Seasonal adaptation phytoplankton

Seasonal temperature changes make interpretation difficult in the case of the phytoplankton also. Steemann Nielsen and Hansen (1961) measured the light saturation onset parameter, Ek, of phytoplankton in the surface layer in Danish coastal waters at the prevailing temperature at intervals throughout a year. It varied about three-fold, being highest in midsummer and lowest in midwinter. A comparison of the variation in Ek, daily insolation and water temperature throughout the year (Fig. 12.16) suggests that most of the seasonal change in the light saturation parameter can be attributed to the change in temperature; however, the value of Ek in the spring seems to be somewhat greater than would be expected on the basis of temperature alone and it may be that the rise in Ek at this time of year is in part due to adaptation of the photosynthetic system to the higher irradiance.

The values of a and P*m for the phytoplankton of a Nova Scotia coastal water were found by Platt and Jassby (1976) to vary about five-fold over a period of 2.75 yr. Measurements were carried out at the prevailing water temperature. Values tended to be highest in the summer and early autumn but subsidiary peaks at other times of the year were also sometimes observed. Statistical analysis indicated that photosynthetic capacity

Fig. 12.16 Seasonal variation of saturation onset parameter (Ek) of phyto-plankton, temperature and total daily irradiance at a Danish coastal station. Plotted from data of Steemann Nielsen and Hansen (1961). The temperature (--O--) and Ek (—•— values are for the surface layer and Ek values were measured at the prevailing temperature. Irradiance values have been converted from lux to quanta m~2s_1 using an approximate factor. The values for total daily irradiance ( ) are for days of average cloudiness throughout the year.

Fig. 12.16 Seasonal variation of saturation onset parameter (Ek) of phyto-plankton, temperature and total daily irradiance at a Danish coastal station. Plotted from data of Steemann Nielsen and Hansen (1961). The temperature (--O--) and Ek (—•— values are for the surface layer and Ek values were measured at the prevailing temperature. Irradiance values have been converted from lux to quanta m~2s_1 using an approximate factor. The values for total daily irradiance ( ) are for days of average cloudiness throughout the year.

(P*m, light-saturated photosynthetic rate per mg chlorophyll) was strongly correlated with temperature but that a (the initial slope of the P versus Ed curve) was not. These findings are in accordance with the known insensi-tivity of the photochemical processes, but sensitivity of carboxylation, to temperature.

The Ek value of the phytoplankton in a Japanese pond was found to vary about three-fold during the year, rising to a maximum in the summer and falling to a minimum in the winter;34 however, these measurements were carried out at the prevailing water temperature.

Even when the effects of temperature are allowed for, there is the problem of determining whether any apparent seasonal change in the photosynthetic behaviour of the population is true ontogenetic adaptation within species, or merely represents changes in which species are present, since there are marked successional changes in the taxonomic composition of the phytoplankton population through the year, both in the sea1115 and in lakes.1120 A study by Durbin etal. (1975) has shown how the relative importance of different types of phytoplankton (separated in terms of size), both as components of total biomass and as contributors to primary production, varied in a northeast American coastal water throughout most of one year (Fig. 12.17). Larger cells (from 20 mm to

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Fig. 12.17 Seasonal variation in the contribution of different size fractions of the phytoplankton to total biomass and to primary production in a north temperate coastal water (by permission, from Durbin, Krawiec and Smayda (1975), Marine Biology, 32, 271-87). The data are for Narragansett Bay, Rhode Island, USA. (a) Cumulative graph of chlorophyll a content of the different size fractions. (b) Upper: cumulative graph of photosynthetic carbon assimilation (per unit volume) of the different phytoplankton size fractions. Lower: photosynthetic carbon assimilation by the different size fractions expressed as a percentage of assimilation by the total population.

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1973 1972

Fig. 12.17 Seasonal variation in the contribution of different size fractions of the phytoplankton to total biomass and to primary production in a north temperate coastal water (by permission, from Durbin, Krawiec and Smayda (1975), Marine Biology, 32, 271-87). The data are for Narragansett Bay, Rhode Island, USA. (a) Cumulative graph of chlorophyll a content of the different size fractions. (b) Upper: cumulative graph of photosynthetic carbon assimilation (per unit volume) of the different phytoplankton size fractions. Lower: photosynthetic carbon assimilation by the different size fractions expressed as a percentage of assimilation by the total population.

