www.elsevier.com/locate/jembe

Journal of Experimental Marine Biolo

Macroalgal-mediated transfers of water column nitrogen to

intertidal sediments and salt marsh plants

Katharyn E. BoyerT, Peggy Fong

Department of Organismic Biology, Ecology and Evolution, 621 Charles E. Young Drive, South, University of California, Los Angeles,

Los Angeles, California 90095-1606, USA

Received 28 May 2004; received in revised form 1 December 2004; accepted 5 January 2005

Abstract

In many temperate estuaries, mats of opportunistic macroalgae accumulate on intertidal flats and in lower elevations of salt

marshes, perhaps playing a role in linking water column nitrogen (N) supply to these benthic habitats. Using a flow-through

seawater system and tidal simulator, we varied densities (equivalent to 0, 1, 2, or 3 kg m�2 wet mass) of 15N-labelled

macroalgae (Enteromorpha intestinalis) on estuarine sediments in microcosms with/without pickleweed (Salicornia virginica)

to assess N transfers from algae. In the 6-week experiment, macroalgal biomass increased from initial levels in the lower density

treatments but all algae lost N mass, probably through both leakage and decomposition. With all densities of algae added,

sediments and pickleweed became enriched in 15N. With increasing mat density, losses of algal N mass increased, resulting in

stepwise increases in 15N labeling of the deeper sediments and pickleweed. While we did not detect a growth response in

pickleweed with macroalgal addition during the experiment, N losses from algal mats that persist over many months and/or

recur each year could be important to the mineral nutrition of N-limited marsh plants. We conclude that N dynamics of intertidal

sediments and lower salt marsh vegetation are linked to the N pools of co-occurring macroalgae and that further study is needed

to assess the magnitude and importance of N transfers.

D 2005 Elsevier B.V. All rights reserved.

Keywords: Enteromorpha; N isotopes; N limitation; Salicornia; Salt marsh

0022-0981/$ - see front matter D 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.jembe.2005.01.005

T Corresponding author. Current address: Romberg Tiburon

Center for Environmental Studies and Department of Biology, San

Francisco State University, 3152 Paradise Drive, Tiburon, California

94920, USA. Tel.: +1 415 338 3751; fax: +1 510 558 0167.

E-mail address: katboyer@sfsu.edu (K.E. Boyer).

1. Introduction

Many studies have documented algal-mediated

linkages between nutrient dynamics of the water

column and sediments in the subtidal zone of

estuaries. For example, phytoplankton and macro-

algae suspended in the water column influence

benthic nutrient processes through interception of

gy and Ecology 321 (2005) 59–69

K.E. Boyer, P. Fong / J. Exp. Mar. Biol. Ecol. 321 (2005) 59–6960

light and water column nutrients (Fong et al., 1993;

McGlathery et al., 1997; Hauxwell et al., 2001).

Macroalgae can also intercept nutrients fluxing from

the sediments (Bierzychudek et al., 1993) and

promote such fluxes by depleting water column

nutrients (Thybo-Christesen et al., 1993; Fong and

Zedler, 2000). In addition, macroalgae have been

found to release nutrients and organic matter to

underlying sediments when decomposing (Hansen

and Kristensen, 1997; Tyler et al., 2001), fueling

remineralization, and nutrient fluxes back to the water

column (Lavery and McComb, 1991; Hansen and

Blackburn, 1992; Enoksson, 1993).

While many of the same processes are at work

in the intertidal zone of estuaries, alternating

immersion and emersion of the sediment surface

may uniquely influence the relationship between

algae and benthic/pelagic nutrient exchanges. For

example, benthic microalgae at the surface of

intertidal flats access both the water column and

sediment nutrient supply when submerged, but

direct deposition of N from the air during exposure

can be several orders of magnitude higher (Paerl,

1997). Further, macroalgae in the intertidal become

desiccated when exposed at low tide, resulting in

reduction of photosynthesis, light attenuation within

the compressed algal mat, and inorganic carbon

limitation (Pregnall, 1983; Pregnall and Rudy,

1985). These stresses are likely to influence

nutrient dynamics both within and outside of the

algal mat.

