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AND AMBIENT CONDITIONS IN A HIGHLY
DYNAMIC EGYPTIAN MARINE BASIN
W Labib
To cite this version:
W Labib. SHORT-TERM VARIATIONS IN PHYTOPLANKTON AND AMBIENT CONDITIONS
IN A HIGHLY DYNAMIC EGYPTIAN MARINE BASIN. Vie et Milieu / Life & Environment,
Observatoire Océanologique - Laboratoire Arago, 2000, pp.29-39. �hal-03186849�
VIE ET MILIEU, 2000, 50 (1) : 29-39
SHORT-TERM VARIATIONS
IN PHYTOPLANKTON AND AMBIENT CONDITIONS
IN A HIGHLY DYNAMIC EGYPTIAN MARINE BASIN
W. LABIB
National Institute of Oceanography and Fisheries, Kayet Bey, Anfoshi, Alexandria, Egypt
SHORT-TERM PHYSICAL VARIABILITY CHEMICAL VARIABILITY PHYTOPLANKTON VARIABILITY MEX BAY COURT-TERME VARIABILITÉ PHYSIQUE VARIABILITÉ CHIMIQUE VARIABILITÉ DU PHYTOPLANCTON BAIE DE MEX (EGYPTE)
ABSTRACT. - The phytoplankton standing crop variability, succession and bio-mass were assessed at a fixed station in a highly dynamic marine basin, Mex Bay, west of Alexandria (Egypt). This station was operated for 49 days between May 26 and September 4, 1992. The bay receives a daily injection of nutrients input from land-based sources. Physical and chemical variability was observed with the massive development of algal épisodes, raising chlorophyll a and oxygen content to abnormal values. The causative species of the différent blooms achieved their maximum occurrence with sharp drop in salinity and différent nutrient levels. A phytoplankton bloom is not necessary to accompany or follow a period of enhanced nutrient concentrations. The phytoplankton progressed differently and there was distinct succession in the dominance of the major species. Statistical analyses were applied to discuss the corrélation between phytoplankton variability and measured physico-chemical conditions.
RÉSUMÉ. - La variabilité, la succession et la biomasse du phytoplancton d'une station fixe située dans un bassin marin très dynamique, la Baie de Mex, à l'Ouest d'Alexandrie (Egypte) ont été évaluées. Cette station a été échantillonnée pendant 49 jours, du 26 mai au 4 septembre 1992. La baie reçoit les rejets quotidiens d'eau chargée en sels nutritifs provenant des activités agricoles. La variabilité physique et chimique observée ainsi que les épisodes de développement algal massif entrainent des valeurs anormales de la chlorophylle a et de l'oxygène. Les espèces du phytoplancton responsables des différentes phases de prolifération atteignent leur maximum lorsque les valeurs de la salinité et des sels nutritifs baissent nettement. Les proliférations du phytoplancton n'accompagnent pas ou ne suivent pas nécessairement une augmentation de la concentration en sels nutritifs. Il existe une succession distincte de la dominance des principales espèces du phytoplancton. Une analyse des correspondances a permis de mettre en évidence les coefficients de corrélation entre la variabilité du phytoplancton et celle des facteurs physico-chimiques.
INTRODUCTION
Round (1971) discussed the rôle of shock e-vents associated with changes in day-length, tem-pérature and overturn conditions, in determining species succession and growth. If the actual pro-cesses of phytoplankton changes to be understood, short term sampling proved to be advisable, instead of weekly or biweekly intervais (Winter
et al. 1975). The time-scale variations in
phyto-plankton abundance, composition and biomass can be circadian (Sournia 1974), seasonal (Harrison & Platt 1980) or vary from a few days to one year (Harris 1980). Harris & Piccinin (1980) found that changes in species
composition/abun-dance tend to average environmental variables over short scales. According to Côte & Platt (1983) physical transient events can dramatically alter the species and structural composition of phytoplankton community, conditions for growth and rate of primary production. Richmond (1986) reported that phytoplankton requires a time from a few hours to several days to adapt a new envi-ronmental condition. Studies dealing with the daily changes in plankton population are rather limited (e.g. Klein & Sournia 1987, Sournia et al. 1987, Abi-Saab 1992, Labib 1994a).
