Changes in metabolic rates under fluctuating salinity regimes for two subtidal estuarine habitats

The metabolic rate of individual habitats can differ significantly in their contribution to the total system productivity of estuaries. Changing envir onmental conditions such as those created by tidal exchange can frequently alter these rates. In an effort to quantify these rate responses, metabolic rates were measured for macroalgal and sediment habitats at different sali nities. Microcosms representing the two habitats were incubated at three sa linity ranges (high: 25 to 31 parts per thousand; moderate: 12 to 18 parts per thousand; and low: 0 to 4 parts per thousand) and production and respir ation rates were estimated. The production rates for both habitats were pro portional to the salinity of the water in the incubation, with the lowest m etabolic rates associated with the lowest salinity. Average macroalgal habi tat net production rates were 879 mg O-2 m(-2) h(-1), 609 mg O-2 m(-2) h(-1 ), and 451 mg O-2 m(-2) h(-1) at high, moderate, and low salinity treatment s, respectively, and the dark respiration rates were -401 mg O-2 m(-2) h(-1 ), -341 mg O-2 m(-2) h(-1), and -333 mg O-2 m(-2) h(-1). Average sediment h abitat net production rates were 60 mg O-2 m(-2) h(-1), 13 mg O-2 m(-2) h(- 1) and 10 mg O-2 m(-2) h(-1) and the respiration rates were -114 mg O-2 m(- 2) h(-1), -55 mg O-2 m(-2) h(-1), and -31 mg O-2 m(-2) h(-1) at high, moder ate, and low salinity treatments. The larger contribution of macroalgal hab itats to system metabolism may account for observed diurnal changes in wate r column oxygen levels in some estuaries. Macroalgal production rates expla ined 83% of the increase in water column oxygen levels during daylight hour s and macroalgal respiration rates explained 65% of the decline in oxygen l evels during the night. The contribution of macroalgal metabolism to the sy stem can be influenced by even short-term changes in water column salinity. Environmental processes that alter salinity levels on hourly time scales m ay moderate the effect of macroalgal metabolism on oxygen levels.