effects of varying nitrogen on brackish submersed aquatics under varying levels of phosphorus in sediments

[From the U.S. Government Printing Office, www.gpo.gov]

Task #9

THE EFFECTS OF VARYING NITROGEN ON

BRACKISH SUBMERSED AQUATICS UNDER

VARYING LEVELS OF PHOSPHORUS IN

SEDIMENTS

J. C. STEVENSON

LORIE W. STAVER

HORN POINT ENVIRONMENTAL LABORATORY
UNIVERSITY OF MARYLAND
CAMBRIDGE, MARYLAND 21613

FINAL REPORT

COASTAL RESOURCES DIVISJION
TIDEWATER ADMINISTRATION
MARYLAND DEPARTMENT OF NATURAL RESOURCES

DAVID BLEIL
PROJECT OFFICER

DECEMBER 1989
QK@
102
S84
1989
.ratioxi of this report was funded by the Coastal Resources Division, Tidewater
1"Imfion, Maryland Deparlmen, of Natural Resources.

mo W. P.

Task #9

THE EFFECTS OF VARYING NITROGEN ON

BRACKISH SUBMERSED AQUATICS UNDER

VARYING LEVELS OF PHOSPHORUS IN

SEDIMENTS

J. C. STEVENSON

LORIE W. STAVER

HORN POINT ENVIRONMENTAL LABORATORY
UNIVERSITY OF MARYLAND
CAMBRIDGE, MARYLAND 21613

FINAL REPORT

COASTAL RESOURCES DIVISION
TIDEWATER ADMINISTRATION
MARYLAND DEPARTMENT OF NATURAL RESOURCES

DAVID BLEIL
PROJECT OFFICER

DECEMBER 1989

Preparation of this report was funded by the Coastal Resources Division, Tidewater
Administration, Maryland Department of Natural Resources.

MD W. P.

BACKGROUND

Since the restoration of submersed aquatic vegetation (SAV) is one of the

objectives in the current efforts to clean up Chesapeake Bay, it is important to target the

water quality pa rameters necessary for its reestablishment. Previous studies during the

EPA Bay Program established that the most probable causes of the Baywide decline

during the 1970s included excess nutrient loadings and reduced light levels in the

shallows (with herbicide damage sporadically exacerbating the problem in areas adjacent
to agricultural fields). However, due ,the ending of the phase I (i.e. research) of the
EPA program, (in order to begin the implementation phase), little was accomplished in

terms of establishing the actual water quality necessary to support SAV. This data gap

was especially problematic in regard to the issue of whether nitrogen or phosphorus was

more a problem in encouraging epiphytic overgrowth on SAV leaves, one of the main

hypothesis' for the decline in the upper Bay.

In 1985 we began an intensive survey of sites in the Choptank River, where

revegetation was re-occurring because of the low runoff patterns associated with redUced

precipitation. (Low runoff produces decreased levels of nutrients, sediment and

herbicides in the eastern shore tributaries of the bay because they are dominated by

non-point source pollution, see Lomax and Stevenson 1982). Several conclusions -

emerged from an intensive field study of the Choptank River with supporting data from
the Wve River and the Severn River. Perhaps most significant was the finding that...
areas supporting the regrowth of SAV during the growing seasons in 1985, 1986 and
1987 could be defined with a small number of key water quality parameters. Generallv,

US PRW-trtmezit of Commerce
No-N.,A coastal services center Library
2234 South Hobson Avenue
Charleston, SC 29405-2413

at protected sites in the mesohaline lower reaches of the estuary (10-20 ppt salinity),

regrowth of SAV was observed when mean ambient growing season (April - September)

concentrations were less than values in Table 1:

Table 1. Water quality values associated with growth of submersed aquatic vegetation in
the Choptank River between 1985 and 1989.

