The loss of seagrass in cockburn sound, Western Australia. III. The effect of epiphytes on productivity of Posidonia australis Hook. F.

Aquatic Botany, 24 ( 1 9 8 6 ) 3 5 5 - - 3 7 1

355

Elsevier Science P u b l i s h e r s B.V., A m s t e r d a m - - P r i n t e d in T h e N e t h e r l a n d s

THE LOSS OF SEAGRASS IN COCKBURN SOUND, WESTERN AUSTRALIA. III. THE EFFECT OF EPIPHYTES ON PRODUCTIVITY OF POSIDONIA A USTRALIS HOOK. F.

K. S I L B E R S T E I N ' , A.W. C H I F F I N G S 2 a n d A.J. M c C O M B

Botany Department, University o f Western Australia, Nedlands W.A. 6009 (Australia) ( A c c e p t e d for p u b l i c a t i o n 25 F e b r u a r y 1 9 8 6 )

ABSTRACT S i l b e r s t e i n , K., Chiffings, A.W. a n d M c C o m b , A.J., 1 9 8 6 . T h e loss o f seagrass in C o c k b u r n S o u n d , W e s t e r n Australia. III. T h e e f f e c t o f e p i p h y t e s o n p r o d u c t i v i t y o f Posidonia australis H o o k . f. Aquat. Bot., 24: 3 5 5 - - 3 7 1 . T h e h y p o t h e s i s was e x a m i n e d t h a t i n c r e a s e d e p i p h y t e g r o w t h was r e s p o n s i b l e for a r e d u c t i o n in seagrass m e a d o w s in C o c k b u r n S o u n d d u r i n g t h e d i s c h a r g e o f n u t r i e n t rich e f f l u e n t . O n e s t u d y site was in a d e t e r i o r a t i n g m e a d o w n e a r a n e f f l u e n t o u t f a l l , t h e o t h e r at similar d e p t h in a n u n a f f e c t e d m e a d o w in m o r e o c e a n i c w a t e r . Seagrass p r o d u c t i o n at t h e first site was less t h a n t h a t a t t h e s e c o n d , w i t h 33% l o w e r g r o w t h per s h o o t a n d 29% less d e n s e m e a d o w . W a t e r at t h e f o r m e r site h a d h i g h e r m e a n c o n c e n t r a t i o n s o f c h l o r o p h y l l a n d p h o s p h a t e t h a n t h e l a t t e r , b u t l i g h t r e a c h i n g t h e seagrass m e a d o w s was n o t s i g n i f i c a n t l y d i f f e r e n t . E p i p h y t e loads (as d r y w e i g h t or c h l o r o p h y l l per u n i t leaf a r e a ) w e r e 2--8 t i m e s h i g h e r at t h e f o r m e r site. S e a s o n a l c h a n g e s in e p i p h y t e l o a d s w e r e well c o r r e l a t e d w i t h p e r i p h y t o n b i o m a s s o n glass slides o r plastic seagrass. P h o t o s y n t h e s i s of leaf s e g m e n t s , w i t h a n d w i t h o u t e p i p h y t e s , was m e a s u r e d u s i n g a n o x y g e n m e t e r in t h e l a b o r a t o r y ; e p i p h y t e p h o t o s y n t h e t i c r a t e s w e r e similar t o t h o s e o f p e r i p h y t o n o n plastic, e x p r e s s e d p e r u n i t c h l o r o p h y l l . T h e p e r c e n t a g e r e d u c t i o n in light b y k n o w n p e r i p h y t o n loads was m e a s u r e d , a n d u s e d t o c a l c u l a t e light r e d u c t i o n b y epip h y t e s in t h e field, w h i c h was e s t i m a t e d t o b e 63% o n average at t h e first site a n d 15% at t h e s e c o n d . P o o l i n g d a t a for sites a n d seasons, t h e r e w a s a n e g a t i v e log-linear r e l a t i o n ship b e t w e e n leaf p r o d u c t i o n a n d e p i p h y t e load. T h e o b s e r v a t i o n s p r o v i d e s u p p o r t for t h e suggestion t h a t seagrass loss in t h e S o u n d m a y b e a t t r i b u t e d t o e n h a n c e d e p i p h y t e loads f o l l o w i n g n u t r i e n t e n r i c h m e n t .

INTRODUCTION

Circumstantial evidence has been presented that, of several hypotheses which may be put forward to explain the loss of seagrass meadows from 1Present address: B o t a n y D e p a r t m e n t , M o n a s h U n i v e r s i t y , V i c t o r i a 3 1 6 8 . 2Also a t D e p a r t m e n t o f C o n s e r v a t i o n a n d E n v i r o n m e n t , 1 M o u n t S t r e e t , P e r t h 6 0 0 0 , W e s t e r n Australia.

0304-3770/86/$03.50

© 1 9 8 6 Elsevier S c i e n c e P u b l i s h e r s B.V.

