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A well-defined nitrogen retention and turnover budget was estimated for a shallow oligohaline lake (Lake Pontchartrain, Louisiana, USA). In 1997 a month-long diversion of the Mississippi River filled the Lake with highly concentrated river water (80 µM nitrate) and lowered the salinity to 0 psu within 2 weeks. After the spillway was closed the Lake mixed with estuarine tidal waters and came to equilibrium over 4 months with the riverine, atmospheric and offshore water nitrogen sources. A flushing rate of 1.78% d−1 was estimated by analyzing a plot of ln salinity versus time for the first 120 days after the diversion ceased. This flushing rate was similar to the loss rate for total nitrogen (1.75% d−1), implying no significant net nitrogen losses or gains were occurring inside the Lake. The percent loss of dissolved inorganic nitrogen was higher than that for TN (4.11% d−1), whereas the loss of organic nitrogen was lower (0.94% d−1), which suggests a net transfer from inorganic to organic nitrogen. These changes occurred steadily as chlorophyll a concentration ranged from 5 to 200 µg l−1. The results demonstrate the potential significance of the organic nitrogen and interconversion of nitrogen forms when calculating estuarine nitrogen retention budgets and the necessity of measuring all nitrogen forms when performing mass balance estimates. The significance of denitrification in nitrogen removal is minimal at the high loading rates observed during this study. An implication to estuarine water quality management is that the relationships between nitrogen loading and retention are not linear and are controlled by factors other than water residence time.

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