Growth and accumulation of macroalgae on

intertidal flats are common in many estuaries through-

out the world, with percent cover up to 100%,

especially where anthropogenic N supply is pro-

nounced (e.g., Pregnall and Rudy, 1985; Sfriso et

al., 1992; Hernandez et al., 1997; Kamer et al., 2001;

Kennison et al., 2003). Opportunistic macroalgae,

typically species in the green (Division Chlorophyta)

genera Ulva, Enteromorpha, Cladophora, and Chae-

tomorpha, have rapid nutrient uptake, high growth

rates, and the capacity to store bexcessQ nutrients for

future growth (Rosenberg and Ramus, 1984; Fujita,

1985; Fong et al., 1994; Peckol et al., 1994). Such

bloom-forming macroalgae represent a large pool of

nutrients potentially available for release, and perhaps

more prone to be released in the stressful intertidal

zone.

Another distinguishing feature of the intertidal

zone of estuaries is the presence of emergent vascular

plants. In many locations, macroalgae can be found

growing or deposited among the bases of the stems of

these marsh plants at their lower elevations. One or

more species of macroalgae can be observed among

culms of smooth cordgrass (Spartina alterniflora

Loisel) in many areas of the US, including the coastal

bays of Maryland and Delaware (J. Gallagher, pers.

comm.), Jamaica Bay, New York (D. Franz, pers.

comm.), Bogue Sound, North Carolina (K. Boyer,

pers. obs.), and Waquoit Bay, Massachusetts (P.

Peckol, pers. comm.). In southern California, the

major bloom-forming genera Enteromorpha and Ulva

are found among Salicornia virginica L. (pickle-

weed), Salicornia bigelovii Torr. (annual pickleweed),

Spartina foliosa Trin. (Pacific cordgrass), Batis

maritima L. (saltwort), and Distichlis spicata L.

(saltgrass) (K. Boyer, pers. obs., H. Page, pers.

comm., Zedler, 1980).

While such associations may be fairly common in

the lower marsh, we know of only one study that has

explicitly examined the effects of macroalgae occur-

ring on sediments among salt marsh plants. Gerard

(1999) found lower biomass of S. alterniflora in plots

where the brown alga Ascophyllum nodosum Scorpi-

odes was removed experimentally; algal nutrient

releases to sediments upon decomposition were

suggested, but not assessed, as the mechanism for

enhancement of cordgrass growth.

If macroalgae release substantial quantities of

nutrients in the intertidal zone, then enhancement of

the nutrient supply to intertidal sediments and the

plants rooted therein is plausible. In view of the fact

that lower marsh plants are typically N limited (e.g.,

Broome et al., 1975; Boyer et al., 2001), access to

water column N formerly sequestered in algal tissues

could be important to marsh plant mineral nutrition

where macroalgae co-occur. However, flooding tides

may wash algal-derived nutrients out of the intertidal

zone, limiting their accumulation in sediments and

their uptake by associated vascular plants. Further,

algal mats in the intertidal may initiate nutrient fluxes

from sediments as can occur subtidally (e.g., Lavery

and McComb, 1991), reducing sediment nutrient

pools. Even if N is being supplied to the sediments

by algal mats, if algal decomposition depletes oxygen

and leads to accumulation of hydrogen sulfide in

K.E. Boyer, P. Fong / J. Exp. Mar. Biol. Ecol. 321 (2005) 59–69 61

sediments, marsh plant productivity may be reduced

(Koch et al., 1990).

We are particularly interested in the fate of macro-

algal N in tidal flats and marshes dominated by coarse

sediments. Where such habitats are created for

restoration purposes, sediments are often sandy due

to terrestrial origin or as a product of dredging (Langis

et al., 1991; Craft et al., 1999). Coarse-grained

sediments can also be found in marshes and tidal

flats in geologically young systems such as those that

form along barrier islands (Tyler and Zieman, 1998),

in areas of estuaries where storms episodically deposit

sandy terrestrial or dune/beach-derived sediments

(Osgood and Zieman, 1993, 1995), or in areas where

there are higher velocity flows (e.g., Bolam et al.,

2000). Sandy sediments lack the nutrient adsorption

capacity of clay particles, and store and supply N

poorly (Brady and Weil, 1999). Therefore, processes

that supply N from external sources, e.g., advective

flux of enriched tidal water through sediments

(Osgood and Zieman, 1998), can be especially

important to nutrient dynamics of coarse sediment

systems. Macroalgal mats on sandy tidal flats could

similarly act as an external N source.

