We saw in the previous chapter that a major factor in limiting efficiency of utilization of incident light in aquatic ecosystems is the removal of a large proportion of the light energy by the aquatic medium. This occurs in waters in which the vertical attenuation of PAR by non-phytoplanktonic material is high (see Table 10.1). We would therefore expect, for example, brown-water lakes with a high concentration of CDOM to be on average less productive than lakes with low background colour. There is some evidence that this is the case, although relatively few systematic studies have in fact been carried out. Jackson and Hecky (1980) found, for water bodies in northern Manitoba (Canada), that primary productivity was inversely related to total dissolved organic carbon (which would be linearly related to CDOM). While these authors did suggest that this was due to the humic substances complexing iron, thus making it unavailable to phytoplankton, rather than by increasing light attenuation, competition by CDOM for light is nevertheless a plausible explanation. For lakes in Wisconsin, Carpenter et al. (1998) observed a weak negative correlation between phytoplankton concentration, and also primary production rates, and the levels of dissolved organic carbon. In the St Johns River, Florida, Phlips et al. (2000) found in time series at two locations over a four-year period that periods of high colour corresponded to periods of low phytoplankton concentration, and vice versa. Jones (1992) points out that increased colour, by promoting absorption of solar radiation in the surface layer, can give rise to a more shallow mixed layer when stratification sets in, thus decreasing the mixing depth and increasing the average light intensity to which the phytoplankton are exposed. He suggests, nevertheless, that this effect is only likely to be of significance in small sheltered forest lakes. Larger lakes with wind exposure do not show a correlation between mixing depth and water colour.
It should be noted that a high total vertical attenuation coefficient need not necessarily be associated with low productivity since that high coefficient may be due to a high concentration of phytoplankton: it is high attenuation by non-algal material that we expect to lower the productivity. So far as seagrasses and freshwater macrophytes are concerned, we have already seen (§11.1) that the depth of colonization is inversely proportional to the vertical attenuation coefficient for PAR, indicating that the more highly attenuating the water, the lower the macrophyte productivity of the water body.
Estuaries are often highly attenuating water bodies, due to the presence of high concentrations of suspended sediment particles, which both absorb and scatter light, and to dissolved humic colour in the river inflow. Turbidity, and consequently attenuation, varies longitudinally down the estuary, rising to a peak at the so-called 'turbidity maximum' the position of which varies with the tide, attenuation and turbidity then diminishing in a seaward direction.246,251,528,650,856,1042
Cole and Cloern (1984, 1987) have found, for a number of estuaries, that phytoplankton daily productivity can be expressed as a linear function of [Chl]Ed (0+)/Kd (PAR) or, equivalently, of [Chl]Ed (0+)zeu. To interpret this, we note that in optically deep waters all the daily surface-incident light, Ed(0+) (apart from the small fraction which is reflected at, or back-scattered through, the surface), is absorbed in the water column, and also that the proportion of this light which is absorbed by the phytoplankton is ap/at (see § 9.4). Given that ap (the phytoplankton absorption coefficient) is linearly related to the phytoplankton concentration, [Chl], and that Kd(PAR) is very approximately proportional to at (where this is understood as a depth- and spectrally averaged total absorption coefficient for the estuarine water), we can see that [Chl] Ed(0+)/Kd(PAR) is an approximate measure of the daily absorption of light by the phytoplankton population. Light limitation of primary productivity due to high turbidity is common in estuaries: examples are the Hudson River estuary (New York State, USA)253 and the Gironde estuary in southwest France.612
Turbid waters in which the turbidity is due to large numbers of mineral particles which are themselves of low intrinsic colour, and in which dissolved colour is also low - such as lakes with suspended glacier-derived mineral flour - represent a special case. Because of the intense scattering, the total vertical attenuation coefficient, Kd(PAR), may be quite high so that the euphotic zone is shallow. This fact alone does limit production by reducing the volume of medium that the phytoplankton can exploit photosynthetically. Nevertheless, within the euphotic zone the relative ability of the phytoplankton to collect light is dependent on its absorption properties compared to the other constituents of the medium (ap/at), and since in the present case the rest of the medium does not absorb the light strongly despite its intense scattering, the phytoplankton is well placed to compete for the available photons. In a medium of this type, the assumption, contained in eqn 10.12, that the relative amounts of the absorbed light captured by phytoplankton and by the rest of the medium are in proportion, respectively, to [Chl]kc and KNP, is no longer even approximately valid. We saw in §10.3 that this assumption can in fact only be made with reasonable accuracy for waters in which vertical attenuation is absorption dominated.
The kind of mechanism we have envisaged, namely effective competition by phytoplankton for the photons within the shallow euphotic zone, may in part account for the surprisingly high productivity of some waters with high inorganic turbidity.1005 The shallowness of such waters (the turbidity often being due to wind resuspension of bottom sediments) will further promote their productivity by ensuring that the circulating cells do not for long remain below the illuminated layer. In addition, the ratio of scalar to downward irradiance is high in turbid waters with a high ratio of scattering to absorption (see Fig. 6.10), and so there is substantially more light available for photosynthesis than the values of downward irradiance alone would indicate.
To fully understand the extent to which light availability limits primary production it is necessary to be aware of the limitations that are imposed at the same time by other environmental factors. Consider, for example, a diatom population photosynthesizing in lake water at a saturating light intensity of ^400 mmol photons m~2 s_1, as illustrated in Fig. 10.1. A small increase in light intensity will bring about no change in photo-synthetic rate and a large increase will even lead to photoinhibition. What is happening within the cells is that the rate of formation of the products of the light reactions of photosynthesis, namely NADPH2 and ATP, is so high that the enzyme system responsible for the dark reactions is saturated so far as NADPH2 and ATP are concerned. Any increase in their steady-state concentration resulting from an increase in light intensity is therefore not accompanied by an increase in the rate of CO2 fixation. This does not mean, however, that the system is necessarily working at its maximum possible rate. The CO2 concentration may be too low to saturate the first enzyme, ribulose bisphosphate carboxylase (Rubisco), in the dark reaction sequence so that it is not functioning at its maximum capacity. Alternatively, or in addition, the temperature may be too low for maximum activity: at a higher temperature the enzymes of the whole dark reaction system might operate more rapidly and thus be able to consume the NADPH2 and ATP at a faster rate.
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