Dinoflagellate 'brawn tides' in Alexandria, Egypt waters ...aquaticcommons.org/19263/1/PJMS9_033.pdf · Pakistan Journal of Marine Sciences, Vol.9(1 & 2), 33-49,2000. DINOFLAGELLATE

Pakistan Journal of Marine Sciences, Vol.9(1 & 2), 33-49,2000.

DINOFLAGELLATE "BROWN TIDES" IN ALEXANDRIA, EGYPT WATERS DURING 1997-1998

WagdyLabib National Institute of Oceanography and Fisheries, Kayet Bey,

Anfoshi, Alexandria, Egypt.

ABSTRACT: The dinoflagellate species, Scrippsiella trochoidea, Gymnodinium catenatum, and Prorocentrum triestinum, caused water discoloration at four intermittent periods during June, July, and August 1997 in Mex Bay, as well as during September ·-1998 in the Eastern Harbour of Alexandria, Egypt Among the environmental variables contributing to their massive occurrence in Alexandria waters was the establishment of a thermo-haline stratified water column. The daily injection of nutrients from land-based sources leads to continuous replenishment, resulting in an uncontrolling growth factor. Zooplankton pressure could inflict losses on the population of S trochoidea, but its bloom was relatively immune to grazing in case of S. trochoidea at its maximum population size. No symptoms of toxicity accompanied the different blooms. However, the appearance of the toxic, Gymnodinium catenalum, can lead to undesirable consequences.

KEY WORDS: Dinoflagellates, Red tide, Ambient conditions, Grazing.

INTRODUCTION

The biological and environmental factors that have been identified as being potentially important for the occurrence of dinoflagellate red tide blooms include hydrodynamic structure (Cullen et a!., 1982), diel vertical migration (Anderson and Stolzenbach 1985; Labib and Halim, 1995), life cycle (Anderson eta!., 1983), tempocature, salinity, nutrients, and/or trace metals effects (Watras eta!., 1982; Labib 1996, 1998), and grazing (Huntley 1982; Labib and Hussein 1994). But the complexity of the problems surrounding the blooms and the numerous species involved, have hindered prediction and modelling efforts.

Grazing by herbivorous zooplankton is often a major loss factor for phytoplankton, and when grazers are abundant, they can suppress bloom development (Uye 1986). Accordingly, when algae are adapted to escape grazing in some way, this is a beneficial· strategy to increase their net growth rate as long as nutrients are available. It has been demonstrated that several phytoplankton species are grazed to a lesser extent than others due to their size and/or shape (Graneli et a!., 1993), or their ability to clump in aggregates (Hansen et al., 1992). It has been also shown that some toxic phytoplankton are toxic to their potential grazers (Uye and Takamatsu 1990).

Eutrophication in Alexandria waters has gradually intensified over the last decades and the progressively heavier eutrophication is associated with problems created by red tides and other noxious blooms, which became regular events in the warm season. Despite the fact that the first red tide bloom in the Egyptian Mediterranean waters was reported in the Eastern Harbour of Alexandria in 1958 (Halim, 1960), this phenomena received very limited attention during the last decades. Yet, the intensive work to verify a variety of biological and ambient factors affecting the massive occurrence of red tide species was monitored during the last few years (Labib 1992, 1994a,b, 1995, 1996, 1997, 1998; Labib and Hussien 1994; Labib and Halirn 1995).

34 Pakistan Journal ofMarine Sciences, Vol. 9(1& 2), 2000.

The Mex Bay, west of Alexandria, receives directly agricultural wastewater, mixed with agrochemical, domestic and industrial wastes (daily average 6.5xl 0 6 m3

). The Eastern Harbour is a semi-enclosed estuarine located in the central part of Alexandria City. The harbour is subjected directly, to municipal wastewater (35.2x10 6m3

, annually). The area between the Mex Bay and the Eastern Harbour has also been affected, intermittently, by additional volume of discharged raw sewage, estimated annually as 90xl0 6 m3

•

The overall objective of the data presented here, derived from daily sampling, is to clarify environmental variables that may be contributory to the occurrence of the dinoflagellate "brown tides" in the Mex Bay during 1997 and in the Eastern Harbour, Alexandria,_ Egypt-during 1998. The grazing suppression was determined to evaluate its impact on the bloom dynamics of Scrippsiella trochoidea in the Eastern Harbour.