356 Cockburn Sound, Western Australia, the most acceptable one is t h a t enhanced growth of epiphytes in nutrient-enriched waters has reduced the a m o u n t of light available for seagrass photosynthesis (Cambridge et al., 1984, 1986). The evidence comes from: (1) observations of the levels of both epiphytes and loosely-attached, blanketing layers of filamentous algae associated with deteriorating meadows; (2) the coincidence of widespread loss of seagrass with the discharge into the Sound of effluents rich in plant nutrients; (3) the observed prolific growth of epiphytes 6n leaves of seagrasses transplanted into the Sound; and (4) the k n o w n importance of epiphytes in other systems. Other proposed mechanisms could n o t satisfactorily account for the general decline in seagrasses, although t h e y m a y be of supplementary importance in some instances, in that they have brought a b o u t local decline (as with scouring or oil refinery effluents) or increased the rate of decline (as with sea urchin grazing and reduction of light transmittance by p h y t o p l a n k t o n blooms). The work described in this paper was designed to obtain more direct evidence about the role of epiphytes, and proceeded along several lines. Two sites were selected in the Sound, one where meadows were declining, the other where meadows were apparently unaffected. Epiphyte loads and seagrass p r o d u c t i o n rates were measured for several months, to find o u t if there was any relationship between the two. Then, as it might be argued that increased growth of epiphytes could be a consequence rather than a cause of physiological deterioration in seagrass leaves, measurements were made of the growth of 'epiphytes' (periphyton) on artificial substrates at the two localities. Finally, more direct investigations were carried o u t into the a m o u n t and significance of shading brought a b o u t by epiphytes. The relation between r a t e of seagrass p h o t o s y n t h e s i s and light intensity was measured and, through their control over light intensity, the effects of k n o w n loads of epiphytes on photosynthesis of the seagrass were determined. MATERIALS AND METHODS

Study sites and measurement o f seagrass growth Investigations were carried o u t in meadows of Posidonia australis Hook. f. on the barrier bank at the n o r t h e r n end of Cockburn Sound (Fig. 1). The first site was towards the eastern end of the bank, close to the o u t l e t of a sewage treatment plant near W o o d m a n Point. Aerial p h o t o g r a p h y confirmed that the area and density of seagrass meadows had been reduced between 1977 and 1980. The second site was of the same depth (3 m), but towards the western end of the bank, near Carnac Island, where water is predominantly oceanic {Chiffings, 1979; Steedman and Craig, 1983). Productivity was measured essentially by the technique of Zieman (1974). Holes 1--2 mm in diameter were p u n c h e d through the upper sheath and enclosed leaf blades with surgical tongue forceps (Kirkman and Reid, 1979),

357

~

o

0.5

~

o...........

.

.),',

.,,,;

/7

.... <"....

-

•

~

I

FocmIf Seagresa Areas

~//~'/'/////JPcmsenl SiIqirass Areas We$1em ALISITIIIa

COCKBURN SOUND

d

Dredged Areas Sampling Site . . - - Sewage Ouifall ~ 5 - - Oeplh Contours In' M@tres ~]+ Beacons •

10

2 km

Fig. 1. The distribution of seagrass on Parmelia Bank, at the northern end of Cockburn Sound, showing the distribution of meadows in 1978 and at the time of the present study, in 1980--81. Study sites 1 (near Woodman Point) and 2 (near Carnac Island) are shown. avoiding damage to t he lower sheath. Sho ot s were m arked within seven plastic-coated steel quadrats, each 0.1 m 2 . T he shoots were harvested after a p p r o x i m a t e l y 14 days, and similar m e a s ur e m ent s were made f r o m J a n u a r y to S ep temb er , 1980. Shoots for growth analysis were decalcified in 10% h y d r o c h l o r i c acid, and epiphytes scraped off. S h o o t material above the p u n c h e d hole in the sheath was washed in sea water and divided into 'fresh growth', consisting o f new leaves and the part of t he older leaves below the p u n c h e d hole, and the remainder o f the shoot, f r om the p u n c h e d hole to the leaf tip. Both c o m p o n e n t s were measured and dried to c o n s t a n t weight (70°C, 48 h). Standing crops were calculated by summing t ot al leaf weight, and were expressed per unit area. Leaf p r o d u c t i o n was calculated by dividing the a m o u n t of new gr ow t h by the n u m b e r o f days elapsing b e t w e e n p u n c h i n g and harvesting, and was expressed pe r unit area (Zieman, 1974). T u r n o v e r rates or c o m m u n i t y r e p l a c e m e n t rates were calculated by dividing standing crop by p r o d u c t i o n rate. A measure of relative leaf growth rate was obtained by dividing p r o d u c t i o n by leaf standing crop (West and L a r k u m , 1979), and expressing dr y weight as mg g-1 d a y - ] .