The objective of our study was to investigate the

potential role of macroalgal mats in influencing N

supply to intertidal sand flats and salt marsh habitats.

We hypothesized that algal mat density influences

algal N transfers, with thicker mats declining more in

underlying layers and releasing a greater pool of

nutrients than thinner mats. We used Enteromorpha

intestinalis Link isotopically-enriched in N to assess

N transfers to sediments and S. virginica tissues.

2. Methods

We performed a 4�2 factorial experiment varying

E. intestinalis (hereafter called macroalgae) mat

density (0, 1, 2, or 3 kg macroalgae m�2, see below)

and presence/absence of S. virginica (hereafter called

pickleweed) in pots subjected to simulated tides (Fig.

1). In April 2000, we collected sediment and

pickleweed from a sandy restoration marsh at Mugu

Lagoon in Ventura County, California. Sediments

were collected from an unvegetated slope at a point

just below the highest tidal elevation. Lower on the

slope, a trowel was used to collect pickleweed

recruits and soil surrounding their roots (~4 cm

diameter, 4 cm deep).

Sediments were added to nursery pots and half were

planted with pickleweed. Five extra pots were planted

for later analysis of initial conditions. To acclimate

plants, pots were set into pans of low nutrient (b3.57

AM NO3, b1.61 AM PO4) seawater in a greenhouse at

the University of CA Los Angeles for three weeks; de-

ionized water was added to counter evaporation.

Macroalgae were also collected at Mugu Lagoon,

cleaned of visible fauna and debris, and cultured in

low nutrient seawater to equalize internal nutrient

stores. After 1 week, 15N-labelled NaNO3 was added

to produce a 200 AM culture solution, to simulate

moderately enriched conditions common to southern

CA estuarine waters (concentrations can exceed 800

AM in upper Newport Bay (Boyle et al., 2004) and

3000 AM at Carpinteria marsh, near Santa Barbara

(Kennison et al., 2003)). Two more equivalent nitrate

additions were made over the next 2 weeks, along

with deionized water to counter evaporation.

The 6-week experiment was conducted outdoors

at the Redondo Beach Generating Station in

Redondo Beach, CA, from early May to mid-June

2000. Algal tissue was spun for 1 min in a lettuce

spinner, then divided into 20, 40, or 60 g portions

(equivalent to 1, 2, or 3 kg algae m�2) to reflect the

range in densities typical of estuaries in southern

CA and elsewhere where algal blooms occur (e.g.,

Pregnall and Rudy, 1985; Hernandez et al., 1997;

Kamer et al., 2001; Tyler et al., 2001). Algal tissues

were spread evenly on the sediment surface and the

pots were placed in a random array on a table. With

5-fold replication, there were a total of 40 pots, each

independently plumbed with seawater using drip

irrigation tubing and emitters (Fig. 1B). A swim-

ming pool timer controlled a pump to simulate

twice-daily tides of equal length. Pots were

scrubbed weekly to reduce algal/bacterial films,

then re-randomized.

At the start of the experiment, water samples were

collected from the seawater system, filtered through

Whatman GF/C filters, and frozen for later analysis of

total Kjeldahl N (TKN), NH4–N, NO3–N, and total P

at the University of CA, Davis Division of Agriculture

and Natural Resources (DANR) laboratory using

standard methods. As the TKN method does not

digest N from oxidized forms (e.g., nitrate and nitrite),

Fig. 1. (A) Schematic of experimental design with four levels of algal addition (0–3 kg m�2) and presence/absence of pickleweed in pots of

sediments. (B) Cross-sectional view; water entered and exited the outer pots through irrigation tubing and emitters, and the inner pot through

holes (covered with mesh to retain sediments) in the bottom. Wooden blocks elevated the shorter inner pots such that water did not reach their

top edges, and so that the water level dropped below the inner pots at simulated low tide. Pump timing permitted approximately equal periods of

submergence and exposure of the sediment surface.

