Mixed layer depth and phytoplankton production

It is instructive to consider under what conditions phytoplankton primary production may be prevented altogether as a result of the mixed depth exceeding the critical depth. Using Tailing's (1957b) model for calculating integral photosynthesis (see §11.5) we can derive an approximate expression for the critical depth where p is the ratio of respiration rate to light-saturated photosynthetic rate in the phytoplankton, Kd is the vertical attenuation coefficient for downwards irradiance of PAR, N is daylength in hours (readily calculated from eqn 2.11), Ed(0) is the average value of downward irradiance just below the surface during daylight hours, and Ek is the irradiance value defining the onset of saturation (§10.2). We can see from eqn 11.1 that the likelihood of the mixed layer depth exceeding the critical depth increases (i.e. zc decreases) as respiration rate (relative to photosynthetic rate), attenuation by the water, or the light-saturation parameter, increase: an increase in incident irradiance or in daylength has the opposite effect.

As an example of a temperate-zone lake with relatively low attenuation we shall take Lake Windermere, England, and use the data of Talling (1957a). For a day in late spring (1 May 1953), with Ed(0) « 170 W m~2, daylength « 15 h, Kd = 0.43 m_1, and the phytoplankton (predominantly the diatom, Asterionella) having Ek = 6.3 W m~2, and p = 0.033, we may calculate that the critical depth was about 177 m. Since the lake itself (North basin) has a maximum depth of only about 63 m, there is clearly no possibility of net primary production being prevented altogether. Even if the lake were much deeper, once the spring-summer thermal stratification had set in, the comparatively shallow depth of the epilimnion (the layer, commonly 5-20 m deep, above the thermocline, within which wind-induced circulation occurs) would prevent the critical depth being exceeded. For Lake Windermere in midwinter, on the other hand, with daily incident light down to a small fraction of the summer value and circulation through the full depth of the lake, Talling (1971) estimated that phytoplankton growth was impossible. In the much shallower Esthwaite Water (zmax = 15 m), in contrast, he concluded that phyto-plankton growth should be possible throughout the year.

Even in shallow water bodies, if the attenuation is high enough the circulation depth can exceed the critical depth. For turbid Lough Neagh

(N. Ireland), with a mean depth of 8.6 m and Kd values commonly 1.5 to 3.5 m-1, Jewson (1976) estimated that the critical depth would be exceeded in winter, thus precluding growth. In spring, despite the lack of stratification in this well-mixed lake, the increase in illumination was sufficient to ensure that zc was not exceeded.

When high attenuation is combined with a requirement for high light intensity for saturation, then the critical depth can be very shallow. In Lake George, Uganda, on the equator, Ganf (1975) observed Kd values in the region of 9 m_1, and the blue-green algal population had a lightsaturation onset parameter (Ek) of about 55 W m~2, on a day (12 April 1968) when the average incident irradiance (Ed(0)) was 360 W m~2. Assuming p to be about 0.1 and the daylength to be 12 h, eqn 11.1 gives a critical depth of 1.4 m. Thus, despite the high irradiance incident on this tropical lake, the mixed depth could often exceed the critical depth. In productive lakes of this type a large part of the attenuation is due to the phytoplankton itself and so the situation is self-correcting. Cessation of growth is likely to be followed by breakdown of the algae, following which attenuation falls and photosynthesis and growth can recommence.

In the marine environment, the relation between the mixed depth and the critical depth is of particular importance, and indeed the first attempt to characterize this relation quantitatively was carried out by Sverdrup (1953) for the Norwegian Sea. In middle and high latitudes the mixed layer is deep at the end of the winter (varying from 100-400 m in March at 66° N 2° E), but in the spring a shallow mixed layer (25-100 m) develops due to thermal stratification. For the 66° N station that Sverdrup studied, he estimated that until the last week of April the mixed layer was much deeper than the critical depth, thus precluding phytoplankton growth, but that after the middle of May the mixed layer depth was smaller than the critical depth so that growth was possible. Springtime stratification can be established very quickly. At a station in the North Atlantic, south of Iceland, Dickey et al. (1994) found that the mixed layer shoaled from ^550 to ~50 m within a five-day period towards the end of April. This was followed by a major phytoplankton bloom, with a ten-fold increase in near-surface chlorophyll in less than three weeks.

