NOAA TECHNICAL REPORT SERIES OCRM/MEMD

BENTHIC PRIMARY PRODUCTION IN THE EELGRASS

MEADOW AT THE PADILLA BAY NATIONAL ESTUARINE

RESEARCH RESERVE, WASHINGTON

by

RONALD M. THOM

U.S. DEPARTMENT OF COMMERCE

NATIONAL OCEANOGRAPHIC AND ATMOSPHERIC ADMiNISTRATION

NATIONAL OCEAN SERVICE

OFFICE OF OCEAN AND COASTAL RESOURCE MANAGEMENT

MARINE AND ESTIJARINE MANAGEMENT DIVISION

WASHINGTON, D.C.

This work is the result of research sponsored by the U.S. Department of Commerce,National Oceanographic and Atmospheric Administration, National Ocean Service, Office

of Ocean and Coastal Resource Management, Marine and Estuarine Management Division,under contract NA86AA-D-CZ027.

Approved

F) ~

Date____________ _____________________ Robert C. Francis, Director
This report was prepared as an account of government-sponsored work and has been

approved for distribution. Approval does not signify that the contents necessarily reflect

the views and policies of the government, nor does mention of trade names or commercial

products constitute endorsement or recommendation for use.
Seattle, WA 98195

(G) NA86AA—D—CZ027

12. Sponsoring Organization Name and Address 13. Type of Report & Period Covered

NOAA/OCEN/MEND Padilla Bay N.E.R.R.

~. . Final Report 6/86—12/87 l82~ Connecticut Ave NW WA State Dept. of Ecology

Universal Building 1043 Bayview—Edison Rd. 14.

Washington, D.C. 20235 Mt. Vernon, WA 98273

15. Supplementary Notes

16. Abstract (Limit: 200 words)

The spatial patterns of standing stock and productivity of benthic autotrophic components (Zostera

marina and Zosterajaponica, epiphytic algae and benthic sediment associated algae) of the eelgrass

meadow in Padilla Bay, Washington, were studied seasonally for one year. Eelgrass extended from

approximately l.Om above MLLW down to subtidal depths. However, standing stock decreased at depthsbelow -0.5rn MLLW. Z. japonica formed a band at upper elevations, and mixed with Z. marina at about0.3m to 0.6m MLLW. Epiphytic algae were abundant in the meadow, and reached greatest standing stock

between 0.3 and -0.4m MLLW. Sediment associated algae were abundant at all elevations as indicated bychlorophyll a concentrations. The standing stock of all components varied seasonally. Sediment algae

reached peak abundance in early spring, followed by rapid growth of epiphytic macroalgae in early sum

mer. Eelgrass biomass was greatest in early to late summer depending upon depth. Annual net primary

productivity (NPP) for the system was 351 gC rn2. Epiphytic algae, Z. japonica and Z. marina wereresponsible for 50,2 and 48% of this production, respectively. NPP by sediment algae was negligible due

to high sediment associated respiration rates. Total annual production from the 3,000ha system wasapproximately 28,000mt dry plant matter.

17. Document Analysis a. Descriptors

eelgrass, primary production, standing stock, seasonal dynamics, epiphyte production, estuarine primary

production

b. ldentiflers/Open.Ended Terms

c, COSATI Field/Group

Security Class (This Report)

EllabliltY Statement 21. No ot Pages

OPTIONAL FORM 272 (4-77)

(Formerly NTIS—35)

Department of Commerce

(See ANSI—Z39,18) See Instructions on Reverse

-Il
Sampling Sites 5

Standing Stock Sampling 5

Productivity Measurements 6

Eelgrass 6Benthic Algae 7

System Productivity Calculations 8

RESULTS 9

Eelgrass Morphometrics 9

Density and Standing Stock 9

Eelgrass 9

Epiphytes 14

Sediment Microalgae 14

Productivity 14Production to Biomass 20

Comparison of Methods 25

DISCUSSION 26REFERENCES 31

ifi
4. Eelgrass mean density at three depths vs time 12

5. Eelgrass mean biomass vs depth 136. Eelgrass mean biomass at three depths vs time 15

7. Epiphyte mean biomass vs depth 16

8. Epiphyte mean biomass at three depths vs time 17

9. Mean sediment plant pigment concentrations vs depth 18

10. Mean sediment chlorophyll a concentration at three depths vs time 1911. NPPvsdepth 2112. Eelgrass shoot weight vs annual system NPP 24

iv
A

(q:J~J}~J) aoj ~qi Ai~mud si~onpoid w ~p Apnis s~is
viding equipment, logistical support and overnight accommodations during field trips. I

especially thank T. Stevens, Director of the Padilla Bay Reserve, for help and support.

