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86 88 90 92 94 96 98 00 Figure 9.8. Growth rate ofphytoplankton in the tidal-freshwater Hudson. The solid circles labeled "actual" show the growth rate calculated from GPP (Fig. 9.7) and biomass. The thin line shows the growth rate at optimum light ("potential"), with no light limitation. In this scenario the light level was 1000 |i Einst. m-2 s-1 throughout the water column for 12 h each day.

and the 14C measures net phytoplankton production (GPP-Ra) in the light, we can provide an estimate of algal R and GPP (Fig. 9.7). In this scenario, we assumed that Ra was constant at 7 percent of P^ax. Actual estimates of phytoplankton R range from 5 to 25 percent of P^ (Falkowski, Dubinsky, and Santostefano, 1985; Geider and Osborne, 1989; Raven and Beardall, 1981).

GPP is useful to know because we can compute the phytoplankton growth rate from GPP and algal biomass (Fig. 9.8). In the Hudson this growth rate (|) ranges from about 0.05 to 0.4 d-1 and is much slower than these taxa are capable of. If we compute the growth rate the phytoplankton would have without light limitation (growth at P^) we see the |max is much higher than | ranging from 0.4 to more than 1.75 d-1, about as fast as these organisms grow in culture. Averaged over the entire data set |max is 5.9 times faster than actual Looking closely at the plot of actual | we generally see slightly faster growth rates in the post zebra mussel years than in the pre-zebra mussel years, at least during the summer. Presumably the high filtration rate of the zebra mussel selects for phytoplankton species that are faster growing. We have seen a significant change in species composition of the phytoplankton in response to the invasion (Smith et al., 1998), perhaps reflecting this grazing pressure.

From GPP and R we can also estimate phytoplankton net production (GPP-R), which is an interesting quantity since it is the C available to be consumed or exported. Note that NPP is much smaller than either GPP or 14C-based estimates of primary production (NDPZP), rarely exceeding 30 mmol m-2 d-1, and is occasionally negative (algal R > GPP). On an annual basis, GPP-R is about ten times smaller larger than measured14 Cprimary production. Had we assumed that 14C in the light measured gross rather than net production, or had we used higher values for algal respiration, GPP-R would be even smaller (Cole et al., 1992). The very large difference between GPP-R and net daytime photic zone primary production is due to the deeply mixed water column. In the Hudson, phytoplankton spend a great deal of time in the dark. For the deeper-water parts of the river, Poughkeep-sie for example, algal R would frequently exceed GPP (Cole et al., 1992). The inference is that phytoplankton, as they pass through these deeper regions, must lose biomass due in part to their own respiration. This is consistent with the observed pattern of less algal biomass in the deeper regions of the river, as long the water column is well mixed (Fig. 9.5). We do not know the actual rate of algal R, so these calculations are only illustrative of what is likely. However, it is unlikely that algal R is much lower than 7 percent of P^ (Beardall and Raven, 1990; Geider and Osborne, 1989). Itis also possible that R is neither constant nor always proportional to P^. (e.g., Laws, 1975; Stone and Ganf, 1981). The major point here is that in the well-mixed and dimly lit water column of the Hudson River, algal respiration is a large and important fate of algal gross primary production. This condition is likely true in other well-mixed turbid rivers and estuaries but has only been considered in a few of them (Peterson and Festa, 1984).

Since the net production of phytoplankton (GPP-Ra) and growth rates of the Hudson River phyto-plankton are low, a small change in a loss term (advection, predation, etc.) can be extremely significant. Even without considering the respiration of phytoplankton, the growth rate of phytoplankton based on GPP is only 0.3 d-1 at its peak. The biological filtration of the water column by zebra mussels is about this magnitude, 30 percent of the water column filtered each day. If Ra is proportional to P^, as we have modeled it, phytoplankton respiration consumes on the order of 80 to 90 percent of Gpp. Thus a change in removal rates of on the order of 5 to 10 percent of the water column per day would still significantly impact the phytoplankton of the tidal-freshwater Hudson. The major crash in phytoplankton biomass in response to the zebra mussel invasion is indeed consistent with this reasoning (Caraco etal., 1997).

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