Mex Bay, west to Alexandria (longitude
29°50'E and latitude 31°10'N) has an average
width of 3 km, total area of about 20 km2 and
from Lake Maryout, through Umum Drain, a daily
of 6-11.8 x 106 m3 of agricultural wastewater,
mixed with chemical and municipal wastes. It is also affected directly by additional volume of wastewater from industrial outfalls at its western part. The discharge water into the bay is largely the cause of man-made eutrophication.
The previous investigations on the phyto-plankton standing crop in Mex Bay stressed its monthly variations in relation to physico-chemical parameters (Dorgham et al. 1987, El-Sherif 1989, Samaan et al. 1992).
The présent study represents an attempt to do-cument the importance of short-time sampling to fully describe the phytoplankton variability and ambient environmental conditions in a highly dy-namic marine basin.
MATERIALS AND METHODS
The station, 5m depth (Fig. 1) was sampled for 49 days between May 26 and September 4, 1992. Surface samples, collected during mid-day, included the déter-mination of the water température, salinity (refractome-ter, S/Mill), oxygen (Winkler method), chlorophyll a and nutrient contents, nitrate, nitrite, phosphate and silicate (Strickland & Parsons 1972). Water density calculated on the basis of température and salinity data (Williams 1962).
The phytoplankton samples were first examined for identification under a research microscope, then preser-ved by the addition of neutral formalin (4 %), and a
few drops of Lugol (acid solution) and counted (Uter-môhl 1958).
In order to summarize in a concise way this large information, corrélation matrix, correspondence analy-ses {cf. Benzecri 1970, Teil 1975) and index of spécifie diversity (Shannon & Weaver 1949) were applied. The later was used as an index of the structural properties of the phytoplankton community, computed at the spe-cies level. The value of the diversity index was expres-sed as bits, individual"1. The relationship between the diversity index and water physico-chemical properties was also calculated to verify the extent of their effect on the structure of the phytoplankton community.
RESULTS
Physical conditions
The short-term physical variations throughout the investigated period are shown in Fig. 2. The corrélation matrix is given in Table I.
The most important factors driving the proces-ses that détermine the modification of température and salinity variations seem to be the wide fluc-tuations of surface heat fluxes (with respect to the limited height of the water column) and the vo-lume of the discharge water (with respect to the whole volume of the basin).
Surface température range normally from 21 °C with the start of the period to 30 °C in late August. Two periods of remarkable température increase were recorded during the first week of June and in early July. Generally, température showed a
SHORT-TERM PHYTOPLANKTON VARIABILITY 31
Table I. - Corrélation coefficient matrix between the measured physical, chemical parameters, standing crop, chlorophyll a, community structure and dominant species from May 26 to September 4, 1992.
T°C. S%0 Sig. / Stability N03 N02 PO„ Si04 Chl a. St. crop Diatoms Dinof. Chloro. Cyano. Euglen. N. c R d. S. c. S. t. G. c. E.g. T. s. P. t.
T°C. s%» Sig. t stab. N03 N02 PO< SiO„ Chl. a st. cr. Diat. Dinof. Chlor. Cyan. Eugl. N. c. R. d. S. c. S. t. G. c. E.g. T. s. P. t.
