Temporal variation in photosynthesis

The short answer to the question 'When does aquatic photosynthesis take place' is that it takes place when and to the extent that the various limiting constraints we have already discussed permit it to take place. Thus, to understand temporal variation in photosynthesis we need to know the manner in which these limiting factors vary with time.

Considering diurnal variation first, there is of course no photosynthesis at night. Photosynthesis begins at dawn and ends at dusk. The total photosynthesis of the whole water column roughly follows the approximately sinusoidal variation of solar irradiance during the day.535,659,1337 Under still conditions there may be a reduction of photosynthetic rate per unit volume near the surface due to photoinhibition. Also, where dinofla-gellates are the dominant organisms, their downward migration to lower light intensities can lower photosynthetic rate per unit volume near the surface in the middle part of the day.1360 There have been many reports of variation in the photosynthetic capacity (i.e. photosynthesis in samples taken from the water body and exposed to saturating light) of phyto-plankton populations and benthic algae during the day (see, for example, the reviews by Sournia (1974), Raymont (1980) and Harris (1980)). This is discussed later (§ 12.6).

In temperate or Arctic/Antarctic latitudes, there is marked seasonal variation in aquatic photosynthesis. This approximately follows, and is also a major contributing factor to, the seasonal variation in plant biomass. For the marine phytoplankton a detailed account has been given by Raymont (1980). In the winter, phytoplankton biomass and total photosynthesis are both very low, partly because of low solar irradiance1157 but even more because, as a consequence of lowered temperature and winter storms, thermal stratification is lost and the mixed layer through which the phytoplankton are circulated comes to exceed the critical depth for net column photosynthesis (see §11.1). In the spring, with increased heating of the surface water resulting from increased daily insolation, thermal stratification sets in, circulation below the critical depth is prevented, and this, combined with the increase in incident PAR and the availability of nutrients in the water, leads to a massive increase in phytoplankton biomass, typically of the order of 20-fold, measured as chlorophyll. Riley (1942) found that for a number of stations over the Georges Bank (Gulf of Maine) during the spring bloom the rate of phytoplankton increase (late March to mid-April) was approximately inversely proportional to the depth of the mixed zone. He concluded that the balance between the effects of vertical turbulence and the increasing vernal radiation determines the beginning of the spring bloom (see also § 11.1). When vertical wind mixing is absent or weak, the spring phytoplankton bloom can begin even before thermal stratification is established.1373

Typically, the spring bloom is followed by a decline to a lower level of biomass and integral photosynthesis in the summer, caused by zooplankton grazing and possibly also by a loss of nutrients from the mixed layer resulting from the sedimentation of zooplankton faecal pellets down below the thermocline. In the autumn, the fall in temperature and increased wind strength lead to intermittent disruption of thermal stratification with consequent transport of nutrients up from below the thermocline. Thus, in oceanic and continental shelf waters in intermediate and high latitudes, there is typically a second phytoplankton bloom and associated increase in areal photosynthesis in the autumn: this is terminated when disruption of the thermocline becomes so severe that stratification disappears, circulation below the critical depth sets in and productivity declines to the low winter values. Figure 11.7 shows the variation in photosynthetic rate throughout the year in North Pacific coastal water illustrating the large spring, and small autumn, peaks.734 Broadly speaking, as latitude increases, the growing season becomes progressively shorter and the spring and autumn phytoplankton peaks tend to merge into one.

The pattern of temporal variation for temperate latitudes described above is by no means universal; it can be greatly modified by local conditions. In the Kattegat (Baltic), lowered salinity (and therefore lower density) in the upper layer maintains stratification throughout the year, and so productivity is substantial at all times of the year, diminishing somewhat in the winter due to reduced light availability.1297 In coastal waters so shallow that the mixed depth rarely or never exceeds the critical depth, production also tends to remain high throughout the year.

Fig. 11.7 Variation of photosynthetic rate per unit volume throughout the year in North Pacific coastal water (after an average curve derived by Koblents-Mishke, 1965, from literature data).

