Plankton community production and respiration rates were examined at 3 stations representmg distinct regions along the estuanne gradient in the main stem of Chesapeake Bay, USA. Rates were measured as in vitro changes in oxygen concentration, as determined by Winkler titration with an automated photometric end-point detection system. At each station rates of both processes exhibited annual patterns which followed that of water temperature. There were distinct differences, however, among the 3 stations in the relative magnitude of metabolic rates measured. Annual rates of daytime net plankton community production were estimated to be 265, 1680, and 2040 g O2 n r 2 yrW1, while annual night-time plankton community respiration rates in the upper water column were estimated as 130, 1090, and 490 g 0; m 2 y r l at the upper, middle and lower Bay stations, respectively. Thus, whereas rates of net daytime production increased substantially moving downbay, highest measured rates of community respiration were, m fact, found in the middle region of the Bay. Annual cycles of production and respiration rates were significantly related to each other at the upper and middle stations, but unrelated at the lower station. Integrated estimates of net plankton community metabolism (production minus respiration) at the 3 stations exhibited seasonal patterns departing from balanced metabolism (production = respiration) during winter-spring and converging on zero net metabolism in summer-fall. During the cooler months net plankton metabolism was negative (net heterotrophic) at the upper station and positive (net autotrophic) at the middle and lower stations. Over the annual cycle, the 3 stations showed a longitudinal pattern of increasing die1 net plankton community metabolism, progressing from a net heterotrophy of -70 g O2 m 2 y r l in the turbid, upper Bay, to shghtly positive metabolism of 160 g 0; m-2 yr-I m the mid-region, to strong net autotrophy of 760 g Oa m 2 y r l in the less turbid, lower Bay.

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