358 E p i p h y t e load on seagrass leaves

Two leaf blades were taken from each quadrat at the time of productivity measurements, rinsed in seawater and frozen at P 1 0 ° C . Most of the epiphytes flaked off the frozen shoots easily, and the remainder were gently scraped off. Shoots were then dried as described above. Epiphytes were washed o n t o glass fibre papers (Whatman GF/C). Some samples were dried to constant weight at 105°C for dry weight determination. Chlorophyll extractions were carried out on other samples; the filters plus epiphytes were ground, extracted in 90% aqueous acetone and optical densities of extracts determined by the m e t h o d s described in APHA (1976) for perip h y t o n . Chlorophyll a and p h a e o p h y t o n were expressed as mg m -2 of seagrass leaf. Complications arose when larger epiphytes were present, and an arbitrary decision was made to include that part of the e p i p h y t e which shaded the leaf under reproducible conditions. Each shoot was placed under a sheet of plastic, and the outline of shoot and macroepiphytes traced. Macroepip h y t e s were removed with forceps,, and their chlorophyll contents determined as described above. The tracings were p h o t o c o p i e d , and areas measured with a digitizer. The total projected area of each epiphyte was measured, along with t h a t part of it which covered the seagrass leaf; the ratio between the two was calculated and used to apportion the total a m o u n t of macroepiphyte chlorophyll. The calculated a m o u n t of chlorophyll was expressed per unit leaf area and was added to the microepiphyte chlorophyll load. On one occasion, the seagrass in a quadrat was m a r k e d as described above, leaf material harvested after 35 days and epiphyte loads on the new growth determined. Ten shoots from each quadrat were rinsed in sea water, measured and divided into new growth and older leaf material; epiphytes were removed for dry weight and chlorophyll analysed as described above. E n v i r o n men ta l data

Water samples were collected at approximately m o n t h l y intervals at the two sites, and analysed for nitrate+nitrite, a m m o n i u m , phosphate and chlorophyll a using standard methods. Water sampling was also being carried o u t on a regular m o n t h l y basis at the sites shown in Cambridge et al. (1986), and data on temperature and light a t t e n u a t i o n were provided for those sites closest to the seagrass sampling stations referred to here. Unpublished incident radiation data for 1980 were provided by the D e p a r t m e n t of Conservation and Environment, and the water depth and a t t e n u a t i o n coefficient used to estimate total" daily radiation at the seagrass beds. Daily wind data were from the Fremantle Port A u t h o r i t y . The total n u m b e r of hours of wind at each range of speeds (10--20, 20--30, 30--40, > 4 0 knots) was calculated for each day, and multiplied by the mid-point of the appropriate

359 range. A mean dally estimate (knot h day -1 ) was calculated for each day from leaf punching to harvest. Each measured environmental parameter was p l o t t e d against time, and the value at the mid-point between day of leaf punching and harvest was used to calculate correlation coefficients between leaf production and environmental parameters, using the SPSS programmes of Nie et al. (1975).

Periphyton on artificial substrates Samplers were constructed of plastic (polyvinyl chloride; PVC) to each hold 6 glass microscope slides. At each site, two samplers were attached by rope to a sub-surface buoy, one with slides orientated vertically, the other horizontally. Samplers were replaced each 2--3 weeks. There was some loss of slides through storm damage. Harvested slides were each placed into vials with filtered sea water and returned to the laboratory. Before p e r i p h y t o n removal, light transmission was measured by placing each slide on a stand made of vertical black PVC tubing containing a light sensor (Li-Cor L a m b d a L1-185, Q u a n t u m Radiometer), which was clamped so t h a t the sensor faced the end of the tube. To reduce optical problems and prevent p e r i p h y t o n dehydration, the stand was submerged in an aquarium filled with seawater. The light source above the p e r i p h y t o n slide was a single beam from a fibre optics instrument (Schott-Mainz KL 150B; 150 watt, 15V Philips bulb). Light readings were taken at 400, 700 and 1,500 pE m -2 s-1 , using a blank reference slide, and the percentage reduction in light measured when a slide carrying p e r i p h y t o n was substituted. Periphyton was then removed from each slide and washed o n t o filter papers for dry weight or chlorophyll determination as described above. Quadrats of plastic seagrass (0.1 m 2) were assembled using a base of plastic-coated, woven steel mesh (25-mm squares). Strips of plastic approximating the dimensions of a typical seagrass r a m e t or shoot with blades 6, 30 and 38.5 cm long and 11 m m wide, were attached to the grid with staples and plastic-coated wire. The quadrats were anchored with steel pegs, two quadrats per site. Ten 'shoots' were harvested after 35 days, and returned to the laboratory in sea water. They were cut into sections a b o u t 7.5 m m long, and two were placed lengthwise on microscope slides for light transmission measurements as described above. Periphyton was then scraped o f f for dry weight and chlorophyll analysis.

Oxygen production Shoots of Posidonia australis or plastic 'seagrass' were transported to the laboratory at 10°C and maintained in an aerated tank of seawater at 20°C. Oxygen evolution by leaf segments was measured using a Clark-type electrode (Rank Bros., Bottisham, England) with an 8-ml vessel. Calibration was

360

with air-saturated sea water at the experimental temperature, 20°C, and with a zero obtained after the addition of sodium dethionite. Before each run, the cell was filledwith filteredsea water which had been largely deoxygenated by bubbling through nitrogen containing carbon dioxide (295 vpm). Leaf segments were held upright by a plastic-coated wire clip. The light source was as described for measuring light transmission through periphyton, directed at right angles to the leaf surface through the transparent walls of the vessel, so that the whole leaf surface was illuminated. The amount of light reaching the leaf at each setting of the lamp was estimated by substituting a half-vesselcontructed of perspex with a light sensor behind it, for the experimental vessel so that refraction due to water and perspex would be taken into account. Segments of seagrass, ca. 2.5 c m long, were cut and placed into the cell, and dissolved oxygen concentration recorded for 20 min after a lag phase (see below). Epiphytes were then removed for chlorophyll measurement and the process repeated with the leaf segments, n o w without epiphytes. Each leaf segment was measured, ground and its chlorophyll a content measured after extraction in 9 0 % aqueous acetone. Investigations were also carried out with plastic seagrass segments carrying different epiphyte loads. RESULTS