R. Nakatani, T. Stevens and an anonymous reviewer provided helpful comments that

considerably improved the manuscript. M. Duke did fmal editing of the manuscript.

The work reported herein documents the distribution and production of the dominant

primary producers in the Reserve. This information is needed to understand the ecological

importance and contribution of the major primary producers within the reserve. The data

compliments work on the distribution of fish prey (C. Simenstad) and Dungeness crab (D.Armstrong) in the Bay, and research on the nutrient dynamics (S. Williams) and food web

support (R. Wissmar) on the flats. Research which documents the total disthbution ofseagrasses in the Bay (T. Mumford, B. Webber) will utilize the present data to calculate the

total primary production in the Bay. FY87 research focusing on primary production in the

remaining benthic habitats, i.e. mucifiats, gravel beds (R. Thom) and marshes (M. Burg),

will be coupled with the seagrass data to document the relative and absolute contribution ofbenthic plants to the food web of the Bay.

v
of autotrophic components of seagrass systems to nearshore food webs is proven in several

areas (e.g., Stoner 1980, Fry 1984). Further, the systems can act autonomously in terms

of production and consumption processes (Asmus and Asmus 1985). Seagrass systems

are utilized as habitat and nursery areas by many animals (e.g., Heck and Thoman 1984:

Heijs 1985: Virnstein and Curren 1986).

Although dominant and ecologically important, fundamental information on eelgrass

distribution and production is limited for the Pacific coast. Recently, Kentula and Mclntire

(1986) showed that Zostera marina L. in Netarts Bay, Oregon, exhibited a mean annual

production rate of 383 gC nr2. The total production from the 176 ha bed amounted to

3,200 mt y1. Of significance was the fact that the biomass was replaced 10 times during

the growing season, which indicated substantial export of organic matter to surroundinghabitats. Thom (1984) estimated that eelgrass accounted for over 50% of the annual

primary production in Grays Harbor, a coastal estuary in Washington.

Seaweeds and diatoms epiphytic or closely associated with seagrasses can form a sig

nificant proportion of the total primary production in the system. Penhale (1977) calculated

an 18% contribution by the epiphytic flora to the total productivity of the eelgrass system in

New Jersey. Kentula (1982) found that maximum standing stock of epiphytes on eelgrassin Netarts Bay exceeded seagrass leaf biomass by 2.3 times during spring. The high

importance of seagrass associated algae has been proven for one food web inhabiting

estuaries of Hood Canal, Washington (Simenstad and Wissmar 1985).

The purpose of this work was to describe the spatial patterns and seasonal dynamics of

standing stock and productivity of the major benthic intertidal plants in Padilla Bay (Fig.

1). An attempt was made to partition the production among the major components (i.e.,

two eelgrass species, seaweeds and sediment microflora), provide data from which

predictions of production could be made in unsampled areas, and calculate a first order

approximation of total annual primary production (i.e., the amount of carbon fixed by

plants during photosynthesis) in the Bay.
Vancouver

Island

PADILLA

BAY

Olympic Peninsula

Figure 1. Location of Padilla Bay.
dissected by shallow channels which drain the flats during falling tides. The mean tide

range and mean spring tidal range for the area are 1.55 and 2.56 m, respectively. Riverine

input is primarily from the Skagit River via Swinomish channel, and secondarily from

small sloughs draining local uplands. Salinity in the Bay ranges from 15 ppt to 30 ppt

depending upon river flow and tidal influences (Cassidy and McKeen 1986 andunpublished data). Water temperature varies from 7°C in December-February to as high as25°C in large poois of standing water on the flats during warm mid-summer (July) days.Inorganic nitrogen values are typically low throughout the year. Nitrate concentration rarelyexceeds 5 jiM and is usually less than 1 jiM. (unpublished data). Low nitrogen to

phosphorous ratios (range 3-6) exist in spring and summer in the water over the seagrass

meadow (Wissmar 1986).

The most conspicuous autoirophic component of the intertidal area is eelgrass, which

occupies approximately 3,500 ha, and represents one of the largest contiguous eelgrassmeadows on the Pacific coast of North America. The eelgrass species present in PadillaBay are Zostera marina and Zostera japonica Aschers. & Graebn. Z. japonica is an annual

species, and is believed to have been introduced to the region inadvertently with the

importation of oyster seed from Japan in the 1930s (Harrison 1976). This latter seagrass

species is generally smaller in size and occupies higher intertidal elevations as compared to

Z. marina (Harrison 1982a).
Figure 2. Padilla Bay and the study sites. The inset shows depths of sites relative to

MLLW. The line delineating channels and flats within the Bay indicates theapproximate seaward extent of the intertidal zone (-l.Om MLLW). The outline

of eelgrass is also approximate. Little information exists on the distribution ofeelgrass in the channels.
subtidal channel. The depths of the sites, relative to MLLW, were 0.8m, 0.6m, 0.5m,