K.E. Boyer, P. Fong / J. Exp. Mar. Biol. Ecol. 321 (2005) 59–6962

dissolved organic N was calculated by subtraction of

NH4–N from TKN. Water nutrients (Table 1) were

extremely low compared to levels in southern CA

estuaries (Fong, 1986; Fong and Zedler, 2000; Boyle

et al., 2004; Kennison et al., 2003).

Initial sediment subsamples were dried (55 8C),ground, and sieved (1-mm sieve), and grain size

distribution, organic matter, and total N and P

concentration were determined at DANR (Table 1).

The high proportion of sand particles was comparable

to that of other marshes formed naturally on sandy

deposits or newly constructed from dredged material

(Osgood and Zieman, 1993, Boyer and Fong,

unpublished data). There had been little accumulation

of organic matter in these sediments (Table 1;

compare to 0.8–3% in sandy marshes N10 years old,

Osgood and Zieman, 1993; Craft et al., 1999). As

expected for coarse sediments, N concentration was at

the low end of the range for low elevation marshes

(often greater than 0.2% dry mass and as high at

0.8%; reviewed in Boyer et al., 2001), but similar to

other constructed and natural coarse grained sites

(Boyer and Zedler, 1998; Craft et al., 1999). Total P

was in the middle of the range found in lower

elevation marshes (0.01–0.14% dry mass: Mendels-

sohn and Marcellus, 1976; Ellison et al., 1986;

Table 1

Initial conditions of (A) incoming seawater for the tidal simulator,

(B) sediments, and (C) macroalgae after culturing, and pickleweed

A. Water (all lM)

DONa 24.29 (3.27)

NO3–N 7.00 (1.24)

NH4–N 7.14 (1.24)

Total P b1.56b

B. Sediments

Total N (% dry mass) b0.02b

Total P (% dry mass) 0.05 (0.00)

Organic matter (%) 0.05 (0.00)

Sand (%) 96 (0)

Silt (%) 1 (0)

Clay (%) 3 (0)

C. Macroalgae and pickleweed

Pickleweed

Macroalgae succulent roots

Total N (% dry mass) 3.41 (0.19) 1.22

(0.08)

1.02

(0.15)15N (atom%) 5.89 (0.17) 0.3704

(0.0002)

0.3695

(0.0002)

y15N (x) N/A 11.1

(0.4)

8.7

(0.6)

Total P (% dry mass) 0.33 (0.01) 0.11

(0.01)

Not

measured

Values are means (S.E.); n=5.a Dissolved organic nitrogen.b Below this detection limit.

K.E. Boyer, P. Fong / J. Exp. Mar. Biol. Ecol. 321 (2005) 59–69 63

DeLaune and Pezeshki, 1988; Craft et al., 1999;

Boyer et al., 2001).

Prior to the experiment, samples of the enriched

algae were rinsed in de-ionized water, dried (60 8C),and ground. Stable isotope ratios of nitrogen were

measured by continuous flow isotope ratio mass

spectrometry at the UC Davis Stable Isotope Facility.

Sample peak areas and isotope ratios were compared

to those of standard samples for calculation of atom%15N. Samples were also analyzed for total N and P

(Table 1). Algal tissue N was at the middle of the

range reported elsewhere for Enteromorpha and Ulva

spp. (~1–5%; Soulsby et al., 1985; Hernandez et al.,

1997; Kamer et al., 2001; Tyler et al., 2001).

Pickleweed from 5 randomly selected pots was

clipped at the sediment surface, rinsed in deionized

water, and sampled for initial conditions. Aboveground

tissues at the start of the experiment were almost

entirely bsucculentQ (see end of experiment methods

below). Sediments were rinsed from the roots over a 1-

mm sieve. Tissues were dried (60 8C), weighed,

ground, and analyzed for 15N and total N; succulent

tissues were also analyzed for total P (Table 1).

Pickleweed N and P concentrations were similar to

those found in other studies (0.9–2.5% N, 0.15–0.17%

P: Sullivan and Zedler, 1999; Boyer et al., 2001; Boyer

and Fong, unpublished data). For the experimental

pickleweed replicates, we recorded initial branch

number and lengths and calculated total branch length

per pot (grand mean=178.4F18.1 cm).

