Impacts of nutrients on saltmarsh: a rapid evidence assessment. Natural England Evidence Report NEER152

Perring, Michael P. ORCID: https://orcid.org/0000-0001-8553-4893; Harley, Joanna ORCID: https://orcid.org/0000-0003-3861-9127; Jones, Laurence ORCID: https://orcid.org/0000-0002-4379-9006; Burden, Annette ORCID: https://orcid.org/0000-0002-7694-1638; Garbutt, Angus ORCID: https://orcid.org/0000-0002-9145-9786. 2023 Impacts of nutrients on saltmarsh: a rapid evidence assessment. Natural England Evidence Report NEER152. Natural England, 78pp.

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Abstract/Summary

•Context: Natural England (NE) wish to understand the impacts of nutrients on saltmarsh vegetation so as to complement existing biological indicators of water quality, primarily to understand how water quality relates to the condition of saltmarsh habitat. This will allow NE to assess features at an appropriate scale. The response of saltmarsh plant species and vegetation communities to increased nutrient availability, not only in relation to water quality, is expected to be context dependent. Nutrients can have complex impacts on systems depending on their source, the species present, and through multiple trophic levels. Response to nutrient enrichment may also depend upon biotic and other abiotic factors. Feedbacks and interactions among these different ecosystem properties can further complicate vegetation response to nutrients. Here, we report the results of a rapid evidence assessment (REA) conducted on behalf of NE by UKCEH. The aim was to explore and interpret the complexity of saltmarsh response to nutrients through synthesizing relevant literature. This will aid NE in its desire to use feature-based assessments, while exposing potentially important knowledge gaps. •Objectives: The REA addresses 3 objectives in the context of species and vegetation communities found, or likely to be found, in UK saltmarsh systems: Objective 1: Collate the evidence on the impact of nitrogen (N) and phosphorus (P) compounds on the condition of typical saltmarsh species present in the UK marine environment Objective 2: Collate the evidence for environmental (abiotic) factors that can affect the impact of elevated nutrients on saltmarsh species present in UK waters Objective 3: Collate the evidence for ecological (biotic) factors that can affect the impact of elevated nutrients on saltmarsh species present in UK waters •Key findings: Objective 1 There was robust evidence (i.e. multiple peer-reviewed sources) that nutrients, particularly N, altered the species composition of different marsh zones in the European context, with both ‘winner’ and ‘loser’ species. However, the identity of these species could depend upon abiotic and biotic context. In general, there was robust evidence that Atriplex portulacoides and Elymus athericus showed positive (or at least neutral) responses to increased nutrient availability. Species characteristic of upper to mid-marsh tend to decrease with additional N, while combined high N and P addition sped up succession through increased biomass of late successional species. There was limited evidence, from a non-UK context, that “saltmarsh squeeze” may be induced by additional nutrients: the pioneer zone can be smothered by Ulva species, and the upper marsh can be invaded by Phragmites australis. However, such responses depend upon context and may not be directly related to nutrient addition. We did not assign critical levels of nutrient concentrations in the marine environment, or nutrient fluxes onto the marsh from terrestrial/atmospheric sources, at which compositional change occurred. This was due to lack of data, with studies not reporting nutrient additions, forms, and/or background availabilities, and/or conflicting results for particular species. Objective 2 Few studies directly showed clear interactive effects in relation to nutrients and abiotic factors. Sediment type, salinity, deposition/accretion, temperature and pH appeared to co-vary with N and differences in vegetation communities. Relationships between salinity, P and vegetation was shown by multiple studies, including through residence time. Objective 3 Some evidence showed grazers can change which species are dominant for a given set of environmental conditions, not always including nutrients. Dynamics of competing autotrophs e.g. Ulva spp. and Enteromorpha spp. at the pioneer zone appear more related to turbidity/bed stress as opposed to interactions with nutrients. •Caveats, knowledge gaps and future research: Some studies did not directly measure nutrients, referencing other work and/or using the vegetation itself to indicate environmental conditions. We did not follow-up such references. Follow-up may reduce uncertainty in the absolute and relative importance of nutrients in driving vegetation change, in relation to other factors e.g. sea level rise. There is a lack of UK research on how environmental changes (e.g. in drought, temperature) could interact with nutrients and alter saltmarsh feature dynamics. There is a lack of evidence on how saltmarsh condition, life-stage, and surrounding vegetation affect response to nutrients. There is a clear need for: Experimental and survey studies at appropriate scales in UK marshes to understand saltmarsh species’ and vegetation community response to nutrients (forms and amounts). This includes studies that consider how multiple trophic levels and marsh stability are affected directly and indirectly by increased nutrient availability over short/long timescales; Research to quantify nutrient drivers of change in saltmarsh ecosystems through consideration of all input pathways. This includes concentrations in the marine environment, and fluxes from terrestrial habitats and the atmosphere. Linked to this, there is a need to develop a metric of nutrient pressure that can account for different nutrient inputs to saltmarsh plants and vegetation communities.

Item Type:	Publication - Report (Project Report)
UKCEH and CEH Sections/Science Areas:	Environmental Pressures and Responses (2025-) Surface Atmosphere Interactions and Effects (2025-)
Funders/Sponsors:	Natural England
Additional Information. Not used in RCUK Gateway to Research.:	Open Access report - full text available via Official URL link.
Additional Keywords:	annex 1 habitat, atlantic salt meadows, coastal habitats, context dependency, marine terrestrial interface, nitrogen, phosphorus, salicornia, salt marsh, sediment quality, spartina, water quality
NORA Subject Terms:	Ecology and Environment Botany
Date made live:	20 Jan 2025 11:29 +0 (UTC)
URI:	https://nora.nerc.ac.uk/id/eprint/537621

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