Seasonal variation in production may follow the temperature cycle if the water is so shallow that light is non-limiting.1471

Data obtained by Dandonneau et al. (2006) on the basis of sea-surface sampling along 12 trans-ocean cruises, over the period 1999 to 2002, between France and New Caledonia via New York, Panama and Auckland, confirmed that variability is strongest at high latitudes where the phyto-plankton biomass and population structure have large seasonal cycles. The spring bloom in the North Atlantic was marked by chlorophyll concentrations often >0.5 mg m~3, and by high concentrations of fucox-anthin, an indicator of diatoms, while in the summer, chlorophyll was low and populations were dominated by Prochlorococcus. Tropical areas had low seasonal variability and were characterized by relatively high divinyl chlorophyll a and zeaxanthin (photoprotective carotenoid) concentrations. Barlow et al. (2007) used pigment analysis to characterize the seasonal succession of phytoplankton, and the associated changes in chlorophyll a and accessory pigments in subtropical waters of the Atlantic, Indian and Pacific Oceans in the southern hemisphere. Under low temperature and irradiance conditions, the photosynthetic carotenoids were prominent, but as temperatures and irradiance increased there was a significant increase in the proportion of photoprotective carotenoids, these eventually becoming the largest component of the pigment pool, exceeding chlorophyll a, at the highest temperature and irradiance values.

With the advent of satellite remote sensing of ocean colour, it has now become possible to monitor seasonal phytoplankton changes from space.

Stramska (2005) used SeaWiFS data to examine the year-to-year variability of timing, intensity and spatial distribution of phytoplankton chlorophyll during spring-summer seasons in the north polar region of the Atlantic, over the period, 1998 to 2004. Each year the phytoplankton bloom varied in onset, temporal evolution and intensity to a degree largely controlled by local weather. Timing of the bloom was delayed in years in which there was a high rate of turbulent kinetic energy (TKE) supply to the ocean in March, by wind stirring. In April, phytoplankton chlorophyll and rate of TKE supply were inversely related to one another. Vertical mixing in the ocean increases with the rate of supply of TKE. Perez et al. (2005) analysed the seasonal variability of phytoplankton in the Equatorial Atlantic from 1998 to 2001, using SeaWiFS-derived chlorophyll a concentration data. They used the remotely sensed chlorophyll a values at monthly intervals to compare phytoplankton changes at locations in the Eastern (10° W) and the Western (25° W) provinces. At both sites chlorophyll a levels rose to a maximum around August, but the concentrations achieved were about twice as high at 10° W as they were at 25° W. Romero et al. (2006) used SeaWiFS-derived monthly chlorophyll data for the seven-year period, 1998 to 2004, to study seasonal to inter-annual variability of phytoplankton over the Patagonian shelf and shelf break, in the western South Atlantic. North of 45° S, chlorophyll a blooms initiated in early austral spring (Sept-Oct), while south of 45° S, blooms began in late spring to early summer (Nov-Jan). Productivity was higher (>4 mg chl m~3) than in the open ocean (<1.5 mg m~3). Chlorophyll a concentrations in the northern mid-shelf sharply decayed in late spring, reaching their lowest values in summer (Feb-March), followed by a secondary maximum in early winter (June). All the highchlorophyll a regions were associated with well-defined fronts.

In the turbid estuarine waters of San Francisco Bay, Cloern (1991) found that the spring phytoplankton bloom each year was associated with the density stratification that resulted from the seasonal increased input of fresh water to the estuary. On a shorter time scale, rapid phytoplank-ton growth occurred during neap tides (low tidal energy, weak vertical mixing), but populations declined during spring tides (high tidal energy, intense vertical mixing).

In tropical oceanic waters where there is no seasonal variation in the hydrological regime, there is usually no pronounced variation in phyto-plankton biomass and photosynthetic production. In regions where there is a seasonal upwelling of nutrient-rich deep water, such as parts of the west coast of Africa, there is an accompanying massive increase in phytoplankton and photosynthetic production. Seasonal variations in river outflow can also cause seasonal variations in primary production in tropical coastal waters.