Chl a 15 Ag 1-1
SESTON 20 mg 1-1
Light (Kd) 2.0 m-1
DIN 0.14 mg 1-1
DIP 0.01 mg 1-1

Among the management values of these concentrations is that they might be Used

as water quality criteria which could be used to set standards for effluents of sewage

treatment plants (STP) discharging into tidal waters or upstream tributaries as well as

for targeting where non-point source cleanup is necessary. The Bay cleanup targets

might be set (at least in part) by the water quality requirements of SAV. However,

there is one limitation in a strictly field approach which needs to be resolved before

these values gain broad acceptance by management agencies. That is the fact that these

concentrations usually co-vary and effects of nitrogen (N) for example can not be readily
distinguished from phosphorus (P). Thus we can not directly answer the question: what
might happen if we just removed P from the STP and left the N at existing levels?

One approach which can be used to circumvent the covariance of field variables

is through the use of microcosm experiments.

Previous studies (Twilley et al. 1985; Staver 1984) used microcosm and mesocosm

experiments to establish the link between N and P enrichment in the water column and

reductions in SAV growth. However, they did not distinguish between the effects of

nitrogen versus phosphorus enrichment. Since SAV are capable of obtaining nutrients

from the sediment pool via their root system (Chen and Barko 1988), these effects could

be partitioned out by making one available in the sediment while increasing the other in

the water column.

Phosphorus is rapidly adsorbed onto sediment particles under field conditions,

becoming entrained in the sediment. Therefore if P inputs to the estuary were reduced,

water column concentrations could become very low, creating P limitation in species

which cannot access the sediment pool. Thus, algal growth may be suppressed, even

though N concentrations could remain very high. This is scenario which seems to exist

in the Potomac River since the Blue Plains STP was upgraded to remove P in 1982, and

at Havre. de Grace, MD, where the water flowing in from the Susquehanna is low in P

due to high sedimentation rates upstream behind Conawingo Dam. Whether or not

similar conditions in brackish water would promote SAV growth is an open question,

and the primary one which we have attempted to address here.

In the current study, mesocosms (large fiberglass tanks) were used to examine the

effects on SAV arowth of increasinu nitro-en in the water column when phosphorus Is

available only in the sediment. The experiment was conducted at ambient salinitv in the

Choptank estuary at Horn Point, which was 7 ppt at the beginning of the study.

METHODS

The recirculating fiberglass tank system located in the Horn Point Environ mental

Laboratory greenhouse (described in detail in Stevenson and Staver 1989) was used to

conduct this study. Each pair of tanks was filled with filtered (5 gm) Choptank River

water and was maintained at 27 � 2'C throughout the study. Salinity was maintained at

7 ppt by the addition of deionized water (to compensate for evaporation) at

approximately two week intervals.

Three native SAV species, Ruppia matitima, Potamogeton perfoliatus and P.

pectinatus were started four weeks before the experiment was begun in fiberglass

raceways in which filtered (Vortex XL diatom filter) river water was recirculated.

Ruppia was grown from seeds, while the two Potainogetons were grown from dormant

tubers.

Phosphorus, in the form of sodium phosphate, was incorporated into Difco Bitek

complete nutrient agar, 100 ml of which was poured into individual 0.9 liter plastic
containers to yield a final P fertilization rate of 15 g p M-2 . A second set of containers

received unamended agar. On top of the agar each container received approximately

0.5 liter of a sediment mixture consisting of one part silty sediment (from the Choptank

Estuary) and 2 parts sand (for a final organic content of 1.5%). The roots of immature

(nonflowering) plants of each species were rinsed free of sediment and the entire plants

were transplanted into these pots. Two cm of clean, commercially available "play-sand"

was added to the top to slow the diffusion of nutrients from the sediment into the water

column. One plant was placed in each pot, with triplicate pots of each species beino
planted for plain agar and P amended agar at each of the six levels of nitrogen

enrichment (for a total of 108 pots).

