This project is structured around the concept that lagoons in good conservation status are dominated by benthic macrophytes rather than phytoplankton blooms.
One of the largest lagoons in Ireland, Lady’s Island Lake, Co. Wexford (350ha) has been shown to be also, one of the most eutrophic lagoons in the country. This state contrasts with the situation of Lady’s Island in the 1980s when clear water was matched by a well-developed benthic sward of Widgeon grass (Ruppia sp.) and charophytes (Healy, 1997). There is little doubt that the lagoon ecosystem switched from a benthic dominated habitat to one dominated by plankton (largely Cyanobacteria) sometime between the 1980’s and the first decade of the 21st C., probably due to excessive inputs of N and P.
As Lady’s Island is a designated SPA and also an SAC for which the Annex I habitat Coastal Lagoon is a qualifying interest. Coastal lagoons are a priority habitat under the Habitats Directive and as a coastal water body it is also protected under the WFD. The Government is obliged under EU law, to restore the site to favourable conservation status. At present, it is unclear what measures must be undertaken to achieve this purpose.
In this project we propose to describe nutrient inputs, cycling and phytoplankton growth, sediment accumulation and chemistry and devise a hydrodynamic model of the lagoon. The results of this research will allow us to accurately describe the origin, quantities and fate of N and P in the lagoon and to investigate how proposed management strategies would affect nutrient supply and salinity. A small saline pond at the neighbouring lagoon site of Ballyteige, which is in a clear water state will be used as a comparison site.
In order to investigate what amount of nutrient loading will result in a shift from macrophyte to phytoplankton dominance we will first use existing lagoon data to redefine Ecological Quality Ratios (EQRs) and nutrient loading. An estimate of the reduction in nutrient loading needed to achieve the proposed values for good or high status can then be made, based on current and proposed winter values of N and P in Lady’s Island. We will also make a box model of N and P flows in Lady’s Island, allowing us to calculate the relationship between N and P inputs, losses, and N and P available for plankton growth. A simple quantitative model will be developed which relates plankton growth to reduction in light availability to macrophytes through shading. This model will help define the conditions which lead to a switch from macrophyte to phytoplankton dominance. We will then review what measures are available to achieve these changes. We will attempt to quantify the effectiveness of techniques already in use in Ireland such as artificial wetlands or nutrient buffer zones and the impact of agri-environment schemes that encourage reduced fertiliser inputs. It will also be necessary to review techniques used abroad to manage lagoon eutrophication and assess their applicability to Lady’s Island.
The initial outcome of the project will be a detailed plan to restore the Lady’s Island site. The project will also result in the production of a manual documenting methods to quantify excessive nutrient inputs and impacts of lagoon salinity change and flushing rates on conservation status for all Irish lagoons. The manual will outline a suite of techniques to restore environmentally degraded lagoons.