GROWTH DYNAMICS AND MORPHOLOGY OF PLANKTON GREEN ALGAE FROM BRACKISH WATERS UNDER THE INFLUENCE OF SALINITY, TEMPERATURE AND LIGHT

Nine unicellular green algal species isolated from the Gulf of Gdansk phytoplankton were studied. The species were cultured within the salinity range of 0-35% at temperatures of 5-38C and at light intensity (PAR) ranging within 20-380 μE/ms. Four species were typical of brackish water, five being freshwater plants. In terms of temperature requirements, groups of three species each belonged to high temperature, mesothermophilic, and low-temperature strains. Most species required relatively high light intensities as growth of six of them was saturated at 120 μE/ms.


INTRODUCTION
Every food chain in aquatic ecosystems begins with planktonic algae which are the major producers of organic matter for the community. As algae are autotrophs, they support all marine life directly or indirectly. Knowledge of biology, and parti cularly the autecology of phytoplanktonic species as well as information on their responses to different environmental factors is therefore of great importance (Round 1977). Identification of the basic environmental factors influencing algal growth and distribution is possible to a certain extent through in situ experiments. However, reliable data can be obtained only under controlled laboratory conditions. The combi· nation of the abovementioned methods affords proper knowledge of phytoplankton (Fogg and Thake 1982).
Strains isolated and cultured in the laboratory do not necessarily represent the total population of species in a water body studied. However, it should be noted rela· tively few data point to differences between laboratory cultures and natural popula· tions (Fogg and Thake 1982).
Ecology of planktonic algae from the Gulf of Gdansk is interesting because of the non-specific features of the physicochemical environment. The relatively low salinity and a steady influx of suspended matter, pollutants and nutrients accelerating eutrop· hication are the main factors which greatly affect changes in the phytoplankton spe cies composition (Plinski et aL 1985). The Gulf oJ Gdansk phytoplankton consists of marine, brackish water and freshwater species. Green algae are one of the important groups, being particularly abundant in late spring and summer.
The experiments presented were aimed at elucidating effects of the major environ· mental factors such as salinity, light, and temperature on growth and morphology of some unicellular green algae isolated from the Gulf of Gdansk phytoplankton.
Most of those species are typical of the Gulf of Gdafisk, i.e. in some periods of the growth season they are found at all sampling sites situated in different parts of the Gulf (Plifiski et al. 1985). Pure algal cultures were isolated from natuhal plankton communities and belong to the Laboratory of Marine Plant Ecology collection, Gdafisk University (Lataia, unpubl. manuscript).
Experiments were carried out in 300 cm 3 Erlenmayer flasks, each containing 150 cm 3 of the medium. The' medium was prepared from the Atlantic water (35%0) or from water taken at station GN 18° 48' E, 54° 32' N) situated in the central part of the Gulf of Gdafisk. As in the F/2 medium (Guillard 1975), the water was enriched with nutrients and microelements (without metasilicate and vitamins). The batch cultures were stirred vigorously once a day. The inoculum was taken from the cultures grown at 18 ± l 0C, 27; µE/m 2 s, ar.d 7.35 % •• The experimental cultures, 5 replicates each, were not pre-adapted to a new temperature, light, and salinity condi t ions.
The light intensity (PAR) measurements were carried out using an FF-01 phyto photometer (made in Poland). The cell number was determined by counting the cells in a Burker chamber. To determine cell morphology of the species examined, the mean cell size of 50 cells at the exponential growth phase was calculated. The cell volume was estimated as recommended by Edler (1979). Regression analysis was performed and correlation coefficients between the cell volume and different light intensity : temperature or salinity were calculated. Statistical significance of regre ssion equations and correlation coefficients was tested using Student's t test and the analysis of varience. All the regression equations and correlation coefficients were significant at ex = 0.05.

RESULTS
To obtain detailed information on requirements of individual species, wide ranges of salinity, temperature, and light intensity were used.
