content was highest near the marsh relative to all other stations [Figure 4(b)]. These results suggest that runoff water from the marshes enriched in dissolved organic matter was the primary source of freshwater to the northern region of Caminada Bay, whereas freshwater in the nearshore zone, especially east of Caminada Pass, was enriched with inorganic nutrients (NO;+NO,), SiO, and POi3 was discharged by the Mississippi River during this time period. in Caminada Bay was IO times lower than that in The average value of (NO;+NO,)-N and 0.282 mg/l, respectively), but the surface near-shore waters south of the bay (0.025 organic-N was four times higher in the bay than in the nearshore waters (0.524 and 0.167 mg/l,

2. Nutrient distribution in the nearshore waters of Caminada and Barataria Bays (17 January, 1973)

TABLE

Sample location’

yiiy m

Caminada S I-I B

nil

0.093

0.024

0.661

O-006

0’330

0.024.

S

Diss. SiOs b-41)

Salinity (%o)

3.13 3’07

14.0 14.6

0.501

3’27 2’33

15.2 17.0

0.362 0.196

2.76 1’54

19’3 30.2

0.019

0’344 0.126

2.76 1.70

19.7 32.0

2.76 2.62

8-r 18.1

1-z

B

S B

0.005

1-3

S B

0.003

I-4

0'021

(NO;+NO;)-N (w/l)

0.003

~778

nil

o.IIg

nil

o6w

0.003

0.032

3’73 2.68

9.8 19.7

0.230 0.176

2.83 2.15

16.0 31.0

0.526 0.171

3’71

15.0

0’011

I.02

32’7

0.860

3’70 3’31

S B

nil

2-3

S B

0.005

2-4

0.027

Quatre Bayou

6-s

4-1

;

0.058 0,065

0.790

sB

o-079 o-037

0.962 0.238

go0

4-2

1.80

7’5 27’9

4-3

fj

0.062 0.042

~071 0.231

3.85 1.30

10.8 30’9

4-4

;

o-39

0.639 0.116

3.10 1.60

32’7

‘3 = surface;

0.051

B = bottom.

8.8

15'2

C. L. Ho & B. B. Barrett

180

respectively). These results further suggest that mixing between waters of the Mississippi River and Caminada Bay via a westerly nearshore flow and during a rising tide was not substantial during this time. Stratification of salinity and nutrients in the nearshore waters was pronounced (Table 2). Salinity generally was higher in the bottom than in surface waters, with the smallest vertical variation occurring at station 4-1 which was more influenced by the Mississippi River than other stations. (NO,+NO,)-N was inversely proportional to salinity and was generally lower in the bottom waters. However, the (NO,+‘NO,)-N level in the bottom water at stations I-I and 1-2 (offshore from Caminada Pass) was significantly higher than at the surface, whereas, the opposite was true at other nearshore stations. The inversion of nutrientenriched surface water to the bottom near Caminada Pass could be caused by the so-called inversed current around the Caminada nearshore region (U.S. Army Corps of Engineers, 1971). NH:-N was generally higher in bottom water than in the top which may have been due to the influence of sediment interstitial water (Table 2).

29 March Ig73---Camilzada and Barataria Bay and the adjacent near offshore area Wind velocity was high (about 55 km/h) d uring this sampling period. In spite of the high wind, distribution patterns of salinity and inorganic nutrients in Caminada Bay and its near offshore waters [Figure 5(a) and (b)] were similar to those in January [Figure 4 (a) and (b)], -24 -22 -20 - 18 -16; - 14 g -12

2

-10; -8

0

8

!

16

24

32

h Caminada Bay

40”

* Caminada Pass

Nearshore zone Distance

m

0

I__;_

Caminada my

8

1 I6

Caminada Puss

24

32

Nearshore zone

from bay to river mouth (km)

Figure 5. Distribution of chemical components at selected stations in Caminada (a) salinity and inorganic-N; (b) Bay and its nearshore zone, March 29 1973: organic-N, dissolved SO,, and PO;s-P.

although their concentrations differed due to vertical mixing by wind and tidal action. Salinities near the upper marshes in Caminada Bay were lower (12%~) than during the January sampling (14%~)which can be attributed to dilution by the extremely high rainfall in March (Table I). Salinities in the nearshore surface waters were uniform but were higher in March than during the January sampling due to vertical mixing. Reduction in Mississippi River discharge between these months (Table I) may also account for the slight increase in

181

Nutrients in Louisiana’s coastal waters

offshore salinities. The higher concentration of NH:-N at nearly all stations as compared to the January trip was probably due to the strong wave action which stirred the NH:-rich interstitial water of the bottom sediments into the water cohunn. The distribution pattern of POi3-P throughout Caminada Bay and its near offshore area in March was similar to that in January, i.e low values in the bay increasing offshore towards the river mouth [Figure s(b)]. Dissolved SiO, was significantly lower in March than in January at all stations, except station 4 in Caminada Bay. The inverse relationship between salinity and dissolved SiO, is well known (Bein et al., 1958). However, the pronounced fluctuation in SiOs at a relatively constant salinity level in the near offshore water could be attributed to adsorption of dissolved SiO, by large amounts of suspended clay particles carried by the Mississippi River during high river discharge and strong wind agitation [Figure s(b)].