0.3m, 0. im, -0. im, -0.2m and -0.4m. The site at -0.2m was located in the middle of a

shallow, narrow (ca. 15m wide) channel, and the site at -0.4m was positioned at the edge

of a wider channel that extended to subtidal depths. Water flowed relatively rapidlythrough these channels during falling tides. Preliminary observations showed that the siteswere representative of the range of seagrass standing stock and morphology in the Bay.Each site consisted of a lOm transect line positioned parallel to the edge of the water. The

sites were marked by wooden stakes placed at each end of the lOm line. Sampling was

carried out using 0.1 m2 quadrat placed at random points along either side of the line.

Density and Standing Stock Sampling

Eelgrass shoots within the quadrat were harvested by cutting at their base. The plant

material was placed in labelled plastic bags. A small plastic tube was used to extract a

sediment core (1cm diameter x 1cm deep) from the quadrat for analysis of chlorophyll a

and phaeopigment concentrations. All samples were frozen in the dark for later analysis.

In general, three quadrats (one was also used for productivity measurements, see below)

were sampled at each site. Sampling was conducted on 9 and 23 June, 22 July, 5 and 18August and 7 October 1986, and 26 January, 25 February, 16 April and 1 May 1987. All

sites were sampled during the June, July, August and May visits. Three sites (i.e., 0.6,

0.1, -0.4m) representative of the two main habitats (i.e., Z. japonica and Z. marina) were

sampled during all visits.

Frozen samples of macrophytes were thawed in the laboratory, rinsed gently in tap

water to remove sediments, and separated by species. The epiphytes, consisting of seaweeds and filamentous and tube dwelling diatoms were carefully removed from the seagrass leaves by scraping with a spatula. The freezing process facilitated easy removal of

the epiphytes. The number of shoots of each eelgrass species was recorded for eachsample. The weight of each species was recorded following drying at 90°C for at least 48

hours. To describe morphological differences in eelgrass populations among the sites, the
Productivity Measurements

Eelgrass

Net shoot primary productivity (NPP) of Z. marina was measured using a modification

of the method of Kentula and Mclntire (1986). A 0.1 m2 area was marked at each sitecontaining this species by anchoring a quadrat using stiff wire (i.e., clothes hanger) pushedinto the sediment. A shoot was marked by poking a hypodermic needle through the leaves

at a point immediately above the youngest sheath. Different diameter needles were used

depending on the width of the leaves not only to minimize damage to the plant but to also

assure relocation of the marks at a later date. The majority of shoots within the quadrat

were marked. All vegetation in the quadrat was harvested during the next visit to the site,

and frozen for later analysis. In the laboratory, the samples were treated exactly like the

samples for standing stock. However, the marked shoots were separated from the

remainder of the sample. The shoot was cut at the level of the mark in the oldest leaf.

Next, the portion of leaf distal to the mark on that leaf was removed. The material betweenthe level of the mark on the oldest leaf and the distal mark consisted of the biomass

produced during the time between the original marking and the harvesting. It was assumed

that no leaf loss occurred during the 13-15 day intervals. Loss of the oldest leaf was

apparent in a few cases in that a mark was found in the basal portion of the outer senescent

leaf, but the upper portion of the leaf was lost. In addition, using the mark in the oldestleaf as a baseline assumed that no growth took place in the oldest leaf. Very little (<10%)of the annual Z. marina NPP was due to growth in the oldest leaf in Netarts Bay (Mary

Kentula, personal communication, 1986). The dry weight of the new material producedduring the growth interval was recorded. Finally, the data on shoot density and biomass ofcomponent plants from the quadrat were included as a third replicate sample of standing

stock from each site. Production experiments were carried out during June, July and

August 1986, and April-May 1987; a period covering the growing season for the plants

(Kentula and Mclntire 1986, Harrison 1982a).
coupled with the relatively short intervals between samplings during the growing season,

minimized the error in estimating NPP using biomass changes. Limited experiments wereconducted with both eelgrass species using oxygen flux (see below) to provide a compari

son of methods measuring NPP using oxygen flux versus shoot marking and biomass

changes. Because of evidence from physiological and environmental work, the use ofoxygen flux to measure net production was strongly discouraged by Zieman and Wetzel

(1980). Therefore, I used leaf marking and biomass changes to estimate eelgrass NPP in

Padilla Bay.Benthic Algae

The NPP of algae epiphytic on the eelgrass was measured using standard light and dark

bottle incubations following the methodology in Littler and Arnold (1985). Healthy

specimens of epiphytic algae were collected from the sites and transported to laboratory

facilities at the Padilla Bay Interpretive Center. Portions of specimens were placed in 300ml BOD bottles containing water collected from the Bay. The initial dissolved oxygen

