Salt marshes are highly productive coastal wetlands that provide important ecosystem services such as storm protection for coastal cities, nutrient removal and carbon sequestration. Despite protective measures, however, worldwide losses of these ecosystems have accelerated in recent decades. Here we present data from a nine-year whole-ecosystem nutrient-enrichment experiment. Our study demonstrates that nutrient enrichment, a global problem for coastal ecosystems, can be a driver of salt marsh loss. We show that nutrient levels commonly associated with coastal eutrophication increased above-ground leaf biomass, decreased the dense, below-ground biomass of bank-stabilizing roots, and increased microbial decomposition of organic matter. Alterations in these key ecosystem properties reduced geomorphic stability, resulting in creek-bank collapse with significant areas of creek-bank marsh converted to unvegetated mud. This pattern of marsh loss parallels observations for anthropogenically nutrient-enriched marshes worldwide, with creek-edge and bay-edge marsh evolving into mudflats and wider creeks. Our work suggests that current nutrient loading rates to many coastal ecosystems have overwhelmed the capacity of marshes to remove nitrogen without deleterious effects. Projected increases in nitrogen flux to the coast, related to increased fertilizer use required to feed an expanding human population, may rapidly result in a coastal landscape with less marsh, which would reduce the capacity of coastal regions to provide important ecological and economic services.

Phosphorus fertilization of a pristine tundra river for four consecutive summers dramatically changed biological processes and populations at all trophic levels. At the primary producer level, both algal biomass and productivity increased and chlorophyll accumulated on the river bottom in the first two summers. Diatom community composition changed little in spite oflarge chlorophyll changes. However, an increase in grazing insects prevented chlorophyll buildup in the third and fourth summers.Some microbial processes were also stimulated by the increased photosynthesis caused by fertilization. Total respiration of the epilithon, acetate uptake, and decomposition of lignin monomers were all stimulated but only in light-grown epilithon. When epilithon was grown in the dark in the fertilized region of the river, there was no increased respiration. Also, phosphorus did not stimulate the decomposition of Carex litter.Although insects grew more rapidly in the fertilized section of the river, there were community interactions that kept total insect production from appreciable change. The four most abundant large insects did increase their growth rates in response to phosphorus addition and there were increases in populations of Baetis lapponicus and Brachycentrus americanus. These increases were offset by the decline in abundance of the dominant species, the black fly Prosimulium martini, perhaps caused by competition for space from Brachycentrus.Growth of both young-of-the-year and adult grayling (Thymallus arcticus) was strongly stimulated by phosphorus addition in years 3 and 4 (not tested in years 1 and 2). Carbon and nitrogen stable isotope tracers indicated that the measured increases in insect and fish growth were largely attributable to increases in the production of epilithic algae. Overall, the results indicate a strong "bottom-up" response of the riverine food web to additions of the limiting nutrient, phosphorus. The response was modified in later years, however, by a strong "top-down" feedback of insects grazing on epilithic algae and by competitive exclusion of black flies by caddisflies.

A long-term stream fertilization experiment was performed to evaluate the potential eutrophication of an arctic stream ecosystem. During 16 years of summer phosphorus (H 3 PO 4 ) fertilization, we observed a dramatic change in the community structure of the Kuparuk River on the North Slope of Alaska. A positive response to fertilization was observed at all trophic levels with increases in epilithic algal stocks, some insect densities, and fish growth rates. After approximately eight years of P fertilization, bryophytes (mosses) replaced epilithic diatoms as the dominant primary producers in the Kuparuk River. The moss impacted NH 4 ϩ uptake rates, benthic gross primary production, habitat structure, and insect abundance and species composition. This study documents the long-term changes in an arctic tundra stream in response to nutrient enrichment. Predicting stream ecosystem responses to chronic perturbation requires long-term observation and experiments.

Understanding the biogeochemical processes regulating carbon cycling is central to mitigating atmospheric CO 2 emissions. The role of living organisms has been accounted for, but the focus has traditionally been on contributions of plants and microbes. We develop the case that fully "animating" the carbon cycle requires broader consideration of the functional role of animals in mediating biogeochemical processes and quantification of their effects on carbon storage and exchange among terrestrial and aquatic reservoirs and the atmosphere. To encourage more hypothesis-driven experimental research that quantifies animal effects we discuss the mechanisms by which animals may affect carbon exchanges and storage within and among ecosystems and the atmosphere. We illustrate how those mechanisms lead to multiplier effects whose magnitudes may rival those of more traditional carbon storage and exchange rate estimates currently used in the carbon budget. Many animal species are already directly managed. Thus improved quantitative understanding of their influence on carbon budgets may create opportunity for management and policy to identify and implement new options for mitigating CO 2 release at regional scales.