TABLE 3. Nutrient distribution in the nearshore waters of Caminada and Barataria Bays (29 March 1973) Sample locations”

NH:-N hdl)

Caminada S

0'112

(NO;+NO;)-N 641)

Diss. SiO, (mdl)

Salinity (%A

0.096

0.483 0*189

1’34 1.36

16.1 26.2

0.075 0*080

0.448 0.166

2’23, 1.31

31’4

0.046 0.044

0.353 0.153

I’31

B S B

0,046 o*og1

0’097 0.080

0.99

S

0.061 0*0.59

0'520 0.465

2.43

16.8 17.0

S

0.066 0.050

0.477 0’454

2’43 2’33

17’4 19.7

I-I

B

I-2

Jj

1-3

I-4

S

S

1’34

1'21

18.7

22'1 30’9 30’9 32.0

Barataria 1.36

2-I

B

2-2

B

S

2-3

B

0.037 0.030

0.276 0.196

1.16 1.71

22.7 28.3

2-4

s B

0.017 0.036

0.153 0.069

0.76 0.89

30.2 31.2

0.040 0.059

0.463 0.412

2.53 2.43

16.0 18.9

Quatre Bayou 4-I

;

4-2

;

0.037 0.032

0.527 0'252

2’33 1.69

15.8 22.7

4-3

;

0.026 0.025

0.317 0.552

2.39 1'21

14’7 27.2

;

0'021[ 0*009

0.130 0.066

0.68

4-4

28.3 29.0

“S = surface; B = bottom.

1’11

C. L. Ho &? B. B. Barrett

182

The nearshore bottom water at stations 2-1 and 4-1 showed similar composition in salinity, (NO,+NO,)-N, and NH:-N as that in the surface, indicating that the water column in these areas was well mixed. However, station I-I near Caminada Pass was an exception; chemical stratification in this region was pronounced (TabIe 3). Furthermore, the quality of bottom water rather than surface water at station 1-1 exhibited some similarity to that of the surface water in Caminada Pass (station 3), thereby indicating a limited degree of water exchange between the nearshore bottom waters and Caminada Pass [Figure 5(a) and

@)I*

Content of dissolved SiO, in the surface and bottom waters of the March sampling was reversed from the January conditions which showed lower dissolved SiO, in the bottom than in the surface waters (Tables 2 and 3). Some of the dissolved SiO, at the surface in March may have been removed from solution by adsorption onto clay particles as a result of strong wind agitation, Wind effect would be reduced considerably at the bottom of the water column which may account for some of the higher dissolved SiOaat the bottom relative to that at the surface (Table 3). The average salinity level in Barataria Bay was lower (13.5%~)than that in Caminada Bay (16.7%~). Much of the freshwater in Barataria Bay was drained from canals, bayous and marshes in the northern region of this bay as indicated by the much lower salinity levels in this region as compared to those in the southern region near the passes (Table 4). The average concentration of (NO, +NO,)-N in Barataria Bay was more than twice as high as in Caminada Bay (oez5g and 0.113 mg/l, respectively), but NH:-N was slightly lower in TABLE

4. Nutrient

distribution

in Barataria

Bay (29 March

(NO,+ NH:-N b-d)

Sample location Southern Region

I

BI Bz

B3 B4 B5 B6 B7 BS Bg BIO BII

0.066 0.038 0.043 0.050 0.057 0.057 0.063 0.047 0.038 0.0.57 0.072

POi3-P k-d)

Org.-P bdl)

0.261

0.005

0.030 0.026 0.034

0.049 o-005 0.048 0.024

2'00

1.5'0

0.028

0'010

1.71

0.231

0.308 0.325 0'373 0.324 0.317 0.036 o-297

0.180

0.208

0.190

0~282

0.240 0.284

0.224 0.108

0.028 0.029 0.027 0.024 0.03r 0.027

16.3 IS.5 15.2 16-3

0.336 0.329

0.166 0.281

0.037 0.036

0.198

0.374

0.015

0'349 0.333 0.438

0.005 0'022

0.023

0'029

2.08

0.026 0.026 0.027

0.033 0.048 0.036 "'044 0.038

130

0.285 0.208

0.247 0'220 0'250

Middle Region BI 5 B21 3

0'011

0.148

0.084

0.297 0.300

4

B22

0'121

B16

0.084

0'201

BI7 BIS BI9

0'102

0.218

0'344 0.326

0'122

0.305 0.384 0.375

0.780 0.605

Bzo

Average Average

0.092 0.068

Salinity (%A

Org.-N (w/l)

0.052

Northern Region

Diss. SiOz @x/l)

NO;)-N (w/l)

Eastern Region B I 2 2 BI3 B14

0.054 0.043

1973)

0.639

0'020 0.019

1.70

2.37 2.37

0'021

2'34

0'000

2'22

0.013

2.28

0.036

2.32 2.70 2.31

0.016

0.042

23'5 15'3 16.1

16.0 15'2 15'0

2.84 2.57

12’2

I.99

15'3

0'052

1.90

0.004

2.30

12.3 17.1 14.8

0.029

0.031 0.049

0.030 0.02.5 0.065 0.338 0.259 Average nutrient contents (IO stations) in Caminada Bay: 0.422 0.013 0.040 0*098 0.113