MATERIALS AND METHODS

The study area and the location of the sampling stations are shown in Figure 1. The sampling stations (about 5m depth), were operated at intermittent periods: during June, July, and August 1997 in the Mex Bay, and during September 1998 in the Eastern, Harbour. The "brown water" was seen to spread over the two areas. Samples were taken at 50 em below the surface. The measured physicochemical parameters include; water temperature (thermometer accurate to± 0.1 oc), salinity (salinity refractometer), dissolved oxygen (Winkler method), dissolved inorganic nitrate, ammonia, phosphate, silicate, and chlorophyll a (hereafter, Chl. a) (Strickland and Parsons, 1972). Salinity and temperature were also measured above the bottom. The phytoplankton samples were frrst examined for identification, then preserved by the addition of Lugol's solution for quantitative estimation (Utermohl, 1958). Zooplankton samples were collected from the Eastern Harbour from 1st to 7th September with a 50!-lmmesh net (50 em diameter). The net was towed at the surface. The filtered seawater volume is about 2m3

• Zooplankton species were identified and counted using an inverted microscope.

RESULTS

Water discolouration was observed at intermittent periods during June, July, and August 1997 in the Mex Bay and during September 1998 in the Eastern Harbour. The measured physicochemical data, dissolved, oxygen, Chl. a content, and the concentrations of the causative species are shown in Figures 2, 3, 4, and 5.

TheMexBay The first bloom (19-27 June) Physicochemical characteristics of the water: The surface water temperature

increased as days went by, reaching 26.5-26.8 °C with the massive occurrence of the bloom on 23-24 June. The surface water was wanrier than the above bottom (maximum difference of 3-3.3 °C with the bloom peak). This difference was reduced to 2.5 oc on the last three d.ays. The surface salinity was always low (21-26 %o). The over bottom salinity fluctuated within a narrow range (38-39.5 %o); the highest values appeared with the decline of the bloom.

Labib: Dinoflagellate "Brown Tides" in Alexandria, Egypt

. ; 30 00

MEDITERRANEAN SEA

c

5 Km. '--'---'--'----'---" 3 0 Q 0 ~

0 ,

3Z 00

35

Sea

Fig. L a. Egyptian Mediterranean coastline. b. Alexandria coastline. c. Mex Bay. Eastern Harbour and location of the sampling stations (· ).

Nutrient concentrations exhibited the same trend: relatively higher levels with the onset of the bloom (36.5, 6.12 and 5.9 11M.l-1 for ammonia, nitrate and phosphate, respectively; noticeable drop on 23 June (5, 4.5 and 4!-lM.l·\ respectively), followed by very fast replenishment on the next day, as well as with the dissipation of the bloom.

Description of the bloom: The water discolouration covered most of the bay. It was weakly noticed in its eastern part on 27 June and no longer visible on the next day. The bloom was attributed to the proliferation of the dinoflagellate Scrippsiella trochoidea (78-91% to the total numerical standing crop). There was a remarkable increase on 20 June (S. trochoidea at 0.75xl06 cellJ-t, ChL a 6.71-lgJ-I, oxygen 5.1 mU-1

). The development of the bloom was very rapid, -s trochoidea attaining its peak on 23-24 June (4.lxl06

- 4.5xl06 celH\ with a maximum Chi. a,_l8.5!lg.I-I, oxygen 8.511 mi.l-1). The

dissipation of the bloom occurred on 27 June (0.17xl06 cell.I-1).

The accompanied, numerically insignificant phytoplankton species with the bloom were: Rhizosolenia delicatula, Nitzschia closterium, Prorocentrum triestinum, Ceratium furca, Ceratium fusus,and Euglena granulata. The first species dominated the community with the dissipation of the bloom on 27 June (0.23xl06 cell.l·t, 73.5% to the total).