Five of the species studied show the best growth in fresh water (Species 2, 3, 4, 5) or at a very low salinity of 2.5 %.(Species 6). They also exhibit low resistance to higher salinity as shown by a decrease in their growth at 15 or 20 % •• Fig. 1 presents the growth curves for one of these species, Monoraphidium contortum. Three of the spe· cies tested (Species 1, 8 and 9) grew most luxuriantly at 7.35 %. (salinity typical of the Gulf of Gdafisk), Stichococcus bacillaris tolerating also the wide range of salinity (0-30%.and even 35%� applied. The growth curves of S. bacil/aris are shown in Fig. 9. 2. Oocystis submarina (Species 7) shows the best growth at a relatively high salinity (11.5-15 %.) and is tolerant of the whole range of salinity used, i.e. 0-35%, (Fig. 3).
Different salinities considerably affect morphology of the green algae studied, except for Oocystis po.rva. An increase in cell volume with increasing salinity was observed. Scenedesmus armatus showed the highest correlation coefficient (0.80) between the cell volume and salinity within 2.5-25 %., R 2 for this species being 64.2% Fig. 4. presents the scatter of data obtained with the regression line. Relatively high correlation coefficients, i.e. above 0.55 R > 30%), were obtained in 5 species (Spe cies 1, 3, 4, 5, and 9).
Effects of temperature on growth of the green algae studied was diversified. Five species (Species 1, 2, 3, 6, and 9) grew best at high temperatures (26-30°C), Scenedes mus acuminatus yielding the most luxuriant growth even at 34°C Typical growth cur ves for Chlorella vulgaris cultures at different temperatures are presented in fig. 5. species mentioned exhibit a poor growth orcease growing at 5-lO°C. The remain ing species examined showed an optimal growth at l8°C and grew also at the lowest temperature used (5°C), but did not Ioterate the highest (38°C). of such relationships can be found in the growth curves of · Oocystis submarina presen ted in Fig. 6. Except for the two Scenedesmus species, effects of temperature on algal morpho· fogy was diverse and of a minor importance only. The cell volume of Scendesmus acu minatus decreased with temperature increasing from 18 to 38"C Fig. 7 presents results of the measurements together with the calculated regression values. The correlation coemdents were r"' 0.65 and R 2 = 42%, Scenedesmus armatus showed also a certain correlation between the cell volume ,md temperature in the range of 10-30°C.
The algal species studied have high light requirements. Six species (Species 1, 2, 4, 5, 6, and 9) grew best at a high radiation intensity (PAR of 120 µ E/m 2 s) and a 16:8 L:D cycle, Le. the minimum quantum illumination causing growth saturation fa equal to 7 E/m 2 s·d. Fig. 8 shows growth curves at different l.ight intensities for one of the species Monoraphidium contortum" The species above grew also at the highest radiation intensity but their was rather poor. Two of the species examined (Species 3 and 8) showed the best growth at a lower ., I !) . , t, · ,  (Species 3, 4, 5, and 6). On the other hand, increasing light inten sity was observed to cause various changes in cell volume in other species (Species  1, 2, 7, 8, and 9). The highest correlation coefficients (t = 0.597; R 2 = 3.565%) were The growth cycle of algal species as well as their photosynthetic or enzymatic acti· vities show a strong dependence on salinity changes. The optimum salinity for growth of many planktonic algae was determined (Braarud 1961;Guillard and Mylestad 1970) . .According to Proskina-Lavrenko (1963), the optimal salinity for a given species is the same as in its natural habitat. Results of the present study show that 3 species only (Species 1,8,9) grew best in a salinity typical of the Gulf of Gdafisk, whereas the majority of the algae cultured prefered fresh watero This fact confirms the opinion on the typical freshwater nature of most planktonic green algae (Starmach 1963).
