Primary production is the formation of organic compounds from inorganic building blocks. The energy required to synthesize these organic products may come from sunlight (photosynthesis); from chemical reactions, (chemosynthesis, e.g., ammonia or sulfide oxidation); or a mixture of the two as in some types of an oxygenic bacterial photosynthesis (Brock, 1979).

In the Hudson River, as in most aerobic aquatic environments, oxygenic photosynthesis is by far the major pathway of primary production. In the tidal-freshwater portion of the Hudson River this photosynthesis is carried out by several functionally different groups of organisms: phytoplank-ton (small, often single celled, eukaryotic algae and cyanobacteria suspended in the water column), periphyton (algae attached to various surfaces), submergent macrophytes (higher plants such as Valisneria [water celery] that grow attached to the bottom with leaves that remain within the water column), and floating or emergent macrophytes (higher plants such as Trapa [water chestnut] whose leaves are partially or completely exposed to the air). These differing groups of plants have different consumer organisms, different sets of regulation and constraints and different effects on dissolved gas dynamics in the river.

This chapter focuses on primary production by phytoplankton andits regulation, in the tidal, freshwater portion of the Hudson from Albany south to Newburgh, New York. Further, we compare phy-toplankton production in this section of the river into the context of the entire river and other groups of primary producers and compare phytoplankton production in the Hudson to other rivers and estuaries of the world.

Why consider primary production in part of a large riverine estuary? First, the conditions in the tidal, freshwater river are substantially different from those in the saline part of the lower estuary. Thus, phytoplankton experience different regulatory factors in these two sections. Second, the invasion of the zebra mussel in the tidal-freshwater section had dramatic effects on the phytoplankton and provided a great deal of insight into how phytoplankton were regulated. Third, the investigative approaches have differed between the lower estuary and tidal-freshwater river. The lower estuary is covered in the chapter by Howarth et al. (Chapter 10).

measurement and terminology

To discuss primary production and its measurement, we need to introduce a few terms.

• Gross Primary Production (GPP) is total photosynthesis, including the portion respired by the autotrophs themselves.

• Respiration (R) is the respiration by all organisms. R is the sum of respiration by autotrophs (Ra) and heterotrophs (Rh).

• Net Primary Production (NPP) is GPP - Ra. NPP is the amount of organic matter available to consumer organisms. That is, NPP is the primary production left after plant respiration has removed that needed to sustain the plants themselves. While NPP is usually >0, it need not be. When NPP is <0, the biomass of autotrophs must actually be declining over time. That is, phy-toplankton are respiring their stored biomass, in excess of new photosynthesis. This condition sometimes occurs for phytoplankton in Hudson River (below).

GPP, or R, or NPP can be studied for a group of organisms (e.g., phytoplankton, macrophytes, etc.) or for an entire community ecosystem. This chapter focuses on these quantities for phytoplankton.

In aquatic systems primary production is usually measured indirectly through changes in dissolved oxygen or dissolved inorganic C (DIC) or, most often, by labeling the DIC pool with 14C and measuring the incorporation of label into phytoplankton or plant tissue. The various methods do not measure exactly the same quantities (see Williams, Raine, and Bryan, 1979; Williams and Robertson, 1991). From the changes in O2 in paired light and dark bottles containing river water, one can measure total planktonic respiration (Ra + Rh in the dark) and the rate of pelagic NPP in the light. By assuming that R in the dark and light are equivalent we can calculate GPP and R. With the oxygen method, there is no direct way to estimate Ra and therefore no direct way to estimate NPP. The 14C method gives, in the light, something between GPP and NPP depending on the length of the incubation, the growth rate of the phytoplankton and the degree of C recycling (Williams et al., 1979). With the 14C method there is no direct way to estimate any of the components of R. The 14C method, because of its high sensitivity, is most widely used and

Figure 9.1. Phytoplankton in the Hudson River are well-mixed vertically and are often swept to depths with light conditions unfavorable for photosynthesis.

usually reported as "NPP." Typically results are integrated over depth and during daylight hours, excluding both nighttime and depths with light too low to sustain photosynthesis. To make clear what is, and is not, included, we will call this type of estimation Net Daylight Photic Zone Production (NDPZP; Cole, Caraco, and Peierls, 1992) to distinguish it from true NPP.

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