12’2

I.09

6.4 5.6 4'1

2.32 2.13

8.0

I'94

8.0

2.15

13'4

I.69

16.7

Nut&&s in Louisiana’s coastal waters

183

Barataria than in CaminadaBay. The (NO,+NOJ-NinBarataria Bay can not be attributed solely to input by the Mississippi River discharge; other sources for (NO;+NO,)-N could have been drainage waters from the surrounding marshes and bayous, washout fertilizer from sugar-cane fields, since stations in the upper bay contained more (NO, +NO,)-N than stations near the passes(Figures I and 2 and Table 4). The fact that water near the passesof Barataria Bay (Table 4) contained higher (NO, +NO,)-N concentrations and lower salinity than near Caminada Pass (Figure 2) suggests that more Mississippi River water enters Barataria than Caminada Bay. The number of passesto the Gulf in each of these bays, the width and angles of the passes with respect to the flow direction of nearshore waters, all contribute to the degree of exchange between nearshore and bay waters. Also, Barataria Bay is closer than Caminada Bay to the discharge points of the Mississippi River. The average levels of POi3-P and dissolved SiO, exhibited similar trends as (NO,+ NOB)-N, being higher in Barataria than in Caminada Bay (Table 4). Run-off water to Caminada Bay was less than to Barataria Bay during the high rainfall period in March, thus accounting for the higher average levels of dissolved organic-N, organic-P and salinity in Caminada. 22 May rg73--Camilzada and Barataria Bays and the near oJf$hwe area from Caminada Pass to Southwest Pass During this sampling period, salinity varied widely throughout the entire nearshore zone and the bays [Figures 2 and 6(a)]. Exceedingly high concentrations of (NO,+NO&N were

i “o~i’y

I

424

02

4 0

0

0.2 ,

-_I_

Cominodo BOY

Cominada POSS

,

Nearshore Distance

from

, 56

, 64

72

I 80

I 88

zone

bay lo river mauih

98 Mouih

(km1

Figure 6. Distribution of chemical components at selected stations in Caminada Bay, its nearshore zone and west of the Mississippi River mouth, 22 May 1973 (a) salinity and inorganic-N; (b) dissolved Si02, organic-N and POi3-P.

:

184

C. L. Ho -3 B. B. Barrett

found in the nearshore waters east of Grand Bayou Pass to the mouth of the Mississippi, ranging from 0.9 to 1.8 mg/l. These values agree with values obtained at the river mouth in Nearshore waters west of Grand Bayou Pass contained little February 1972 by Ho (1972). (NO, +NO,)-N. NH:-N content was low in all nearshore surface waters. The elevated NH:-N concentration adjacent to Grand Bayou Pass may be the result of local drainage water and/or upwelling of bottom waters in this area. Inorganic-N species were in a depleted state of less than 0.02 mg/l near the marsh (station 9) in Caminada Bay, but were noticeably higher in the pass (station 3) [Figure 6(a)]. Depletion of inorganic-N in the bay was observed previously by Barrett (1971) and Ho (1971) and could logically be attributed to more rapid phytoplankton uptake but a slower rate of replenishment. The slightly higher (NO,+ NO,)-N in the pass and its vicinity [Figures 2 and 6(a)] may have been caused by waste disposal from local communities. The reduction of (NO, +NO,)-N accompanied by the absence of a substantial increase in organic-N [Figure 6(b)] in the nearshore waters between Caminada Pass and Grand Bayou Pass cannot be explained solely by phytoplankton assimilation. Displacement of (NO,+ NO,)-N enriched Mississippi water by low nutrient, high salinity seawater during this period may have taken place as a result of a shift in current direction, Evidence to support

23 -

22- @\

21 20 19 18 17 161514 13

Borotorio

shore

I

Quotre,Boyou shore

t

2

4 6 8 IO Distonce from shoreline (km)

I2

Figure 7. Distribution Bayou Pass influenced

of chemical components in nearshore water west of Grand by coastal current: (a) Salinity; (b) (NO; +NO,)-N.

Nutrients in Louisiana’s coastal waters

18.5

this reasoning was the decrease in surface salinity accompanied by an increase in (NO,+ NO,)-N seaward within the II-km (7 miles) transect investigated [Figures 7(a) and (b)]. Concentrations of (NO, +NO,)-N, organic-N, POi3-P and dissolved SiO,remained high in nearshore waters between Grand Bayou Pass and Southwest Pass relative to the area west of Grand Bayou Pass because of prevailing influence of the Mississippi River discharge in this region [Figure 6(b)]. Water inside Caminada Bay showed variations in organic-N, POi3-P and dissolved SiO,, depending on distance to the source. Organic-N was the highest near the marsh but was low in the center of the bay as well as in the pass. Dissolved SiO, was lowest in the pass where salinity was highest; stations near the marsh (station 9) and the center of the bay (station 4) contained similar amounts of dissolved SiO,. The slightly higher POi3-P in the pass coincided with an increase in (NO, +NO,)-N in this area, indicating that both components were probably derived from waste disposal in the vicinity [Figure 6(b)]. Comparison between average nutrient levels in Caminada and Barataria Bays showed that (NO; +NO,)-N was higher in the former than the latter. The opposite was noted for levels of NH:-N and organic-N (Tables 5 and 6). The sharp rise in organic-N accompanied by TABLE 5. Nutrient Sample location

distribution NH:-N 6-4) 0.007 0.040

CI

in Caminada Bay (NO,+NO;)-N 641) 0.006 0.084 0.129

(22

May

1973)

Org.-N 6-w/l)

Salinity (%I 13.6

C8

0’000

C9

0.017 0.023

0'010

I.179 0.067 0’228 0.268 0.082 0.321 0.628 1.019 0.681

0'022

0.354

17'3

Average

0'022

0.053

0.483

IS.5

TABLE 6. Nutrient

distribution

in Barataria

Bay (22 May 1973)

c2

C3 C4 C5 C6

0.028

C7

0.028

0'011 0'011 0.051

CIO

Sample location BI B3 J37

BIO B12 BI4 BI5

B16 BIS B20 B22 Average

. NH:-N (mdl)