28 Temperature 40 Salinity __ __.

/~ 37

~ ~_____. 34

u 24 Surface ~ 31

0

,

~ 28

25 • /

Bottom 22 /~ .. 20 19

40 Ammonia Nitrate

8

jfl;

30 7 ,/ .-

';" I

0 20 \ ~~ i 6 E ::::!... E

~ :::l1

10 5

0 4

7 !Phosphate Oxygen

12

6

~A~ 10 j\ 5 8

0 E "E jfl;~ \ =\ 4 6

/ ~

3 ""-

4 • 2 2

20 Chlorophyll a 5

S. trochoidea

4 (\ 15 "' 0

-

I "'": 3 o,

~ iii 2 10 u

J " 5 0

19 20 21 22 23 24 25 26 27 19 20 21 22 23 24 25 26 27

June June

Fig. 2. Physicochemical parameters and density of Scrippsiella trochoidea in the Mex Bay, Egypt, between 19 and 27 June 1997.

Labib: Dinoflagellate "Brown Tides" in Alexandria, Egypt 37

Temperature 40

Salinity 29

~ ~ 35

0 26 % 0

30

23 25

Ammonia Nitrate 20 11

18 9

16 -=--:....:

0 14 0 7

E §. r :a.. 12 5

10

8 3

Phosphate Oxygen 6 7

5

/)\ 6

0 4 E e ~ I 5

3

2 4

Chlorophyll a G. catenatum 16 2

12 "' 0

~ "":

0, 8 ~ ~ a;

0

4

~ 0 I I 0 9 •10 11 12 13 14 9 10 11 12 13 14

July Juiv

Fig. 3. Physicochemical parameters and density of Gymnodinium catenatum in t:6.e Mex Bay, Egypt, between 9 and 14 July 1997.


Temperature Salinity 30 40

29 38

36 0 28 ';;F.

0

34

27 Bottom 32

26 30

Ammonia Nitrate 30 13

25 11

I .. 20 0 0 E 9

E 15 :::1

~ 10 7

5 5

0 3

8 Phosphate Oxygen

10

~

8 0 E 4 ::!._ e

6

0 4

Chlorophyll a P. trfestinum 40 12

32

.. 8 0 - 24 '"': 01 ~:....

~ 16 -;

0 4

8

0 0 3 4 5 6 7 8 9 3 4 5 6 7 8 9

August August

Fig. 4. Physicochemical parameters and density of Prorocentrum triestinum in the Mex Bay, Egypt, between 3 and 9 August 1997.

Labib: Dinoflagellate "Brown Tides" in Alexandria, Egypt

31 Temperature 40 Salinity

~~ f /\ S"rtaoe :: ~ u2si~ .. ~ 37 ~

0 271 /~ ~ 36 J/ v 26 ~ Bottom 35

25 34

24 ~ I I i f I I ! i I 33 I l I l f' I 1 1--+-!

4

20

15

34

30

26

'L; 22 Cl 18 ~

14

Nitrate

Silicate

ChioroplhyU a

10

6+4~;-4--r-r-r~~-+~

29 30 31 1 2 3 4 5 6 7 August - September

4

3

0 2 E ::1

C!J

1

6

5

3

u 2

Phosphate

Oxygen

S. trocho!dea

1

0+-+-+-+-+-4-~~~~~

. 29 30 31 i 2 3 4 5 6 7

August ~ September

39

Fig. 5. Physicochemical parameters and density of Scrippsiella trochoidea in the Eastern Harbour, Alexandria, Egypt, between 29 August and 7 September 1998.


-------- Scrippsiella trochoides -<>-Zooplankton

:J 80

60 "'o '"'o -: """":

3 "! 40 e

'Oj ciJ u 2 ....

20 0

0 0 1 Sep. 2 3 4 5 6 7

Days

Fig. 6. Zooplankton population (org.m3.103), and density of Scrippsiella trochoidea

(cell.l-1.106) in the Eastern Harbour of Alexandria, Egypt, from 1st to 7th

September 1998.