Inland waters in intermediate and high latitudes show marked seasonal variation in biomass and total photosynthesis. Both are low in the winter due to a decrease in irradiance (sometimes accentuated by ice or snow cover) and temperature and, in the deeper waters, circulation of the phytoplankton through a depth greater than the critical depth. Increased irradiance and temperature in the spring, together with the availability of nutrients and the onset of thermal stabilization, lead to a phytoplankton bloom and associated increase in areal photosynthesis. The behaviour during the rest of the seasons tends to be both complex and highly variable. Production may remain consistently high, or there may be wide fluctuations in populations and production caused by zooplankton grazing, fluctuations in nutrient content due to influxes of nutrients from the hypolimnion associated with temporary wind-induced disruption of the shallow (by marine standards) thermocline, or major changes in species composition resulting from the seasonal changes in water quality (e.g. higher pH values in late summer tend to favour blue-green algae rather than diatoms).

Discrete spring and autumn blooms separated by a summer minimum are often observed in lakes,602,1120 but this temporal pattern is more commonly found in eutrophic than in oligotrophic water bodies.870 Figure 11.8 shows the variation in phytoplankton biomass, and in daily phytoplankton photosynthesis through one-and-a-half years in mesotrophic Lake Constance (Germany-Austria-Switzerland). In some tropical lakes such as Lake George, Uganda, there may be no seasonal variation and primary production and phytoplankton biomass remain high throughout the year.431 On the other hand, where, as in the case of Lake Chad (Chad, Africa), there are alternating dry and rainy seasons, there can be seasonal variation in temperature, insolation and water level, with associated changes in areal photosynthesis.795

Benthic macrophytes, being fixed in position, are not, unlike the phyto-plankton, directly affected by the seasonal onset or disappearance of thermal stratification. They do, however, show seasonal variation in photosynthetic production in response to the yearly cycles of irradiance and temperature. Figure 11.9 shows the variation in net photosynthetic rate (per unit area of thallus) of the sublittoral brown alga Laminaria longicruris (a kelp) in Nova Scotia, Canada, coastal waters through a 12-month period, together with values of the total amount of PAR

Seasonal Variation Pyramids Biomass

Fig. 11.8 Seasonal variation in phytoplankton biomass and photosynthetic production. Redrawn from data of Tilzer (1983) for the mesotrophic Lake Constance (Germany-Austria-Switzerland). The middle curve represents the estimated proportion of incident PAR that is captured by the phytoplank-ton. The areal (integral) photosynthetic rate is the hourly average for the middle four hours of the day.

Fig. 11.8 Seasonal variation in phytoplankton biomass and photosynthetic production. Redrawn from data of Tilzer (1983) for the mesotrophic Lake Constance (Germany-Austria-Switzerland). The middle curve represents the estimated proportion of incident PAR that is captured by the phytoplank-ton. The areal (integral) photosynthetic rate is the hourly average for the middle four hours of the day.

received per day at the sea bottom (10 m depth) on which the algae were growing, and the prevailing water temperature.543 Photosynthetic rate was highest in summer, declined through the autumn to approximately zero in November-December, became significant once more in late winter and spring, and then rose steeply in early summer. The temporal variation in photosynthesis corresponded roughly to the variation in irradiance. By multiple regression analysis, Hatcher et al. (1977) found that 61% of the variance in daily photosynthesis could be accounted for by irradiance. Variation in temperature (from 1.5 to 13°C in these cold waters) did not account for any significant part of the variation in daily photosynthesis. Temperature did account for 56% of the observed variation in light-saturated photosynthetic rate (measured at ambient water temperature,

Temperature Data Photosynthesis

Fig. 11.9 Seasonal variation of the photosynthetic rate of the kelp Laminaria longicruris and of total daily irradiance (PAR) and water temperature at 10 m depth in a Nova Scotia, Canada, coastal water (St Margaret's Bay) (from data of Hatcher et al., 1977). Photosynthesis measurements were made in situ at the prevailing water temperature. The graph of total daily irradiance was obtained by drawing a smoothed curve through the data points of Hatcher et al.