Potomogeton perfoliatus and Ruppia plants were grown as part of an SAV

propagation project (Stevenson and Staver 1989), while P. pectinatus plants were

obtained from the experimental ponds at Horn Point. Plants of comparable size were

chosen, with each of the propagation plants having been grown from a single seed or

tuber. The P. pectinatus plants were harvested as plugs, placed in plastic pots and left in

the pond for approximately three weeks before being transplanted into the experimental

pots. All of the experimental plants were allowed to acclimate for one week before the

addition of nitrogen to the aqueous medium began. Five levels of nitrogen fertilization

were maintained by weekly additions of ammonium nitrate dissolved in water (taken

from the tanks) to final concentrations of 10, 20) 40, 80, 160 AM. A sixth pair of

unfertilized tanks was the control. Beginning on day 28 additions were made twice

weekly after nutrient analysis of the tanks showed that water column concentrations

were declining to low levels very rapidly, presumably due to plant uptake.

Nutrient concentrations in the tanks were monitored weekly, and additionally JUSt

prior to and shortly after fertilization in the beginning of the study. Samples were

analyzed for N02, N03, NH4 and P04 on a Technicon Auto Analyzer 11 using standard

techniques. Chl a was measured on a weekly basis from 60 ml samples filtered thrOLIgh

Whatman GFC glass fiber filter. Determination of chl a was made fluorometrically

(Standard Methods). The tanks were not filtered, cleaned or disturbed in any way

during the experiment, so estimates of chl a and epiphytes reflected actual algal

populations.

The experimental plants were harvested after eight weeks of growth. Epiphytes

growing on the plants were estimated by the method of Staver (1984). Three

undisturbed segments of plant stems were collected in a 500 ml bottle underwater and

shaken vigorously to remove attached epiphytes. The plants were removed and dried at

60 'C, then weighed on a Mettler AT 250 balance. The epiphyte/water mixture was

homogenized in a Waring blender, and two 10 ml aliquots were filtered onto a pre-

weighed, pre-ashed Whatman GFC glass fiber filter (nominal retention 1.2 Arri).

Estimates for all plant species at each treatment level were made.

The plants were subsequently harvested and divided into aboveground (AG) and

belowground (BG) material and dried to constant weight in a large forced draft oven

(PGC Aminco) at 60 'C. They were weighed on a top-loading Obaus balance and

aboveground material was ground in a Wiley Mill (Arthur H. Thomas Co.) with a I mm

mesh sieve. The three aboveground replicates were combined for subsequent analysis

after it was determined that there was not sufficient plant material in some samples to

do all of the desired tests. The following analysis were conducted on the combined

replicates. Ash-free dry weight was determined by combustion at 500 'C for four hours.

Carbon and nitrogen content were determined by combustion in a Control Equipment

modified Perkin-Elmer 240B C-H-N Analyzer. Phosphorus content was measured by

acid digestion followed by colorimetric determination using the molybdate-vanadate

method.

RESULTS AND DISCUSSION

Actual water column N concentrations slowly built up over the course of the

experiment (Figure 1). DIN concentrations remained close to zero in the control and 10

gM tanks but ranged up to over 700 uM in the highest enrichment tank. A buildup of

N occurred in the higher enrichment treatments apparently because nitrogen additions

exceeded plant and sediment uptake.

Growth of Potomogeton perfoliatus and Ruppia inatitima responded positively to

increasing nitrogen concentrations in the water column. Increases over controls in both

total aboveground (AG) biomass (Figures 3a and 4a) and ash free dry weight (A.FDW)

(Table 2) were observed from the 20 uM N treatment to the 40-80 AM N treatments

and then levelled off at the highest levels of enrichment. These data contrast sharply

with the results obtained in brackish ponds by Twilley et al. (1985) in which increasing-, N

and P was added in the water column resulting in declines in SAV biomass.

In addition to Twilley et al. (1985), Staver (1984) showed that when the water

column is enriched with both N and P, epiphyte communities develop which reduce the

amount of light reaching SAV leaves, ultimately resulting in reductions in SAV biomass

and reproductive potential. In our experiment, P was introduced to the root system of

the plants via agar in the bottom of the culture vessels, and was not added directly to

the water column. A small amount of P diffused through the sediment into the water,

but generally concentrations were very low, thus limiting the growth of algae in the

systems.