>100 mm), mainly diatoms, dominated the winter-spring bloom, while small cells (<20 mm), mainly flagellates, dominated the summer population. Even within given size classes the species composition changed with time: for example, different diatom species were dominant in each of the three abundance peaks during the February to May period.

The photosynthetic capacity of the phytoplankton population in a Canadian Pacific coastal inlet was found by Hobson (1981) to rise rapidly many-fold from a very low value in March to April up to a peak in the June to August period, and then to decrease rapidly in the mid-August to September period, down to a very low value again in October. Although the measurements were carried out at the prevailing water temperature, and although Pm for any given species tends to increase with temperature (§11.3), it seemed that only a small part of the variation (~17%) was accounted for by the changes in temperature. The major changes in Pm in early April and August/September coincided with changes in the taxo-nomic composition of the phytoplankton, this being dominated by unidentified nanoflagellates at times when photosynthetic capacity was low, and by diatoms (Chaetoceros, Thalassiosira) or dinoflagellates (Gymnodinium, Peridinium) when capacity was high. Hobson suggested that the nanofla-gellates, which dominated the phytoplankton in the winter, were genetically adapted to conditions of short days and low irradiance.

Situations like these, with a continually varying species composition of the phytoplankton population, are probably the rule rather than the exception in fresh as well as marine waters and so any apparent seasonal adaptation of phytoplankton photosynthetic characteristics (after allowing for temperature effects) is in most cases attributable to changes in the algal species present, i.e. is phylogenetic rather than ontogenetic adaptation. It is possible, of course, that species such as Skeletonema costatum, which in many coastal waters are present in significant amounts throughout the year, do indeed adapt to the changing irradiance values, as they are known to do under laboratory conditions, but this is hard to demonstrate in the presence of all the other species. A further complication is that even within the population of a given phytoplankton species at a given geographic location there can be marked genetic heterogeneity, and changes in genetic composition through the seasons. By analysis of isoenzyme patterns and physiological characteristics of individual clones of S. costatum isolated from Narragansett Bay (RI, USA), Gallagher (1980, 1982) was able to show that the winter bloom populations of the diatom were on average genetically different from the summer bloom populations, although neither were genetically homogeneous.

In high-latitude and high-altitude lakes, the phytoplankton in the dim light under the ice in winter have a higher cellular chlorophyll content and a lower light-saturation intensity than the phytoplankton present in the summer.660,668,1366 In one such case, Lake Paajarvi, Finland, different algal species were dominant at the different times of the year.660 In an Austrian alpine lake, however, the species composition did not vary greatly through the year, being mainly dominated by the dinoflagellate species Gymnodinium uberrimum,1360 and Tilzer and Schwarz (1976) attributed the much higher (two- to four-fold) chlorophyll content, and lower Ek values, of cells under the ice to actual adaptation within the species.

Evidence for comparatively short-term photosynthetic adaptation of phytoplankton, over periods of a few days, has come from a number of field studies.656,1063,1178 Platt and Jassby (1976) found the value of a for phytoplankton in Nova Scotia coastal waters to be correlated with the average solar irradiance over the previous three days. In Lough Neagh (N. Ireland) Jones (1978) found that the light saturation onset parameter, Ek, for the dominating blue-green algal population was positively correlated with the average daily irradiance over the previous five days, i.e. the higher the average irradiance in the previous five days, the higher the light intensity required to achieve saturation. The changes in Ek appeared to be due more to changes in a (the initial slope of the P versus Ed curve) rather than in the light-saturated rate (Pm) per mg chlorophyll (a = P*m/Ek). Since a = df*fm, these changes can plausibly be attributed to an increase in the specific average absorption coefficient per unit chlorophyll, associated with a decrease in the cellular chlorophyll content at higher light intensity (§12.2). In view of the short time period during which the changes occurred, we may reasonably assume that they correspond to true onto-genetic adaptation within the species rather than to changes in the population structure.

Thus it may be that while there are substantial seasonal changes in the photosynthetic properties of the phytoplankton population as a whole, due to the succession of species, there are also, on a shorter time scale, adaptive changes in the photosynthetic properties of individual species, during the few weeks that they persist as significant members of the population.

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