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1 Yale University, New Haven, CT
2 Lafayette College, Easton, PA
3 Boston University Marine Program, Woods Hole, MA
Nitrogen supply is a major control on growth of coastal macroalgae (1, 2, 3). Top-down effects in which grazing significantly 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 influence 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 flow. 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-1y-1; Quashnet River, 350 kg ha-1y-1; and Childs River, 601 kg ha-1y-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 µM 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 field 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 significantly 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 significantly different, although they contained lower grazer abundances than the 1-mm and 4-mm mesh cages (Fig. 1a). Predatory fish 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.
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Top-down effects caused by grazers were small compared to the effects of nutrients (Fig. 1b). Across all estuaries, net growth in the 1-mm (fewer grazers) cages was higher than growth in 4-mm (more grazers) cages, but these differences were statistically insignificant. In contrast, there was more than a 200% increase in percent net growth caused by the nutrient effects. Taken together, these results suggest that the direct and indirect bottom-up effects associated with nitrogen loading were much larger than the top-down effects of grazing on the net growth of U. lactuca.
Increased nutrient inputs might affect the composition of the grazers found in the three estuaries (Fig. 1c). Shrimp (Palaemonetes sp.) and amphipods (Gammaridea) accounted for over 85% of grazers in all cages. Shrimp were far more abundant in high nitrogen load conditions (Childs River), while amphipods were more plentiful in lower nitrogen load conditions (Sage Lot Pond; Fig. 1c). Shrimp are predators as well as grazers (7), so they could have fed upon amphipods. Similar shifts in species composition have been reported elsewhere (3, 8). Increased anthropogenic supplies of nitrogen might therefore not only change the growth rates of U. lactuca directly, but also alter the relative abundances of consumers and lower abundances of grazers. This effect is possibly linked to more frequent anoxic and hypoxic conditions in the more nitrogen-loaded estuaries such as Childs River (9).
To further assess possible grazer effects, we plotted percent net growth separately versus the abundance of amphipods and shrimp. The difference in abundances of these two groups is evident in Figure 1c, d, and e. Despite the shift in grazer composition among the estuaries, there was no significant effect of either amphipod or shrimp number on percent net growth of U. lactuca (Fig. 1d, e). Even though the treatments significantly varied the abundance of grazers, we could find no compelling evidence of significant top-down control of U. lactuca net growth by grazers in Waquoit Bay estuaries. This differs from the results of other studies, which show that under eutrophic conditions, top-down effects play a significant role in controlling macroalgal biomass (10). Our results corroborate the conclusions of previous studies that bottom-up effects may overwhelm top-down forces in nutrient-enriched estuaries (3, 4).
This research was supported by internships to J.M. from a grant from the National Science Foundation’s Research Experience for Undergraduates #OCE-0097498, and to A.A. from the Woods Hole Marine Consortium. This work was also supported by a grant from the National Oceanic and Atmospheric Association/NOS, ECOHAB #NA16OP2728, and is ECOHAB publication #77. Special thanks to Paulina Martinetto for help with statistical analysis.
Literature Cited
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