0.045 0.046 0.063 0.075 0.05 I

(NO;+NO,)-N @w/l) I

Org.-N b-4)

0.104 0.085

0'01

0.068 0.018

0'000

0'022

0.066

0.843 0.829

0'002

0'021

1’044

0.004

0.026 0.038 0.063 0.019 0'002

0.996 1’044 0.918 0.908 0.594

0.027

0.841

0'002

0.017 0.023 0'010 0.032

0.034 0'021

0.603 o-799 0.676

28.2 22.7 20'0 16.2 19.6 27.2 8.1 11.3

Salinity (%a) 19.8 19'9 21.9 16.0

IO.5 10.6 5’8 1’2 0.8 I'2 12'0

10.9

depletion of (NO, +NO&N in Barataria Bay relative to Caminada Bay may be indicative of high productivity of plankton as a.result of more nutrient input from drainage water from

186

C. L. Ho @ B. B. Barrett

marshes and sugar-cane fields; export of organics from marshes; and/or waste disposal. Organic-P, POi3-P, and dissolved SKI, were similar in both bays (Figure I ; Tables 5 and 6). The diversion of nutrient-enriched Mississippi water seaward east of Grand Bayou Pass suggests that river water, as a nutrient supply source to Caminada and Barataria Bays, also may subside during this time. This contention is supported by the fact that concentrations of in waters near the passes of Barataria Bay (Table 6) and their nearshore (NO,+NO,)-N zones [Figures 7(a) and (b)] were indeed lower than in areas inside the bay.

.ZO September rg73---Caminada

and Barataria area from Camilzada Pass to Southwest Pass

Bays and the near oflshore

Salinity and nutrient levels in the entire nearshore zone changed drastically as compared to values obtained previously [Figures 8(a) and 6(a)]. Salinity was high throughout the entire

16 !2 + b" 2 -

0.4

8

0.2

4

n

2 \ g :t ::

f-7

0. IO 0.08 0.06 0.04

JO k Camina!ZZAnada Pass Bay

Nearshore zone

4 Mouth

Distance from bay to river mouth (km1

Figure 8. Distribution of chemical components at selected stations in Caminada Bay, its nearshore zone and west of the Mississippi River mouth, 20 September 1973 : (a) inorganic-N and salinity; (b) dissolved SiO,, organic-N and POi3-P.

nearshore zone studied. Salinity inside Caminada Bay, however, remained lower than that in the nearshore zone. Contrary to previous observations, concentration of (NO,+NO&N in the nearshore waters in the vicinity of the Mississippi River mouth were depleted to an exceedingly low level, whereas nearshore waters west’of Grand Bayou Pass and east of Caminada Pass showed

Nutrients in Louisiana’s coastal waters

rg7

higher concentrations of (NO; $-NO,)-N. NH:-N constituted the dominant fraction of the total inorganic-N throughout the entire study area. The ratio between (NO, +NO&N and NH:-N was especially striking in Caminada Bay. (NO;+NO,)-N was depleted to only a trace amount, whereas NH:-N remained relatively high [Figure 8(a)]. The depletion of (NO;+NO,)-N accompanied by an increase in levels of NH;-N is indicative of utilization of the former by phytoplankton resulting in organic-N synthesis followed by subsequent decomposition of some of the organic remains to NH:-N at the end of phytoplankton growth period. The supply of (NO, +NO,)-N by the Mississippi River to the entire nearshore zone and associated estuaries became essentially nil during this time probably because of low run-off from farm lands in the upper Mississippi Valley. High coastal rainfall in September 1973 apparently did not bring about a significant contribution of (NO;+NO,)-N to the nearshore water. Levels of organic-N, dissolved SiO, and POi3-P provided additional evidence on the importance of marshes surrounding the bays as nutrient supply sources. Concentrations of these components in nearshore waters east of Grand Bayou Pass all decreased to low levels characteristic of seawater influence during this sampling period. Waters in the nearshore zone west’of Grand Bayou Pass showed higher concentration of dissolved SiO, and organic-N but similar POi3-P content as compared to water east of the Grand Bayou Pass. Concentrations of these components in Caminada Bay were higher than those in the nearshore zone. The simultaneous enrichment of organic-N, NH;-N, POi3-P and perhaps SiO,in Caminada Bay and its nearshore water further supports the explanation that these components were derived from decomposition products of organic detritus, thus providing the ecosystem with a natural mechanism for regeneration of nutrients. Some of these components are exported offshore, therefore accounting for the substantially higher levels of nutrients in waters adjacent to these bays relative to that found near the river mouth [Figure 7(b)]. Salinity near the passeswas higher than in the upper marshes in both bays. Barataria Bay showed a consistently wider range of salinity than Caminada Bay (Tables 7 and 8). This TABLE 7. Nutrient Sample location CI c2

C3 c4 C5 C6 c7 C8 C9 CIO

Average

NH:-N 6-d)

distribution

(NO, +NO;)-N bg/l)

0.030 0.084 0.066 wo90 0.078 0.080 0.073 0.078 0.090 0.091

0.007 0.003 0.004 0.004

0.076

0.006

0.037 nil 0.007 0'001 o*ooj

nil

in Caminada

Org.-N bdl)