0.38 -0.51 0.40 0.37 -0.13 0.99 -0.98 -0.98 -0.40 -0.44 0.42 ■0.50 -0.44 0.49 0.40 -0.20 -0.52 -0.45 0.49 0.24 0.85 0.22 0.39 0.13 0.35 ■0.19 ■0.12 0.11 0.36 0.29 0.25 0.14 •0.23 0.18 0.39 •0.02 0.27 ■0.44 -0.45 ■0.25 -0.30 0.04 0.02 •0.05 •0.19 0.35 ■0.10 ■0.14 0.35 0.48 0.27 ■0.04 0.04 0.47 ■0.33 0.69 0.10 •0.16 •0.12 0.11 ■0.27 •0.20 -0.20 •0.06 -0.11 0.07 0.02 ■0.17 -0.20 0.00 -0.03 ■0.17 -0.12 ■0.06 -0.10 ■0.11 -0.16 0.12 0.11 ■0.11 -0.15 0.16 -0.08 0.06 -0.06 •0.07 -0.09 0.15 -0.04 0.03 -0.09 0.15 0.14 0.09 -0.09 0.10 -0.04 ■0.11 -0.13 0.15 0.07 0.40 0.12 0.07 0.09 -0.01 0.52 ■0.05 0.04 0.58 0.80 0.46 0.14 0.23 0.31 -0.12 ■0.18 -0.17 -0.02 0.62 0.30 -0.11 0.16 0.00 0.26 -0.01 0.06 0.00 -0.15 •0.13 -0.28 0.32 0.13 0.26 -0.13 -0.11 -0.02 ■0.11 -0.02 0.35 0.65 0.70 -0.11 0.42 0.19 0.14 0.02 0.06 0.50 0.53 0.69 -0.05 0.13 -0.07 0.16 0.13 0.01 0.08 0.25 0.31 0.51 -0.07 -0.03 -0.03 0.14 0.10 0.48 0.13 0.17 0.30 -0.13 0.99 -0.10 -0.02 -0.11 -0.10 0.11 -0.04 0.18 -0.10 -0.11 0.04 -0.07 0.01 -0.16 -0.09 ■0.11 -0.24 0.32 0.12 0.25 -0.13 -0.13 -0.02 0.99 0.13 -0.05 -0.08 -0.22 0.08 -0.01 0.04 -0.03 -0.09 -0.08 0.35 0.02 -0.04 -0.08 0.41 0.06 0.10 -0.05 -0.06 -0.05 0.58 0.47 0.00 ■0.04 ■0.17 0.40 •0.21 0.07 •0.12 •0.12 ■0.11 ■0.11 0.44 ■0.21 •0.11 0.14 ■0.09 0.05 0.16 ■0.07 ■0.09 0.14 0.07 -0.11 -0.10 0.23 -0.07 -0.06 ■0.10 -0.11 -0.16 0.01 -0.15 0.33 •0.03 -0.10 0.58 0.43 Number of variables: 23 Number of samples: 49 Probability level: 95% = 0.281; P< 0.05; 99% = 0.363; P< 0.01; 99.9% = 0.44; P< 0.001
Nitzschia closterium = N.C.; Rhizosolenia delicatula = R. d.; Skeletonema costatum = S. c; Scrippsiella trochiodea = S. t.; Gymnodinium catenatum = G. c; Euglena granulata = E. g.; Thalassiosira subtilis = T. s.; Prorocentrum triestinum = P. t.
Température
M26 31 J5 J15 J30 Ju10 20 31 A9 19 30 S4
Days
Fig. 2. - Physical measurements from May 26 to Sep-tember 4, 1992.
tendency to an increase by days. Température is positively correlated to the numerical standing crop and chlorophyll a content (r = 0.13, P > 0.05 and 0.35, P < 0.05, respectively). The multiple-régression équation is :
') = 1.029 + 0.436 = 0.3, P < 0.05) Chlorophyll a (ug. 1
* température (r
Salinity exhibited wide range of fluctuations. Ge-nerally, salinity values are lower than that assu-med for the inner boundary of the Mediterranean neritic waters of Alexandria (El-Maghraby & Ha-lim 1965). Exceptionally, salinity can be high as 39 %c, but values between 20 and 33 %c are
com-mon. Such high values suggest the latéral advec-tion of the marine water from the open basin. Salinity is insignificantly correlated to the phyto-plankton counts and chlorophyll a, negatively with the latter parameter (r = 0.04 and - 0.25, P > 0.05, respectively). The régression équation is :
Chlorophyll a (ug. T1) = 15.527 - 0.111 * salinity (r = 0.1, P > 0.05)
Sigma t is strongly correlated with température, in excellent relation with salinity variations (r = -0.51, 0.99, P < 0.001, respectively), indicating that stratification of the sea is resulted predomi-nantly variations in the latter parameter.
