Jonathan J. Cole and Nina F. Caraco abstract Photosynthesis is the main process by which new organic matter is synthesized. In many aquatic ecosystems, phytoplankton are the major pho-tosynthetic organisms and are responsible for most of the organic C input. In the tidal-freshwater Hudson, primary production by phytoplankton is maintained at relatively low values by a combination of high turbidity and deep mixing (which lowers light availability), advective losses downstream and consumption by grazers. Limitation by nitrogen or phosphorus, the most common plant limiting nutrients, is not an important regulatory factor in the tidal-freshwater Hudson. Respirationby the phytoplankton themselves is the major fate of phytoplankton-derived organic matter (gross primary production), leaving relatively small amounts available to higher trophic levels. Thus, small increases in grazing pressure could have large impacts on phytoplankton. Phytoplankton biomass and gross primary production were dramatically reduced by the 1992 invasion of the zebra mussel, and phytoplankton have not yet recovered to pre-invasion levels. We estimate that phytoplankton gross primary production was 331 gC m-2 y-1 in theyears prior to the zebra mussel invasion and 82gC m-2 y-1 in the years following. This is from about one-half to one-eighth as large as the input of terrestrial organic C from the watershed.
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