Fig. 11.9 Seasonal variation of the photosynthetic rate of the kelp Laminaria longicruris and of total daily irradiance (PAR) and water temperature at 10 m depth in a Nova Scotia, Canada, coastal water (St Margaret's Bay) (from data of Hatcher et al., 1977). Photosynthesis measurements were made in situ at the prevailing water temperature. The graph of total daily irradiance was obtained by drawing a smoothed curve through the data points of Hatcher et al.

back in the laboratory), but since for most of the time the plants in situ would be photosynthesizing at subsaturating irradiance values, the effect of temperature on the saturated rate is irrelevant.

Kirkman and Reid (1979) studied the seagrass, Posidonia australis, growing in shallow water - from low-water mark to a depth of 3 m below this - at Port Hacking, Australia (34° S). Figure 11.10 shows the variation in relative growth rate (mg C g-1 day-1, which should be closely related to photosynthetic rate), and water temperature during a 12-month period. The growth rate was found to be strongly correlated (r = 0.79) with water temperature. In shallow seagrass beds in the northern Gulf of Mexico, also, the seasonal growth cycle of the dominant species, Thalassia testudinum, correlates more closely with water temperature than with solar irradiance.840 It is possible that in these shallow, well-illuminated environments, light intensities were at or near saturation for much of the time, so that temperature became a limiting factor.

Hanisak (1979) carried out a combined field and laboratory study of the growth pattern of the siphonous green alga Codium fragile, growing in shallow water (average depth 1.5-2.3 m below mean low-water mark) on the northeastern (41° N) coast of the USA. The main period of growth (as dry matter accumulation, therefore closely related to photosynthesis) was

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May Jun? July Auq Sep! Oci Hw Dec Jan Feb Mai Apr 1977 1573

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Fig. 11.10 Seasonal variation in relative growth rate of the seagrass Posidonia australis, and in surface water temperature, at Port Hacking, NSW, Australia (by permission, from Kirkman and Reid (1979), Aquatic Botany, 7, 173-83).

May Jun? July Auq Sep! Oci Hw Dec Jan Feb Mai Apr 1977 1573

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Fig. 11.10 Seasonal variation in relative growth rate of the seagrass Posidonia australis, and in surface water temperature, at Port Hacking, NSW, Australia (by permission, from Kirkman and Reid (1979), Aquatic Botany, 7, 173-83).

from late spring (April-May) through to early autumn, the maximum rate being in July. It appeared that the total daily irradiance levels were above saturation for growth from April to August/September so that temperature was the major limiting factor, but that in the autumn, when temperatures were still high, the reduced light levels were limiting for growth. In the winter it seemed that both temperature and light were limiting.

In a two-year study of seasonal change in the benthic macroalgae/ seagrass community in Puget Sound (Washington, USA), Thom and Albright (1990) found that while water temperature appeared to correlate best overall with standing stock, changes in solar irradiance appeared to trigger the onset of biomass build-up in the spring, and die-back in the autumn. During periods when neither light nor temperature were limiting, nitrate level in the water became the limiting factor for growth.