Except for higher salinity at times, the water was well stable with maximum (14.84-16.11
du-ring 8-14 August), associated with lower salinity over the whole period (20 %c). High values of Sigma t imply small values of water stability and vice versa. Insignificant corrélation was found between the standing crop, chlorophyll a and sta-bility, négative with the first parameter (r = - 0.04 and 0.27, P > 0.05, respectively). The multiple-régression équation is :
Chlorophyll a (ug. I"1) = 7.453 + 0.586 * stability (r = 0.1, P > 0.05)
However, Chlorophyll a variations seem depen-ding upon température and stability combination :
Chlorophyll a (ug. T1) = 0.656 + 0.307
* température + 0.438 * stability (r = 0.37, P < 0.01)
Despite the very weak corrélation found between salinity and the standing crop as well as chloro-phyll a, most of the phytoplankton blooms took place with reduced salinity. This may simply re-flect the stimulation of algae by stabilization (sa-linity dépendent), associated with their massive occurrence, rather than salinity variations.
Nutrient conditions
Nutrient concentrations in Mex Bay (Fig. 3) are mainly governed by their input from land-ba-sed sources, water exchange with the open sea, the exhaustion by phytoplankton blooms at times and others.
Nitrate varied dramatically throughout the who-le period. Lower concentrations (1.26-2.8 umol. H) were measured during 9-19 June, following a red tide bloom period Scrippsiella trochoidea, the causative organism), and accompanying a minor bloom (prédominance of microflagellate species). On the other hand, 3 major nitrate peaks were
detected on 26 June (12.78.umol 1_1), 12 August
(14.79 umol.
H),
associated with distinct drop inthe standing crop around 0.013 x 106 cell. H) as
well as on 28 August (15.78 umol. l_l), with a
moderate phytoplankton increase (0.77 x 106 cell.
H,
Skeletonema costatun, Gymnodiniumcatena-tum and Nitzschia closterium dominated). Nitrate concentrations are very weakly correlated with the standing crop and chlorophyll a. The régression équation is :
Chlorophyll a (ug. L1) = 10.885 + 0.23 * NO3
(r = 0.10, P > 0.05)
Nitrite concentrations ranged between 0.35 and 3.9 umol. H.
Phosphate levels, except for its highest on
Sep-tember first (7 umol. h1, with the bloom of G.
catenatum), were always low, exhibiting a narrow range of variations. Phosphate is very weakly
cor-Nitrate (N03)
M26 31 J5 J15 J30 Ju10 20 31 A9 19 30 S4
Days
Fig. 3. - Chemical measurements from May 26 to Sep-tember 4, 1992.
related with the standing crop and chlorophyll a. The régression équation is :
Chlorophyll a (ug. L1) =10.69 + 0.85 * PO4 (r = 0.12, P > 0.05)
However, phosphate is highly positively correla-ted with the dinoflagellates (r = 0.46, P < 0.001), particularly with Scrippsiella trochoidea (r = 0.48, P < 0.001). The significant rôle played by phos-phate for the maintenance of dinoflagellate red tide species in the neritic waters of Alexandria was proved by Labib & Halim (1995), Labib (1996).
Silicate concentrations showed a wide range (7.31 - 64.93 umol. 1_1), never fell down influen-cing the phytoplankton growth. The diatom peaks in July and August occurred with enhanced sili-cate concentrations. The régression équation is :
Chlorophyll a (ug. T1) = 11.49 + 0.03 * SiO4 (r = 0.07, P > 0.05)
The very weak corrélation between the standing crop and chlorophyll a with nutrients is not sur-prising in the view of the fact that the growth of algae will resuit in consumption of nutrients, but their high daily injection to the bay leads to con-tinuous replenishment of their concentrations.
SHORT-TERM PHYTOPLANKTON VARIABILITY 33
Meanwhile, the concentrations were negatively, significantly correlated with salinity, indicating the discharge water to be their origin.