(DO) was measured using an oxygen meter (YSI dissolved oxygen meter model 58). The

bottles were placed in a water bath held at ambient sea temperature, and incubated under

ambient mid-day light for 1-3 hours. Bottles containing water only were run to account for

plankton metabolism. Following the incubation, the fmal DO was recorded. The specimens were removed from the bottles, dried and weighed. NPP was calculated for each

species using the formulas in Littler and Arnold (1985). Two-four replicates were run for

each species. The experiments took place on 9 June, 23 June, 22 July and 18 August

1986, and 30 April 1987. Shoots of both seagrass species were incubated in a similar

manner during some experiments.

Sediment NPP was measured in clear plexiglass cylinders. The cylinders were 20cm

long and had an inside diameter of 10 cm. An undisturbed sediment core (5cm deep) wascarefully removed from within one random 0.1 m2 standing stock quadrat at each site using

the cylinder. The bottom was sealed with a rubber bung. The top of the cylinder was

closed with a plexiglass plate except for a hole (1.3 cm diameter) through which the oxygen
The Z. marina growth data was extrapolated to NPP per unit area by multiplying mean

NPP per shoot by the mean shoot density. Z. japonica production was calculated as thedifference in mean biomass between successive collection dates. Epiphyte production perunit area was determined by multiplying mean production per g of tissue by mean g dry

weight of tissue per unit area. Sediment production was directly measured per unit area of

sediment. Epiphyte and sediment hourly production was expanded to daily rates by

multiplying hourly rates by 8 to simulate the duration of non-light limited photosynthesis

(unpublished data). Total daily NPP for a site was the sum of per unit area rates for thespecies occurring at the site during the sampling period. A factor of 0.38 gC = 1 g dry wt

was used to inter-convert plant dry weight with carbon content (Westlake 1963),
japonica (Table 1). Shoot length and leaf area of both species increased with increasing

depth, although a relatively wide range in shoot lengths for Z. marina especially at the

deepest sites resulted in a relatively weak relationship (Table 1). Non-linear models appliedto the data did not yield appreciably greater R2 values over that of the linear model.

Density and Standing Stock

Eelgrass

Shoot density for both species showed a strong relationship to depth during the

growing season (Fig. 3a, b), with Z. japonica reaching peak density between 0.5 and 0.8m

depth and Z. marina having peak density at -0.2m depth. Based on the density vs depthcurves, the depth of maximum overlap of the two species was at about 0.3rn.

The dramatic seasonality ofZ. japonica is typified by the change in mean density from a

maximum in June of approximately 3200 shoots rn-2 to a minimum of less than 100 shootsrn-2 in winter at the site at 0.6rn depth (Fig. 4). Seasonal changes in the density of Z.

marina was much less pronounced (Fig. 4).

Mean biomass was strongly related to depth also (Fig. 5a, b). For Z. japonica,

biomass was greatest at depths between 0.3 and 0.8m. An interesting shift in biomass is

evident during the growing season, however. At 0.6m, biomass was lowest in May,

greatest in June and intermediate in August. The difference between May and August

values at 0.5rn is also relatively great. Seasonal differences at the other elevations are lesspronounced. Peak biomass of Z. marina occurred at the lowest depth (-0.4m) in May and

June, and at a much shallower site (0.lm) in August. The relatively great leaf area of

plants at the lowest elevation explains the high biomass at relatively low density (Fig. 3b)

condition at the lowest elevation. As with density, the greatest overlap of the two specieswith regard to biomass was at the 0.3rn depth (Fig. 5a, b). Although peak densities werequite dissimilar between the two species (Fig. 3a, b), the maximum mean biomassmeasured was similar (Fig. 5a, b).
leaf area (cm2) length (cm) 1.89 2.50 0.46

shoot wt. (mg) leaf area (cm2) 1.75 -3.00 0.32

length (cm) depth (m)a -6.43 7.26 0.55

Z. marina shoot wt.(mg) length (cm) 43.86 -362.61 0.74

leaf area (cm2) length (cm) 19.31 -120.41 0.78

shoot wt. (mg) leaf area (cm2) 2.00 -59.10 0.85

length (cm) depth (m)a -13.84 15.81 0.26

aRelative to MLLW.
c3(n\In

I

00

IIn

0

z

B) Z. MAl~INA

a

inI.00IIn

0

z

2

1

0—0. 5

I.

I.

I.

I.