1000-
0-0 0 AM DIN
800-- 10 AM
20AM
600-- 40AM
z 13- E3 80 AM
0
160 AM
400-
Of

z CL
LIJ
C) 200-- CL Er
z 40
0
W 4j
0

-200
0 20 40 60

1.000
0.800-- Phosphate

0.600--
z

0.400
cy-

0.200--
z

0.000 Ar-

-0.200
0 20 40 60

DAYS
Fl(-,Lire 1. Dissolved inorganic nitrogen and phosphorus concentrations (UM) in the
experimental tanks during the growing period.

800-

Nitrate
600- 8-8 OUM
10'UM
A* 20 pM
400-- 40 pm
200-- IGO Jim .9*
06

-200 0 2i0 4 i0 60
100- Ammonium
80--

z 60--

40..

z
W 20--
0
z -CL
0 -A.
U

-20
0 20 40 60

140-- Nitrifta

100--

60-- N
20-- Ne
Cr

-20 i
0 20 40 60

DAYS
Figure 2. Nitrate, ammonium and nitrite concentrations (gM) in the experimental tanks
during the growing period.

17@ 0-0 Low P
High P

V)
(A ------------------
< 10__

T
< 5--

0-0 Low P
4-- High P

(A 3--
V)

E-n 2-..-

1 pe

0-
0 40 80 120 160

NITROGEN AMENDMENT (/,Lmol/1)

Figure 3. Total aboveground (a) and belowground (b) biomass (g dry weight) of
Potamogeton pectinatus over the range of water column nitrogen concentrations.

25-.

0-0 Low P
4-J a
20-- High P
T

V)
V)
<
:2
C
M 10--

<
5

A.; 0-0 Low P
High P

2--

0
M

M 0

0
40 80 120 160

NITROGEN AMENDMENT (jLmol/l)

Flutire 4. Total aboveground (a) and belowground (b) biomass (g dry weight) of Ruppi(l
CD
//i I

Inaritima over the range of water column nitrogen concentrations.

25-

171

20-- 0-0 Low P
L
High P
(n 15--
V)

10__
M
CD

T
0
5 - - - - - -

171
i-j 0-0 Low P
3:
"ON 4-- High P
C

C',
V) 3-- /*
V)
< T
0 T1*
Co 2Jk
U
M
0

0
0

0
0 40 80 120 160

NITROGEN AMENDMENT (ALmolll)

Figure 5. Total aboveground (a) and belowground (b) biomass (g dry weight) of
Potamogeton perfoliattis over the range of water column nitrogen concentrations.

Tabl e 2. Ash free dry weight (g) of experimental plants grown with high or low sediment phosphorus
over a range of water column N concentrations.

Species

N addition P. perfoliatus P. pectinatus Ruppia maritima
(jumol I-,)

High P Low P High P Low P High P Low P

0 3.32 2.42 2.19 2.27 4.55 3.56

10 3.04 1.98 1.76 1.72 5.10 3.58

20 3.97 1.50 4.16 4.43 8.13 6.97

40 2.94 2.70 6.07 5.77 13.08 9.25

80 2.37 7.11 7.34 8.77 14.60 9.01

160 5.96 3.22 4.85 8.28 9.81 9.74

Ref. ashfree.tab

Chl a concentrations (Figure 6) remained below 5 gg 1-1 throughout the
fertilization period. They rose to 15 and 20 gg 1-1 in the 40 and 160 tanks, respectively,

late in the study, but fell off again shortly thereafter. Nutrient samples from this period

which are still being analyzed should explain this brief excursion. Epiphyte loadings on

the plants ranged from a low of 0.002 gdw/gdw plant on Potomogeton pectinatus to 0.836

gdw/gdw plant on P. perfoliatus. These values are in the lowest range of those reported

by Staver (1984), who found epiphytes ranging from 0.26-3.10 gdw/gdw of plant

material. Furthermore the highest values in our study, which occurred on P. perfoliatus,

were inflated by the presence of CaCO, on the leaves.
Thus increases in N alone appear to cause increases in SAV biomass, provided P

is available in the sediment and is low enough in the water column to preclude algal

growth. The question remains, then, what caused the levelling off of biomass in our

experiment at high nitrogen concentrations? Actual water column nitrogen

concentrations suggested that some factor other than nitrogen limitation was responsible

for preventing further increases in growth. Nitrogen accumulated to concentrations of

several hundred AM toward the end of the experiment under the highest N treatments