Phytoplankton variability
The phytoplankton standing crop, the spécifie diversity index, chlorophyll a and oxygen contents exhibited well marked variations (Fig. 4). The bay is a site of repeated heavy phytoplankton blooms, which resulted in abnormal biomass increase and high dissolved oxygen. The gênerai pattern is the low diversity in this eutrophicated area. Also, an inverse trend, generally, was seen between the standing crop, despite its magnitude, and the spe-cies number with the diversity index.
The standing crop attained an average of 3.55 x 106 cell. I"1, reflecting a clear sign of heavy eutrophication, with a pronounced down shift in the phytoplankton community structure. Diatoms
(31 spp.) contributed an average of 1.67 x 106
cell. 1_1, 47 % to the total, followed by dinofla-gellates (17 spp, 26.2 %). The fresh-water forms are numerous, including 9 chlorophycean species (15 %, Ankistrodesmus falcatus, Crucigenia qua-drata, Scenedesmus dimorphus and S. quadricau-da, were the major species), 6 euglenophyceans (8 %, mainly, Euglena acus, E. caudata and E. granulata), and 6 cyanophyceans (3.8 %, Lyng-bya, Merismopedia, Oscillatoria and Spindina spp.).
The phytoplankton progressed differently du-ring the investigated period. There was a distinct succession in the dominance of the major species (Fig. 5, Table II).
The dinoflagellate, Scrippsiella trochoidea, for-med a red tide bloom in the first week of June. This species was previously reported a numerical-ly important constituent of the community in the Eastern Harbour of Alexandria, culminating its major peak in late spring (Labib 1994 b), 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 symp-toms of toxicity accompanied the présent S. tro-choidea bloom, as well as elsewhere (Koray
1992). However, wild and cultured fish kills have been associated with its blooms through the gé-nération of anoxie conditions (Hallegraeff 1989). The centric diatom, Thalassiosira subtilis, contri-buted its peak on 11 June. The dinoflagellate, Gymnodinium catenatum, became leading on 8 July. This species was reported a toxic red tide species in the neritic Mediterranean waters of Spain (Estrada et al. 1988). The occurrence of G. catenatum and ambient environmental conditions were followed during the 4 years survey 1993-1996 (Labib 1998). Its dissipation was then fol-lowed immediately by the prédominance of the
Table II. - Top, maximum density of the major species, chlorophyll a content, dissolved oxygen and ambient environmental conditions during the investigated pe-riod. Bottom, corrélation coefficient matrix between the measured physical, chemical parameters and the diver-sity index from May 26 to September 4, 1992.
Peak Density Chl.a D,02 T Sal Sig.; Stab. NO: i N02 1>04 SiO„ day Species Cell. 1 "'xlO 6 MB 1" 1 mg. 1 -'«c %o umol. 1 3 June S. trochoidea 5.10 22.83 5.1 26 25 5 15.9 10.4 9. 5 2 2 5 23 11 June T. subtilis 0,76 6.74 5.3 24.6 38.2 26 1 1 1.3 0.63 1 8.7 26 July S. coslaluni 0 70 18 1 7 5 29 24.5 14.3 114 5 1.2 1. 18.4 28 July R. delicatula 4.28 35 7 6,9 30 25.5 14.8 11.3 5.4 1.3 2. 52.7 31 July N. cloterium 3 90 144 4 5 29.5 30.2 18 2 7.8 5.5 1.2 1.4 266 20 Aug. P. triestimim 3.22 33,4 6 30.6 26 14.8 114 8.6 18 2 1 25 3 21 Aug E. granulata 0.61 20.89 5 29.8 27.3 15.8 10.3 4.9 1.4 1.2 119 1 Sep. G. catenatum 0,73 23.21 7.2 29.3 26 15.1 11.2 3.4 0 35 1.1 11.9 D. index r*C Stab. N03 SiO„ Chl. s
D. index 1.00 T°C 0.34 1.00 S%. -0.19 -0.34 1.00 Stability 0.18 0.36 -0.98 1.00 NOa 0.26 0.29 -0.44 0.46 1.00 PO, -0.17 -0.24 -0.50 0.47 0.39 1.00 SiO, 0.06 0.20 -0.42 0.45 0.68 0.39 1.00 OlI 3 -0.09 0.36 -0 21 0.23 0,07 0.10 0.06 1.00 Number of variables: 6 Number of samples: 49 Probability level: 95% = 0.281; P< 0.05; 99% = 0.363; P< 0.01; 99.9% = 0.44; P< 0.001