Within any water body, essentially all the infrared component (1 > 700 nm) of incident solar radiation is absorbed within the uppermost 0.5 m by water itself. The depths at which the remainder of the solar flux, the 45 to 50% which is PAR, is absorbed and converted to heat is determined by the inherent optical properties of the water in question. Phytoplankton blooms, by increasing the absorption of solar radiation near the surface, can significantly increase sea surface temperature, and in this way give rise to stronger near-surface thermal stratification, and shallower mixed layers.1304

If, in early spring, with increasing solar irradiance, vertical wind mixing is weak or absent, phytoplankton blooms can develop even before stratification is established.1373,599 Townsend et al. (1992) observed such a phenomenon in the Gulf of Maine, in March 1990. In locations where shallow coastal waters are permanently well mixed by tidal action, such as the coast of Brittany (France) in the western English Channel, a stratification-induced spring bloom does not occur, and the light intensity is such that the euphotic depth becomes comparable to the water depth.1474 In some aquatic systems strong prevailing wind, by causing deep mixing, can become the main limiting factor for primary production even after the phytoplankton growing season has commenced. This has been observed for the Atlantic sector of the Southern Ocean1398 and also for a deep lake, Loch Ness (average depth 132 m), in Scotland.661

Density stratification brought about by vertical variation in salt concentration, rather than by solar heating, can also induce a phyto-plankton bloom. This has been observed in the case of lower salinity upper layers resulting from freshwater inflow in estuarine and coastal waters,247,1042,1374 and from melting ice in the marginal ice zones around Antarctica,915,1259 or from intrusion of dense continental slope water beneath less saline coastal water.1374 In the Southern Ocean Marginal Ice Zone, Fitch and Moore (2007) found that in the Antarctic summer, phytoplankton blooms were largely suppressed at high wind speeds, due to breakdown of the vertical stratification. At low wind speeds (~5 m s-1), blooms covered about one third of the Marginal Ice Zone.

Strictly speaking, these various interactions between mixed depth and critical depth apply to non-motile forms such as diatoms. Motile algae such as dinoflagellates can to some degree escape these effects by migrating again up to regions of higher irradiance if the circulating water moves them down. Nevertheless the evidence is clear that for the total phyto-plankton population, circulation of water within the mixed layer is a major factor influencing primary production, and frequently determines whether such production takes place at all.

In the sea, the mixed layer, once thermal stratification has been established (spring to autumn in moderate and high latitudes, all year in the tropics), is typically 20 to 100 m deep. Within and below the thermocline there is comparatively little circulation of the water. Since the euphotic zone will frequently be deeper than the mixed layer (for marine waters

Photosynthesis (mg C m 3 day 1) Chlorophyll (mg m-3 x 10-1)

Photosynthesis (mg C m 3 day 1) Chlorophyll (mg m-3 x 10-1)

th 50

10 12 14 16 18

m th 50

10 12 14 16 18

Temperature (°C)

Fig. 11.1 The deep chlorophyll maximum in the Northeast Pacific Ocean (after Anderson, 1969). The temperature profile shows that the mixed layer is about 25 m deep.

with Kd values of 0.03, 0.05, 0.11, 0.16 m-1 (Table 6.1), zeu « 153, 92, 42 and 29 m, respectively), there is, throughout much of the oceans and coastal seas, a substantial layer of water which combines sufficient light intensity to support photosynthesis, with a stable water column.

When the vertical profile of phytoplankton chlorophyll is measured in the (stratified) ocean, a distinct peak of chlorophyll concentration is normally found close to the bottom of the (nominal) euphotic zone. An example of this deep chlorophyll maximum (DCM) as it is often called, in the northeast Pacific Ocean is shown in Fig. 11.1. The peak of the chlorophyll distribution is either a little above or a little below (as in Fig. 11.1) the depth at which Ed is 1% of the subsurface value. It will be noted that there is a layer of increased photosynthetic activity in this region indicating that the algal cells are active, not moribund. This layer of deep phytoplankton is very widespread in the world's oceans: it appears, for example, to extend right across the Pacific.1220 It occurs only where the water column is stabilized by a pycnocline (density gradient).24,1408 In the Atlantic subtropical gyres, the DCM was found by Perez et al. (2006) to extend

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