(Figure 1a), showing that N uptake was not keeping up with inputs. Tissue N

concentrations of all three species increased with water column N enrichment over the

range tested, but increases slowed considerably above the 40 AM N treatment (Figure

7). This corresponds well with actual water column N values, which increased markedly

above the 40 AM N treatment (Figure 1). It can be inferred from this data that N was

limiting in the control through 40 AM N treatments, but not at higher levels of
enrichment (which correspond to actual water column N concentrations exceeding 100

40-- 0-0 C) AM
1 OAM
30-- 20 AM
,L- -A 40 AM
z3L 20-- BOAM
160AM

10--

0--

-10
0 20 40 60 80 100

DAYS

Fi,,Ltre 6. Chlorophyll a concentrations (gg 1-1) in the experimental tanks during the
C
growing period.

LOW PHOSPHORUS

3-5

2.5

0
Q: 2
t
z

1.5

0.5

0 7- -7
0 20 40 60 80 100 120 140 160

NITROGEN (-jM)
0 P. perfoliatus + Ruppio 0 P pectinatus

HIGH PHOSPHORUS

3.5

2
-5

1.5

0,5

0
0 20 6'0 so 100 120 140 160

NITROGEN (uM)
C P perfoliatus + RuPDIG 0 P. pecunatus

Figure 7. Tissue nitrogen concentrations (%) in the species tested under low (a) and
higli (b) sediment phosphorus conditions over the range of water column nitrogen
concentrations.

uM from day 19 and 30 for the 80 gM and 160,4M treatments, respectively). These

data agree with the results of Van Wijk (1989) which indicate that a N concentration of

57 gM N03 (the maximum tested) was limiting to P. pectinaw growth under lab

conditions. However, Gerloff and Krombholz' (1966) critical value for tissue N, 1.3%,

was exceeded in all but the control plants (low P) and control and 10 4M plants (high

P), suggesting that this value is not valid for the plants studied here.

Several factors could have contributed to limit growth at the highest N levels,

including self shading, which occurs when stem density becomes very high; carbon

limitation, which occurs when high densities of plants are grown in confi ned systems; or

a shortage of micronutrients resulting from plant uptake. Self shading undoubtedly

occurred toward the end of the study when plant densities were at their maximum. In

addition, pH shifts above 9 were recorded in the latter half of the study during mid-day

despite our attempts to minimize carbon limitation by bubbling the tanks continuously

with ambient air; therefore carbon limitation may have come into play. Micronutrient

limitation was probably not an issue due to the small demand by aquatic plants

compared to the large volume of water in which they were grown.

Belowground (BG) biomass trends reflect the different reproductive strategies of

Potomogeton pectinaw and Ruppia mafitima (Figure 3b and 4b). While Ruppia

belowground biomass shows little apparent influence of N concentration, P. pectinatus

BG biomass increases up to 80 gM N at both P concentrations. Presumably this is dL]e

to a buildup of tubers in the sediment (although tubers were not counted), which results

from enhanced nutritive status at higher N concentrations.

Ruppia overwinters as both seed and rhizomes. While overwintering rhizomes

probably have a head start over seeds in the spring, they are more susceptible to the

ravages of the weather, which can be quite severe in the shallow embayments where

Ruppia typically occurs. Winter winds create wave action which can dislodge the

rhizomes, which are generally located within the top few centimeters of sediment, and

low winter tides can cause exposure of sediments resulting in freezing or dessication.

Ice scouring can also occur, also causing disruption of surface sediments containing

rhizomes. Seeds of this species are particularly resilient to harsh conditions (compared

to soft seeds like Zostera marina which can be easily damaged -R. Orth pers. com. 1989),

making reproduction by seed essential to help prevent possible winterkill.