PO;%-P 641) --

0.676

0.021 0.019 0'010 0.016

Bay (20 September Org.-P (mdl) 0.071

0.930 0.139 0’579

0'020 0.014

1.067 1.019 1.321

0.026 0.03 I o-047

0,029 ~024 0.039 0.036 0.03 I 0.029 0.026 0.035 0.072

0.827

0.023

0.039

0.521 1.150 0.871

0.027

1973)

Diss. SiO, k-cdl) 2.90 2.48 2.13

Salinity (%o) 18.5

24’9 24.8

3’rr 2.66 2.83 2.73 3’41 3.08

13’5 20.3

2.18

18.8

2’75

18.0

16.5 17.2

12-j

13.2

trend was similar to that observed during the May trip, indicating that Barataria Bay had continued to receive more fresh drainage waters from the upper marshes than did Caminada Bay in September 1973. Associated with the drainage waters into Barataria Bay was a noticeably higher input of (NO;+NO,)-N relative to Caminada Bay. Contrary to the May trip, the average concentration of organic-N in September was higher in Caminada Bay (a.83 mg/l) than in Barataria Bay (0.57 mg/l). It is conceivable that more dissolved organic-N

C. L. Ho @ B. B. Barrett

188

TABLE 8. Nutrient Sample location

0.081

B3

0.069 0.064 0.073 0.065 0.064 0.069 0,075

B7 BIO Br2

BI4 BIS Br6 BIS B20 B22 Average

Bay (20 September

1973)

PO ;s-P (mail)

Org.-P (mg/l)

Diss. SiO, b-4

0’030 0’030

0.582

0.019

0.474

0.018

0’010 0.033

0.017 0'01 I 0'021 0'022

0'210

0.027

0'020

0'020

0.027

0.026 0.029

0.028

0.013 ,o.org

19’3 25’4 25’5 26.2 26.6 13’9 11’1 7’9

0'012

0.037 0’034 0’045 0,004 0.035 0.065

1.78 1’43 1’44 1’50 1.86 2.30 2’54 2.69 2.44 2’94

0'022

0.03 I

I.96

16.1

0.064 0.075

0.040 0.009 0.030

0.521 0.420 0.764 0.645 0.782 0.346 0.730 0.834

0.071

0'022

o-574

0.082

in Barataria

Org-.N (w/l)

NHi-N (mdl)

BI

distribution

0.006 0'021

0.036 0.025

Salinity (%o)

4’6 5’2 IO.8

in Barataria Bay could have been exported to the nearshore zone via Barataria Pass, but less through Caminada Pass during high rainfall and low tide in September (Table I). This is indicated by the correspondingly higher organic-N in the nearshore waters of Barataria Pass (1-07mg/l) and lower organic-N in the nearshore waters of Caminada Bay (0.6 mg/l) [Figure S(b)]. Th e average content of dissolved SiO, was also higher in Caminada Bay than in Barataria Bay. NH:-N, POi3-P and organic-P were about the same in both bays (Tables 7 and 8). Contribution

of nutrients

by the Mississippi

River and marshes

During 1973, the average monthly and mean’ discharge of the Mississippi River from January through July was 26 ooo m3/s. This value was used to estimate the quantities of selected chemical components discharged by the Mississippi River based on the following assumptions. I. During high river discharge, surface water at station M4 (Figure 2) represented river water. Concentrations of the components in the river water would not change significantly during high discharge of the river, i.e. January through July of 1973. The basis for this assumption is the proximity of Station M4 to the river mouth, the similarity in concentrations of (NO; +NO,)-N obtained in February of 1972 at Southwest Pass and May 197-j at station M4 (1.8 mg/l), and the higher levels of POi3-P and dissolved SiO, and low salinity at station M4 compared with other stations sampled in the vicinity of the river mouth in May 1973. 2. Surface concentrations of nutrients at station M4 in September 1973 represented the ambient nearshore concentrations relatively free of run-off water from agricultural lands in the upper Mississippi River valley. Discharge in September 1973 was 6280 m3/s, whereas 38 800 m3/s was discharged during May 1973. 3. The net nutrient input of the Mississippi River into the Gulf of Mexico represents the difference between the values obtained at station M4 in May and the corresponding values at this station in September. The magnitudes of selected inorganic and organic components introduced by the river between January and July of i973 are presented in Table 9. The combined weight of (NO;+NO,)-N, POi3-P, dissolved SiO,, and organic-N, totalled 3.06 billion kg. Because of the unusually high river discharge during 1973, the chemical components transported by

Nutrients in Louisiana’s coastal waters

189

the river were about 60% higher than the long-term average of previous years (Table 9). Dissolved organic-C was not analyzed from samples, but was calculated from a C/N ratio of 12. Thus, the calculated total dissolved organic-C during the first seven months of 1973 was TABLE 9. Estimated quantities of selected chemical components discharged by Mississippi River into the Gulf of Mexico from January through July [in billions (IO’) of kilograms]

1973 Long-term

(NO;+NO,)-N

POi3-P

Diss. SiO,

Org.-N

Org.-C

0%89

0.036

1.320

0.821

9.84

0’544

0.023

0.816

0’503

6.08

average

(1940-1973)

945 billion kg as compared to 6.08 billion kg estimated for the long-term average for these months. Fresh organic matter may also be added to the nearshore Gulf waters by photosynthesis of phytoplankton in response to the introduction of excessive amounts of available nutrients by the river. Reduction of dissolved 0, in the nearshore water due to decomposition of organic matter may follow when water circulation slows down. In fact, low oxygen content

6 !? z

0.55 050 0.45 0.40 0.35 0.30 0.25

I’

I< \\ \\

Safnity

,A1

25

0.20 0.15 0 IO 0.05 0

JC2”. Feb. Mar. Apr. May Jun. Jul. Aug. Sep. Sampling time 1973

Figure 9. Seasonal changes in inorganic-N and salinity in nearshore zone (average of o.Sandrr.zkm) of: (a)Caminada Pass; (b) Barataria Pass. -, surface; - - - -. bottom.

