Relative influence of grazing and nutrient supply on growth of the green macroalga Ulva lactuca in estuaries of Waquoit Bay, Massachusetts.

Abstract

Nitrogen supply is a major control on growth of coastal macroalgae (1, 2, 3). Top-down effects in which grazing signifi cantly affects macroalgae (4, 5), and nutrient-grazer interactions (3) have also been described. In this paper we describe an experiment in which we measured net growth of a common macroalga, Ulva lactuca, in treatments that allowed different numbers of grazers to access fronds as well as incubation of fronds in estuaries with demonstrably different nutrient supplies. These treatments were intended to assess the relative infl uence of grazer and nitrogen supply on net growth rates of a coastal producer. To examine the effect of grazing on growth of U. lactuca, we constructed acrylic plastic cages with sides of 1-mm, 4-mm, or 18-mm mesh. The different mesh openings were intended to allow entry to different numbers of grazers, which we took as a proxy for grazing pressure. The cage design also allowed for light penetration and horizontal water fl ow. The 18-mm mesh permitted larger size classes and a greater number of grazers to enter the cages, while the 1-mm mesh excluded larger size classes and allowed fewer grazers. The 4-mm mesh was intended to furnish an intermediate grazer treatment. To evaluate the effect of nitrogen supply and grazing on algal growth, cages with the three mesh sides were placed in three estuaries in Waquoit Bay, Massachusetts. These three estuaries experience different nitrogen loads—Sage Lot Pond, 14 kg ha 1 y 1 ; Quashnet River, 350 kg ha 1 y 1 ; and Childs River, 601 kg ha 1 y 1 —from their watershed (6). These nitrogen loads led to different mean nitrate concentrations measured in the estuaries during July 2002, one year prior to the time of our experiments: 0.04, 6.1, and 11.75 M for Sage Lot Pond, Quashnet River, and Childs River, respectively (G. Tomasky, Boston University Marine Program, unpubl. data). To minimize effects of differences between estuaries other than our treatments, we chose sites similar in salinity, depth, and algal composition. In each estuary we placed four replicates of each of the three grazing pressure treatments, for a total of 36 cages. Three fronds of U. lactuca, each approximately 300 mg (blotted wet weight), were suspended inside each cage. To measure the effect of top-down versus bottom-up factors, we measured net growth as the dependent variable. Net growth was the growth achieved by the fronds minus the biomass consumed by grazers. To determine net growth, the U. lactuca fronds were weighed initially (blotted wet weight) and again after 10 days of fi eld incubation. First, we assess the successed of the treatments. To roughly measure the grazing pressure, we sorted and counted the potential grazers found in the cages for two replicates of each treatment at the end of the incubation. The grazers were sorted into four groups, amphipods, shrimp, crabs, and isopods. The total number of grazers in the 1-mm mesh cages was signifi cantly lower in all three estuaries than the number in the 4-mm cages (Fig. 1a; ANOVA F 31.0, P 0.0014). The number of grazers found in the 18-mm mesh cages was also signifi cantly different, although they contained lower grazer abundances than the 1-mm and 4-mm mesh cages (Fig. 1a). Predatory fi sh and large shrimp entered the cages with 18 mm mesh and likely fed on the smaller grazers, thus decreasing grazer abundances. This possible effect of predators on grazers suggests that there might be important top-down cascade effects in this system waiting to be studied. The difference in nitrogen load in the three estuaries provided quantitatively different nutrient supplies, as evident in the nitrate concentrations cited above. Bottom-up effects from these different nitrogen supplies on net growth of U. lactuca were dominant factors. Rates of net growth were higher in estuaries receiving