Shoot density, standing crop and leaf production The mean leaf standing crop at Woodman Point, near the sewage works outfall, was consistently less (by 65%) than at Carnac Island, the m o r e oceanic site (Table I). The difference is partly explained by shoot densities, TABLE I O b s e r v a t i o n s o n s t a n d i n g c r o p s a n d p r o d u c t i v i t y o f Posidonia australis a t t h e t w o s t u d y sites. All w e i g h t m e a s u r e m e n t s are in d r y w e i g h t , a n d m e a n s a r e a c c o m p a n i e d b y s t a n d a r d errors

Measured property

Carnac Island

W o o d m a n Point

L e a f s t a n d i n g c r o p (g m -2 ) S h o o t d e n s i t y ( n o . m -2 ) F l o w e r s ~ ( n o . m -2 ) (g m -2 ) L e a f p r o d u c t i o n ( g m -2 d a y -~ ) Leaf turnover (day) G r o w t h p e r s h o o t (rag d a y -~ ) G r o w t h r a t e (rag g-~ d a y -] ) Y e a r l y leaf p r o d u c t i o n ( g m -2 ) L e a f + f l o w e r ~ p r o d u c t i o n ( g m -2 ) ( L e a f + f l o w e r ) ~ p e r s h o o t (g)

215 522 225 286 2.7 75 5.1 12.9 949 1235 2.37

95 368 12 16 1.4 65 3.4 15.4 475 490 1.32

-+ 32 * 32

±

0.3

_+ 0.8 -+ 1.2

-+ 23 +- 11

+- 0.3 -+ 0.7 -+ 1.3

Mean for August and September, including young fruits and embryos.

361

which were on average 29% lower at W o o d m a n Point than at Carnac Island. There were no distinct seasonal trends. Inflorescences were observed in August and September at both sites, with 220--230 flowers m -2 on the two occasions at Carnac Island, but only 10--14 at W o o d m a n Point. In contrast to standing crop, leaf p r o d u c t i o n showed a distinct seasonal pattern at both sites (Fig. 2), with high production in the summer m o n t h s , and low levels in winter and spring. The fall in p r o d u c t i o n in February--April was more rapid at W o o d m a n Point. Mean leaf p r o d u c t i o n and growth per shoot were significantly lower at W o o d m a n Point (50 and 33%, respectively) than at Carnac Island.

O----

~"

CARNAC ISLAND WOODMAN POINT

4,

E

\

J

F

M

/

A

M

J

J

A

S

O

N

D

J

Month

Fig. 2. L e a f p r o d u c t i o n o f P o s i d o n i a australis at t h e t w o sites s h o w n in Fig. 1. Vertical bars are s t a n d a r d errors.

Shoots at Carnac Island generally consisted of 3--4 blades (0.37 g dry wt. shoot-I), whereas at W o o d m a n Point the older blades tended to senesce and break off earlier under a heavy e p i p h y t e load, and there were rarely more than two blades per shoot (0.2 g dry wt. shoot-I). New growth, therefore, made up a higher proportion of the shoots at W o o d m a n Point, and at Carnac Island the total leaf biomass included a higher proportion of senescent material. As a result of this, leaf replacement times were shorter at W o o d m a n Point than at Carnac Island, while relative leaf growth rates (calculated as increase in dry weight divided by total standing crop) were n o t significantly different at the two sites. The a m o u n t of leaf dry weight p r o d u c e d annually was calculated, and the dry weight of a single crop of flowers and fruits was added to this; more than twice as much production occurred at Carnac Island when compared with

362 W o o d m a n Point, and t he difference, although reduced, was still seen w hen results were expressed on a 'per s h o o t ' basis (Table I). In subsequent calculations, th e leaf p r o d u c t i o n by the seagrass m e a d o w , calculated f r o m leaf density and average g r ow t h rate per leaf has been used as t he basis for comparison.

Environmental data T h e mean c h l o r o p h y l l level in the water was significantly higher (P < 0.01; t test) at W o o d m a n Point than at Carnac Island, and so t o o was t he c o n c e n t r a t i o n o f p h o s p h a t e (Table II). However, c o n c e n t r a t i o n s o f the inorganic nitrogen c o m p o n e n t s were low, and n o t significantly d i f f e r e n t bet w e en th e sites. Ratios b e t w e e n nitrogen and p h o s p h o r u s were m u c h lower t h a n th e ratio o f a b o u t 1 0 : 1 - - 2 0 : 1 , required to sustain high p h y t o p l a n k t o n g ro w th rates, suggesting possible nitrogen limitation in t he water c o l u m n ( R y t h e r and Dunstan, 1971; G o l d m a n , 1976). As p o i n t e d o u t elsewhere (Chiffings and McComb, 1981), low levels o f inorganic nitrogen are consist e n t with rapid utilization o f available nitrogen in the Sound, while high p h o s p h a t e c o n c e n t r a t i o n s , like high c h l o r o p h y l l levels, o c c u r in waters enriched b y e f f l u e n t f r o m i n d u s t r y and sewage t r e a t m e n t . TABLE II Properties of the water column at the two study sites. Data are means (with standard errors) for February--September 1980 (n = 8) Parameter

