Estuaries are among the most productive of marine environments, although food abundance does fluctuate greatly over space and time. This extraordinary productivity is a product of the large amounts of nutrients that enter the estuary seasonally and their extensive recycling between the overlying water and the biologically active sediments. The Hudson has some interesting complications that create exceptions to these generalizations. Most importantly, the Hudson bears a large sediment load, coming from its drainage system and consisting of materials ranging from clays derived from erosionof glacially derived deposits to organic particles derived from substances such as leaflitter. The combination of materials reduces light penetration in the water column, which in turn reduces photosynthesis ofphytoplanktonandrestricts sub-aquatic attached vegetation to very shallow depths. The high particle concentration is complicated by strongvertical mixing, owing to tidal and wind mixing. Thus, phytoplankton cells spend much of their time in suboptimal light conditions.
Owing mainly to light limitation, the Hudson is not a river with high primary production in the water column (Chapter 9). Primary production is seasonal, with a peak in spring. Respiration, however, is a dominant process and little production is available for higher trophic levels in the Hudson. The freshwater phytoplankton are not limited by nutrients, but by light. In recent years the invasion of the zebra mussel has strongly reduced phytoplankton populations, which has further reduced the potential for oxygen production from photosynthesis. The respiration of the zebra mussels has lowered oxygen concentrations in the freshwater Hudson, which may be stressful to active organisms such as fish under some circumstances.
In the saline part of the Hudson in the vicinity of New York Harbor nutrient concentrations increase greatly as a result of dissolved sources from sewage. Nutrients are so concentrated that phytoplankton production is not limited by nutrients, but by light and temperature. Nutrient input in the saline part of the Hudson is among the highest of any coastal water body in America. Before the 1990s, organic matter input from untreated sewage resulted in anoxic or strongly hypoxic waters in New York Harbor. Since then, water quality has improved greatly, as has oxygen levels (Chapters 10, 23).
Zooplankton are abundant in both the freshwater and saline parts of the Hudson estuary, but in neither part of the estuary do they exert major grazing effects on the phytoplankton (Chapter 16). The zooplankton in the freshwater part of the Hudson are dominated by copepods, rotifers, and cladocera. Occasionally ciliates and flagellates are very abundant. While they may not be important in the cycling of nutrients, the zooplankton, nevertheless, are crucial food sources for larval and juvenile fish. It is notable, therefore, that the invasion of the zebra mussel resulted in strong decline of some zooplankton, particularly rotifers. It is not clear whether the decline was due to direct consumption by the mussels or by a shortage of phytoplankton food caused by zebra mussel feeding. In the saline portion of the Hudson, copepods dominate the zooplankton and feed mainly on phytoplankton.
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