Salt marsh restoration surprise: A subordinate species accumulates and shares nitrogen while outcompeting salt marsh dominants

Selectively planting native species could guide ecosystem development toward wetland restoration targets, once we understand how influential species function, alone and in combination. Knowing that Triglochin concinna (arrow grass, Juncaceae) accumulates N in its perennial roots, we asked how it would influence N dynamics on an excavated salt marsh plain at Tijuana Estuary, in southern California. We hypothesized that it would (a) accumulate N in roots and shoots, (b) reduce biomass of other marsh plain plants or, alternatively, (c) share N with neighbors as its litter decomposed and released N. We used 15N stable isotope enrichment to quantify N transfer between Triglochin and the marsh plain’s seven-species halophyte assemblage in field and greenhouse experiments. We also examined the effect of Triglochin on individual marsh plain species’ biomass and N accumulation. Triglochin had low shoot biomass (0.96 ± 0.5 g m−2 in field plots and 17.64 ± 2.2 g m−2 in greenhouse pots), high root:shoot ratios (4.3 in the field and 2.0 in the greenhouse), and high tissue N content (1.9 ± 0.2% in the field and 1.7 ± 0.1% in the greenhouse). Two productive perennials, Sarcocornia pacifica (pickleweed) and Frankenia salina (alkali heath), outgrew Triglochin; yet these biomass dominants produced 44%–45% less shoot biomass in greenhouse pots with Triglochin than without. However, we did not find this reduction in the field where roots were unconfined. In the greenhouse, δ15N values were higher for species grown with 15N-enriched Triglochin, indicating that this species made N available to its neighbors. The δ15N values for plants grown in the field exceeded background levels, also indicating that the marsh plain assemblage took up N released by Triglochin. We conclude that Triglochin can influence the restoration of salt marsh vegetation by accumulating N and releasing its tissue N to neighbors as leaves and roots decompose, while simultaneously reducing the biomass of neighbors. The seasonally deciduous Triglochin is low in shoot biomass, yet competitively superior in N uptake. Because this often-ignored species has limited tidal dispersal, we suggest restoration plantings, including tests of its ability to facilitate diversity where S. pacifica, the marsh plain dominant, might otherwise form monocultures

Restored saltmarshes lack the topographic diversity found in natural habitat

Saltmarshes can be created to compensate for lost habitat by a process known as managed realignment (MR), where sea defences are deliberately breached to flood low-lying agricultural land. However, the vegetation that develops on MR sites is not equivalent to natural habitat. In natural sites, surface topography and creek networks are drivers of vegetation diversity, but their development on restored sites has not been well studied. We investigate the topographic characteristics of 19 MR areas, and compare these to nearby natural saltmarshes (representing desired conditions) and to coastal agricultural landscapes (representing conditions prior to MR). From high-resolution LiDAR data, we extracted values of elevation, six measures of surface topography (although two were later excluded due to collinearity), and three measures of creek density. MR and natural marshes differed significantly in all surface topographic indices, with MR sites having lower rugosity and more concave features, with greater potential for water accumulation. MR sites also had significantly lower creek density. MRs and coastal agricultural landscapes were more similar, differing in only one topographic measure. Importantly, there was no relationship between age since restoration and any of the topographic variables, indicating that restored sites are not on a trajectory to become topographically similar to natural marshes. MR schemes need to consider actively constructing topographic heterogeneity; better mirroring natural sites in this way is likely to benefit the development of saltmarsh vegetation, and will also have implications for a range of ecosystem functions

Limited Vegetation Development on a Created Salt Marsh Associated with Over-Consolidated Sediments and Lack of Topographic Heterogeneity

Restored salt marshes frequently lack the full range of plant communities present on reference marshes, with upper marsh species underrepresented. This often results from sites being too low in the tidal frame and/or poorly drained with anoxic sediments. A managed coastal realignment scheme at Abbotts Hall, Essex, UK, has oxic sediments at elevations at which upper marsh communities would be expected. But 7 years after flooding, it continued to be dominated by pioneer communities, with substantial proportions of bare ground, so other factors must hinder vegetation development at these elevations. The poorly vegetated areas had high sediment shear strength, low water and organic carbon content and very flat topography. These characteristics occur frequently on the upper parts of created marshes. Experimental work is required to establish causal links with the ecological differences, but other studies have also reported that reduced plant β-diversity and lower usage by fish are associated with topographic uniformity. Uniformity also leads to very different visual appearance from natural marshes. On the upper intertidal, sediment deposition rate are slow, water velocities are low and erosive forces are weak. So, topographic heterogeneity cannot develop naturally, even if creeks have been excavated. Without active management, these conditions will persist indefinitely

Evaluating the effects of Southern Resident orcas recovery actions and external threats in the marine ecosystem of Puget Sound