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.

Salt marshes buffer coastlines and provide critical ecosystem services from storm protection to food provision. Worldwide, these ecosystems are in danger of disappearing if they cannot increase elevation at rates that match sea-level rise. However, the magnitude of loss to be expected is not known. A synthesis of existing records of salt marsh elevation change was conducted in order to consider the likelihood of their future persistence. This analysis indicates that many salt marshes did not keep pace with sealevel rise in the past century and kept pace even less well over the past two decades. Salt marshes experiencing higher local sea-level rise rates were less likely to be keeping pace. These results suggest that sea-level rise will overwhelm most salt marshes' capacity to maintain elevation. Under the most optimistic IPCC emissions pathway, 60% of the salt marshes studied will be gaining elevation at a rate insufficient to keep pace with sea-level rise by 2100. Without mitigation of greenhouse gas emissions this potential loss could exceed 90%, which will have substantial ecological, economic, and human health consequences.

The clearing of moist, lowland tropical forest for cattle pasture represents a widespread land use change that has consequences for soil biogeochemical cycles, as well as the links between soil processes and the concentrations of dissolved and particulate materials in rivers and streams. We examined how conversion of tropical forest to actively grazed cattle pasture in the Brazilian Amazon influenced the concentrations of different forms of nitrogen (N) and phosphorus (P) in soil solution and stream water. We compared two pairs of watersheds containing second-order streams that drained land in either forest or pasture at Fazenda Nova Vida, a cattle ranch in central Rondô nia. Measurements were made during the dry season (low flows) and the wet season (high flows). Forest soil solution had higher NO 3 Ϫ concentrations than pasture, but similar concentrations of NH 4 ϩ and PO 4 3Ϫ . Higher solution NO 3 Ϫ led to higher ratios of dissolved inorganic N:P in forest soils. Pasture streams had higher concentrations of total suspended solids, particulate organic carbon (POC), and particulate organic N (PON) during the dry season, but not during the wet season. Pasture streams also had lower concentrations of NO 3 Ϫ than forest streams. This was consistent with previous studies that found lower extractable NO 3 Ϫ concentrations and lower rates of net N mineralization and net nitrification in the soils of the pasture watersheds compared with forest watersheds. Dissolved organic N (DON) dominated stream water dissolved-N concentrations in forest (53-76%) and pasture (67-84%). Higher dissolved inorganic N (DIN) concentrations in forest streams coupled with a trend toward higher DON and PON concentrations in pastures led to small differences in total N (TN) concentrations between land uses. Lower ratios of inorganic and total dissolved N:P in pasture streams suggested a switch from P limitation in forest streams to N limitation in pasture streams. Periphyton bioassays in forest and pasture streams confirmed that N limited algal growth in pasture streams where light was available. These results suggest that links among deforestation, soil biogeochemistry, and the stoichiometry of N and P reaching streams in small watersheds have the potential to influence the structure of these aquatic ecosystems.

We used 15 NH 4 tracer additions to determine travel distances of ammonium (NH 4 ) and suspended particulate organic nitrogen (SPON) in six streams ranging from second to fifth order located within a single watershed on the North Slope of Alaska. Based on the distribution of 15 N stored in stream bottom compartments (primary producers or grazers), we estimated NH 4 travel lengths. We used a two-compartment model to estimate the travel length of SPON based on the distribution of source 15 N on the stream bottom and SPO 15 N in the water column. Both NH 4 and SPON travel lengths (S w and S p , respectively) increased with discharge primarily due to changes in depth and velocity. Variation in the vertical mass transfer coefficient ( f ) of both NH 4 and SPON did occur among the streams but was not related to stream size and was relatively small compared to the change in physical characteristics. Thus, in the Kuparuk watershed, physical gradients outweighed biological or chemical changes as controls on NH 4 and SPON travel length. The one exception was the Kuparuk fertilized reach, where phosphorus fertilization greatly increased biological activity and NH 4 processing compared to unaltered streams. Longitudinal gradients in major biological driving variables such as litter inputs, debris dams, and shading are absent in the Arctic, perhaps explaining the relatively uniform NH 4 -f . Watersheds in other biomes may show differing degrees of physical versus biological/chemical controls. A conceptual model is presented for comparing the relative strength of these controls among different watersheds. Strong relationships between discharge and travel length should greatly aid development of watershed models of nutrient dynamics.