I’

7”

‘.4-

0 0.5

DEPTH Cm)

1

-*- MAY

CX 100

1 -.4- JUNE - AUGUST

4

0—0. S 0 0.5

DEPTH Cm)

Figure 3. Eelgrass mean density vs depth. A) Z. japonica; B) Z. marina.
CX 1000)

d(I)(I) I—00IU)

4

d 1

-*- ZJ -‘-O.Sm

•+ ZM -‘-0. im

J F M A M

MONTH C19B6—1~87)

Figure 4. Eelgrass mean density at three depths vs time. ZJ Z. japonica; ZM = Z.

marina.

S2 -- ZM —O.Bm

+

+

0

J J A S 0 N D
UU)

150

>-a:o 100

50

0

—0. 5

300250

I

200

UU) \

150

>-a:o 100

50

0

—0. 5

0 0.5 I

DEPTH Cm)

B) Z. MARINA -~- MAY

•+ JUNE

-. AUGUST

0 0.5

DEPTH Cm)

Figure 5. Eelgrass mean biomass vs depth. A) Z. japonica; B) Z. marina.
p

p

y

Maximum mean epiphyte biornass, which included both seaweeds and filamentous and

tube dwelling diatoms epiphytic on eelgrass and associated with the base of the eelgrass

shoots, occurred at the site at 0. lm depth in June (Fig 7). There was a pronounced peak of

relatively short duration of epiphyte biornass during early summer (June) at the three

intensively studies sites (Fig. 8). The June peak was dominated by diatoms, the red alga

Ceramium pacjficum and the green alga Ulvafenestrata. Diatom biomass was greatest at

0. lm (248.9 g dry wt rn-2). The biornass of C. pac~cum was greatest at -0.2m (45.6 g

dry wt rn2). U.fenestrata biomass was greatest at -O.lm (34.2 g dry wt rn-2). The green

alga Enteromorpha intestinalis was abundant in late June, and reached at peak mean

biomass of 113.6 g dry wt rn-2 at -0.4m during this period. Other species recorded in the

epiphyte samples included the green alga Cladophora sp., the brown alga Laminaria

saccharina and the red alga Gracilaria pac~fica. C. pac~ficum, U. fenestrata and G. pac~fica

were common members of the understory assemblage in the Z. marina portion of the bed.

Sediment Microalgae

There was no apparent trend in sediment chlorophyll a concentration relative to depth

during any of the samplings in spring-summer (Fig. 9a). Concentrations did change

among the months, and this change appeared most pronounced at the highest site.

Phaeopigment concentration was greatest at 7 of the 8 sites in May as compared to June andAugust (Fig. 9b). Phaeopigment concentration showed little relationship to depth duringthe latter two months.

Chlorophyll a concentration was highest at the three intensive study sites in late winter

through early spring (i.e., February-April) (Fig. 10). This peak precedes the early summer

increase in seagrass (Fig. 6) and seaweed (Fig. 8) biomass.

Productivity

Calculation of NPP was divided into spring (i.e., April-June) and summer (i.e., JulyAugust) to cover the growing season and to emphasize apparent seasonal differences in
Zi -*-0.Sm

I.

I.

I.

0 /4 — I. .— — — — / - — — — J J A S 0 N D J F M A M

MONTH <1980—1967)

Figure 6. Eelgrass mean biomass at three depths vs time. ZJ Z. japonica; ZM Z.

marina.

+ ZM -s-0. lrn

ZM —O~.4m

T

I

U,\

I—>

0

300

250

200

1. 50

1 00

Sc

0

+

I

I

‘Ii.

+

0~

+
EPIPHYTES —N— MAY

•+ JUNE

300

AUGUST

1•

250

200

0U)

150

0

-4.

50

DEPTH Cm)

Figure 7. Epiphyte mean biomass vs depth.
EP I PHYTES

•+ -‘-0. lm

300 — -- —O.4m

-~

250 — I

ciU) \

150 —.

>-a:0 100 —;

:,~,—I ••~..L j

50—.~ — ~ Tb — — — — +

0 ..~ t.1 i~i—_ ~I ~

J J A S 0 N D J F M A M

MONTH C1~B6—1~B7)

Figure 8. Epiphyte mean biomass at three depths vs time.
1U

U)(51

200

100

0

—0. 5

1

1200

1000

$00

f-IU) 500\(5I

400

200

0

—0. 5

B) PHAEOPIGMENTS

0 0.5 1

DEPTH Cm)

Figure 9. Mean sediment plant pigment concentrations vs depth. A) Chlorophyll a; B)

Phaeopigments.

O 0.5

DEPTH Cm)

-II- MAY

.4. JUNE - - AUGUST

+
I

ciU) \

I

CHLOROPHYLL ~

MONTH C1~6e—1~a7)

4-0. 8m

•+ -1-0. im -- —O.4m BOO

800

400

200

0

—7’. — ‘ — — 74

/

/

/

S.

S.