The second bloom (9-14 July) Physicochemical characteristics of the water: Compared with the June bloom, the

record one maintained higher temperature and salinity range (27-28°C and 30-37.5%o, respectively), with differences of about 3°C and 5-8.8%o between the surface and over the bottom; lower ammonia (11-18.5!-lM.P, and phosphate (2.5-4!-lM.l-1

), and almost unchanged nitrate. The dissipation of the bloom occurred with higher salinity, and reduced difference of 1.5%o between the surface and over the bottom.

Description of the bloom: The discoloured water appeared suddenly due to the growing, Gymnodinium catenatum. This species contributed 0.25xl06 cell.P, 24% to the total on 9th July. The euglenophycean, Euglena granulata contributed 0.48x106 celU\ 22% to the total. The bloom peak on 12th July, G. catenatum at 1.7xl06 cell.!-\ consumed about 50%, 39% and 55% of ammonia, nitrate and phosphate concentrations, respectively, of the day before. The accompanied Chl. a with the peak was 13.8!-lg.I-\ and oxygen 6.5 ml.l-1

• However, the second peak on 13th July (G. catenatum, l.lxl06

cell.l-1), occurred at higher nitrate of 6.5!-lM.l-1

.

Other minor species with the bloom period were: Protoperidinium diabolus, P. depressum, Prorocentrum micans, Scenedesmus bijugatus, S. dimorphus, Rhizosolenia fragilissi'ma, Skeletonema costatum and Euglena granulata. The last three species dominated the community with the dissipation of the bloom on 15th July (0.23xl06 cell.!-\ 30%; 22% and 9.5%, respectively).

The third bloom (3-9 August) Physicochemical characteristics of the water: The bloom triggered at higher

temperature (29•C), lower salinity than July (23%o), but higher levels of ammonia


(21).1M.l-1), nitrate (12.4).1M.l-1

), and phosphate (6.5).1M.l-1). The bloom peak on 7th

August took place at 29.8"C, 30%o, difference of 2.3°C and 8.3%o between surface and over bottom, moderate ammonia (9).1M.l-1

), high nitrate (7.8).1M.P), and low phosphate (2.9).1M.l-1

). Salinity and nutrient concentrations increased by the dissipation of the bloom on 9th August.

Description of the bloom: 'fhe brown to reddish water covered aU the bay. The causative species was Prorocentrum triestinum. The community with the onset of the bloom showed predominance of the diatom Skeletonema costatum (0.8xl06 ceUJ-r, 63.5% to the total, Chl. a 5.5).lg.l-1

). Prorocentrum triestinum started to be an active component of the community by 5th August (0.85xl06 celiJ-1, 33.5%, Chl. a at 6.5jlg.l-1

).

The bloom peak took place on 7th August (P. triestinum, 11.6xl06 celUt, 90%, Chl. a, 3l.2jlg.l-1 and oxygen, 9.8 mU-1

• The accompanied, S. costatum, formed 8% to the total. The bloom started to dissipate on 9th August (P. triestinum at 0.65xl06 ceU.I-1, 12%). Again, S. costatum dominated on the next day (l.lxl06 ceUJ-\ 61%, Chl. a, 6.6).lgJ-1

).

The minor phytoplankton species with the bloom period were mainly the dinoflagellate species: Protoperidinium depressum, P. diabolus, P. steinii, S. trochoidea, and G. catenatum (collectively contributed 1.1% to the total on 7th August).

The Eastern Harbour (29 August- 3 September 1998): The causative species was Scrippsiella trochoidea. Another followed the dissipation

of this bloom within a few days, due to the overwhelmingly dominance of the centric diatom, Cyclotella nana, contributing about 9.1x106 cell.!-\ 98.25% of the total community.

