Experimental addition of marine-derived nutrients affects wildflower traits in a coastal meta-ecosystem

Organismal movement can bring individuals, resources and novel interactions across ecosystem boundaries and into recipient habitats, thereby forming meta-ecosystems. For example, Pacific salmon ecosystems receive large marine-derived nitrogen subsidies during annual spawning events, which can have a wide range of effects on aquatic and terrestrial plant species and communities. In this study, we evaluate the effects of cross-ecosystem nutrient subsidies on terrestrial plant growth and reproduction. We conducted a large-scale field experiment with four treatments: (i) addition of a pink salmon (Oncorhynchus gorbuscha) carcass, (ii) addition of the drift seaweed rockweed (Fucus distichus), (iii) addition of both salmon + rockweed, and (iv) a control. We examined treatment effects on leaf nitrogen and fitness-associated floral traits in four common estuarine wildflower species. We found elevated leaf ∂¹⁵N in all plant species and all sampling years in treatments with salmon carcass additions but did not observe any differences in leaf per cent nitrogen. We also observed larger leaf area in two species, a context-dependent increase in floral display area in two species, and a limited increase in plant seed set in response to both salmon carcass treatments. In sum, our study suggests that marine nutrients can affect terrestrial plant growth and reproduction.

Accounting for direct and indirect cumulative effects of anthropogenic pressures on salmon- and herring-linked land and ocean ecosystems

Salmon and herring support both land and ocean predators and are critical to ecosystem resilience. Their linkages across land and sea realms make them highly susceptible to human activities, which can have flow-on effects up the food web. We quantify and compare the potential cumulative effects of human-driven pressures on interdependent species in salmon- and herring-linked ecosystems of western Canada using a risk assessment methodology. Adding indirect risks resulted in 68% greater total risks for land species than for direct risk alone, versus 15% for marine species. Inclusion of climate change pressures resulted in the greatest change in risk for low trophic marine species and habitats (greater than 25% increase). Forestry-related pressures accounted for the highest risk to all species and projected management of these pressures resulted in a total reduction of risk across all ecosystem components that was more than 14% greater than management of fisheries pressures. Ignoring land food web linkages and pressures underestimated cumulative risk by more than 40% for salmon and herring. This simple framework can be used to evaluate potential risk of existing human uses and future change to inform immediate management of linked land-sea ecosystems and help species avoid the ‘death by a thousand cuts'.

This article is part of the theme issue ‘Nurturing resilient marine ecosystems’.

Relationships between Pacific salmon and aquatic and terrestrial ecosystems: implications for ecosystem-based management

Pacific salmon influence temperate terrestrial and freshwater ecosystems through the dispersal of marine‐derived nutrients and ecosystem engineering of stream beds when spawning. They also support large fisheries, particularly along the west coast of North America. We provide a comprehensive synthesis of relationships between the densities of Pacific salmon and terrestrial and aquatic ecosystems, summarize the direction, shape, and magnitude of these relationships, and identify possible ecosystem‐based management indicators and benchmarks. We found 31 studies that provided 172 relationships between salmon density (or salmon abundance) and species abundance, species diversity, food provisioning, individual growth, concentration of marine‐derived isotopes, nutrient enhancement, phenology, and several other ecological responses. The most common published relationship was between salmon density and marine‐derived isotopes (40%), whereas very few relationships quantified ecosystem‐level responses (5%). Only 13% of all relationships tended to reach an asymptote (i.e., a saturating response) as salmon densities increased. The number of salmon killed by bears and the change in biomass of different stream invertebrate taxa between spawning and nonspawning seasons were relationships that usually reached saturation. Approximately 46% of all relationships were best described with linear or curved nonasymptotic models, indicating a lack of saturation. In contrast, 41% of data sets showed no relationship with salmon density or abundance, including many of the relationships with stream invertebrate and biofilm biomass density, marine‐derived isotope concentrations, or vegetation density. Bears required the highest densities of salmon to reach their maximum observed food consumption (i.e., 9.2 kg/m² to reach the 90% threshold of the relationship’s asymptote), followed by freshwater fish abundance (90% threshold = 7.3 kg/m² of salmon). Although the effects of salmon density on ecosystems are highly varied, it appears that several of these relationships, such as bear food consumption, could be used to develop indicators and benchmarks for ecosystem‐based fisheries management.

Salmon nutrients are associated with the phylogenetic dispersion of riparian flowering-plant assemblages

A signature of nonrandom phylogenetic community structure has been interpreted as indicating community assembly processes. Significant clustering within the phylogenetic structure of a community can be caused by habitat filtering due to low nutrient availability. Nutrient limitation in temperate Pacific coastal rainforests can be alleviated to some extent by marine nutrient subsidies introduced by migrating salmon, which leave a quantitative signature on the makeup of plant communities near spawning streams. Thus, nutrient-mediated habitat filtering could be reduced by salmon nutrients. Here, we ask how salmon abundance affects the phylogenetic structure of riparian flowering plant assemblages across 50 watersheds in the Great Bear Rainforest of British Columbia, Canada. Based on a regional pool of 60 plant species, we found that assemblages become more phylogenetically dispersed and species poor adjacent to streams with higher salmon spawning density. In contrast, increased phylogenetic clumping and species richness was seen in sites with low salmon density, with steeper slopes, further from the stream edge, and within smaller watersheds. These observations are all consistent with abiotic habitat filtering and biotic competitive exclusion acting together across local and landscape-scale gradients in nutrient availability to structure assembly of riparian flowering plants. In this case, rich salmon nutrients appear to release riparian flowering-plant assemblages from the confines of a low-nutrient habitat filter that drives phylogenetic clustering.