+

.. -1-.

+

J J A S 0 N D J F M A M

Figure 10. Mean sediment chlorophyll a concentration at three depths vs time.
elevations as compared to spring. In contrast, maximum NPP for Z. japonica shifted to alower elevation as compared to spring. NPP declined at all sites between spring and

summer except at 0.5 and -0.2m. NPP declined to zero at the two highest elevations (Z.

japonica dominated sites) in summer.

Annual NPP varied with depth, with a maximum NPP at -0.lm (Fig. 1 lc). Seaweed

NPP predominated, on an annual basis, at intermediate elevations. Maximum NPP by Z.

marina and Z. japonica occurred at -0.2 and 0.5m elevations, respectively.

Mean NPP by sediment associated microflora was negative during all samplings and is

not presented. Very high oxygen demand (unpublished data) associated with the sediments

accounted for the negative values.

Total annual NPP estimated from the Padilla Bay eelgrass system was 10,557 x l0~ kg

C (Table 2). Ninety-seven percent of this production occurs in the Z. marina bed atelevations below 0.3m. A total of 50% of the production in the system was attributed to

epiphytes. The average annual NPP for the macrophyte component of the system was 351

gC m2 (= 924 gdry wt m2).

Production to Biomass

Epiphyte annual NPP to average annual biomass ratio was greatest among all

components at all sites except the highest two (Table 3). These high values are explainedby the relatively low annual biomass of epiphytes due to their markedly seasonal

appearance (Fig. 8). There was little variation in NPP:B for the seagrass species over the

depth gradient. The turnover rate for the entire system, calculated by dividing total average

annual NPP by total average annual biomass, was 9.2. This means that the entire biomass

of the system was replaced approximately 9 times during the year of study.

The relationship between mean shoot weight (regardless of species) for the sampling

having the greatest mean eelgrass standing stock (August) and annual NPP is linear for

shoot weights up to approximately 300 mg (Fig. 12). Higher shoot weights appeared to
IaLi

a)

>.

ci\

1a14

0

-0. 5

C) ANNUAL NPP

Figure 11. NPP vs depth. A) April - June; B) July - August; C) Annual NPP.

0—0.5 0 0.5

DEPTH Cm)

9) NPP JULYAUO.

—*— Z. Japonica.* Z. marina — - Epip~~yt

—~— TatOl

4

3

0

—0. 5 0 0.5

DEPTH Cm)

•oo

—W— Z. Joponica

• 4. Z. mar t no - — Epiphyt..

—4.— Total

400

200

O 0.5

DEPTH Cm)
Z.japonica (0.8 to 0.3m) 44.7 481 215 2

Epiphytes (0.8 to 0.3m) 21.3 481 102 1

Z. marina (0.3 to -0.4m) 199.7 2526 5044 48

Epiphytes (0.3 to -0.4m) 205.7 2526 5196 49

Total= 10557
(m) Biornass NPP Annual

(g rn2) (gdry wt rn-2) NPP:B

ZJ ZM B Tot. ZJ ZM B Tot. ZJ ZM E

0.8 2 0 0.2 2.2 15.8 0 0.8 16.6 7.9 - 4.0

(95) (0) (5)

0.6 22 0 0.7 22.7 138.9 0 0.5 139.4 6.3 - 0.7

(100) (0) (<1)

0.5 49 0 0.7 50.6 230.0 0 17.1 247.1 4.7 - 353.0

(93) (0) (7)

0.3 24 33 8 65.0 85.5 159.0 206.1 450.6 3.6 4.8 25.8

(19) (35) (46)

0.1 0 93 21 114 0 520.2 746.8 1267.0 - 5.6 35.6

(0) (41) (59)

-0.1 0 102 13 115 0 698.4 1037.1 1735.5 - 6.8 79.8

(0) (40) (60)

-0.2 0 115 13 128 0 807.7 271.1 1078.8 - 7.0 20.9

(0) (75) (25)

-0.4 0 118 15 133 0 440.6 443.4 884.0 - 3.7 29.6

(0) (50) (50)
900

*

L

600

N\ *

U

3) 400 *

a. *

0~z

I< 200

I- *

0

F0 .111111 I Iii 1111111 1111111

0 2 4 8 8 10 12

CX 100)

ZOSTERA MEAN SHOOT WT Cm9/h~~t)

Figure 12. Eelgrass shoot weight vs annual system NPP. Shoot weights are means from

samples collected in August 1986.
Limited comparisons of oxygen flux data with shoot marking and biomass change

estimates of NPP showed that oxygen flux resulted in greater NPP values for both eelgrass

species and epiphytic algae. Daily NPP forZ. marina (at 0.lm MLLW) estimated by

oxygen flux was 4.37 and 1.30 times higher than the estimate from shoot marking in June

and August, respectively. For Z. japonica (at 0.5m MLLW) in August, oxygen flux

produced a daily NPP value that was 3.66 times that calculated on the basis of change in

biomass. In comparison, total daily epiphyte NPP estimated by oxygen flux at 0.6mMLLW was 2.50 times that estimated by biomass change during the period of April-June.The ratio of estimates based on the two methods at 0.lm MLLW was 2.78 during the same

period.
present study are roughly applicable to the system in general.