We apply two ecosystem models, the Atlantis Model for Puget Sound and the Salish Sea Atlantis Model, to simulate the recommendations of the Southern Resident Orca Task Force for recovery of the endangered Southern Resident Killer Whales (“Southern Residents”) in the Salish Sea. The downturn of the Southern Residents has been attributed to multiple, co-occurring anthropogenic and ecological pressures that are being addressed through the recommendations of the Orca Task Force. Atlantis is a spatially-explicit ecosystem modeling platform that simulates oceanography and biochemistry, food-web interactions, fisheries, and dependence of species on biogenic and physical habitat. We are evaluating the ecosystem-level impacts of recovery actions aimed at enhancing population growth and long-term sustainability of the Southern Residents and the future cumulative impacts from human population growth, oil spills, and climate change. We are addressing three objectives by simulating long-term dynamics using the two ecosystem models built in the Atlantis framework that span the full Salish Sea range of Southern Residents: (1) Analyze whether recovery actions for Southern Residents will support or hinder other conservation objectives; (2) Reveal potential tradeoffs inherent in the proposed recovery actions, both via direct effects and indirect (trophic) pathways; (3) Examine future cumulative threats from population growth, ocean warming, and oil spills. We are evaluating these scenarios in terms of abundance, size, diets, catch, and ecosystem indicators. We are generating information on ecosystem-level tradeoffs, the probability of success of recovery actions, and economic impacts, that can help managers and policy makers reconcile potentially conflicting unintended consequences that are likely to arise in response to bold conservation actions for Southern Resident recovery and from future cumulative threats

An Atlantis Ecosystem Model for the Gulf of Mexico supporting Integrated Ecosystem Assessment

The Gulf of Mexico supports a high biological diversity and biomass of fish, seabirds, and mammals; in this region, multiple commercial and recreational fishing fleets operate providing economic resources for local populations. The Gulf is also the site of important oil and gas production and tourism. As a result of intensive human use, the Gulf is subject to various impacts, including oil spills, habitat degradation, and anoxia. Management of this Large Marine Ecosystem requires an ecosystem-based management approach that provides a holistic approach to resource management. The Gulf of Mexico is managed as part of NOAA\u27s Integrated Ecosystem Assessment Program (IEA). This program considers the development of ecosystem models as a tool for ecosystem-based fisheries management (EBFM) and to support the different stages in the IEA process, particularly testing the effects of alternative management scenarios. As part of this program, we have parametrized an Atlantis ecosystem model for the Gulf of Mexico, including major functional groups, physiographic dynamics, and fishing fleets. The Gulf of Mexico (GOM) Atlantis model represents a collaboration between the University of South Florida, the University of Miami, the Southeast Fisheries Science Center, the National Coastal Data Development Center, and other contributors --Executive summary

An Atlantis Ecosystem Model for the Gulf of Mexico supporting Integrated Ecosystem Assessment

The Gulf of Mexico supports a high biological diversity and biomass of fish, seabirds, and mammals; in this region, multiple commercial and recreational fishing fleets operate providing economic resources for local populations. The Gulf is also the site of important oil and gas production and tourism. As a result of intensive human use, the Gulf is subject to various impacts, including oil spills, habitat degradation, and anoxia. Management of this Large Marine Ecosystem requires an ecosystem-based management approach that provides a holistic approach to resource management. The Gulf of Mexico is managed as part of NOAA\u27s Integrated Ecosystem Assessment Program (IEA). This program considers the development of ecosystem models as a tool for ecosystem-based fisheries management (EBFM) and to support the different stages in the IEA process, particularly testing the effects of alternative management scenarios. As part of this program, we have parametrized an Atlantis ecosystem model for the Gulf of Mexico, including major functional groups, physiographic dynamics, and fishing fleets. The Gulf of Mexico (GOM) Atlantis model represents a collaboration between the University of South Florida, the University of Miami, the Southeast Fisheries Science Center, the National Coastal Data Development Center, and other contributors --Executive summary

Ecosystem-Based Harvest Control Rules for Norwegian and US Ecosystems

Management strategy evaluation (MSE) provides a simulation framework to test the performance of living marine resource management. MSE has now been adopted broadly for use in single-species fishery management, often using a relatively simple “operating model” that projects population dynamics of one species forward in time. However, many challenges in ecosystem-based management involve tradeoffs between multiple species and interactions of multiple stressors. Here we use complex operating models, multi-species ecosystem models of the California Current and Nordic and Barents Seas, to test threshold harvest control rules that explicitly address the linkage between predators and prey, and between the forage needs of predators and fisheries. Specifically, within Atlantis ecosystem models we focus on how forage (zooplankton) availability affects the performance of harvest rules for target fish, and how these harvest rules for fish can account for environmentally-driven fluctuations in zooplankton. Our investigation led to three main results. First, consistent with studies based on single-species operating models, we found that compared to constant F = FMSY policies, threshold rules led to higher target stock biomass for Pacific hake (Merluccius productus) in the California Current and mackerel (Scomber scombrus) in the Nordic and Barents Seas. Performance in terms of catch of these species varied depending partly on the biomass and recovery trajectory for the simulated stock. Secondly, the multi-species operating models and the harvest control rules that linked fishing mortality rates to prey biomass (zooplankton) led to increased catch variability; this stemmed directly from the harvest rule that frequently adjusted Pacific hake or mackerel fishing rates in response to zooplankton, which are quite variable in these two ecosystems. Thirdly, tests suggested that threshold rules that increased fishing when productivity (zooplankton) declined had the potential for strong ecosystem effects on other species. These effects were most apparent in the Nordic and Barents Seas simulations. The tests of harvest control rules here do not include uncertainty in monitoring of fish and zooplankton, nor do they include uncertainty in stock assessment and implementation; these would be required for full MSE. Additionally, we intentionally chose target fish with strong mechanistic links to particular zooplankton groups, with the simplifying assumption that zooplankton biomass followed a forced time series. Further developing and testing of ecosystem-level considerations can be achieved with end-to-end ecosystem models, such as the Atlantis models applied here, which have the added benefit of tracking the follow-on effects of the harvest control rule on the broader ecosystem