Water-quality data for nine constituents collected from January 1976 through March 1984 in upper Charlotte Harbor were examined for evidence of trend (change) with time. Average seasonal patterns and moving averages were described to aid initial descriptions of variation. Multilinear regression models were developed using independent variables that are known to or that may affect variation of constituent. A nonparametric analysis (Seasonal Kendall test) was used as a different approach to determine trends. These results were generally consistent with the parametric analyses. Evidence suggested an increasing trend for temperature and orthophosphate and a decreasing trend for near-surface dissolved oxygen. The temperature trend was related to low winter temperatures early in the period. The orthophosphate trend was related to a major source, the Peace River. The source of the dissolved oxygen trend was not identified, but a relation with primary producers (phytoplankton) was likely. Evidence of a trend was not found for total phosphate, reactive silica, and organic nitrogen. Trend analysis for chloride appeared complicated by strong serial correlation, and the tentative conclusion was that no trend occurred. Nitrate plus nitrite and ammonia data contained too many values that were less than detection limits, and trend analysis could not be made.
