Assessment of seagrass health in response to water quality in the Solent & Poole Harbour

Background to Project

Concerns around water quality (elevated nutrients) in our estuaries is currently leading to agencies to seek for improvements to agriculture, wastewater treatment works and nutrient neutrality in development.

Environment Agency data for Dissolved Inorganic Nitrogen, phytoplankton and opportunistic macroalgae covering littoral sediment, is currently used a proxy indicator to assess the water quality-nutrients attribute for intertidal and subtidal seagrass. Evidence is currently largely based on the impacts on mudflats from opportunistic macroalgae. However, there is concern that this current indicator of water quality is not taking account sufficiently the wider ecosystem effects of elevated nutrients in our estuaries. Water quality sampling can vary greatly throughout the year, assessments of these using these indicators are not always close to where the Zostera beds are present and there is literature that suggests seagrass health can be affected at lower nutrient concentrations than indicated by high densities of macroalgae over littoral sediment.

Throughout the Solent and Poole Harbour there have been significant declines in the extent of seagrass and some loss at least has been linked to eutrophication through the smothering of plants by macroalgae. Moreover, wider literature highlights that the health of seagrass can be impacted directly through an increase in indicators of poor health e.g. incidence of Labyrynthula infection and excessive epiphytic growth, with indications that this effect occurs at lower nutrient loadings than that leading to dense macroalgae biomass on mudflats.

Despite these effects of elevated nutrients on seagrass being highlighted as a concern in global literature little in the way of studies have been undertaken in the UK. This project proposes a study of the seagrass in estuaries in the Solent and Poole Harbour area over Summer 2021 in order to better understand the relationship between the health of seagrass and elevated nutrients in the Southern estuaries and whether a more suitable indicator can be found that represents how water quality - nutrients is affecting seagrass.

With respect to the 'health' of seagrass (and depending on budget) the project may also include an assessment of carbon storage through analysis of sediment core samples for carbon. There is some evidence that in eutrophic waters the capability of seagrass to store carbon within their sediment is impaired (Lima et al, 2020) and their function as a blue carbon habitat diminished.