19”

C. L. Ho &3 B. B. Barrett

in northern Gulf of Mexico waters have been reported since the early 1900s (Hedgpeth, 1957) and could be attributed to introduction of organic matter and nutrients by the Mississippi River. The sharp increase of (NO, +NO,)-N in both nearshore surface and bottom waters south of the bays coincided with the rise in river discharge before the end of May 1973 [Figures 9(a) and (b)]. NH:-N d i d no t constitute a major component in the river water; therefore, its concentration was subjected to little influence by river discharge. Instead, the increases in NH:-N, especially in bottom waters and in water column during March, was probably caused by wave action which stirred up the NH:-N rich interstitial water in bottom sediments. Other components of river,origin such as PO, -3-P and dissolved SiO, would follow the same seasonal trend as that shown by (NO5 +NO ,)-N. The fact that (NO,+NO,)-N, POi3-P and dissolved SiO, concentrations in the nearshore waters between Caminada Pass and the mouth of the Mississippi River from January to May were of Mississippi River origin can be shown by the inverse relationships of these components with salinity [Figures IO(a)-rz(c)].

(b)

.

t t A--

d.. 1I.0 *.

.

0.20

I I I I I 0.015 0.025 0.035 PO-4j_P (mg /II

0.40 (NO;+ NO;)-N (mg/l)

-3

I

. .1 l

I 0.045

I

d 0.055

35 *.‘.-:.

7&(c)

. 6 2 2521 20 f fj 15-

\

cnIO5 00

0.5

I.0

. 4

. l:\q. . z ** . **. .@

I.5 2.0 2.5 3.0 3.5 4.0 4.5 Diss. SiOe (mg /II

Figure IO. Relationship between salinity and’ (NO;+NO;)-N and dissolved SiO, (c), in nearshore waters south of Barataria 1973. 8, Surface Water; A, bottom water.

(a), PO,s-P (b), Bay, 17 January

inorganic form of nitrogen in Caminada and Barataria Bays also appeared to coincide.with the high discharge of the Mississippi River [Figures 13(a) and (b)]. Nevertheless, this coincidence alone does not provide sufficient evidence to suggest that nutrients in the bays, especially (NO; +NO,)-N, were supplied solely by the Mississippi River. The present evidence indicates that effective mixing between the waters of the Mississippi River and Barataria Bay was limited primarily to the southern region of this bay. The mixed water

Nutrients in Louisiana’s coastal waters

191

POi3-P, and was characterized by slight variations in salinity (IS--17x0), (NO,+NO,)-N, dissolved SiO, [Figure q(a)-(c)]. Salinity and nutrient concentrations in the northern region of Barataria Bay fluctuated more than those in the southern region due to different sources of drainage water and lack of mixing in the upper bay.

-Y. +

35

I2

30 -

(NO:+

(b)

NO 2 tN (mg/l)

Diss. Si02(mg/l)

‘,*cr, l

e

0

.a3

*

25 \

l l

20 -

,oIIII 0I0.010 15-

0020

0.030

POiriP

0.040

0,050

0.060

(mg/l)

Figure II. Relationship between salinity and (NO;+NO;)-N (b), and dissolved SiOp (c), in nearshore waters south of Barataria 1973.

(a), PO;QP Bay, zg March

POZ-P (mg /I 1

(NOitNO’;>N(mg/i)

” 15 a&? IO 5 s

5 0

0

0.5

I.0

I.5

2.0

2.5

Diss. SiO,(mg/l

3.0

3.5 4.0

I

Figure 12. Relationship between salinity and (a) (NO;+NO,)-N, and (c) dissolved SiOz in nearshore waters west of Southwest Grand Bayou Pass, 22 May 1973.

(b) POi3-P, Pass and east of

C. L. Ho, &f B. B. Barrett

192

Jan. Feb. Mar. Apr. May Jun. Jul. Aug. Sep. Oct. Sampling time, 1973

Figure 13. Seasonal changes in inorganic-N (b) Barataria Bay.

24

and salinity:

20

7

. . . . . . . . . .. . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2 16 7.

of bay l . . *a* .. . . . . . . .. . . . . . . .. . . . . .. . . . . . . . . . . . . . . . . ..

c

Southern

t

0

0.05

region

0.10

. .. .

0.35

0.40

.

. . . . . . . . . .. . . . . . . .. . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . , . . . ,. . . . . . * . . . . . .i . . . . . . * . . . . . . . . . . . . . Southern region T Q . l Miss. Input -e ‘5 . . . . . . .?‘..b!Y . . . . . . . . . . . ...? . . . .t?r..:. . .... ..... . . e .. . 2 Drainage water :E IOl *

2

:

5I 0

0.30

. . .

(cl

(b)

0

*Miss. in put

0.15 0.20 025 (N03+N02)-N(mg/l)

20 -

I 0.010

I

I 0.020

I

POi?P (mg/l)

.

Southern region of bay : . . . . . . . . . . . . .