pennate diatom, Nitzschia closterium. The
domi-nance of diatoms (Rhizosolenia delicatula,
Nitzschia closterium and Skeletonema costatun) extended during July-early August. The euglenoid, Euglena granulata, shared the dominance to a lesser degree. It is a common species in the East-ern Harbour (Labib 1994 b) and Mex Bay (e.g. El-Sherif 1989), considered as a biological indi-cator for organic pollution (Munawar 1972). Again, the dinoflagellates regained their important contribution in late August (Prorocentrum triesti-nium), and in early September (G. catenatum). The first species represented a well known red tide species in Alexandria coastal waters during the warm seasons (Labib 1994 a,b), culminating
in a maximum population size at 71 x 106cell. H
in the Eastern Harbour during April 1993 (Labib 1996), under similar physical and chemical con-ditions to the présent ones. P. triestinum blooms is characteristically found in semi-enclosed areas, subjected to land drainage, where the supply of NH4+ and NO3" is plentiful (Iizuka 1976). It is not toxic, and any occasional mass occurrence of fish mortality could be an oxygen deficiency (Labib
1990, Koray 1992).
The diversity index was at the end of the spec-trum of observed values on June 5 (0.147 bits.
individual-1), coincided with the lowest species
number (8 species) when Scrippsiella trochoidea formed its massive monospecific red ride bloom. Although, the species number was almost unchan-ged on the next day (9 species), the diversity index value increased considerably (1.27 bits, in-dividual _1) resulting from the sharing of the
o r■ ■ ■ ■ t■ • ■ • i■ ■ • ■ i■ ■ ■ ■ i■ -■ ■ i■ ■ ■ • i■ • • ■ i■ ■ ■ • i■ • • ■ i■ ■ ■ ■ i• ■ ■ ■ i■ ■ • ■ i■ ■ ■ ■ i""i'
Days
Fig. 4. - Dissolved oxygen, standing crop, diversity index and chlorophyll a from May 26 to September 4, 1992.
Dunnanella sp. and Thalassisira rotula). On the other hand, the highest diversity index (2.2 bits.
individual-1), was determined on 20 July,
associa-ted with the richest species number (24 species), despite it represented a red tide bloom period. The
six species {Euglena granulate, E. gracilis,
Nitzschia closterium, Pyramimonas sp., Rhizoso-lenia delicatula and Skeletonema costatum), were responsible for such period. This was also clear on 20-21 August. Ail thèse species were
pre-viously recorded as indicators of pollution
(Mihnea 1985).
The diversity index (average 1.17 bits,
indivi-dual-1), reflected signs of pollution. The
corréla-tion coefficient matrix between the measured physical, chemical parameters and the diversity index is given in Table II. Négative corrélation
was found with salinity, P04 and chlorophyll a.
There exists a considérable amount of literatures on the négative relationship between biomass and the diversity index in various organism
commu-nities (e.g. Ghilarov & Timonin 1972). Tempéra-ture, an allogenic process, also seems affecting the community structure (r = 0.34, P < 0.05), as previously reported by Goldman & Ryther (1976). According to Reynolds et al. (1984) stability (dri-ves from température and salinity gradients) in-fluences community changes. Connell (1978) described how environmental instability might play an important rôle in maintaining non-equili-brium diversity, depending upon the frequency of disturbance. Harris et al. (1983), Harris & Trim-bee (1986) attributed the high diversity of phyto-plankton community to the fréquent perturbation and vice versa. The présent community structure is still best explained by a non-equilibrium and allogenic processes, similar to that reported by Reynolds (1984).