Physicochemical characteristics of the water: Comparing the present data to that of June 1997 in the Mex Bay, when the same species, S. trochoidea, formed its massive occurrence, this bloom triggered at higher surface temperature (28"C) and salinity (36.5%o), but much lower nutrient concentrations. Surface temperature showed a gradual increase with the bloom development till its peak on 2nd September. This was followed by a sharp decrease of about 2.5°C on the next day. The bloom maintained a temperature difference of 2-2.6°C between surface and over bottom (5m). Surface salinity exhibited a reverse trend: its value increased by 2.5%o on 3rd September, reaching 39 .5o/oo two days later. The bloom peak occurred within 2%o between surface and over bottom. This was followed by another reduction, and salinity fall down to its minimum of 35.5%o on 7th September, accompanying Cyclotella nana, red tide bloom.

Nitrate was relatively high with the start of the bloom, and was almost exhausted two days before the bloom peak. However, S. trochoidea achieved its peak with 2.86j1M.l-1 of nitrate. On the other hand, phosphate was relatively low with the beginning of the bloom, reaching its highest with its peak on 2th September (3.15~M.l-1). Silicate showed a wide range of fluctuations, falling to 0.5 ).lM.l-1 on 7th September.

Description of the bloom: The discoloured water was seen to cover most of the harbour, more dense in its southwestern area. Scrippsiella trochoidea, contributed 77.32-98.17% of the total community between 29th August and 3rd September. There was a


gradual increase during the first four days of the bloom. This species attained its population peak of 5.64xl06 cell.l-I, raising Chl. a to 28.3!-lg.l-1 on 2nd September. However, Chl. a was maximum on the next day (32.1!-lg.l-1

), due to the active sharing of the accompanied diatom, Bellarochea malleus (0.196x106 cell.l-1

). The density of S. trochoidea dropped dramatically by 4th September (0.17xl06 cell.l-I, 29.77% of the total), and the bloom was no longer visible. Another water discolouration was then observed on 7th September, due to Cyclotella nana.

Other phytoplankton species within the bloom period were: C. nana, R. delicatula, Chaetoceros affine, B. malleus, Phacus triqueter. G. catenatum, P. triestinium, P. micans, P. minimum, and Euglena spp.

Zooplankton population: The zooplankton dynamics in the Eastern Harbour from 1st to 7th September 1998

are shown Figure 6 and Table 1. Zooplankton population was relatively dense on lst September, mainly represented

by Protozoa spp., and to a lesser extent by copepods. Then, a drastic drop occurs: the zooplankton density fell down to its minimum, coincidentally with the bloom peak of S. trochoidea. Protozoa were still dominant. A sharp rise observed on the next day (about a 5-fold increase). The community changed, Ostracoda spp. with copepods and spionid larvae were majors. Polychaete larvae shared an active role (12.1% to the total population). There was a steady zooplankton increase on the following two days with a maximum on 5th September (62030 org.m3

). Copepod nauplii and cirripid larvae were the main constituents, followed by spinoid larvae on the latter day. The density decreased on the last two days (6-7th September). However, the community composition was almost unchanged.

DISCUSSION

The land-runoff discharges, rich in organic substances and nutrients from human settlements, certain industries and agricultural activities are largely the cause of manmade eutrophication in the investigated area. According to the trophic classification of OECD (1982), the study area was considered as eutrophic. However, the present data declared hypertrophic conditions (chlorophyll a > 30!-lg.l-1

) in August 1997 and September 1998.

The present red tide blooms appear to be driven by physicochemical forcing, the continuous external nutrient input, and a quasi-permanent stable stratification of the water column. Water stability, which determines not only the magnitude of phytoplankton production, but ultimately leads to community change or species succession (Estrada et al., 1988), proved to be a crucial factor affecting the development of red tide blooms in the neritic waters of Alexandria (Labib 1998; Labib and Halim 1995). - Nutrient concentrations changed considerably within the bloom periods and between the two sites. Their daily injections from land-based sources led to continuous replenishment. These blooms triggered at enhanced nutrient concentrations and their peaks of S. trochoidea in the Eastern Harbour took place with a high phosphate level, in

Tab

le 1

. Z

oo

pla

nk

ton

pop

ulat

ion

(Org

anis

m.m

'3 ), a

nd

th

eir

freq

uenc

y p

erce

nta

ge(

%)

to t

he t

ota

l po

pula

tion

in

the

Eas

tern

Har

bo

ur

from

1 t

o 7

Sep

tem

ber

199

8.