In contrast, P. pectinatus is extremely plastic in its reproductive strategies and the

allocation of resources into tubers or seeds is heavily influenced by environmental

conditions (Kautsky 1987). It often relies on tubers located up to 10 or more cm deep

in the sediment, making them less susceptible to the problems associated with winter

weatherthan the shallow Ruppia rhizomes. In our study seed production generally

app eared to be much lower in P. Pectinatus than in Ruppia, although we did not quantify

either seed or tuber production.

There was no apparent trend in the response of P. perfoliatus growth to increasing
N concentrations at either P concentrations (Figure 5a). There was a large build-up of
calcium carbonate (CaC03) on the leaves of P. perfoliatus in all tanks, and a comparison
of dry weight to ash free dry weight shows the exaggerating effect that this material can

have on biomass estimates (Table 2).

Calcium carbonate deposition occurs when the leaves of P. perfoliatus become

polarized during photosynthesis, creating micro-zones of very high pH close to the upper

surfaces of the leaves (Prins and Elzenga 1989). Increased temperature at the upper

leaf surfaces, which occurs when sunlight irradiates them, also helps create conditions

favorable for CaC03 precipitation.

Patterns of belowground biomass were similar to AG biomass in P. perfoliatus

(Figure 5b). As- with P. pectinatus, tubers are an important method of reproduction

(Stevenson and Confer 1978) and their production is directly related to AG biomass

(Goldsborough and Kemp 1988). Therefore the similarity is expected.

The response of AG plant growth to the two sediment P concentrations varied

with species (Figures 3a, 4a and 5a). Ruppia growth was consistently higher at high P

concentrations, while P. pectinatus growth was similar up to 40 AM N, but was greater

under low P concentrations at higher N concentrations. This suggests that Ruppia was P

limited at high N concentrations, while P. pectinatus was not. P. perfoliatus growth

showed no clear trend with regard to sediment P.

Tissue P concentrations (Figure 8) were highest in P. pectinatus, lowest in P.

perfoliatus, and were not distinctly different between the two sediment P concentrations

except in P. pectinatus at the highest N concentrations. Mean tissue P concentrations in

Ruppia, P. pectinatus, and P. perfoliatus were 0.17 %, 0.22 % and 0.15 %, respectively,

which indicates no P limitation based on Gerloff and Krombholz' (1966) critical tissue

concentration of 0.13 % and Van Wijk's (1989) 0.15 %.

Tissue carbon concentrations were consistent within each species over all N and P

treatments (Figure 9). A rather large difference existed, however, between species, with

POTAMOGETON PECTINATUS

0-110

0.25

0 0.20

0
0.15

0.10

0. 0 0
0 10 20 40 ao 160
L- PNMZOCEN "i'@, P

RUPPIA MARITIMA

0.35

0,30 -

0.25 -

0.20 -
0

0.,5 -

0.10 -
x

0.05 \,N
x
0.00 -
0 10 20 40 80 60
'EN -iO P
P

POTAMOGETON PERFOLIATUS

0.35

0.30

020

F-7711

0.'0

'N@

0 '0 4()
L.. 11"IMOCIEN @;qh P

FOUre 8. Tissue phosphorus concentrations in Potamogeton pectinatus (a), Ruppia
X

M(WItIlIza (b) and Potaniogeton perfoliatus (c) grown under low and high sediment
phosphorus concentrations over the range of water column nitrogen concentrations.

LOW PHOSPHORUS

45 -

40 -
X
35 -

3 0

M
/X
25

2 0
Ax,
x,
15 -

0 10 -.0 :30 50

@.JTPCGEN (uM)
P. perfolictus LL@j RUPPIC P. pectinctus

HIGH PHOSPHORUS

45 -

.40 -

30)

0 0 -0 8 0 i6o

tNMOGEN (uM)
P. perfoliatus Ruppla P. pectinatus

171@,Lire 9. Tissue carbon concentrations of the species tested Linder low (a) and hioh
A\
X N

(h) sediment phosphorus concetrations over the range of water COILImn nitrogen
conce ntra lions. 21

P. perfoliatus C concentrations being, lower than the other two species under all

treatments. P. pectinatus and Ruppia were similar under all treatments, ranging between

35 and 40 % C.