At the end of the experiment, redox potential (Eh)

was measured at 2- and 6-cm sediment depths (two

replicates each, averaged) using platinum electrodes.

The potential of a calomel reference electrode against a

standard hydrogen electrode (+245 mV) was added to

each measured value.

Algae were removed, rinsed, spun in a lettuce

spinner for 1 min, weighed, dried at 60 8C, and

reweighed. Dried samples were processed and ana-

lyzed for 15N (as atom%) and total N. The mean

wet:dry mass ratio (9.3) was used to estimate initial dry

mass of algae, which was used to calculate the initial N

mass as %N�100�1�dry mass. Final dry mass and N

content were used to calculate percentage change in N

mass from initial values. Initial and final 15N mass

were calculated as atom% 15N�100�1�N mass and

used to calculate percentage change in 15N mass.

Sediments cores (2.5-cm diameter�6 cm deep)

were subdivided into 0–2 cm and 2–6 cm sections,

dried, weighed, ground, and analyzed for 15N, total N,

and total P. Soil salinity was estimated using a

refractometer on saturated soil pastes prepared from

dried soils. Organic matter (loss on ignition) was

determined for the 0–2-cm depth; deeper sediments

were not expected to change in 6 weeks.

Final pickleweed branch lengths and numbers were

recorded and tissues were divided into bsucculentQ(fleshy tissue on stems) and bwoodyQ (older stems

over which the fleshy covering had senesced and

shed). Roots were rinsed of sediments, and all tissues

were analyzed as for the initial samples.

Two-factor ANOVAwas used to test for the effects

of algal density, presence/absence of pickleweed, and

interaction. When one factor was not significant, mean

squares were pooled and re-evaluated through one-

factor ANOVA. One-factor ANOVA was also used to

compare algal treatment effects on N and P concen-

tration in pickleweed tissues. Following a significant

0.5

1

1.5

2

2.5

3A

conce

ntr

atio

n (

atom

%)

p = 0.0001

a

b

bb

K.E. Boyer, P. Fong / J. Exp. Mar. Biol. Ecol. 321 (2005) 59–6964

one-factor ANOVA, multiple comparisons were made

using Fisher’s PLSD method. Data not meeting

assumptions of ANOVA after transformation were

analyzed using the non-parametric Kruskal–Wallis (K–

W) test. When one factor was not significant, mean

squares were pooled for analysis by K–W or Mann–

Whitney U tests. Non-parametric multiple compari-

sons by simultaneous test procedure followed signifi-

cant K–W tests. All errors presented are F1 S.E.

0

0

0.5

1

1.5

sedim

ent

15N

0 1 2 3

kg algae m-2

B

p = 0.0001

ba

c

c

Fig. 3. 15N (atom%) of soils at (A) 0–2 cm depth and (B) 2–6 cm

depth, with p values from Kruskal–Wallis tests on pooled

(F pickleweed) data and results of simultaneous test procedure

indicated by letters (bars with shared letters are not different).

3. Results

3.1. Changes in macroalgae

There was no effect of pickleweed presence on any

of the algal response variables. During the 6-week

experiment, wet mass of macroalgae increased from

initial values in the 1- and 2- kg algal density

treatments (Fig. 2A), by 39 and 6%, respectively. In

contrast, the 3-kg treatment decreased in algal wet

mass by 33%. We observed small patches of bleached

tissue in all the treatments; these were most evident on

the underside of mats in the 3-kg treatment.

A

-8

-6

-4

-2

0

chan

ge in

15N

mas

s (m

g po

t-1)

1 2 3

kg algae m-2

D

a

b

c

p = 0.0001

-150

-100

-50

0

chan

ge in

N m

ass

(mg

pot-1

)

C

p = 0.0001

a

b

c

-30

-20

-10

0

10

chan

ge in

wet

mas

s (g

)

b

b

a

p = 0.0001

B

-2.5

-2

-1.5

-1

-0.5

0

chan

ge in

N c

once

ntra

tion

(% d

ry m

ass)

1 2 3

kg algae m-2

p = 0.0010ab

b

Fig. 2. Change from initial macroalgal (A) wet mass, (B) N

concentration, (C) mass of N, and (D) mass of 15N. Panels B–D are

based on dry mass. p values are shown from pooled (Fpickleweed)

analysis by one-factor ANOVA, with results of multiple compar-

isons indicated by letters (bars with shared letters are not different).