I

I 0.040

.

Miss. input

we** b l . . . .. . . . . . . . . . . . . . . . . . . . * . . .

.

.

Drainage

Northern region of bay I 0.030

Bay;

1

.

(a)

(a) Caminada

. . I

I 0.5

I I.0

I I.5

0. 0 I 2.0

water

Northern region of bay I 2.5

I 3.0

I 3.5

I 4.0

Diss. SiO, (mg/l)

Figure 14. Relationship between salinity and (a) (NO; +NO;)-N, and (c) dissolved Si02, in Barataria Bay, z,g March 1973.

(b) PO;3-P

(NO;+NO,)-N, POi3-P and dissolved SiOz of river origin entering Caminada Bay from the nearshore waters was restricted to the narrow Caminada Pass, even during high flooding stage of the river in March. The water mass of low salinity, (NO,+NO,)-N, POi3-P and

Nutrients in Louisiana’s coastal waters

dissolved SiO,, but higher NH;-N [Figure 13(a)] in Caminada Bay, was characteristic of drainage water from marshes rather than from the Mississippi River (Ho & Schneider, 1975). Moreover, the persistent presence of a water mass of higher salinity (zo-23x,,) was noted between two water massesof lower salinities (below 17%~)in the northern and southern regions of both bays. This phenomenon further suggests that mixing between waters in the southern and northern regions of the bays may be a very slow process; therefore, much of the nutrients in the bays were probably derived from the marshes and drainage areas surrounding the bays. The average distribution of organic-N in the nearshore waters south of Caminada and Barataria Bays is shown in Figures Is(a) and (b). High organic-N contents in the nearshore waters in March were found during high flooding stage of the Mississippi River, whereas low

0.1

L.4 I Apr.I May’ Jun’ Jul.I Aug.1 Sep.’ Oct.I

’ ’ ’ ’ ’ ’ ’ ’ Jan.Feb. Mar. Apr Moy Jun. Jul. Aug. Sep.Oct :Jan.Feb.Mar. Sampling time, 1973

Figure 15. Seasonal changes in organic-N adjacent nearshore waters: (a) Caminada water; - - - - , bottom water.

in Caminada and Barataria Bay; (b) Barataria Bay. -,

Bays and surface

values of organic-N were found in the bays during this time. This indicates that run-off waters from the upper Mississippi River drainage basin not only carried soluble fertilizers such as nitrate and phosphate, but also organic matter to the river. Heavy rainfall prior to the March sampling, however, accounts for the dilution of organic-N in the bays. At the end of May, organic-N in the nearshore waters decreased sharply [Figure Is(a) and (b)] due to displacement of Mississippi River water by seawater as a result of an apparent shift in current direction. Organic-N remained exceedingly high near the river mouth during this time [Figure 6(b)]. On the contrary, organic-N in the bays increased sharply from April to late May which was probably due to phytoplankton production as a result of nutrient enrichment during early spring. The increase in organic-N was especially pronounced in Barataria Bay [Figure Is(b)] as compared to Caminada Bay [Figure Is(a)]. At the end of September, the average content of organic-N in the nearshore waters adjacent to Caminada and Barataria Bays was higher than that in May, and the increase was greater in the nearshore waters south of Barataria Bay (0.28-0.52 mg/l from May to September) than was found south of Caminada Bay (0.35-0.44 mg/l) during the same time period. Contrary to the trend shown in the nearshore waters, organic-N in Caminada Bay showed a continuous increase during the period of March to September, but a sharp decrease occurred in Barataria Bay from May to September [Figure Is(a) and (b)]. This phenomenon may be the result of more organic-N exported from Barataria Bay to its nearshore zone during the high coastal rainfall period in September 1973, whereas much less exportation took place in

C. L. Ho & B. B. Barrett

Caminada Bay. This explanation is supported by the estimation made in August 1947 by Marmer (I@), showing that Barataria Pass drained about eight times more water to the Gulf of Mexico than Caminada Pass during each tidal cycle. This estimate may still hold for the passes at the present time. Therefore, the relative differences of organic matter drained to the Gulf from the two bays are expected. Conclusions The distribution of inorganic nutrients and organic components in Louisiana coastal and estuarine waters depends on many factors. The major contributing factors are the total volume of Mississippi River water discharged, coastal current and wind direction, local rainfall and proximity to marshes and sugar-cane fields. During the major flooding period of the river (January through July, 1973) and heavy rainfall, Louisiana coastal waters within the zone of river influence showed pronounced reduction in salinity, accompanied by enrichment of land-derived inorganic nutrients (NO;-N, PO,-P, and dissolved SiO,) and organicN. Influence of the Mississippi water via its western flow along Barataria and Caminada Bays during the major flooding period was limited to the lower region of these bays in spite of rising tide. Heavy local rainfall resulted in considerable drainage of freshwater from marshes and sugar-cane fields via canals and waterways to the upper bays during this period. During late May, influence of the Mississippi River discharge on coastal waters west of Grand Bayou Pass subsided as a result of an apparent shift in current direction. Hence, nutrients, organic content and salinity in coastal waters between Caminada and Grand Bayou Pass were characteristic of seawater influence during late May. Nevertheless, nearshore waters between Grand Bayou Pass and the Mississippi River mouth which continued to receive Mississippi River water showed exceedingly high concentrations of inorganic nutrients and organic-N, but low salinity during late May. During late September, when the Mississippi River discharge was lowest, accompanied by low tide, the entire nearshore waters between Grand Bayou Pass and the mouth of the Mississippi River were dominated by low nutrient, high salinity seawater. High local rainfall did not exert noticeable influence in nutrient content through the Mississippi River discharge in this area. This was not the casein nearshore areas surrounded by coastal marshes and swamps west of Grand Bayou Pass. Heavy rainfall washed out considerable amounts of inorganic nutrients and organic-N from swamps and marshes to Caminada and Barataria Bays which, in turn, drained some of the components to their adjacent nearshore water, especially through the wide Barataria Pass. It was estimated that Mississippi River discharged vast amounts of inorganic-N, PO,s-P and dissolved SiO, totaling 3.06 billion kg to the Gulf of Mexico during the major flooding period of January through July 1973. However, much of the inorganic nutrients and organicN in estuaries such as Caminada and Barataria Bays were derived from the surrounding marshes and perhaps swamps and sugar-cane fields. Contribution of nutrients and organic matter to these bays and their nearshore water was especially significantduringlowdischarge of the Mississippi River as accompanied by high local rainfall and low tide. Acknowledgements This report was the result of a combined effort by the Louisiana State University Center for Wetland Resources and the Division of Oysters, Waterbottoms and Seafoods, Louisiana Wildlife and Fisheries Commission. Mr Harry Schafer, chief of this division, was instrumental in the co-operation provided by the Commission. Mr Ralph Latapie, Supervisor of

