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Nine localities in a small tropical lagoon of coastal Mexico that were sampled during the summers of 1967 and 1972 provide data for comparison of ostracod production under two differ¬ent sedimentation regimes. “Normal” conditions prevailed in 1967; a flood regime dominated in 1972. Counts of ostracods contained in these quantitative (100 cc) bottom samples include: (1) absolute number dead, (2) absolute number live, (3) total (live plus dead), and (4) percent living. Measurements of change in water chemistry included dissolved oxygen, pH, salinity, and tempera¬ture. Depth variations were also measured.

Changes in the ostracod standing crop from 1967 to 1972 show net average decreases per station of 928 dead individuals, 71 live individuals, and 960 total individuals. As a percent of total population, there was a net average gain of 40% living individuals. These figures comprise net losses per year of 186 dead, 14 live, and 192 total individuals, and a net average gain per year of 8% living individuals. However, these are averages comparing assemblages under two different con¬ditions of sedimentation, and should not be construed as valid estimators of rate of change over the 5-year period. Relative differences in the samples between the total population and absolute num¬ber dead coincide closely, indicating that absolute number dead in Holocene samples may be a close approximation of total number (thanatocoenotic number) as determined in fossil samples. However, relative differences between the absolute number live and the percent living are highly variable, suggesting that actual counts of living individuals are far better estimators of the biocoe-nosis than living percentages. The low populations of living versus dead individuals also illustrate how little of the biocoenosis is actually present at any given time. The primary environmental vari¬able causing these changes appears to be increased bottom-current velocity at times of flood. Other related variables, thought to be of secondary importance, include salinity, temperature, and depth. Changes in dissolved oxygen fluctuate too erratically for any obvious relationships; pH changes behave similarly.

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