Day

s

1 Se

p.

2 Se

p.

3 Se

p.

4 Se

p.

5 Se

p.

6 Se

p.

7 Se

p.

Dom

inan

t Org

anis

ms

Cou

. %

C

ou.

%

Cou

. %

C

ou.

%

Cou

. %

C

ou.

%

Cou

. %

Cir

ripi

d la

rvae

-

. -

-27

32

7.81

81

50

19.6

14

366

23.2

28

73

11.3

18

84

10.1

Cop

epod

s 59

46

19.9

15

70

22.2

97

17

26.2

22

704

54.7

25

812

41.6

87

13

33.7

73

96

39.5

Cop

epod

nau

plii

20

60

6.7

628

8.89

68

30

19.5

10

645

25.7

16

815

27.1

61

23

24.1

35

32

18.8

Cop

epod

ite

stag

es

589

1.97

47

1 6.

67

1554

4.

32

5700

13

.7

5888

9.

49

565

2.22

23

6 1.

2

Ost

raco

da s

pp.

1460

4.

88

864

12.2

70

65

20.2

89

5 2.

16

1743

2.

81

307

1.21

28

2 1.

5

Pol

ycha

ete

larv

ae

2178

7.

28

275

3.89

42

39

12.1

14

1 0.

3 -

-13

19

5.19

16

49

8.79

Pro

tozo

a sp

p.

8184

27

.4

1766

25

22

14

6.33

38

15

9.2

1460

2.

35

1178

4.

46

659

3.52

Rot

ifer

a sp

p.

3003

10

19

6 2.

77

1178

3.

36

--

612

1.0

--

471

2.51

Spi

onid

lar

vae

2296

7.

68

118

1.66

64

35

18.4

70

7 1.

7 12

717

20.5

29

20

11.5

30

62

16.3

Tot

al P

opul

atio

n 29

910

7065

34

995

4149

5 62

030

2538

7 18

746

----·-·-·-

r ~

& u s· 0 t::!l

j:>)

(Jq

(]

> - -j:>) (t

td

>-j

0 ~ ..., ~ {/)~

s· >

~ § §:

_p

tr:l ~ .-+ ..j

::,.

\N

44 Pakistan Journal of Marine Sciences, Vol. 9(1& 2), 2000.

agreement with other red tide data (Labib, 1992, 1996, 1998; Labib and Halim, 1995). Several authors (Pybus, 1980), demonstrated that dinoflagellates, during their bloom peaks, can produce extracellular organic phosphorus and its conversion into inorganic phosphate as these blooms declined. The great variability of the accompanied nutrient concentrations with the different blooms could be attributed to their different nutrition, which leads to difficulties in associating their mass occurrence in nature, with any particular nutritional mode (Passche et al., 1984). The present blooms occurred with environmental conditions similar to that reported by others from elsewhere (e.g., Silva, 1985).

The accompanied phytoplankton species with these bloom periods had a very low diversification, mainly represented by dinoflagellates. Diatoms usually dominated the community with the termination of these blooms. Different mechanisms have been suggested to explain causes for repression or exclusion of algal competitors: the release of inhibitors (Hellebust, 1974), success of grazing, trace metal requirement, the NIP ratio in the sea and the shortage of Phosphate (Dahl eta!., 1989). Arzul et al. (1993) observed that cell free sea water from a bloom of the harmful dinoflagelalte, Gyrodinium aureolum, inhibited the growth of the diatom Chaetoceros gracile.