The study sites spanned a gradient in seagrass leaf dimension, density and plant

biomass and NPP that correlated with depth. Peak standing stocks of Z. japonica and Z.

marina were centered at about 0.5 m and MLLW, respectively. The densities and depth

distribution at Padilla Bay was similar to that reported for an expansive eelgrass system inRoberts Bank in southern British Columbia (Harrison 1982). In contrast, the upper limitof depth distribution of Z. marina in Netarts Bay, Oregon, was much shallower. Kentula

and Mclntire (1986) sampled that system at 1.1 to 1.4 m MLLW; an elevation range thatspanned the intertidal portion of the bed. The upper limit of Z. marina at Padilla Bay wasabout 0.3 m MLLW (i.e., 0.8 m deeper than at Kentula and Mclntire’s deepest study site).

A possible explanation is higher temperatures on the flats in Padilla Bay during low tides in

summer (Bulthius 1987). Harrison (1982b) showed experimentally that the upward limit

of distribution of Z. marina is regulated by low resistance to desiccation in comparison to

Z. japonica, and that Z. japonica can outcompete Z. marina for this reason at upper eleva

tions. Kentula (1982) did not report Z. japonica from Netarts Bay, although it is known tooccur in estuaries north and south of Netarts Bay (Harrison 1982b; Thom 1984 and

personal observation).

Ponds form on Padilla Bay flats during low tides. Ponding was most evident in sum

mer when standing stocks of eelgrass was greatest, which suggests that the mass of plantmaterial functions as a sponge in inhibiting the rate at which the flats drain during ebbingtides. The deepest site (at -0.4m) was under 0.3 m of water even during extreme low tidesof -l.3m MLLW. Sites located in areas of ponding at upper elevations (i.e 0.3 to 0.6m)contained the densest stands of Z. japonica. The site (0.8m) that was dry at low tide had a

very low standing stock of plants in late summer; a time when NPP was negligible. In

winter, when standing stock in the entire system was the least, very little ponding was evident. This further supports the sponge effect hypothesis. Poncling was noted by Harrison

(1982a) on Roberts Bank, Kentula and Mclntire (1986) in Netarts Bay and in England by
minimum near zero. Some individuals of Z. japonica were present during winter, andthese were most evident at the sites at the lower end of its distribution. The biomass ofboth species changed with season. The most rapid increase occurred between April andJune, when NPP was greatest. During this time light energy reaches maximum and water

temperature is increasing rapidly in the region. Light energy at noon in mid-June (i.e., thetime of the summer solstice) can be as high as 4000 p.E rn-2 s1. Mid-winter noon values

are commonly below 100 ~iE rn-2 sd-. Several previous studies (Sand-Jensen 1975, Jacobs

1979, Kentula and Mclntire 1986, Dennison 1987) have concluded that light energy was an

important factor controlling shoot net production. The experimental work of Dennison andAlberte (1982) showed that variations in light intensity exhibited significant control ofeelgrass photosynthesis. Peak shoot biomass occurred in June-August, which is the same

period as reported for other north temperate systems (Phillips 1972, Sand-Jensen 1975,Wyer et al. 1977, Jacobs 1979). In addition, peak shoot density coincided with peak

biomass. In Padilla Bay, seasonal biomass peaks varied by depth. Seasonal shifts in thelocation of these peaks may be related initially to how light energy requirements are met atthe various depths.

The increase in biomass, especially evident in Z. marina, was due largely to an increase

in leaf area. Plants had relatively narrow leaves in mid-winter and noticeably wider leaves

in summer. Kentula and Mclntire (1986) and others have shown that Z. marina plants havemore and larger leaves in spring and summer as compared to winter. This latter conditionis in response to increased light energy and photosynthetic rate, and perhaps decreased leafexport rate, in summer.

