Daytime Photic Zone Primary Production

Traditionally phytoplankton production (NPP, see introduction) is measured by the 14C method and is integrated over the daylight period and to the depth of the photic zone, and does not consider respiration. Thus, these values are NDPZP (see introduction). For these calculations we computed primary production for each 0.1-m slice of the water column for each half hour during daylight, for each week for which we have chlorophyll-« data. Potential irradiance was calculated from the equations of Iqbal (1983) using an albedo of 10 percent and the average degree of cloudiness for the region 40 percent reduction in potential radiation (IES 2001). NDPZP ranges from extremely low values in winter (~1 mmol C m-2 d-1) to values as highas300mmolCm-2 d-1 during summer bloom conditions (Fig. 9.7). The seasonal pattern mirrors that of chlorophyll-« and shows the dramatic reduction following the invasion of the zebra mussel (Fig. 9.7).

Since NPP, based on14 C measurements, does not include an estimate of algal respiration, NPP in daylight is potentially smaller than algal GPP by algal respiration in the light. In the dark part of the water column, and at night, algal respiration consumes algal C that is not accounted for at all (Banse, 1976). Using the approach outlined in Caraco et al. (1997) if we assume that algal R is proportional to P

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