Carnac Island

Woodman Point

Chlorophyll a (~g 1-1) Phosphate (#g 1-1) (Nitrate + nitrite)-N (~g 1- ~) Ammonia-N 0~g 1-1) N:P ratios'

2.1 13.0 5.2 9.1 5.6

4.0 23.7 6.0 8.4 5.3

± 0.8 ± 4.7 ± 1.8 ± 2.4 ± 1.8

± 1.0 ± 9.2 +- 1.8 ± 1.7 ± 2.6

' Calculation by atom for inorganic nutrients. Water t e m p e r a t u r e was at a m a x i m u m o f 24°C in J a n u a r y , and a m i n i m u m o f 15.5°C in J u l y and August; W o o d m a n Point had a slightly higher temperat u r e th an Carnac Island (at most 1°C) b e t w e e n May and July. Irradiance reaching th e seagrass beds was at a m a x i m u m o f 21 X 106 J m -2 d a y - ' in J a n u a r y , and a m i n i m u m o f 5 X 106 J m -2 day -1 in June. T h e levels b e t w e e n the experimental sites were n o t consistently different; on average, t he light reaching th e m e a d o w at W o o d m a n Poi nt was 92.4% o f t h a t at Carnac Island, which was 78.3% o f t h a t at the surface. Correlations were sought bet w e e n leaf p r o d u c t i o n and the following variables: c o n c e n t r a t i o n s o f nutrients and chlorophyll, wind, light and temperature. Significant correlations were f o u n d only b e t w e e n p r o d u c t i o n and

363 t h e i n t e r r e l a t e d variables o f light (r = 0 . 7 4 7 , P = 0 . 0 0 2 ) and t e m p e r a t u r e (r = 0 . 7 3 6 , P = 0 . 0 0 2 ) .

Epiphytes and periphyton E p i p h y t e s r a n g e d f r o m m i c r o s c o p i c f o r m s such as d i a t o m s , t o larger p l a n t s o f w h i c h t h e m o s t c o m m o n g e n e r a w e r e Ectocarpus and Myrionema ( P h a e o p h y t a ) , Polysiphonia, Laurencia, Ceramium and Melobesia ( R h o d o p h y t a ) , Ulva and Enteromorpha ( C h l o r o p h y t a ) . With few e x c e p t i o n s , similar t a x a were e n c o u n t e r e d as e p i p h y t e s o n leaves o r as p e r i p h y t o n o n glass and plastic substrates. Ulva and Myrionema w e r e n o t f o u n d o n t h e glass slides, while Calothrix was o n l y f o u n d o n glass. Ulva, Enteromorpha and Calothrix were f o u n d o n l y at W o o d m a n P o i n t . F o r a m i n i f e r a n s a n d h y d r o i d s were f o u n d o n all t h r e e substrates, b u t t h e r e w e r e v e r y few h y d r o i d s at Carnac Island. E p i p h y t e loads, e x p r e s s e d as c h l o r o p h y l l p e r u n i t leaf area, are s h o w n in Fig. 3. T h e r e is a seasonal t r e n d , w i t h low loads at t h e e n d o f s u m m e r , and an increase in m i d - a u t u m n ; t h e loads were m u c h h i g h e r at W o o d m a n P o i n t , as was t h e a m p l i t u d e o f t h e g r o w t h curve. T h e r e was a m a r k e d increase in e p i p h y t e load in mid-April at this site, and this c o r r e s p o n d e d w i t h the s u d d e n fall in leaf p r o d u c t i o n at t h a t t i m e (cf. Fig. 2). E p i p h y t e d r y w e i g h t s h o w e d similar t r e n d s t o c h l o r o p h y l l at Carnac Island, b u t w e r e m o r e erratic at W o o d m a n P o i n t , w h e r e a high load in A u g u s t - - S e p t e m b e r is a t t r i b u t e d 126-

/

105-

\

/

\

/ E

\

/

84-

~ ~

/

\

/ ~ o Z

63-

~

42-

/

\ \

/

\

/ /

\

/

\,~/

/

21-

J

F

CARNAC ISLAND W O O D M A N POINT

-

M

A

M

J

J

A

\\ S

O

j N

D

J

Month

Fig. 3. Epiphyte toad (as epiphyte chlorophyll expressed per unit area of seagrass leaf) at Woodman Point, near to the sewage effluent, and Carnac Island, in more oceanic water (Fig. 1).

364 to sand particles trapped by macroepiphytes and hydroids. On one occasion, a comparison was made between mean e p i p h y t e loads on new growth and the remainder o f the harvested leaves; on new growth at W o o d m a n Point chlorophyll epiphyte load was 0.28 (SE 0.02) pg cm-:, as compared with 0.12 (0.03) at Carnac Island; on the older leaves the figures were 6.57(1.7) at Woodman Point, and 1.65(0.50) at Carnac Island (n = 5). Rates o f p e r i p h y t o n biomass showed generally similar seasonal trends to epiphyte biomass (Fig. 4), with higher loads at W o o d m a n Point. Changes in p e r i p h y t o n dry weight were more erratic, again because of sand grain accumulation. 1.8Z~ O-----