The unarmoured, chain-forming dinoflagellate Gymnodinium catenatum was reported to produce paralytic shellfish poisoning (e.g., Ikeda eta!., 1989). A few years ago, this species was first recorded from the estuarine waters of Alexandria, Egypt, during the four year survey 1993-1996 (Labib, 1998). Apparently no cases of toxicity accompanied its blooms in Alexandria waters. Hallegraeff et al. (1989) reported the occurrence of G. catenatum to be closely linked with nutrient input from land-runoff, forming a bloom in the southern Tasmanian waters, Australia, at a temperature range 12-l80C and salinity of28-38%o. This species disappeared from the water column when the water temperature fell below ll-12oC. A cultural experiment (Labib, in preparation), showed the optimal growth of G. catenatum at a temperature of 25°C, salinity 30%o, and light intensity of 500!-i-E mz- s·1 (12:12 LD cycle). Limited growth was observed at the low temperature (15°C) and the high salinity (35%o). The toxicity, in case to be detected for G. catenatum in the Alexandria waters, can lead to undesirable consequences. These effects, in turn, have consequences for the economics of this region, of which the sea represents a valuable resource, as for the public health. The accumulation arid retension of the toxins of the dinoflagellate Gonyualax excavata by the two herbivorous zooplankton, the copepods, Acartia clausii, and barnacle nauplii, were demonstrated by White (1980).

Scrippsiella trochoidea was previously reported a numerically important species in the Eastern Harbour of Alexandria, culminating its major peak in late spring (Labib 1994b), with a combination of high nutrient concentrations and·a density stratified water column. It is a well known red tide species (e.g., Park, 1991 ). No symptoms of toxicity accompanied the present S. trochoidea bloom, as well as elsewhere (Koray 1992). However, wild and cultured fish kills have been associated with its blooms through the generation of anoxic conditions (Hallegraeff, 1992). This species induced no mortality of Artemia larvae at any time (Demaret et al., 1995).

Prorocentrum triestinum represented a common red tide species in Alexandria coastal waters during the wam1 seasons (Labib, 1994 a,b), contributing a maximum


population size at 71xl06 cell.l-1 in the Eastern Harbour during April1993 (Labib, 1996), under similar physical and chemical conditions to the present one. Prorocentrum triestinum blooms are characteristically found in seri:ri-enclosed areas, subjected to land drainage, where the supply ofNH4

+. and NOJ- is plentiful (Iizuka, 1976). This species had the ability to undergo diel vertical movement crossing steep temperature and salinity gradients (Labib, 1990; Mikb.ail, 1997). The interaction of the phased cell division and the diel vertical migration of P. triestinum, under its bloom condition, was investigated (Labib, in preparation). Cells in different division stages were seen most of the time, yet, a distinct division gate was observed before dawn. An experimental study (Labib, 1995), suggested a relationship between the growth stimulation of P. triestinum and the release of substance( s) from the bottom sediments under poor oxygen and high ammonia levels above the bottom, and the essential role for the combination of factors, nitrogen, phosphorus and trace metals, rather than their individual effects. These mechanisms could influence the occurrence of P. triestinum. According to Eppley and Harrison (1975), dinoflagellates forming blooms in stratified waters could continue their growth on the basis of nitrogen obtained during night sojourns in nitrate-rich water at the thermocline.

The considerable increase in nutrient concentrations with the termination of the different blooms may lead to a conclusion that nutrients seem to be an uncontrolling factor. The change in the water stability, a result of temperature reduction, but mainly salinity increase, could share dissipating processes. Thus, other biological factors, such as zooplankton pressure and catch of filter feeding fish, could be affective.

The zooplankton community was well diversified, exhibited considerable changes in their species composition, with the dominance of herbivorous zooplankton at times. The data confirm the contribution of zooplankton as an affecting factor terminating the bloom of S. trochoidea. Thus, grazing activity can inflict losses on the population of S. trochoidea. However, this bloom was relatively immune to grazing in case of S. trochoidea maximum population size (5.6x106 cell.l-1