N concentration in algal tissues declined during the

experiment in all treatments (Fig. 2B). The greatest

decrease (68%) occurred in the 1-kg treatment, at least

partly due to dilution of N over increased mass. The 2-

and 3-kg treatments decreased significantly less, by 56

and 50%, respectively (not significantly different from

each other). The N pool in the algae (N mass)

decreased in all algal addition treatments (Fig. 2C).

Both mass of N and mass of 15N in the algae (Fig. 2D)

decreased significantly more with each increasing

level of initial algal density. Algal 15N mass decreased

by 2, 4, and nearly 8 mg/pot in the 1-, 2-, and 3-kg

treatments, respectively.

Algal 15N concentration increased in all density

treatments, suggesting a preferential loss of the lighter

isotope, 14N. Isotopic enrichment of macroalgae was

similar (~14%) regardless of mat density, increasing to

6.7F0.1 atom% (grand mean) from an initial value of

5.9 atom%.

3.2. 15N transfer to soils and plants

The presence of pickleweed had no effect on

sediment 15N concentration (atom%) at either sediment

depth (Fig. 3). With all densities of algae added, 15N

concentration at the surface (0–2 cm) was 4� greater

K.E. Boyer, P. Fong / J. Exp. Mar. Biol. Ecol. 321 (2005) 59–69 65

than natural abundance levels (0-kg treatment) (Fig.

3A). There was no significant difference in sediment15N concentration in the 1- to 3-kg treatments. 15N

levels at depth (2–6 cm) were 2� lower than at the

surface, but increased significantly with increasing

algal density (Fig. 3B). 15N ranked highest in the 3-kg

treatment, but was not significantly different than in the

2-kg treatment.

There was a significant positive effect of algal

density on concentration of 15N (atom%) in all

portions of pickleweed tissues (Fig. 4). The stepwise

pattern of enrichment resembled that of the sediments

in the rooting zone (2–6 cm), and was opposite that of

algal N mass declines. In all portions, 15N concen-

tration of pickleweed in the highest density algal

treatment was 250–300% greater than in the 0-kg

treatment. Effects of algal density on pickleweed 15N

labeling were more pronounced in the succulent and

root tissues than in the stem tissues.

3.3. Pickleweed and sediment responses

Biomass of pickleweed succulent, woody, and root

portions did not differ by algal density treatment

(grand means=1.5F0.2, 1.0F0.1, and 1.9F0.3 g dry

mass pot�1, respectively), nor did total length or

number of branches. Total branch length increased by

22% overall from initial measures, although change

per individual plant was highly variable (range=�29

cm to 126 cm). The number of branches per pot

0

0.2

0.4

0.6

0.8

1

1.2

1.4

15N

con

cent

ratio

n (a

tom

%)

210 3

kg algae m-2

roots

woody

succulent p = 0.0008

p = 0.0018

p = 0.0020

Fig. 4. 15N concentration (atom%) of pickleweed tissues, with p

values from Kruskal–Wallis tests. Non-parametric multiple compar-

isons were not conducted (n=5; nN8 required).

stayed approximately the same despite the new

growth we observed on all plants.

Macroalgal additions had no effect on N concen-

tration of pickleweed tissues. Mean N levels dropped

from initial concentrations by 31% in succulent

tissues and by 13% in root tissues. P in succulent

tissues did not differ by algal treatment or change

from initial levels.

Pots with pickleweed had significantly higher soil

N concentrations at both the 0–2 and 2–6 cm depths

than pots without pickleweed (Fig. 5A,B). At the 0–2-

cm depth, there was no significant effect of algal

treatment. However, at 2–6 cm, soil N concentration

was significantly lower in the two higher density

treatments than in the 0- or 1-kg m�2 treatments

(PLSD, pb0.05). Sediment P did not differ by

treatment (grand mean=0.05F0.00%).