Decreasing salinity brings about a significant decrease in the algal metabolic rate and, in consequence, dwarfish organisms in brackish ecosystems are observed (Re mane and Schlieper 1958)0 The present results conform with this statement The cell volume of each species tested, except for Oocystis parva, decreased gradually with decreasing salinity. The correlation coefficient and R 2 values indicated that the salini ty fluctuations are responsible for 30% of the cell volume changes in 6 species, the contribution of salinity variations reaching 64% in Scenedesmus armatus. Marked dif ferences were found to exist in the cell size before and after cell division. In spote of the high variability of cell volume, the correlation coefficient and R 2 values show a significant relationship between the cell volume and salinity.
Temperature is doubtless one of the most important factors controlling growth of many algal species. At low temperatures, all biochemical processes proceeed at a slow rate, low temperatures inhibiting the metabolic activity and growth of the organism. On the other hand, high temperatures could result in enzyme inactivation and soluble protein coagulation. However, during evolution, organisms developed certain mecha nisms relieving them from strict temperature dependence (Bayliss 1960 ). In most Chlorococcales, the optimal growth temperature is in the range of 20-30°C (Felfoldy 1961;Jankowski 1964). Komarek and Ruzicka (1969) divided the strains of chloro coccal algae into several groups according to their temperature optima. Following this division, Chiarella vulgaris (Species 1), Scenedesmus armatus (Species 2), and S. acumi natus (Species 3) belong to the high-temperature strains with temperature optima between 30 and 38°C. Monoraphidium contortum (Species 5) and M. griffithii (Spe cies 6) belong to the mesothermophilic strains with temperature optima of 20-30°C, whereas Scenedesmus acutus (Species 4), Oocystis submarina (Species 7), and 0.parva (Species 8) belong to the low-temperature strains with temperature optima below 20°C. This classification would place Stichococcus bacillaris (Ulotrichales) among the meso thermophilic algae. To supplement the classification with the results obtained in this study, one can say that the mesothermophilic species do not grow at l0°C and below, whereas the low-temperature species tolerate S°C but do not grow at 38°C.
Temperature affects also the cell size. Species living in cold waters attain greater dimensions than their warmer water relatives. Laboratory experiments demonstrated the cell size to often decrease with increasing temperature. Such a relationship was shown for the diatom Skeletonema cos ta tum (Jorgensen 1968) and the green alga Scene desmus quadricquda (Komarek and Ruzicka 1969). In the present study, the correla tion between the cell volume and a wide range of temperature was noted in two Scenedesmus s p ecies (Species 2 and 3) as well. The largest temperature effect on cell volume was observed in Scenedesmus acuminatus, 42% of changes in the cell volume being ascribed to temperature.
Algae, particularly the planktonic ones, are able to rapidly and actively adapt to different light conditions. The process involves an increase in the chloroplast pigment content, changes of the pigment ratios, and chloroplast movements within the cell (Steemann Nielsen 1975). When the increasing light intensity limits the cell division rate, the algal biomass increases up to the light saturation of growth. Light intensities saturating the growth of different algal species are generally in the range of 40-100 µE/m 2 s (Chang 1980;Durbin 1974;Paasche 1968). Sorokin and Krauss (1958) examin ed several green algae and reported that growth of Chlorella vulgaris was saturated at 250 ftc: (about 50 µE/m 2 s). Similar results were obtained in the present study, Le. 45-120 µE/m 2 s. Most species cultured here showed relatively high light demands. In 6 species, growth was saturated at 120 µE/m 2 s. High light intensities inhibit algal growth the relevant literature data ranging within 120-600 µE/m2 s. Sorokin and Krauss (1958) found that the values in green algae were between 200 and 600 ftc (120-400 µE/m 2 s).
Komarek and Ruzicka (1969), however, demonstrated that growth of Scenedesmus qua dricauda was saturated at about 600 µE/m 2 • Most species studied here (except for Oocystis submarina) tolerated the highest light intensity used (380 µE/m 2 s), but their growth was not so pronounced as that at lower light intensities.
The results obtained here confirm such a relationship only in five of the species stu· died. The highest correlation was obtained in Oocystis parva with increasing light intensity being respoonsible for 35% of the C<"ll size increase.