on average from 67 to 126 m depth. It seems likely that the increase in chlorophyll concentration in this deep layer, relative to that in the mixed layer, is due to an increase in chlorophyll concentration within the cells as well as to an increased population of phytoplankton.24,268,687,1022 Kiefer et al. (1976) found the chlorophyll per unit biomass in the deep phytoplankton layer to be about twice the value near the surface. At five stations in the eastern subtropical Atlantic, Veldhuis and Kraay (2004) found DCMs at depths (position of maximum chlorophyll) ranging from 80 to 130 m. The small prokaryote, Prochlorococcus, dominated the surface waters, but eukaryotic species were the major component of phytoplankton biomass in the DCM. The cellular chlorophyll to carbon ratios increased with depth in both the prokaryotes and the eukaryotes: in the case of Prochlorococcus the increase was as much as 20-fold. In the Gulf of Aqaba (Eilat), Red Sea, Stambler (2006) observed that a DCM characterized by high concentrations of Prochlorococcus developed at ~80 m after summer stratification was established. There was a marked increase in the chlorophyll content per cell with depth.
The mechanism (apart from shade adaptation) by which the deep chlorophyll maximum becomes established is uncertain. A commonly held view is that it is related to the distribution of nutrients. In the well-illuminated mixed layer the phytoplankton rapidly consumes the nutrients in the spring, and the population then decreases. Below the thermocline nutrient concentrations are much higher. Furthermore, additional nutrients can diffuse upwards from the deep water below. The continued availability of nutrients in this region, possibly combined with a diminution in the sinking rate of the phytoplankton once it reaches these nutrient-rich waters, may account for the establishment and maintenance of this phytoplankton layer.24,619,1294 At stations in the Adriatic Sea the DCM has been found to form well below the pycnocline, and is associated rather with the nutricline, and close to the depth at which PAR is 1% of that at the surface.1119,993
The deep phytoplankton layer makes a significant contribution to total primary production in those waters where it occurs. In the Azores Front (Eastern N. Atlantic), Lorenzo et al. (2004) found that photosynthesis in the DCM accounted for 54 ± 17% of the total depth-integrated primary production. In the Atlantic subtropical gyres, Perez et al. (2006) found the contribution to be 46 ± 2%. On the north slope of the Dogger Bank in the North Sea, which is a highly productive continental shelf sea, Weston et al. (2005) found, in a strongly stratified region, that 58% of water column productivity occurred within the thermocline associated with the DCM.
In shallow, tidally energetic shelf seas, there can be two turbulent mixed layers, the surface mixed layer and the bottom mixed layer, separated by a thermocline. At a station in the western English Channel in August 1999, Sharples et al. (2001) found such a region with a thermocline at ~30 m. In the lower half of the thermocline there was a narrow DCM with a vertical thickness of ~5m. The average chlorophyll a concentration was 26 mgm"3, but reached a peak of 80 mg m~3. The turbulence generated by tidal mixing both aided phytoplankton growth by supplying bottom-layer nutrients into the DCM, and simultaneously depleted phytoplankton numbers by entraining them into the bottom mixed layer.
In most inland waters the euphotic depth is less than the mixed depth, and so the conditions for the development of a deep chlorophyll maximum do not occur. However, Fee (1976) found a very narrow layer with very high phytoplankton chlorophyll concentration in the region of, or below, the thermocline in a series of lakes in northwestern Ontario (Canada), which were both clear and thermally stratified. This layer was found between 4 and 10 m depth (depending on the lake) where the irradiance was 0.3 to 3.5% of the subsurface value. Phytoplankton samples from this layer were always dominated by large colonial chrysophycean flagellates of the genera Dinobryon, Synura, Uroglena and Chrysosphaerella, just one species normally being dominant in the deep chlorophyll layer of any given lake. A curious and interesting feature of these chrysophycean phytoplankton layers is that, as well as photosynthesizing, they derive a substantial proportion of their carbon by phagotrophic ingestion of bacteria.118 In the very clear, colourless waters of Crater Lake and Lake Tahoe, comparable optically to ocean water, a deep chlorophyll maximum develops at considerable depth (~75 m) in the summer.769,1364 In Lake Michigan a deep chlorophyll layer develops below the thermocline, after seasonal stratification sets in. This appears to be due to in situ growth but also, to a lesser extent, to sedimentation and shade adaptation.373
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