The usefulness of correspondence analysis in biological oceanography has been shown (e.g. Malmgren et al. 1978). The présent data (23 va-riables, 49 observations, the dominant species :
SHORT-TERM PHYTOPLANKTON VARIABILITY 35 6000 T 4000 4 2000 0 S. trochoidea 'I 1 l1I 'I ' i 1 " i 1 I 1 I 1 I 1 I1I 1 I 1 ! 1 I 'I' I1 I 'I 'I 1I1 I 'I 1 I 1 I1 i 1 I 1 I 1 I 1I ' I 1 I 1 I 1 I'' ' I ' I 1 I ' I'1 ' I ' I ' T. subtilis 800 600 400 200 0 W
, ^ A.ra
, , | , | , | | , ; , | , | , , , | , ; , i , | , , , | | , | , , , , , , r| <^ , , , , , ,,t ; | |^fj»fl , | , |, | , | Prorocentrum thestinum f» i1 4000 T 2000 4 0 I 'l't't'l ' I ' I ' I ' i ' I 't't't i ' I ' I '1 'I 'I 'I 1111 'I ' I ' 111 ' I ' I ' I 'I 'I N 't'i ' I -11 i ' M-fTTI ' 111 ' I ' I ' I 'I 'I IV. closterium 6000 -c 4000 | . 2000 1 AA
À rj 800 600 400 200 -f 0 G. catenatum i111111 ■!1:11 M111 [ ' i111 i1i111 r i i1 r i t11111 r i1111111 r i1 r i '1 800 ~ 600 400 200 0 5000 2500 0 E. granulata 11111 r i ■ i ' M *M1111 ' i1 i-l1M111111 ' r l >V r-PFW 1 A"i R. delicatula i111111 ' i1 i1111111 r l1 ■f i11111111111 r i111111 ■ rftf^l 5. costatum 800 600 400 200 0 H 'I'I' I ' 11 i ' I ' I ' I 'I T* 11 i111111 ' 11111■!11 ■ i 111111111111 "r* l't 26 31 5 10 15 20 25 30 5 10 15 20 25 30 4 9 14 19 24 29 3 M Ju M. Au SFig. 5. - Dominant phytoplankton species from May 26 to September 4, 1992.
Nitzschia closterium, 1 ; Rhizosolenia delicatula,
2 ; Skeletonema costatum, 3 ; Scrippsiella
trochoi-dea, 4 ; Gymnodinium catenatum, 5 ; Euglena
gra-nulate,
6 ;
Thalassiosira
subtilis,
1 ;
and
Prorocentrum triestinum, 8), were subjected to
correspondence analyses to correlate the
phyto-plankton standing crop, community structure and
dominant species with environmental variables.
The corrélation between the axes, the
environ-mental and the biological variables is shown in
Fig. 6 a,b. The analysis indicated that axis 1 and
2 were responsible for 43.55 % and 37.76 % of
the total variability of phytoplankton classes and
dominant species, respectively. The data suggests
that salinity, sigma t, but mainly water stability
are the main contributors, affecting the
phyto-plankton variability. Stability controlled the
varia-bility of dinoflagellates, it was less with
cyano-phytes (Fig. 6 a), in agreement with the matrix
corrélation. However, it seems to have no effect
on the dominant phytoplankton species, slightly
influenced the dinoflagellate, S. trochoidea (Fig. 6
b). Nevertheless, stability was weakly, positively
correlated with the diversity index (r = 0.18, P >
0.05).
The data also suggests that température seems
influencing chlorophyll a, as also proved from the
corrélation matrix. The standing crop and its main
constituent, diatoms, are positively, significantly
correlated with each other (r = 0.8, P < 0.001),
but not affected with the 3 above mentioned
com-ponents. The nutrients are very closely related
with each other. Their négative, significant
corré-lation with salinity suggests the discharge water
1.0 0.8 0.6 0.4 * 0.2 0.0 0.2 0.4 a. - (a) • st crop _ • Chl.a -#Chlo. m • T°C Eug. ^Stab. •«* — #SiOi, Sa Sigma t 1 1 Di" N03*#P0* • -N02 1 1 1.2 -0.8 -0.4 0.0 0.4 0.8 1 Ax i s 1
Fig. 6. - Correspondence analyses (a, community structure; b, dominant species).
to be their main origin. The occurrence of both cholorophytes and euglenophytes seems also cor-related (Fig. 6 a).