The generally lower tissue C levels in P. perfoliatus may be attributable to less
efficie.ni bicarbonate uptake by this species., As plant biomass increased in the tanks,

pH was driven up during daylight hours due to photosynthesis. At pH 9, which was

recorded on sunny days, most dissolved inorganic carbon (DIC) is in the form of HCO 3*

The inability to use HC03 efficiently could lead to depressed tissue C concentrations.

An interesting aside in this experiment was buildup of nitrite (NO 2) in the water

column (Figure 2). Ammonium nitrate was added, but immediately following the onset

of fertilization nitrite appeared in the tank receiving 160 gM N and to a much lesser

extent. in the 80 gM tank. Ammonium was apparently being oxidized to nitrite more

quickly than it could be assimilated by the plants in those tanks, while at lesser dosing

rates ammonium uptake equalled inputs.

MANAGEMENT IMPLICATIONS

The debate over whether N or P is limiting in coastal waters has been going on

since Redfield's (1958) work. As Ryther pointed out (1959) this is not only an academi c

question, and it has recently become important in efforts to manage nutrient inputs for

the benefit of the Bay's living resources. Data from the Choptank River (Stevenson et

a]. 1989) has suggested that for the mesohaline to oligohaline portions of Chesapeake

Bay tributaries, both nutrients can be limiting to algal growth at different times of the

year and in different parts of the river. The variability in which nutrient is limiting is

due in part to the source of inputs, with N entering the estuary primarily by diffuse

source loading from agricultural land and residential sewage while P is entering

primarily from sewage treatment plants (STP) (Fisher 1988).

In contrast to the Choptank Estuary, the tidal fresh section of the Potomac River

and the mouth of the Susquehanna near Havre de Grace, MD are characterized by high

N and low P inputs, leading to. P limitation in the water column most of the year. Both

of these areas support large and healthy populations of submersed aquatics.

Low water column P concentrations were achieved in the Potomac through

reductions in effluent P concentrations at the Blue Plains STP (Carter and Rybicki

1986). The resurgence in SAV following those reductions imply that they were crucial in
reversing the competitive balance so that SAV were favored over algal species.
Although management of tributaries like the Choptank may be based on similar

objectives, there has been debate over the possibility that comparable conditions in

brackish water would not lead to the same results.

Our study was designed to assess the impact on SAV of reduced water column P

concentrations over a range of N concentrations. We have dembnstrated here the

effectiveness of keeping P inputs to the water column low (basically only diffusion in this

experiment) in limiting algal growth in brackish water. When N inputs exceeded SAV

requirements, the excess N was not taken up to any large extent by algae because their

production was limited by P. In fact, without the presence of a large pool of phosphorus

in the water column, SAV growth is enhanced by nitrogen enrichment, up to a point.

The nitrogen "saturation" point, the concentration above which further increases

in SAV biomass are small or zero, can also be determined from the data collected here.

We found that above actual water column N concentrations of about 100 gM there was

little increase in AG biomass. Thursby (1984) and Dennison et al. (1987) found similar

saturation points for Ruppia mmitima and Zostera marina, respectively. Thus N loading

rates which produce concentrations exceeding 100 pM are not beneficial to SAV.

Based on the data produced here, it appears that reductions in phosphorus inputs

would be an efficient way to restore water quality to a level which will support healthy

SAV populations. Since P inputs in the Choptank are dominated by point sources which

can be cost effectively removed by precipitation, P reductions could be achieved more

readily than N reductions, which will involve a more complex array of strategies to deal

with the various sources, and may not ultimately be as important to the submersed

aquatics. In the particular cases of estuaries on the eastern shore of Chesapeake Bay
which have extremely high N concentrations, special attention should be paid to P
removal. This underscores the necessity for advanced P treatment at STP receiving

wastewater from rapidly growing towns such as Easton which has already impacted SAV

growth in the Choptank Estuary.

REFERENCES

Carter, V. and Rybicki, N., 1986. Resurgence of submersed
,aquatic macrophytes in the tidal Potomac River, Maryland,
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