Are estuaries traps for anthropogenic nutrients? Evidence from estuarine mesocosms

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Abstract
A series of estuarine mesocosms is described, where nutrient budgets were used to determine rates of nitrogen and phosphorus trapping and export as a function of nutrient input level, season, and presence or absence of sediments. Regardless of treatment or season these experimental systems exported most of the N and P that they received. Control systems with sediments retained none of the inflowing N and P during summer, and 5% of N and 25% of P inputs during winter. Eutrophied systems with sediments initially retained 30% of added N and P due to increases in water column and sediment nutrient standing stocks in response to daily inorganic nutrient additions; however, after 6 mo of daily nutrient loading, these treatments retained only 5 to 15% of nutrients added. Results of this study suggest that well-mixed estuarine systems may export to offshore waters most of the nitrogen and phosphorus that they receive. For the small percentage of nutrients that were retained, there was more storage during winter than summer, more storage in treatments without sediments, and more retention of P than N. Nitrogen losses through sediment denitrification accounted for 10 to 20% of the N input to controls, and loss than 10% of the N input to eutrophied treatments. The addition of nutrients to the eutrophied treatments resulted in increases in the N and P content of surface sediments, and the rapid deposition of an N and P-rich detrital layer on the bottom of the treatments without sediments.