1.5-

E

/

1.2-

/

f

A/

/

\

/

/

J

/

\

/

~. 0.9o

CARNAC ISLAND WOODMAN POINT

/ ~

0.6-

0.0

J

F

M

A

M

J

J

A

S

O

N

D

J

Month

Fig. 4. Periphyton load (as chlorophyll expressed per unit area on glass slides). Gaps in the data were due to loss of samples. The two sites are as shown in Fig. 1. For pooled data there was a high correlation between epiphyte and perip h y t o n chlorophyll a levels, and between epiphyte and p e r i p h y t o n dry weights (Table III). This provides strong circumstantial evidence that the factors controlling p e r i p h y t o n growth and epiphyte load are the same. Correlations were sought between e p i p h y t e or p e r i p h y t o n chlorophyll on the one hand, and water column n u t r i e n t and chlorophyll concentrations, wind, light and temperature on the other. The only significant correlations were with water column chlorophyll concentrations; for the epiphytes, r = 0.811, P = 0.007; for the p e r i p h y t o n , r = 0.900, P = 0.019. This is consistent with the suggestion t h a t the factors controlling p h y t o p l a n k t o n biomass are similar to those which control the loads of p e r i p h y t o n and epiphytes.

365 TABLE III Correlations between epiphyte and periphyton loads

Epiphyte chlorophyll a Epiphyte dry weight

Periphyton chlorophyll a

Periphyton dry weight

Correlation coefficient r

Significance

Correlation coefficient r

Significance

0.835

0.003

0.683

0.020

0.883

0.010

0.830

0.020

Light reduction by epiphytes T h e r e was a clear r e l a t i o n s h i p b e t w e e n light r e d u c t i o n a n d p e r i p h y t o n load (Fig. 5), t h e d a t a a p p r o x i m a t i n g t o t h e f o l l o w i n g f u n c t i o n (n = 35; r = 0.725;P<: 0.001):

f(x) = 100[1-e(-0.5x)] The r e l a t i o n s h i p f o r d r y w e i g h t (n = 23; r = 0 . 7 3 1 ; P <: 0 . 0 0 1 ) was:

f(x) = 100[1-e(-°.2x)] L i g h t r e d u c t i o n b y p e r i p h y t o n o n plastic seagrass gave a similar c u r v e t o t h a t o f p e r i p h y t o n o n glass slides. Using t h e curves f o r light r e d u c t i o n b y k n o w n p e r i p h y t o n loads, o n e m a y

l°°t J

80

J

60-

40-

20-

0

o.o

0'.2

11o

l'.s

2'.0

PerJphylon C h l o r o p h y l l

2'.s

31o

3'.2

4'.0

a /~g c m - 2

Fig. 5. The relationship between light reduction and periphyton chlorophyll on glass slides. The slides were collected from the field, and light reduction, as proportion of incident PAR, measured in the laboratory.

366

estimate the light reduction by epiphytes on seagrass leaves. The mean epip h y t e chlorophyll load at Carnac Island was 0.62 gg cm -2, which represents a mean light reduction of 31%. At Woodman Point the mean load was 5.41, which converts to a light reduction of 96%. It m u s t be remembered, however, that epiphyte loads are relatively low on new leaf material. Using the loads recorded above for this new growth, light reduction would be 7% at Carnac Island, and 15% at Woodman Point. Using this figure for new growth, and the above figures for the older leaf material, the overall reduction in light was a b o u t 35% at Carnac Island, and 63% at W o o d m a n Point.

Oxygen production The trapping of oxygen within leaf lacunae would lead to underestimates o f p h o t o s y n t h e t i c rates by the m e t h o d used here. When an experiment was started there was a lag phase, usually lasting 10 min, after which oxygen production rate increased to a constant level; presumably the lag phase was the time taken for the severed leaf lacunae to become saturated with oxygen. The rate of p r o d u c t i o n of oxygen after the lag phase was used in calculations. There was no lag phase when measurements of photosynthesis were made using p e r i p h y t o n on segments of plastic seagrass. Figure 6 shows p h o t o s y n t h e t i c rates before and after e p i p h y t e removal, plotted against light intensity. Both are typical photosynthesis versus irra-

......

200

150-

/

V

/

.E E 100-

/

?

-I

i

I

I I #,t

so0

-I

tI

//

/I

J'

//

-

50 ~

- 100

I//I

680

12100 Par

18=00

24=00

30=00

36v00

(/~E m -2 sec -1)

Fig. 6. Rates o f p h o t o s y n t h e s i s o f segments f r o m seagrass leaves, w i t h ( a b o v e ) and w i t h o u t (below) epiphytes, at different light intensities. Vertical bars are standard errors;

n=5.

367 diance curves (e.g. Steemann Nielsen, 1975; Drew, 1979), with light saturation at a b o u t 1500 p E m -2 s-2. Similar curves were obtained when rates were calculated on the basis of seagrass chlorophyll weight, rather than dry weight. Rates of p h o t o s y n t h e s i s of seagrass segments which naturally lacked epiphytes were similar to those from which epiphytes had been scraped, suggesting t h a t e p i p h y t e removal did not damage the leaves. Oxygen production rates at light saturation were measured for segments of plastic seagrass carrying k n o w n p e r i p h y t o n loads, and the relation between the two p l o t t e d (Fig. 7); this gave a linear correlation (regression through the origin) of 0.88 (P < 0.05). K n o w i n g the a m o u n t of chlorophyll present in the epiphytes, their oxygen p r o d u c t i o n at saturating light intensities could be estimated from the graph.