Generally, chlorophyll a runs in parallel with the numerical standing crop, highly significant, correlated with each other. Several peaks were
recorded (maximum of 38.9 ug. 1_1 on 22 July).
Déviations are mainly due to species composition. The régression équation is :
Chlorophyll a (ug. T1) = 9.06 + 2.03 * St. crop (r = 0.52, P < 0.001)
DISCUSSION
The présent data showed Mex Bay, subjected to daily input of a huge volume of discharge water, to be characterized by distinct physical,
chemical and biological structural properties. The repeated algal outbursts raised chlorophyll a con-tent and oxygen to abnormal values, reflecting in clear way signs of eutrophication. The low diver-sity of community structure supports such conclu-sion.
The daily injection of the nutrients from land-based sources and the establishment of water sta-bility probably accelerated the phytoplankton growth in Mex Bay. Yet, a bloom is not necessary to accompany or follow a period of enhanced nutrient concentrations and even intermediate va-lues are sufficient to trigger a phytoplankton peak. Thus, it is difficult to discuss quantitatively the extent of their effect on phytoplankton standing crop, biomass and composition, because of the variability in the environmental gradients, in agreement with the finding of Mukai & Takimoto (1985) and Richmond (1986). On the other hand, the applied statistical analyses stressed the
signi-SHORT-TERM PHYTOPLANKTON VARIABILITY 37
ficant importance of water stability to control phytoplankton variability, particularly dinoflagel-lates, while no a particular factor was defined to regulate the time variability and succession of the dominant species, due to the différent response of the différent species to changes in the surrounding environment. The fast replenishment of nutrients limited their controlling effect. However, it is from importance to mention that any statistical results must remain tentative for différent reasons. According to Margalef (1978) and Levasseur et al. (1984), land-runoff favors algal blooms, not only for its nutrient contents but also for its po-tential rôle stabilizing the water column. Accor-ding to several authors (e.g. Perry et al. 1983), water stability détermines not only the magnitude of phytoplankton production, but ultimately leads to community change or species succession. Har-ris (1983) and HarHar-ris et al. (1983) showed that fluctuations in stability at scales of about 10 days was responsible for the diversity of a plankton community. The data agrée with other results in the Egyptian Mediterranean neritic waters (Labib 1992, 1994 a,b, 1996, Labib & Halim 1995).
The phytoplankton species seem to have dif-férent nutrient requirements. The pennate diatoms (Rhizosolenia delicatula and Nitzschia closterium, the causative species in July and August), required low nutrients to dominate the community, as poin-ted out by several authors (e.g. Ishizaka et al. 1983). The species, Nitzschia frigida, predomina-ted under similar conditions in the Eastern Har-bour of Alexandria (Labib 1994 a). On the other hand, dinoflagellate species in the présent study achieved their maximum occurrence under nu-trient enriched water masses.
The success of the pennate, Rhizosolenia deli-catula, to grow well under the dinoflagellate bloom of G. catenatum agrées with other obser-vation in Alexandria waters (Labib 1994 a,b).
The présent phytoplankton community consis-ted of a relatively small number of species, which could affect the diversity index, but the low di-versity resulted mostly from unequal relative abundance and a situation typical of eutrophic waters with massive monospecific blooms or of a small number of species.
The présence of major events, as the sudden triggering of red tide blooms and the rapid change in community structure, at scales of a few days déclares that weekly or bi-weekly sampling, poin-ted out by Horn (1984) to be essential, may only be used to display the broad patterns of seasonal succession of species. It is insufficient in a highly dynamic marine ecosystem subjected to distinct daily variations. Attempts to actually describe the underlying process require much higher frequency sampling.
It is concluded that short-time scale sampling in an transient area of wide environmental fluc-tuations is necessary to détermine fully, its phy-sical, chemical, the standing crop variability, and the rate of community changes.
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