Latitudinal variation in top-down and bottom-up control of a salt marsh food web

The shrub Iva frutescens, which occupies the terrestrial border of U.S. Atlantic Coast salt marshes, supports a food web that varies strongly across latitude. We tested whether latitudinal variation in plant quality (higher at high latitudes), consumption by omnivores (a crab, present only at low latitudes), consumption by mesopredators (ladybugs, present at all latitudes), or the life history stage of an herbivorous beetle could explain continental-scale field patterns of herbivore density. In a mesocosm experiment, crabs exerted strong top-down control on herbivorous beetles, ladybugs exerted strong top-down control on aphids, and both predators benefited plants through trophic cascades. Latitude of plant origin had no effect on consumers. Herbivorous beetle density was greater if mesocosms were stocked with beetle adults rather than larvae, and aphid densities were reduced in the “adult beetle” treatment. Treatment combinations representing high and low latitudes produced patterns of herbivore density similar to those in the field. We conclude that latitudinal variation in plant quality is less important than latitudinal variation in top consumers and competition in mediating food web structure. Climate may also play a strong role in structuring high-latitude salt marshes by limiting the number of herbivore generations per growing season and causing high overwintering mortality

Salinity and Simulated Herbivory Influence Spartina alterniflora Traits and Defense Strategy

Sea level rise is expected to push saline waters into previously fresher regions of estuaries, and higher salinities may expose oligohaline marshes to invertebrate herbivores typically constrained by salinity. The smooth cordgrass, Spartina alterniflora (syn. Sporobolus alterniflorus), can defend itself against herbivores in polyhaline marshes, however it is not known if S. alterniflora’s defense varies along the mesohaline to oligohaline marsh gradient in estuaries. I found that S. alterniflora from a mesohaline marsh is better defended than plants from an oligohaline marsh, supporting the optimal defense theory. Higher salinity treatments lowered carbon content, C:N, and new stem biomass production, traits associated with a tolerance strategy, suggesting that salinity may mediate the defense response of S. alterniflora. Further, simulated herbivory increased the nitrogen content and decreased C:N of S. alterniflora. This indicates that grazing may increase S. alterniflora susceptibility to future herbivory via improved forage quality. Simulated herbivory also decreased both belowground and new stem biomass production, highlighting a potential pathway in which herbivory can indirectly facilitate marsh loss, as S. alterniflora biomass is critical for vertical accretion and marsh stability under future sea level rise scenarios

Geographic variation in plant community structure of salt marshes: species, functional and phylogenetic perspectives.

In general, community similarity is thought to decay with distance; however, this view may be complicated by the relative roles of different ecological processes at different geographical scales, and by the compositional perspective (e.g. species, functional group and phylogenetic lineage) used. Coastal salt marshes are widely distributed worldwide, but no studies have explicitly examined variation in salt marsh plant community composition across geographical scales, and from species, functional and phylogenetic perspectives. Based on studies in other ecosystems, we hypothesized that, in coastal salt marshes, community turnover would be more rapid at local versus larger geographical scales; and that community turnover patterns would diverge among compositional perspectives, with a greater distance decay at the species level than at the functional or phylogenetic levels. We tested these hypotheses in salt marshes of two regions: The southern Atlantic and Gulf Coasts of the United States. We examined the characteristics of plant community composition at each salt marsh site, how community similarity decayed with distance within individual salt marshes versus among sites in each region, and how community similarity differed among regions, using species, functional and phylogenetic perspectives. We found that results from the three compositional perspectives generally showed similar patterns: there was strong variation in community composition within individual salt marsh sites across elevation; in contrast, community similarity decayed with distance four to five orders of magnitude more slowly across sites within each region. Overall, community dissimilarity of salt marshes was lowest on the southern Atlantic Coast, intermediate on the Gulf Coast, and highest between the two regions. Our results indicated that local gradients are relatively more important than regional processes in structuring coastal salt marsh communities. Our results also suggested that in ecosystems with low species diversity, functional and phylogenetic approaches may not provide additional insight over a species-based approach

Growth of roach Rutilus rutilus (L.) In the River Odra estuary

The back-calculation method was applied to determining growth rate of roach caught in the Międzyodrze, Lake Dąbie, Szczecin Lagoon, and the Pomeranian Bay. Samples were collected from April 1995 to October 1997. The Pomeranian Bay and Międzyodrze individuals showed a faster growth rate than that attributed to other stocks. In each of the areas studied, older females grew faster than males. The Odra estuary roach population can be regarded as one of to the fastest growing populations of the species

Nutritional status of roach Rutilus rutilus (L.) in the River Odra estuary

Nutritional status of roach inhabiting the River Odra estuary was studied with both a traditional (morpho-physiological) approach and a bioenergetic one. A total of 999 individuals caught from the Międzyodrze, Lake Dąbie, the Szczecin Lagoon, and the Pomeranian Bay were examined. The Pomeranian Bay individuals were found to be in the best nutritional condition. None of the traditional condition coefficients was able to clearly reflect seasonal changes over the year. Those changes could be observed instead by analysing bioenergetic indices

Effect of single superphosphate fertilizer on survival and respiratory dynamics of Nile tilapia, Oreochromis niloticus (Actinopterygii: Perciformes: Cichlidae)

Background. Increasing global usage of inorganic fertilizers, including phosphate-based fertilizers has its negative consequences on the aquatic environment. Effects of single superphosphate fertilizer (SPF) remain unknown, particularly its influence on the respiratory dynamics of fish under continuous exposure. We investigated the effects of single SPF on the survival and respiratory dynamics of Nile tilapia, Oreochromis niloticus, under laboratory conditions. Materials and methods. Nile tilapia fingerlings (of mixed sex) (5.40 . 0.03 g) were exposed to various concentrations of the fertilizer in five treatment regimes (in triplicate): 0.88, 1.75, 3.50, 7.00, 14.00 g . L–1 (and 0.00 g . L–1 for control). Each replicate was carried out in a 30-L circular plastic tank based on 20 fingerlings. The study involved: the mortality estimation, the oxygen consumption, the histopathological effects on fish gills, and the activities of lactate dehydrogenase (LDH), alcohol dehydrogenase (ADH) in liver of fish exposed to sublethal concentrations (0.44, 0.22, 0.11, 0.06, and 0.03 g . L–1) of single SPF for eight weeks under laboratory conditions. Results. Acute concentrations of SPF had serious adverse effects on mortality, oxygen consumption and opercular ventilation rates of exposed fish. All variables showed a dose-dependency. A mean value of 96-h LC50 of the SPF to the test fish was calculated to be 3.76 g . L–1. At various acute concentrations, oedema and hyperplasia of gill lamellae were observed in exposed fish. Exposure of the fish to sublethal concentrations of the SPF resulted in reduction in the levels of lactate dehydrogenase and alcohol dehydrogenase activities in liver. Conclusion. Concentrations of SPF in natural water bodies are deleterious to aquatic fauna. With rapid global economic development and need for more food production, pollution from agricultural fertilizers remains a major threat to the aquatic ecosystem. Therefore it is ultimately important that a balance is struck between achieving economic excellence and environmental protection through good pollution management strategies

Appendix C. Details for analyses and full data table from mesocosm experiment.

Details for analyses and full data table from mesocosm experiment

Appendix A. Study system and species.

Study system and species

Appendix B. Expanded methods and additional analyses.

Expanded methods and additional analyses