/

360-

~

300-

E

240-

.~

180-

•

•

-

-

o~ c 120× 0 60-

0

Chlorophyll

a (/J.g c m -2)

Fig. 7. The relationship between oxygen production at saturating light and periphyton chlorophyll a load, both expressed per unit area. For example, on one occasion the difference between o x y g e n p r o d u c t i o n by a seagrass leaf segment before and after epiphyte removal, was 235.2 pg cm -2 min -1 at saturating light. The epiphyte load on the seagrass leaf was 3.74 pg cm -2, which according to Fig. 7 might be expected to produce oxygen at a rate o f 236 + 26 (SD) pg cm -2 min -1. This suggests t h a t the photosynthetic performance o f the p e r i p h y t o n is similar to t h a t o f the epiphytes. DISCUSSION A recurring trend in these results is t h a t the seagrasses at Carnac Island are more vigorous than those at W o o d m a n Point, where a 33% lower growth per

368

s h o o t and a 29% less dense m e a d o w resulted in 50% less leaf production. There is evidence from aerial p h o t o g r a p h y that the seagrass m e a d o w s at W o o d m a n Point were once denser than t h e y are now, and it is possible that t h e y once resembled more closely those at Carnac Island. There are higher epiphyte loads at W o o d m a n Point, and higher chlorophyll and phosphate levels in the water column there. While these observations are consistent with the suggestion that increased epiphyte loads are associated with increased eutrophication in Cockburn Sound (Cambridge et al., 1986), and that nitrogen is more critical to algal growth than is phosphorus in this system (Chiffings and McComb, 1981), t h e y do n o t in themselves provide evidence for a causal connection b e t w e e n eutrophication, increased epiphyte growth and reduced seagrass production. Leaf p r o d u c t i o n rate was significantly correlated with light and temperature, which are intercorrelated, so that their separate effects are difficult to disentangle from such analysis. However, the laboratory studies show that light intensities in the Sound at the level of the seagrass m e a d o w s would be barely sufficient to light-saturate seagrass photosynthesis, especially when it is born in mind that illumination in the laboratory was at right angles to the leaf surface; that there is m u c h mutual shading in a seagrass m e a d o w ; that there are non-photosynthesising tissues to be supported; and that there are periods of darkness and low light intensities in the field. Even in these shallow waters, light must be of critical importance and any substantial increase in shading will presumably affect the growth of the seagrasses. Cambridge et al. (1985) have already suggested that shading by phytoplankton was n o t responsible for the widespread loss of seagrass from Cockburn Sound. The a m o u n t of light reaching the seagrass m e a d o w s at the t w o shallow (3 m) sites studied here did not differ significantly, and differences in seagrass p r o d u c t i o n were n o t correlated with chlorophyll levels in the water. The ability o f 'epiphytes' to colonise artificial substrates as well as living seagrass, the high correlations between the biomass of the t w o and their similar p h o t o s y n t h e t i c performance are consistent with observations elsewhere that submerged plants act largely as neutral substrates for epiphyte growth (Catteneo and Kalff, 1979; B o r u m and Wium-Andersen, 1980), although transfer o f materials can occur b e t w e e n host and epiphytes, and m a y be especially i m p o r t a n t at low ambient nutrient levels (for reviews and other references see Harlin, 1980; Bulthuis and Woelkerling, 1983; Orth and Van Montfrans, 1984). The enhanced growth of epiphytes on deteriorating meadows in Cockburn Sound, and on seedlings transplanted into them (Cambridge et al., 1986) cannot, therefore, be reasonably ascribed to leakage of nutrients from senescing seagrass leaves. The present w o r k shows that periphyton (and epiphytes) grow more rapidly and build up larger standing stocks in nutrient-enriched waters, and this is presumably, like phytoplankton growth, a response to nutrient enrichment. A further complexity is the possibility of reduced grazing of epiphytes in nutrient-rich water

369

(Orth and Van Montfrans, 1984) which we have not investigated quantitatively. However, the periphyton on glass slides was not affected to any extent by grazing, and the few slides which showed gastropod tracks were discarded. The shading of leaves by epiphytes is clearly significant, and is higher at Woodman Point (63% overall) than at Carnac Island (35%). F r o m Fig. 5, this would bring about a decrease of 80% at PAR 500 ~E m -2 s-1 at W o o d m a n Point, while at Carnac Island the reduction would be 35%. These results can be compared with those of Sand-Jensen (1977), who estimated the reduction in photosynthesis of Zostera marina L. by epiphytes to be 31%. The results are also consistent with the observations of Bulthuis and Woelkerling (1983), who have shown that epiphyte growth markedly reduces the time during which positive photosynthesis is possible for developing leaves of Heterozostera tasmanica (Martens ex Aschers.) den Hartog in Western Port and Port Phillip Bays, Victoria. When leaf production observed in the field was graphed against observed epiphyte load (Fig. 8), the resulting curve showed a negative log-linear correlation (r = 0.87; P < 0.001), and a similar curve was obtained when leaf standing crop was plotted against e p i p h y t e load. The relationship m a y be attributed to that between epiphyte loads and their attributed light reduction. Indeed the relation between leaf p r o d u c t i o n and light reduction due to epiphytes (Fig. 8) gave a correlation coefficient of - 0 . 9 0 (P < 0.01), and with a m o u n t of periphyton growing on glass slides gave a correlation o f - 0 . 7 9 (P < 0.01). It is clear that high e p i p h y t e loads are correlated with low leaf production. 3.6-

3.0- I)0

"o

2.4-

~.