Nutrients

in Louisiana’s

coastal waters

19.5

the Commission’s Marine Laboratory, provided personnel and equipment, including boats, for the sampling trips. University support was provided by the Louisiana Sea Grant Program, part of the National Sea Grant Program administered by the National Oceanic and Atmospheric Administration of the U.S. Department of Commerce. References Alberts, J. 1970 Inorganic controls of dissolved phosphorus in the Gulf of Mexico. Ph.D. Thesis, Florida State University, 89 pp. Barrett, B. B., Tarver, J. W., Latapie, W. R., Pollard, J. F., Mock, W. R., Adkins, G. B., Gaidry, W. J., White, C. J. & Mathis, J. S. rg7r Cooperative Gulf of Mexico estuarine inventory and study, Louisiana. Phase II : Hydrology. Louisiana Wildlife and Fish, New Orleans. Bein, G., Contois, D., Thomas, W. 1958, Removal of soluble SiOz from entering the sea. Geochimica et Cosmochimica Acta, 14, 35-54. Bremner, J. M. 1965 Total nitrogen, inorganic forms of nitrogen. In Methods of Soil Analysis (Black, C. A., ed.). American Society of Agronomy, Part II, pp. 1149-1237. Golterman, H. L. 1971 Methods for Chemical Analysis of Fresh Wuters. IBP Handbook No. 8, Blackwell Scientific Publishers, Oxford and Edinburgh. Gunter, G. 1967 Some relationships of estuaries to the fisheries of the Gulf of Mexico. In Estuaries (Lauff, G. H., ed.), pp. 621-637. Hedgpeth, J. W. 1957 Treatise on Marine Ecology and PaZeoecoZogy, Vol. I, pp. 215-217. Waverly Press, Baltimore, MD. Ho, C. L. rg7r Seasonal changes in water and sediment chemistry in Barataria Bay. Coastal Studies Institute Bulletin No. 6 (special Sea Grant issue). Center for Wetland Resources, Louisiana State University, Baton Rouge. Ho, C. L. 1972 Preliminary investigation of water quality at the mouth of the Mississippi River. Center for Wetlands Resources, Louisiana State University (unpublished data). Ho, C. L. & Schneider, S. 1975 Water and sediment chemistry. In Environmental Assessment of a Louisiana Offshore Oil Port, Vol. II, Section II; Technical Appendix VI. I. Center for Wetland Resources, Louisiana State University, Baton Rouge. 71 pp. Kohl, D. H., Shearer, G. B. & Commoner, B. 1971 Fertilizer N; Contribution to NO 2 in surface water in a corn belt water shed. Science 174,1331-I334. Marmer, H. A. 1948 The Current in Barutariu Buy. Texas A & M Research Foundation Publication, College Station. Riley G. A. rg37 The significance of the Mississippi River drainage for biological conditions in the Gulf of Mexico. Journal of Marine Research I, 60-74. Strickland J. D. H. & Parsons, T. R. 1965 A manual of seawater analysis (2nd ed., revised). Fisheries Research Board of Canada Bulletin 125, 83-87. Truesdale, V. 1971 A modified spectrophotometric method for the determination of ammonia (and amino acids) in natural waters with particular reference to seawater. Analyst 96, .@4-590. U.S. Army Corps of Engineers 1973 Stages and Discharges of the Mississippi River and Tributaries and Other Watersheds in the New Orleans District, I940--1973. U.S. Army Corps of Engineers 1971 National Shorline Study: Inventory Report-Lower Mississippi Region. New Orleans District. U.S. Department of Commerce 1973 CZimatoZagicaZ Data, Louisiana, 1955-1973. Vogel. A. I. 1960 A Text-book of Quantitative Inorganic Analysis, Theory and Practice, 2nd ed., 643 pp. - ‘- Longman, New Hork. Warren, E. C. rg7r Kinds of water pollution. In Biology and Water Control, Chapter 6, pp. 57-62. W. B. Saunder Co., Philadelphia, PA. Willrich, T. & Smith, G. 1970 Agricultural Practices and Water Quality. Iowa State University Press, Ames.