Uptake and Release of Nitrogen by the Macroalgae Gracilaria Vermiculophylla (Rhodophyta)1

Macroalgae, often the dominant primary producers in shallow estuaries, can be important regulators of nitrogen (N) cycling. Like phytoplankton, actively growing macroalgae release N to the water column; yet little is known about the quantity or nature of this release. Using 15N labeling in laboratory and field experiments, we estimated the quantity of N released relative to assimilation and gross uptake by Gracilaria vermiculophylla (Ohmi) Papenfuss (Rhodophyta, Gracilariales), a non‐native macroalgae. Field experiments were carried out in Hog Island Bay, a shallow back‐barrier lagoon on the Virginia coast where G. vermiculophylla makes up 85%–90% of the biomass. There was good agreement between laboratory and field measurements of N uptake and release. Daily N assimilation in field experiments (32.3±7.2 μmol N·g dw−1·d−1) was correlated with seasonal and local N availability. The average rate of N release across all sites and dates (65.8±11.6 μmol N·g dw−1·d−1) was 67% of gross daily uptake, and also varied among sites and seasons (range=33%–99%). Release was highest when growth rates and nutrient availability were low, possibly due to senescence during these periods. During summer biomass peaks, estimated N release from macroalgal mats was as high as 17 mmol N·m−2·d−1. Our results suggest that most estimates of macroalgal N uptake severely underestimate gross N uptake and that N is taken up, transformed, and released to the water column on short time scales (minutes–hours).