Abstract. Salt marsh ecosystems have been considered not susceptible to nitrogen overloading because early studies suggested that salt marshes adsorbed excess nutrients in plant growth. However, the possible effect of nutrient loading on species composition, and the combined effects of nutrients and altered species composition on structure and function, was largely ignored. Failure to understand interactions between nutrient loading and species composition may lead to severe underestimates of the impacts of stresses. We altered whole salt marsh ecosystems (;60 000 m 2 /treatment) by addition of nutrients in flooding waters and by reduction of a key predatory fish, the mummichog. We added nutrients (N and P; 15-fold increase over ambient conditions) directly to the flooding tide to mimic the way anthropogenic nutrients are delivered to marsh ecosystems. Despite the high concentrations (70 mmol N/L) achieved in the water column, our annual N loadings (15-60 g NÁm À2 Áyr À1 ) were an order of magnitude less than most plot-level fertilization experiments, yet we detected responses at several trophic levels. Preliminary calculations suggest that 30-40% of the added N was removed by the marsh during each tidal cycle. Creek bank Spartina alterniflora and high marsh S. patens production increased, but not stunted high marsh S. alterniflora. Microbial production increased in the fertilized creek bank S. alterniflora habitat where benthic microalgae also increased. We found top-down control of benthic microalgae by killifish, but only under nutrient addition and in the opposite direction (increase) than that predicted by a fish-invertebrate-microalgae trophic cascade. Surprisingly, infauna declined in abundance during the first season of fertilization and with fish removal. Our results demonstrate ecological effects of both nutrient addition and mummichog reduction at the whole-system level, including evidence for synergistic interactions.

The influence of herbivorous snails on freshwater macrophytes was examined both in the laboratory and the field. In the laboratory, 14 macrophyte species grown in the absence of herbivores had growth rates ranging from 1-10%/d. When grown with four densities of herbivorous snails, species that grew fastest in the absence of herbivores were, in general, most negatively influenced by grazing. In food choice tests, snails typically preferred the plant species that grew fastest in the absence of herbivores. Snail densities were manipulated in a small mesotrophic lake, Christmas Lake. In areas where high snail densities were maintained, macrophyte species richness decreased, and the plants remaining were the species least preferred in laboratory choice tests. In surveys of eight additional lakes having a range of snail densities, the relationship between snail density and macrophyte species composition and diversity was consistent with the laboratory and Christmas Lake results. The levels of herbivore damage in the lake were measured for one macrophyte species. For the leaves examined, from 6 to 13% of the total leaf area was lost to herbivores. These results demonstrate that herbivorous snails can strongly influence the distribution, abundance, and diversity of freshwater macrophytes, and suggest that the role of herbivores in freshwater macrophyte communities may be similar to their role in terrestrial and marine communities.

No abstract

We have evaluated the potential of a North American aquatic weevil, Euhrychiopsis lecontei, to serve as an agent of biological control for an exotic weed, Eurasian watermilfoil (Myriophyllum spicatum), which is currently found throughout much of the United States and in some southern provinces of Canada. We have found this weevil on M. spicatum in lakes where populations of the exotic weed have declined. We introduced weevils into enclosures in two lakes dominated by M. spicatum. In both lakes there was 50% less M. spicatum biomass in enclosures with weevils than in enclosures without weevils. Also, in control enclosures, M. spicatum formed a canopy on the water surface as it did outside the enclosures, while there was no plant canopy in any of the weevil enclosures. In laboratory feeding trials we quantified the effects of weevils on ten native aquatic plants including a native watermilfoil, Myriophyllum sibiricum. Weevils did not have a significant effect on the increase in plant length or final dry mass of any native plant, however the weevils did feed on the native watermilfoil. All native plants added new leaves, leaf whorls, or side branches under all weevil densities. The results from these studies suggest that a North American insect may be a suitable control agent for this introduced aquatic weed. Native biological control agents, when they can be found, offer potential advantages over classical biological control agents; they may have little impact on non‐target native species that have coexisted with the control agent, and may save the time and expense of foreign research and quarantine procedures.

The Eurasian watermilfoil (Myriophyllum spicatum) population in Brownington Pond, Vermont, declined between 1986 and 1989. Watermilfoil covered ≈10‐11 ha of the littoral zone in 1986. Less than 0.5 ha remained in 1989. An herbivorous weevil (Euhrychiopsis lecontei), which is native to North America, was found associated with this watermilfoil population and we hypothesized that this weevil played a role in the decline. We monitored watermilfoil and E. lecontei populations in Brownington Pond from 1990 through 1992 by (1) mapping the location and extent of beds in the pond, and (2) determining watermilfoil biomass along permanent transects. The abundance of weevil eggs, larvae, pupae, and adults were monitored by sampling individual watermilfoil stems. Watermilfoil cover increased to ≈ 2.5 ha by 1991 and then declined again to ≈1 ha by 1992. The reduction in watermilfoil biomass from 1991 to 1992 ranged from 4‐ to 30‐fold depending on location. Mean weevil abundance increased from 1990 (<1/stem) through early 1992 (3‐4/stem) and then began to decline. The number of weevil eggs/stem also peaked in early 1992 and then declined. These survey results are consistent with the hypothesis that the weevil played an important role in producing both observed declines. The effect of weevils on watermilfoil was evaluated in two experiments. In an aquarium experiment, we found that the viability of stem fragments damaged by weevils was reduced compared to stem fragments without weevil damage. Watermilfoil commonly spreads by producing fragments so the spread of watermilfoil beds by fragmentation may be reduced when weevil abundances are high. In a pond enclosure experiment, weevils suppressed the production of new watermilfoil biomass. The biomass of lateral stems and roots of watermilfoil plants damaged by weevils was significantly less than those of control plants. There was no significant difference in the biomass of the stems originally planted in the enclosures. Weevil damage did have a negative effect on the buoyancy of these original stems, however. These experimental results also support the hypothesis that this native weevil played an important role in the two watermilfoil declines observed at Brownington Pond.