Organic matter in surface sediments was signifi-

cantly higher with pickleweed present (grand

mean=0.22F0.02%) than without (grand mean=

0.13F0.02%), reflecting sediment N patterns. Sedi-

ment organic matter did not differ by algal density

treatment.

Sediments at the end of the experiment ranged

from well oxygenated to moderately reduced (Patrick

et al., 1996). Redox potential at both the 2- and 6-cm

depths decreased significantly with pickleweed

present (Fig. 5C,D). At the 6-cm depth, the 3-kg

algae treatment led to more reduced conditions than

both the 0- and 1-kg treatments (PLSD, pb0.05),

while the 2-kg treatment had an intermediate poten-

tial. The presence of pickleweed appeared to influence

the degree to which macroalgal mats led to declines in

redox potential; however, we found no significant

interactions at either depth to support this.

The flow-through seawater system kept sediments

well flushed of salts. Grand means of soil salinity

were 34F1 ppt at the 0–2 cm depth and 31F0.5 ppt

at 2–6 cm depth. There were no differences by

treatment.

4. Discussion

The use of 15N as a tracer allowed us to document

that macroalgae play a role in transferring N from the

water column to sediments and vascular plants of the

intertidal zone. Algal N lost from all mat density

0

0.01

0.02

0.03

N c

once

ntr

atio

n (

% d

ry m

ass)

A

0

0.01

0.02

0.03

0 1 2 3

kg algae m-2 kg algae m-2

B

p = 0.0001

pic p = 0.0118

alg p = 0.0007

int p = 0.5635

0

100

200

300

400

Eh (

mV

)

C

+ pickleweed

- pickleweed

0

100

200

300

400

0 1 2 3

D

pic p = 0.0001

alg p = 0.0504

int p = 0.2586

pic p = 0.0080

alg p = 0.0027

int p = 0.1736

Fig. 5. Total N concentration of soils at (A) 0–2 cm depth (with p value from Mann–Whitney U test on pooled algal treatment data) and (B) 2–6

cm depth, with p values from two-factor ANOVA. Redox potential (Eh) of sediments at (C) 2-cm depth and (D) 6 cm depth, with p values from

two-factor ANOVA.

K.E. Boyer, P. Fong / J. Exp. Mar. Biol. Ecol. 321 (2005) 59–6966

treatments was discernible through isotopic enrich-

ment of sediments and pickleweed in our experimen-

tal units. Further, patterns of 15N labeling of both the

sediments and pickleweed suggest that algal mat

density is important to N transfers, with thicker mats

supplying a greater quantity of N.

While we hypothesized that N would be released

from decomposing algal tissue, our data suggest that

N entered the sediment/plant pool from live macro-

algae as well. Leakage of dissolved organic N can

occur during macroalgal growth (e.g., Tyler et al.,

2001; Fong et al., 2003), and thus may have

contributed to N releases in the 1- and 2-kg density

treatments, in which algae grew during the experi-

ment. The large decreases in algal biomass in the 3-kg

treatment suggest that decomposition was probably

most important as a mechanism for N loss in this

treatment.

Isotopic enrichment of the algal tissue in all

treatments during the 6-week experiment indicates

that the low nutrient supply of the seawater and

sediments could have supported little uptake of bnewQ

nitrogen as 14N to dilute the 15N concentration.

Several processes could have led to preferential loss

of the lighter isotope, 14N, and thus the enrichment of

macroalgal 15N during the experiment. Leakage

across cell walls is related to diffusion rates, which

may be faster for the lighter than heavier isotope

(Flynn and Berry, 1999). In addition, portions of the

algal mats undergoing decomposition, nitrification,

and denitrification could have been enriched in 15N

through preferential release of 14N associated with

these bacterial transformations (Macko and Ostrom,

1994). As isotopic enrichment of algae during the

experiment was similar across algal addition treat-

ments, it should not have influenced our ability to

detect transfers of N through isotopic enrichment of

sediments and pickleweed.