Annual NPP in Padilla Bay was 351 gC m2 was similar to data from other temperate

systems (see Zieman and Wetzel 1980). However, the epiphytes were responsible for 50%

of the NPP, indicating that NPP by eelgrass was, on average, relatively low (ca. 175 gCrn2). Epiphyte NPP has been noted to be significant in many seagrass systems. Typical

values for epiphyte production are between 7 and 58% (Morgan and Kitting 1984). The

high relative importance of the seasonal appearance of the highly productive epiphyte taxawith substantial turnover rates to animals associated with seagrass was proven by stable
employed must be done with caution. Substantially higher values were obtained usingoxygen flux methods for eelgrass and epiphytes as compared to values obtained using

biomass changes. The average difference factor for eelgrass during the growing season

was 3.11. Using this factor, the estimate for eelgrass annual NPP would be increasedfrom 175 to 546 gC m-2. Total annual NPP for the system would be 722 gC m-2, of

which epiphytic algae would comprise 24% instead of 50% of the total. Recently, the

arguments of Zieman and Wetzel (1980) against the use of metabolic techniques for

measuring NPP in rooted aquatic macrophytes was examined by Kemp et al. (1986) using

Potamogeton perfoliatus L. They found that the primary reasons given for not using

metabolic techniques (i.e., problems associated with the storage, recycling and

translocation of metabolic 02 and C02) were not significant sources of error. Obviously,these types of studies need to be carried out for eelgrass in order to better estimate eelgrass

system productivity.

Epiphyte biomass, although substantially exceeding seagrass biomass at some sites

during early summer, comprised only 11% of the total average annual biomass in the

system. Values reported for seagrass systems in New Jersey (Penhale 1977), Netarts Bay

(Kentula 1982) and south Texas (Morgan and Kitting 1984) were 23, 25 and 47%,

respectively.

Greatest total system and epiphyte annual NPP was recorded at -0.lm depth. This site

was located in the center of a small channel through which water from surrounding flatsdrained at during ebbing tides. This site also had the greatest epiphyte production tobiomass ratio within the Z. marina portion of the meadow. I noted on several visits thatwater currents were swifter through this channel as compared to the surrounding flats, andthat floating matter was rapidly removed. Based on limited data from near these sites

(unpublished) inorganic nitrogen concentrations are quite low during the growing season.

It may be that the rapid transport of water past the study site in the channel is supplying the

plants in the channel with a constant, although low, amount of inorganic nutrients. Thisrelatively high flux of nutrients past the epiphytic algae and seagrass may be responsible for
1987). Nutrient flux rates are are known to be important with regard to total nutrient

supply available to support algal growth (see review by Hanisak 1983).

Sediment associated microalgae, as measured by chlorophyll a concentration, were

present. A rapid increase in chlorophyll a during late winter and early spring, prior to theperiod ofrapid macrophyte growth, suggests that the sediment microflora is better adapted

for rapid growth during the low temperatures and lower light conditions during the late

winter. Competition for nutrients, and light, and photoinhibition, along with species

succession may be factors slowing the growth of this assemblage in summer (Mills and

Wilkerson 1986). High sediment associated oxygen demand resulted in low or negative

calculations of NPP. These data suggest that gross primary productivity (GPP) for sediment associated assemblages may be substantial, and a significant source of carbon to the

local sediment food web, and that the sediments are highly heterotrophic. GPP in a Z.

marina bed in Chesapeake Bay by the sediment was 225 gC rn2 on an annual basis, which

accounted for 14% of the annual GPP of the bed (Murray and Wetzel 1987). In compari

son, respiration measurements made on eelgrass indicate that respiration is low relative to

NPP during the growing season (unpublished data). Respiration rates of epiphytes on the

seagrass were usually <10% of the NPP for these plants. Recent research in Padilla Bay(Thom, in preparation) focused on the production dynamics of sediment microflora on

mudflats in Padilla Bay, and will couple the work from seagrass sediment production

studies in evaluating the importance of this component to total production in the Bay.

The values for sediment chlorophyll a concentration measured in Padila Bay were on

the order of those reported for Netarts Bay (Davis and Mclntire 1983) and higher than

those reported for the Columbia River estuary (Mclntire and Amspoker (1986). Sediment

physical characteristics, and biological and physical processes have a strong but poorly

understood influence on the benthic microflora (Davis and Mclntire 1983).

Mean shoot weight, regardless of eelgrass species, in June (as an indicator of leaf area)

and depth appeared to be good predictors of annual NPP for the system. Of significance

was the threshold of annual NPP reached at approximately 300 mg dry wt shoot1 and a
quences of the invasion of the introduced species. Of note is the fact that Z. japonica is

very new to the area in geological and evolutionary time scales.

Finally, a total of approximately 28,000 mt of dry plant matter was produced in Padilla

Bay seagrass system during the one year study. Values from Netarts Bay and GraysHarbor were 3,200 mt (Kentula and Mclntire 1986) and 98,000 mt (Thom 1984), respectively. Although the estuaries differ in size and distribution of eelgrass, these values indi

cate that the amount of organic plant matter produced on an annual basis and potentially

available for utilization in estuarine and nearshore marine systems is substantial.
q

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