Microzooplankton grazing and nitrogen excretion across a surface estuarine-coastal interface

The role of the microzooplankton community in regulating phytoplankton biom ass was examined across a gradient from a river-dominated estuary to an oce anic-influenced coastal zone. Three stations located along a salinity gradi ent from the central region of Mobile Bay to 10 km off the coast were sampl ed from May 1994 to August 1995, Microzooplankton herbivory rates on phytop lankton and microzooplankton excretion of nitrogen derived from phytoplankt on were estimated using the dilution technique. Microzooplankton grazing ra tes (range of station means = 0.57-1.10 d(-1)) and phytoplankton growth rat es (0.70-1.62 d(-1)) both increased across the salinity gradient from the b ay station to the offshore station. However, the percent of primary product ion grazed per day was highest at the bay station (mean = 83%) and decrease d to a low at the offshore station (mean = 64%). Excretion of phytoplankton -derived nitrogen by the microzooplankton was greatest at the bay and bay m outh stations. Excreted nitrogen could potentially supply 39%, 29%, and 20% of phytoplankton nitrogen demand at the bay, bay mouth, and offshore stati ons, respectively. These results support the idea that herbivorous minozoop lankton are important in mediating nitrogen flow to both lower and higher t rophic levels.