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J.

exp. mar. Biol. Ecol .

1976, Vol. 22,

pp

161-169; 0 North-Holland Publishing Company

GRAZING BY TALORCHESTZA LONGZCORNZS ON AN ALGAL MAT

IN A NEW ENGLAND SALT MARSH

DAVID BRENNER, IVAN VALIELA, CHARLENE D. VAN RAALTE

Boston University Marine Program Marine BiologicalLaboratory Woods Hole Massachusetts U.S.A.

and

EDWARD J. CARPENTER

Woods Hole Oceanographic Institution Woods Hole Massachusetts U.S.A.

Abstract: Grazing experiments using %Zand an analysis of fecal pellets and gut contents established

that the gammaridean amphipod, Tulorchestia longicornis Say, ingests blue-green algae on algal

mats in a Massachusetts salt marsh. This grazing had a measurable effect on the lower algal mat,

where the density of T. longicornis was high. Exclusion of amphipods resulted in increases in chloro-

phyll a content, carbon incorporation, and nitrogen fixation. This effect was not seen on the upper

mat where T. longicornis was less abundant. The assimilation efficiency of T. longicornis feeding on

a diet consisting mainly of blue-green algae was surprisingly high (67 A) considering that blue-green

algae are usually considered as a poor quality food for herbivores. The population of T. longicornis

seems to be annual, with growth of the overwintered juveniles in spring and early summer.

INTRODUCTION

Great Sippewissett marsh, West Falmouth, Massachusetts, has an algal mat,

covering z

1695 m2 and located on the lee of the barrier beach dunes between the

marsh and Buzzards Bay. The mat consists principally of three blue-green algae,

Lyngbya aestuarii Liebm., Microcoleus ChthonoplastesThuret and Calothrix contarenii

Born & Flah. Two distinct habitats can be found within the mat, the first somewhat

below the mean high tide level where the mat is moist and soft and the dominant algae

are Lyngbya aestuarii and Microcoleus chthonoplastes. The second habitat is found at a

slightly higher elevation and is inhabited by Calothrix contarenii Microcoleus chthono-

plastes and to a lesser extent, Lyngbya aesttlarii; this area shows a dark and asphalt-

like surface which is at times dry and quite firm. Diatoms and flagellates are also

present in both high and low mat.

A gammaridean amphipod, Talorchestia Iongicornis Say, was observed to feed

nocturnally on the blue-green algae making up the mat. Various insect larvae.

mites, and nematodes live within the mat and may also feed on it. During the day

T. Zongicornis burrows 2-5 cm deep in the sand dunes surrounding the algal mat.

After dusk the amphipods emerge from burrows and move onto the algal mat,

aggregating in large groups. The number of grazing amphipods was always much

greater on the lower mat than the upper mat.

1 Contribution No. 3610 from the Woods Hole Oceanographic Institution.

161

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162 DAVID BRENNER ET AL.

The purpose of this work was to determine 1) whether the amphipods were feeding

on the blue-green algae, 2) the effect grazing has on the mat, 3) the assimilatory

efficiency of the amphipods, and 4) annual population dynamics of the amphipods.

METHODS

ANALYSIS OF FECAL PELLETS AND GUT CONTENTS

T. Zongicornis were collected on the algal mat at night and were killed immediately

in 70 % ethanol. The gut contents were dissected out and placed on a glass slide and

examined under a microscope. Amphipods were also brought to the laboratory alive

and left inside jars in the dark on moist tissue paper. Fecal pellets were collected on

glass slides after several hours and examined under a compound microscope.

GRAZING EXPERIMENT

Surface cores of the algal mat were taken with a cylindrical plastic corer with a

sharpened edge and a radius of 2.5 cm. The cores were placed inside uncapped jars

on the mat and incubated for 3-4 h in the middle of the day in 20 ml of marsh water

containing 7.806 ,uCi or NaH14C03.

After incubation, the cores were washed

repeatedly in marsh water to stop the incorporation of 14C, and then left

in situ

until

nightfall. Amphipods were captured by hand after they emerged at night and put

into the jars. The jars were loosely capped, and the amphipods were allowed tc

graze on the radioactive mat for the entire night. At sunrise the amphipods were

transferred to new containers. Fecal pellets from a first group were collected periodi-

cally over a 5-h period in the dark, after which they were killed with dilute formalin.

Each amphipod in a second group was placed individually in a sealed Erlenmeyer

flask with an open vial containing hyamine hydroxide to collect 14C0,. The second

group was killed after 2.5 h in the dark, and its fecal pellets were collected.

The dead amphipods were rinsed with 2.3 % HCI to remove any 14C adsorbed on

their surface, and then placed individually in liquid scintillation vials to which suffi-

cient Protosol (New England Nuclear Co.) was added to cover the animal. The vials

were left at 55 C for several days, until all of the amphipod tissue had been dissolved.

Approximately 10 ml of Aquasol (New England Nuclear Co.) was then added to

each vial. Each fecal pellet sample was homogenized and placed in a liquid-scintilla-

tion vial to which 3.5 ml of distilled water and 11.5 ml of Aquasol were added. The

total ingested by the amphipods was obtained by summing the decays per min (dpm)

in the feces, CO,,

and body. The amount of radioactive fecal pellets produced in

24 h was estimated by using the results of the first group of amphipods and extra-

polating for a whole day. We did not measure passive adsorption of radioactivity

but Hargraves (1970) estimate of passive adsorption accounted for less than 10 7;

of the counts per min (cpm) per amphipod.