In some coastal regions the diminution in underwater light availability in the winter due to the lowered solar elevation and shorter days is further aggravated by a marked increase in the vertical attenuation of light resulting from increased water turbidity caused by the stirring up of sediments in winter storms. At Banyuls-sur-Mer (western Mediterranean), Kd rose from a minimum value of about 0.075 m_1 in the summer to a maximum of about 0.19 m_1 in the winter,1447 and off Helgoland (North Sea) also, water transparency was much lower from October to March than in the rest of the year.833

These data suggest the tentative generalization that primary production of marine macrophytes is limited in deep water by light availability all the year, but in shallow water is usually limited by temperature except that in higher latitudes light becomes limiting in the winter, and that the temporal variation in primary production is in large part a function of the seasonal variation in these two physical parameters. It is likely that macrophyte photosynthesis (other than in deep water) is, in addition, always and everywhere limited by CO2 availability, but this does not vary with the seasons. Growth, and therefore photosynthesis, are also determined by the availability of nutrients in the water, particularly nitrate, and in higher latitudes this varies on a seasonal basis being high in the winter but decreasing greatly during the spring phytoplankton bloom. This is less of a limiting factor for many macrophytes than it is for phytoplankton, however. Seagrasses, for example, being rooted plants, can obtain nutrients from the sediments in which they are growing. In the summer, when nitrate concentration in the water is low, the kelps (Phaeophyta) continue to photosynthesize and accumulate carbohydrate reserves for later use, in the winter.543 In the tropics we might a priori expect relatively little seasonal variation in macrophyte photosynthesis except where there is variation in the hydrological regime, associated for example with seasonal variation in river outflow.

The extent to which seasonal variation in marine macrophyte photosynthesis is influenced by internal, as well as external, changes is unclear. King and Schramm (1976) in their study of many green, brown and red algal species in the Baltic, found, for example, that light compensation points were lower in the winter than at other times of year, and that the light-saturated photosynthetic rates (per g dry mass) varied markedly within any species (but not in the same way in all species) according to the season. However, their measurements were in all cases carried out at the prevailing seasonal water temperature (which, of course, varied during the year) and so they were unable to conclude whether the seasonal adaptation was simply a direct response to temperature change or whether other factors were involved. An example of a seasonal change in photosynthesis caused by an internal physiological change is, however, provided by the seagrass Posidonia oceanica. Drew (1978, 1979) found that in plants of this species growing off Malta the photosynthetic rate (per unit area of leaf) declined markedly in the summer below the spring value, whereas in another seagrass species, Cymodocea nodosa, growing nearby, the photosynthetic rate remained high. The decline in P. oceanica photosynthesis appeared to be due to senescence (leaf chlorophyll content declined markedly in the summer), possibly triggered by daylength changes.

In fresh waters, the typical pattern of benthic macrophyte primary production in temperate regions is that biomass accumulation does not occur in the winter, that it begins in the spring as a consequence of the increase in solar irradiance and temperature, and rises to its maximum rate (g C or dry matter per m2 of bottom per day) in the early summer. In the macrophyte-dominated Gryde River (Denmark), Kelly et al. (1983) found that the daily primary productivity of the whole plant community throughout one year closely followed the daily insolation. As a consequence, presumably, of light saturation, there was a clear tendency for the efficiency of production within a given day to decrease with increasing irradiance during the day. The relative importance of light and temperature in determining the seasonal pattern of freshwater macrophyte photosynthesis is not well understood. We may reasonably suspect that it is largely a matter of depth, temperature being the more important variable in shallow, and light in deeper, water. In the late summer and in the autumn, the rate of accumulation decreases for a number of reasons including a general deterioration in the vigour of the plant due to disease, grazing damage and excessive temperatures.1456 The rise in pH late in the summer, due to high phytoplankton photosynthesis, can also be inimical to some macrophytes. Since macrophytes can derive nutrients from the sediments in which they are growing, their productivity is not likely to be limited by the seasonal variations in levels of phosphorus and mineral nitrogen in the water. Figure 11.11 shows the seasonal changes in primary production rate in a mixed macrophyte community in a Canadian lake.830 Internal as well as external changes may influence the seasonal variation in photosynthesis: the photosynthetic capacity of freshwater macrophytes is typically low in the winter and high in the spring.1455 Variations in photosynthetic capacity could influence the rate of photosynthesis in plants growing at saturating intensities in shallow water but would be of little consequence for plants growing at subsaturating intensities in deep water.

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