At both the surface and deeper sediment depths,

elevated 15N concentrations trace the path of algal N

into the sediment N pool. Especially great isotopic

enrichment of the surface layer may have been due to

proximity to the algal N supply, and perhaps to the

incorporation of isotopically-enriched macroalgal

K.E. Boyer, P. Fong / J. Exp. Mar. Biol. Ecol. 321 (2005) 59–69 67

detritus into the sediment surface layer. Ammonia

volatilization, which results in preferential release of14N (Wada et al., 1981), may have further enhanced15N concentration in the surface sediments. Condi-

tions suitable for ammonia volatilization were perhaps

provided by the heightened pH that can result in water

during algal photosynthesis and the position of the

NH4–N source (the algal mat) on the sediment surface

as it dried during simulated low tide (Brady and Weil,

1999). If so, this process was not differentially

affected by algal mat density, but rather led to

statistically similar isotopic signatures across algal

addition treatments.

With increasing macroalgal mat density, a striking

stepwise 15N enrichment of the deeper sediment zone

suggests that sediments intercepted a greater supply of

N from denser macroalgal mats. Our data are not

supportive of an alternative explanation for this

pattern, that denitrification beneath thicker mats led

to isotopic enrichment of sediments (Handley and

Raven, 1992). Redox potential fell to levels associated

with denitrification (b250 mV; Patrick and Jugsujinda,

1992) in some of the pickleweed pots with higher

density algal additions, but sediment Eh explained

little of the variation in 15N (regression on sediments

from pickleweed pots with 1–3 kg/m2 of algae; in

surface sediments, n=15, p=0.8740, R2=�0.002; at

depth, n=15, p=0.1290, R2=�0.168).

Pickleweed enrichment in 15N with Enteromorpha

present at all three densities suggests that N transferred

from this alga may contribute to the plant’s mineral

nutrition whenever algal mats occur on sediments in

close proximity. Increased isotopic enrichment with

increasing mat density provides evidence that a greater

quantity of N was available to pickleweed in the

presence of the thickest algal mats.

As macroalgal presence/density did not lead to

differences in pickleweed total branch length or tissue

N concentration, the contribution of macroalgal N to

pickleweed productivity over short time periods

remains uncertain. In this study, pickleweed declined

in N concentration due, in part, to growth, which

would have bdilutedQ N across biomass. However,

growth was highly variable among individuals, with

some even decreasing in total length. In a study at

Mugu Lagoon, Onuf (unpublished manuscript) found

that only ~30% of new branch tips survived for N3

months and that biomass loss by shedding of branches

could be up to 4� the amount of new biomass

produced. Greater sediment N and organic matter

where pickleweed was present further suggests that

the plant contributed detritus to the sediments. With

high variability in growth and uncertainty about tissue

losses, we are unable to adequately assess pickleweed

productivity or, therefore, to estimate the absolute

contribution of algal N to it.

In conclusion, this experiment provides evidence

that mats of an opportunistic green macroalga link

water column and benthic N pools through uptake of

water column N and transfers to underlying tidal flat

or vegetated saltmarsh sediments. Transfers of N

occurred from algal mats that were both growing and

declining in biomass, suggesting that macroalgae may

contribute N to sediments and marsh plants whenever

mats are present. In addition, the persistence of algal

blooms in the intertidal zone of estuaries over periods

of months may have a cumulative effect on N pools of

both sediments and emergent vascular plants, perhaps

over many years of recurring algal blooms. As has

been suggested previously for a US Atlantic coast

marsh (Gerard, 1999), the N dynamics of intertidal

sediments and marsh plants appear to be linked to the

N pools of co-occurring macroalgal mats, and further

study is needed to assess the significance of N

transfers.

Acknowledgements

We thank Larry Elkins for providing space at the

Redondo Beach Generating Station, Wayne Shipman

for help in developing the tidal simulator, and Karleen

Boyle for sampling assistance. Phil Rundel, Christy

Tyler, Rick Vance, Rich Ambrose, and anonymous

reviewers provided helpful comments. Funding was

provided by the United States Environmental Protec-

tion Agency (fellowship #U915399 to K. Boyer and

grant #R827637 to P. Fong), the Society of Wetland

Scientists, the California Native Plant Society, and the

American Association of University Women. [SS]

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