Algal mat cores were placed on a Millipore HA 0.45 filter and rinsed in dilute HCI

to remove surface adsorbed 14C. The filters with the remains of each core were then

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GRAZINGONANALGALMAT

163

transferred to an Erlenmeyer flask to which 20 ml of concentrated HNO, was added.

The sample after leaving overnight to digest the core was centrifuged until the super-

natant was clear (from 3-5 min at low speed). 1 ml of the supernatant was added to

9 ml of 0.15 M Tris buffer and 1 ml of this solution was then transferred to a liquid

scintillation counter vial with = 10 ml of Aquasol. Quenching curves for this machine

were determined.

Two 1 mz enclosures were constructed on the high mat and two on the low mat.

Enclosures consisted of four stakes sunk into the sediment surrounded by a vertical

wall of fiberglass screening 25 cm in height. The screening was buried 3 cm deep

into the sand and a rim of either electrical tape or aluminum flashing was attached

at the top to prevent amphipods from entering the enclosures. Adjacent to each

enclosure was a control plot of similar size which was exposed to amphipod grazing.

Six weeks after the enclosures had been built, chlorophyll a 14C incorporation rate,

and nitrogen fixation rate were measured in cores taken from inside and outside the

enclosures.

CHLOROPHYLLUCONTENT

Five cores of 1.3 cm radius were taken from inside each enclosure and five from

the adjacent controls, and the amount of chlorophyll a measured (Estrada,

Valiela & Teal, 1974; UNESCO, 1966). In order to minimize any partial shading

by the fiberglass screening of the enclosures, core samples were always taken from

the middle of the plots where the effect of shading were least. Each core was ground

with a mortar and pestle in 90 % acetone solution to which a small quantity of MgCO,

had been added. The ground sample was left overnight at 5 C in the dark. The sample

was then filtered using a glass fiber filter (Gelman type A). The total volume of the

filtrate was then recorded and an aliquot was centrifuged for 10 min at 5000 rpm.

Optical densities were measured at 630, 645, 665, and 750 nm on a Beckman DU

Spectrophotometer. For the last two wavelengths the readings were repeated after

acidification with one drop of concentrated HCl. The measurements at 750 nm

reflected the turbidity of the sample and was subtracted from the other readings.

The chlorophyll a concentration was calculated using the equation derived by Strick-

land & Parsons (1968, p. 189).

14C INCORPORATIONBY THE ALGAE

14C incorporation was measured as described by Van Raalte, Stewart et al. (1974)

for marsh muds. Six cores of 1.3 cm radius were taken for each enclosure, three from

inside and three from the adjacent controls. Each was placed in a glass jar containing

9.5 ml of marsh water and 0.5 ml NaHX4C0, (4.68 &i) and incubated for 4 h in

the middle of the day near the site of collection. In each set of three cores, one was

covered with aluminum foil to act as a dark control. The algal mat cores were killed

with formalin after the incubation period.

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DAVID BRENNER ET AL

In the laboratory, the cores were rinsed with dilute HCl using a Millipore HA 0.45

filter, digested overnight in 10 ml of concentrated HN03, and centrifuged. 1 ml of

supernatant was added to 9 ml of tris buffer and 1 ml of this solution transferred to

a liquid scintillation counter vial containing 10 ml of Aquasol. The amount of carbon

incorporated during the incubation period was calculated as by Van Raalte, Stewart

et al

(1974).

NITROGEN FIXATION

Van Raalte, Valiela et al . (1974) measured nitrogen fixation in blue-green algal

mats using the acetylene-reduction method. The same procedure was used here. Six

surface cores 2 mm deep with a radius of 0.4 cm were taken, three from inside the

enclosure and three from the adjacent controls. Each core was transferred to a 15 ml

serum bottle to which 0.5 ml of sea water was added. The bottle was sealed with a

rubber stopper and incubated for 4 h in acetylene. The ethylene was measured using

a Packard Becker 417 gas chromatograph with a 0.3 cmx 2.7 m Poropak R column.

A conversion factor of 1.5 moles of ammonia/mole ethylene was used (Stewart,

Fitzgerald & Burris, 1968).

AMPHIPOD POPULATIONS

The total number of amphipods was estimated using a capture-recapture method

(Southwood, 1966, pp. 57-74). Amphipods were marked by dyeing in a solution of

neutral red stain. The amphipods remained well-stained for four days in the laboratory

and the color disappeared by the tenth day. Several hundred amphipods were hand-

captured at night on the algal mat, counted, soaked in the neutral red solution for 3 h

and then released later that night and re-captured after l-2 days. Since T. Zongicornis

is very active over the relatively small area of an algal mat it was considered that one

or two days were sufficient time for adequate mixing of the stained specimens with

the population at large.

On three dates the length of the head plus two thoracic segments was measured for

large samples of individuals to determine the size classes. Individuals were also dried

at 60 C and weighed for the calculation of a length-weight regression.

RESULTS AND DISCUSSION

The fecal pellets and gut contents of the amphipods consisted of an amorphous

mass mixed with strands of blue-green algae. The algae found in the fecal pellets

and guts showed a range of degradation ranging from intact trichomes to empty

sheaths. Amphipods captured on the upper mat contained an array of algae similar

to that found in the upper mat in their guts Calothrix contarenii with some Lyngbya

aestuarii).

Amphipods caught on the lower mat had gut contents resembling the species

assemblage typical of low mat L. aestuari i with some M icrocoleus cht honopl astes).

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