). The decrease of the zooplankton density with the bloom peak was attributed also to the mono specific nature of the bloom, i.e., the absolute concentration of S. trochoidea and the inadequate other algal nutritional food. The copepod, Acartia hudsonica, and larvae of the polychaete Polydora spp_, do consume Gonyaulax tamarensis in the presence of other phytoplankton species (Anderson et al., 1983). The tintinnid, Flavella ehrenbergii, also feeds on the same species within rather narrow size limits (Stoecker et al., 1981). There was an active avoidance of copepods of the dinoflagellate Prorocentrum triestinum in the Eastern Harbour, when this species formed its peak of 71x106 cell.P (Labib and Hussein, 1994), similar to the results of Huntely (1982). Reasons for the dissipation of the previous bloom included the break down of the thermohaline stratification of the water column. Hansen et al. (1992), in a culture experiment, postulated that F. ehrenbergii was able to graze on the toxic dinoflagellate Alexandrium ostenfeldii when its cell concentrations was less than 2x1 06 ceU.l-1

, but higher concentrations caused backward swimming of the ciliate followed by swelling and lysis of cells_ Verity and Stoecker (1982) reported that the growth rates of the tintinnids, Tintinnopsis tubulosoides and Flavella spp., were reduced in the presence of 102-103 Olisthodiscus luteus cells_ml-1 in multialgal treatment. Lethal effects were observed for both tintimiid spp. at 0. luteus concentration of 5xl03


cells.ml·1• Growth rate inhibition was proportionately greater at higher 0. luteus

densities. Adult rotifers and copepods show reduced filtration rates when present with 0. luteus or in combination with acceptable food species (Tomas and Deason, i 981 ).

The rapid change in the zooplankton community could be attributed to the overwhelmingly dominance of S. trochoidea that can disrupt the plankton food chain, altering the abundance and species composition of microzooplankton communities. The ability of zooplankton to feed selectively (Chotiaputta and Hariyama, 1978), suggests that grazing on other phytoplankton may be intensified during S. trochoidea blooms. Thus, energy transfer into marine food chains may not be simple functions of phytoplankton concentration and zooplankton abundance. The size spectrum, abundance, and species composition of both groups may have a significant impact on the dynamics of phytoplankton and zooplankton populations.

The present study confirms a noticeable contribution of the zooplankton pressure on the dynamics of a red tide species. Considering the occurrence of the toxic dinoflagellate species, G. catenatum, and others .in the Alexandria waters; Alexandrium minutum (Labib and Halim, 1995); A. ostenfieldii (Mikhail, 1997), and the fact that planktonic herbivores may act as vectors of the dinoflagellate toxins (White, 1980), the eutrophication in the coastal areas of Alexandria must be brought under control to avoid toxic algal impacts on humans and on the economy. However, the grazing pressure in the present study was based on numerical data. Grazing impacts are particularly difficult to estimate because of the spatial and temporal variabilities in the densities of zooplankton and S. trochoidea population and diel variability in grazing activity. At the same time, the vertical migration behaviour of S. trochoidea, if present, could be. very effective in minimising losses due to grazing.

In summary, the present study emphasises the importance of ambient factors to the changes of the different bloom dynamics. However, factors controlling these processes still remain obscure. Clearly, the understanding of the development and succession of blooms of many dinoflagellates will require more emphasis on life cycle stages and systematic investigations into the subtle factors that regulate their occurrence in natural waters. The interaction of nutritional factors with hydrodynamic conditions and the sediment release and exchange also require further evaluation for the predictability of algal blooms. Man made eutrophication in Alexandria waters is still beneficial to marine life due to activation of the food chain through increased algal biomass and production. However, the occurrence of toxic dinoflagellate species can affect the marine processes that alter the natural dynamic equilibria and the biotic composition of the respective ecosystems, leading to undesirable consequences. Although, the data declared zooplankton pressure an active dissipating factor of the occurrence of S. trochoidea bloom, its rapid termination mechanism has not been clarified yet. Other biological factors, such as the catch of the filter feeding fish must deserve attention.

ACKNOWLEDGEMENTS

The author deeply appreciates the great help offered by Mrs. Nabila Kodb, Institute of Oceanography and Fisheries, Alexandria, in the analysis of zooplankton samples.


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(Received: 20 April, 1999)

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