Primary Productivity Rates and Controls

Nutrient concentrations and loadings in the saline Hudson River estuary are very high, and primary productivity is regulated largely by physical factors (Malone, 1977; Howarth and Swaney et al.,

2000). Perhaps the most important of these physical factors is water residence time. Most species of phytoplankton have maximum growth rates that result in a doubling of the population in roughly one day, with larger species having somewhat longer doubling times ranging from two to fifteen days (Banse, 1976; Malone, 1977, 1980; Chan,

2001). Given the short residence times, much of the time phytoplankton are flushed from the Hudson estuary about as rapidly as they grow, limiting the size of populations and keeping rates of primary productivity fairly low.

Figure 10.1. Hudson River basin and saline Hudson River estuary. As defined in this chapter, the Hudson River estuary begins at the Battery at the southern tip of Manhattan and runs north. The boundary between the mesohaline and oligohaline estuary varies with freshwater discharge but is generally near river kilometer 36. The oligohaline estuary boundaries also vary with salinity but generally fall between river kilometer 36 and 66. See text for further discussion.

Figure 10.1. Hudson River basin and saline Hudson River estuary. As defined in this chapter, the Hudson River estuary begins at the Battery at the southern tip of Manhattan and runs north. The boundary between the mesohaline and oligohaline estuary varies with freshwater discharge but is generally near river kilometer 36. The oligohaline estuary boundaries also vary with salinity but generally fall between river kilometer 36 and 66. See text for further discussion.

Both the tidal cycle and freshwater discharge into the Hudson affect water residence times in the estuary. When freshwater discharge at the Green Island gauging station is greater than 300 m3 sec-1, water residence times in the photic zone are less than one day (Fig. 10.2A). At lower rates of discharge, water residence times vary greatly, with at least some of this variation related to the tidal cycle; shorter residence times are more prevalent during the spring-tide period of the month when tidal mixing is greater, and neap tides favor longer water residence times.

In a series of cruises in the mid 1990s, we found that rates of gross primary production (GPP) were always less than 2 g C m-2 d-1 and generally less than 1 g C m-2 d-1 when the freshwater discharge (measured at Green Island) was greater than 300 m3 sec-1, leading to short water residence times (Fig. 10.2B). Substantially higher rates of production, up to 18 g C m-2 d-1, were found during some neap tide cycles when freshwater discharge was low and water residence times in the photic zone exceeded two days. A similar effect of discharge on 14C-production in the lower Hudson estuary during the 1990s has beenreported by Taylor, Way, and Scranton (2003). In addition to increased water residence times, greater light penetration into the water column may have favored high rates of production during periods of low discharge and low tidal mixing (Fig. 10.2C). The greater light penetration likely resulted from lessened input of sediment from up river and decreased resuspension of

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