1.8-

!

Z :

t+2.

0.6-

0.0

~0

,5

~0

8'0

t~0

Epiphyte Chlorophyll a (mg Ill- 2 Lear-~)

t~0

t~0

~o

,~

io

io

t~o

% Light Reduction by Eplphytes

Fig. 8. The relation b e t w e e n seagrass leaf p r o d u c t i o n m e a s u r e d in the field and e p i p h y t e load; left, load expressed as e p i p h y t e c h l o r o p h y l l per unit leaf area; right, using light red u c t i o n brought a b o u t by d i f f e r e n t e p i p h y t e loads as e s t i m a t e d f r o m the relationship in Fig+ 5. Data bulked f r o m b o t h sites over the period J a n u a r y - - O c t o b e r .

370

The calculations reported in this paper necessarily contain a number of approximations, but there seems little doubt that shading by epiphytes has a marked effect on the productivity of seagrasses, sufficient to account for the reduced growth of seagrasses in nutrient-enriched waters, and providing support for the suggestion (Cambridge et al., 1986) that enhanced epiphyte growth following nutrient enrichment has been responsible for seagrass decline in Cockburn Sound. ACKNOWLEDGEMENT

We are indebted to the Department of Conservation and Environment, Western Australia, for financial assistance and continuing interest. REFERENCES APHA, 1976. Standard methods for the examination of water and wastewater. American Public Health Association, Washington, 874 pp. Borum, J. and Wium-Andersen, S., 1980. Biomass and production of epiphytes on eelgrass (Zostera marina L.) in theq)resund, Denmark. Ophelia, suppl., 1: 57--64. Bulthuis, D.A. and Woelkerling, Wm. J., 1983. Biomass accumulation and shading effects of epiphytes on leaves of the seagrass, Heterozostera tasmanica, in Victoria, Australia. Aquat. Bot., 16: 137--148. Cambridge, M.L. and McComb, A.J., 1984. The loss of seagrass in Cockburn Sound, Western Australia. I. The time course and magnitude of seagrass decline in relation to industrial development. Aquat. Bot., 20: 229--243. Cambridge, M.L., Chiffings, A.W., Brittan, C., Moore, L. and McComb, A.J., 1986. The loss of seagrass in Cockburn Sound, Western Australia. II. Possible causes of seagrass decline. Aquat. Bot., 24: 269--285. Catteneo, A. and Kalff, J., 1979. Primary production of algae growing on natural and artificial aquatic plants: A study of interactions between epiphytes and their substrate. Limnol. Oceanogr., 24: 1031--1037. Chiffings, A.W., 1979. Cockburn Sound Environmental Study, Technical Report on Nutrient Enrichment and Phytoplankton. Department of Conservation and Environment, Perth, Western Australia, Report No. 2, 59 pp. Chiffings, A.W. and McComb, A.J., 1981. Boundaries in p h y t o p l a n k t o n populations. Proc. Ecol. Soc. Aust., 11: 27--38. Drew, E.A., 1979. Physiological aspects of primary production in seagrasses. Aquat. Bot., 7: 139--150. Goldman, J., 1976. Identification of nitrogen as a growth-limiting nutrient in wastewaters and coastal marine waters through continuous algal assays. Water Res., 10: 97--104. Harlin, M.M., 1980. Seagrass epiphytes. In: R.C. Phillips and C.P. McRoy (Editors), Handbook of Seagrass Biology: an Ecosystem Perspective. Garland STPM Press, New York, London, pp. 117--152. Kirkman, H. and Reid, D., 1979. A study of the role of the seagrass Posidonia australis in the carbon budget of an estuary. Aquat. Bot., 7 : 173--183. Nie, N.H., Jenkins, C.H., Steinbrennea, K. and Bent, D.H., 1975. Statistical Package for the Social Sciences, 2nd edition, McGraw-Hill, New York, 675 pp. Orth, R.J. and Van Montfrans, J., 1984. Epiphyte--seagrass relationships with an emphasis on the role of micrograzing: a review. Aquat. Bot., 18: 43--69. Ryther, J.H. and Dunstan, W.M., 1971. Nitrogen, phosphorus and eutrophication in t h e coastal marine environment. Science, 171: 1008--1013.

371 Sand-Jensen, K., 1977. Effects of epiphytes on eelgrass photosynthesis. Aquat. Bot., 3: 55--63. Steedman, R.K. and Craig, P.D., 1983. Wind driven circulation of Cockburn Sound. A u s t J. Mar. Freshwater Res., 34: 187--212. Steemann Nielsen, E., 1975. Marine Photosynthesis with Special Emphasis o n the Ecological Aspects. Elsevier, Amsterdam, 141 pp. West, R.J. and Larkum, A.W.D., 1979. Leaf productivity of the seagrass P o s i d o n i a australi$ in eastern Australian waters. Aquat. Bot., 7: 57--65. Zieman, J.C., 1974. Methods for the study of the growth and production of turtle grass Thalassia t e s t u d i n u m K6nig. Aquaculture, 4: 139--143.